Published by João Reis, Department of Economics, Management, Industrial Engineering and Tourism, GOVCOPP, Aveiro University, Campus Universitário de Santiago, 3810-193, Aveiro, Portugal. Email: reis.joao@ua.pt
Abstract: In the last years, most European countries have developed strategies to implement the use of electric vehicles. This paper uses a qualitative case study research in the automotive industry to evaluate the efficiencies concerning the implementation of the first electric light truck produced for public services. The results indicate that electric light trucks in public services are more efficient, economically reliable and contribute to the reduction of carbon dioxide emissions. Moreover, the strategy of using these vehicles is suitable for nocturnal collection of urban waste, to the extent that it reduces the daily traffic and, at the same time, drastically reduces the noise caused by diesel engines during night hours, thus, improving the quality of life on residential areas. By investing on such strategy, European governments are giving a step further to accomplish the European Commission requirements, which is stimulated by the reduction of the carbon dioxide footprint.
Keywords: electric vehicles; electric light trucks; case study; renewable energies; carbon dioxide; efficiency; public services.
1. Introduction
Vehicles moved by renewable energies are urgently needed in Lisbon, Portugal, due to the requirements imposed by the European Commission to reduce the carbon dioxide emissions, but also to decrease the economic impact of regular activities, e.g., the collection of urban wastes. However, the implementation of electric vehicles is not straightforward, although electric vehicles are commercially available, they are still not welcomed by most users mainly because of battery limitations (Høyer, 2008). The high prices of electric vehicles (EVs) are attributed to their expensive green components, as the battery is typically the most expensive component, which almost takes up 30–40% of the entire production cost of these vehicles (Fu et al., 2018). Therefore, a study conducted by Delang and Cheng (2013) has revealed that citizens from China, one of the most developed economies, recognise the positive environmental, economic and social benefits that electric vehicles bring but the aforementioned citizens also prefer not to purchase these vehicles due to the high costs. According to several studies (Black, 2000; Delucchi and Lipman, 2001; Weinert et al., 2008) the price of electric cars is higher than the internal combustion engine vehicles (ICEVs), again due to the high cost of the batteries, large investment involved on the research and the small numbers of electric vehicles that are still produced. However, these limitations are not consensual. An example of this is the United States (USA), where Xie et al. (2018) claim that the promotion of these electric vehicles is considered an effective solution helping the country reducing its dependency on imported oil and also allows to improve its competitive position in the emerging era of the renewable energy market. The empirical evidence of such strategy is visible by the sales of Nissan Leaf and the plug-in hybrid Chevy Volt of General Motors, which were introduced in the US market (Fu et al., 2018).
The electric vehicles in public services are a relevant discussion theme, well-noticed by the increasing of sales and production, which has aroused the interest of the academic community. This interest is also verified by the increasing promotion of special issues on hot topics, such as, alternative fuel vehicles (AFVs) or electric vehicle batteries, in order to build and promote scientific breakthroughs. Some of these examples are from top tier journals, that have recently opened call for proposals: (1) the Journal of Transportation Research Part D, has opened recently a call for paper with the topic “Advances in Alternative Fuel Vehicles”; and/or the (2) the Journal of Energy Storage, with the special issue “Second Life of Electric Vehicle Batteries in Stationary Applications”.
Currently, all major foreign manufacturers and suppliers have located themselves in China during the last few years, alongside a large number of Chinese automotive manufacturers, and are already focusing their sourcing in particular on global markets (Proff, 2012). However, some manufacturers are changing this tendency, as an international corporation, responsible for the commercialisation of electric vehicles i.e. full electric-powered light trucks, started the production at the original equipment manufacturer (OEM) in Portugal. Currently, the Portuguese OEM is producing electric light trucks called eTruck, which will become relevant to the Portuguese economy, since that country is going to export these vehicles worldwide and allows to expand market shares despite the increasing Chinese competition. The corporation has recently started the project in six cities around the world, which are: Lisbon, London, Berlin, Amsterdam, New York and Tokyo. Lisbon municipality has acquired 10 units for public services purposes (e.g., collection of urban waste) before acquiring a larger volume of units. At this time, the eTruck is a pre-series production vehicle, which is expected to achieve the full production and availability to the markets in 2019–2020 to embrace sustainable and reliable transport solutions.
Motivated by the above, this study focuses on the urban waste collection in Lisbon, due to its exponential increase in tourism in the last years. Consequently, the municipality has been facing an accelerated growth of urban waste. By studying the pros and cons a question remains: Are the electric light-duty trucks a reliable alternative to public services?
This paper is structured as follows: first, the author has reviewed the literature by presenting the state of the art; second, the author describes the methodological approach; third, it discusses the results regarding the studied real-life phenomenon; finally, it provides conclusions, implications and suggestions for future research.
2. Literature review
This section addresses the theoretical background, which the author considers essential to understand the phenomenon.
2.1 The scope of electric vehicles
In the West, the ambiguous term electric vehicles (EVs) is commonly used and mostly associated with battery EVs (BEVs) (Chen and Midler, 2016). The EVs are classified into three major categories by their fuel consumption technology: hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs), while both PHEVs and BEVs are also referred to as plug-in electric vehicles (PEVs), since they are designed to be recharged by plugging into the power grid (Zhu et al., 2018). Hence, Hybrids (HEVs) are vehicles with an electric drive system and an internal combustion engine running on either petrol or diesel (Milowski et al., 2018), while PHEVs are vehicles equipped with at least two energy sources to propel them (Xu et al., 2018), usually by combining electric and conventional propulsion (Plötz et al., 2018) and BEVs are considered vehicles that operate on batteries that have a limited life as well as specific charging and discharging patterns (Pelletier et al., 2017). In short, the new generation electric vehicles (EVs) are likely to become increasingly popular for city travellers and are expected to feature prominently in ‘Smart Cities’ of the future (Milowski et al., 2018). Although there are other conceptualisations, Mahmoudi et al. (2014) corroborates the earlier definitions and argues in favour of a broader classification, accordingly the degree of electrification (Figure 1).
Figure 1. Degree of electrification
Mahmoudi et al. (2014) defines each degree as:
BEV, uses high capacity batteries and electric motor for propulsion
HEV uses mechanically a combination of electric motor (EM) in low speeds dedicated for in-city traffic and a conventional internal combustion engine (ICE) for use outside urban areas
REEV are range extended electric vehicles, which are vehicles in which the propulsion is driven only by an electric motor powered by high capacity batteries
FCEV (fuel cell electric vehicle) introduced to perform long distances because it uses a fuel cell system to power its on-board electric motor
SEV are solar electric vehicles, which are directly or complementarily powered by direct solar energy.
Similar to other research papers (e.g., Chen and Midler, 2016), the author of this paper ended up on focusing on the EVs, namely the BEVs. He did not consider other fuel sources, such as the fuel cell EVs (FCEVs), due to the low maturity of this technology and its demanding supporting infrastructure in terms of cost, when compared with BEVs. According to Bansal (2005), the EVs have a much longer history than most people realise, since this technology was mentioned soon after Joseph Henry introduced the first DC-powered motor in 1830 and the first known first electric car was a small model built by Professor Stratingh in the Dutch town of Groningen in 1935 (Bansal, 2005). Bansal (2005) also states that, the first EV was built by in 1834 by Thomas Davenport in the USA, but there were no rechargeable electric cells batteries at that time, the EV did not become a viable option until the Frenchmen Gaston Plante and Camile Faure respectively invented and improved the storage battery.
2.2 The strategic implementation of electric vehicles
In the last years the urban transport has raised specific issues and brought attention to political, social and environmental prejudice of pollution, noise and stress (Racicovschi et al., 2007; Schiffer and Walther, 2018). Racicovschi et al. (2007) stressed that, the European Union (EU) is encouraging scientific and technological research activities to develop clean and efficient transport, mainly by incorporating limits to greenhouse effect with efficient solutions as the EVs. In addition, some EU countries are encouraging a sustainable market for EVs, by combining high taxes on high emissions and zero tax for zero emissions vehicles (ZEVs). Due to this practice, Norway has the highest number of electric vehicles per capita in the world by achieving in January 40,000 electric vehicles in a country of 5 million inhabitants (Haugneland and Hauge, 2015).
Although emissions do result from fossil fuelled generation of electricity, these emissions are removed in both space and time from the point of operation of a EV (Santini, 2011). In Portugal, the Portuguese Association of Renewable Energies (APREN) argue that, from January to August 2018, renewable energy sources have played a leading role in the electricity production, contributing with 55.3% to the total electricity generated (37,451 Gigawatt hours).
The results from Figure 2 are mainly driven by the availability of renewable resources, such as aeolian and hydraulic energy. Although the coal (18.62%) and natural gas (17.72%) are still relevant, to our best understanding, the overall result is still positive, since the production of renewable energy surpasses half of the national production of energy.
Figure 2. Renewable and fossil energy sources (see online version for colours)
2.3 European regulation for carbon dioxide emissions (CO2)
Cities consume over two-thirds of the world’s energy and account for more than 70% of global CO2 emissions (Kuppusamy et al., 2017). A salient characteristic of EVs is their cleaner environmental impacts relative to conventional fuel vehicles, as they have either zero or much less tailpipe greenhouse gas (GHG)/CO2 emissions than conventional cars (Manjunath and Gross, 2017). As EVs have no combustion engine, there are no oil changes, tune-ups, or timing and there is no exhaustion (Bansal, 2005). The EU legislation sets mandatory emission reduction targets for new vehicles. According to the European Commission, cars are responsible for around 12% of total EU emissions of CO2, the main greenhouse gas, as 2021 targets represents a reduction of 40% compared with the 2007 fleet average of 158.7 grams of CO2 per kilometre (COM, 2014). On 8 November 2017, the European Commission presented a legislative proposal setting new CO2 emission standards for passenger cars and light commercial vehicles in the EU for the period after 2020, the proposal also includes a mechanism to incentivise the up-take of zero- and low-emission vehicles, in a technology-neutral way (COM, 2017). The objective of this proposal will contribute to the achievement of the EU’s commitments under the Paris agreement. According to the European Commission the CO2 greenhouse gas most commonly produced by human activities is responsible for 64% of man-made global warming, a part of the causes for rising emissions are due burning coal, oil and gas which produces carbon dioxide and nitrous oxide (EC, 2018). The prospect of reducing CO2 emissions has also raised the obligation of European governments to embark on substantial programs to achieve a cleaner mobility. In particular, the Portuguese government is following that trend, by investing on upstream renewable energy sources that allows recharging of existing public service electric vehicles downstream.
3. Methodology
This paper reports on the results of a case study carried out in a Portuguese OEM. The objective was to understand the pros and cons of the implementation of electric vehicles to achieve clean practices of waste collection. This case study research uses multiple sources of data collation for triangulation purposes. The sources of data collection consisted of semi-structured interviews, direct observation and documental analysis. The author decided about using multiple sources of data collection as a form of triangulation to prevent exclusive reliance on a single data collection method and thus, aid to neutralise any bias inherent to a particular data source (Given, 2008).
The study builds on 11 semi-structured interviews conducted with employees from the OEM that is operating in Portugal and seven semi-structured interviews with the drivers of the eTrucks. The researcher made use of his personal contacts network to identify the respondents who were best positioned to provide answers to the interview protocol. Convenient and snowball sampling was also used to select the respondents, as well as the recommendations and directions from the respondents of the first round of interviews. Prior to the interviews, the author obtained the participants’ consent, which included the consent for audio recording. The direct observation involved observations to enable a better understanding about the real life phenomenon, thus, these observations were recorded in a research diary, that had notes from random visits to the sites and informal discussions about the company strategy and the successful of such implementation. The institutional documents were generally produced by the company for communication or record-keeping purposes and were sources of exceptional data collection (Mills et al., 2010), because most of the records were available at the official website.
The data analysis was examined according to the technique of content analysis, which is frequently recommended by scholars when the case research is qualitative by nature (Mills et al., 2010). The author categorised the transcript into codes and categories in order to identify patterns and relations between variables with the help of a qualitative data analysis software (NVivo11). The reliability and validity of the case study was achieved by a well-designed interview protocol and improved by double-checking the transcripts with the participants in order to avoid misinterpretations.
4. Results and discussion
This section provides an empirical summary of the case study research, presenting the pros and cons of implementing electric light trucks in European capitals for an effective public service with reduced CO2 footprint.
4.1 Environmentally friendly policy
As an EV, the eTruck reduces the impact of exhaust and noise emissions on city centres, while it is also environmentally attractive when compared to diesel engines.
The environmental impact has been widely discussed in the academic literature, but no consensus has been reached yet. The papers of Zivin et al. (2014) and Yuksel and Michalek (2015) describe the impact of temperature on EVs’ efficiency, range and emissions, while McLaren et al. (2016) considers different charging scenarios and travel profiles of EVs and analyses their impact on the associated carbon emissions. However, there are a lot of questions that still remain open and one of them is “what happens at the end of the lifecycle of a battery and an electric motor?”, as the number of sold electric vehicles will increase the amount of electric motors and battery waste will increase too, leading to a greater impact on the environment (Racz et al., 2015).
On the other hand, current research shows that EVs may have a role in reducing air pollution and its consequences for health. The study “How clean are electric vehicles? Review of the environmental and health effects of electric mobility” conducted by Requia et al. (2018) provides a comprehensive review of the effects of EV adoption on air quality, greenhouse gas emissions, and human health. Requia et al. (2018) have resumed relevant published papers, from up to 4734 studies, mostly carried out in the USA and China, out of which 65 papers fulfilled the inclusion criteria, showed consistently reductions in greenhouse gas emissions and emissions of some criteria pollutants. The respondents of our case study also emphasised the eco-friendliness of the eTruck, which is expected to become one of the most important criteria to the Portuguese public administration acquisitions. The efficiency of electric light trucks fleet allows the possibility to balance the electricity usages rates and the electricity demand, since some of these vehicles are operating during the night hours. Moreover, the Portuguese government provides dedicated infrastructures of easy access to fast charging that reduces the eTruck downtime.
4.2 Economically reliable alternative to diesel engines
Frequently, the fuel used by electric trucks is cheaper, when compared with ICEVs (Delang and Cheng, 2013). However, it is dependent on the weather conditions and low temperatures (0°C or below), which can have negative impacts on the battery’s performance. This creates ineffectiveness and increases on-road energy consumption, which further limits the vehicle’s range (Kim et al., 2008). Due to the temperate climate of Portugal, the aforementioned limitations are not observed, and do not have a real impact on the use of electric vehicles.
The limitations of Lisbon are quite the same as other cities around the world (e.g., Hong Kong), due to its compact and dense streets, most of trips are short in distance; therefore, the short range of electric vehicles (mostly within the 100–200 km range) would not represent a problem to the needs of public services (Delang and Cheng, 2013). Consequently, electric light vehicles might be a reliable choice, since the other forms of EVs (e.g., hybrid electric vehicles), as motor vehicles, are more credible to long distances (Racicovschi et al., 2007).
The eTruck has a range of 100 kilometres and load capacity up to three and a half tons. This light-duty truck contains up to six high voltage lithium ion batteries of 13.8 kWh each. According to the respondents, the batteries are charged with regenerative braking and deceleration which reduces the battery consumption. In addition, reports and direct observation have showed that, in comparison to diesel engines, the Portuguese public administration will have 30% lower maintenance costs and potential fuel savings of 1000 euros per 10,000 km (estimated values).
The cons the author has recorded are associated to acquisitions costs because of the low production, since the eTruck is still being in pre-test. On the other hand, for the most sceptical respondents, the autonomy of these vehicles can always be optimised with the research and development (R&D) of more efficient batteries and green technologies. These findings are in line with the literature, as scholars argue that the relatively high costs of purchase and low durability of key components (e.g., batteries) are significant barriers to the wider use of electric vehicles (Delucchi and Lipman, 2001; Schiffer and Walther, 2018). Besides major environmental advantages, the EVs have not yet managed to penetrate massively into the car market and convinced possible users (Racicovschi et al., 2007), despite the limitations the Portuguese government is trying to push and accelerating the adoption of AFVs as ecological alternatives.
4.3 Improving the quality of life in capital cities
To improve the quality of life in Lisbon, the eTruck is identified by the respondents as the most suitable vehicle to collect urban residues. The eTruck mainly works during night hours and it can quietly travel the city without disturbing the local inhabitants. Despite the mentioned advantage, the respondents had also identified some constraints:
The eTruck cargo boxes are traditionally made of metal, and are the same as the combustion trucks, which are not zero-noise efficient; therefore, unless these boxes are adapted to keep a silent propulsion, the EV zero-noise advantage will be lost or reduced.
The eTruck also requires an independent front suspension to improve the driving quality, as well as a greater cushioning in the cabin. The eTruck drivers stated the manufacturer should take into account that the city of Lisbon has many areas where the road surface is in poor condition, and this affects the driver experience. While this truck is in pre-test and before going to production on large scale, these limitations should be taken in consideration.
Studies also refer that drivers of EVs experience range anxiety or worry about the limited driving range of these vehicles (Eberle and Von Helmolt, 2010). To avoid the driver range anxiety, the instrument panel of the eTruck only shows the percentage of charging of the batteries, whether if the drivers need to know how many kilometres are left, they have to access the central screen and select the vehicle information.
Besides the zero-noise, the issue of poor air quality is also a pressing problem in many urban areas as it directly affects the health of people and as a result the life expectancy of citizens (Quak et al., 2015). At long term, despite the mentioned constraints, the respondents agreed the electric light trucks will definitely contribute to a reduction of the poor air quality that is felt in Lisbon area, and consequently will have a positive impact on the environment.
4.4 Pros and cons of implementing electric light trucks at public services
This section addresses the motivations which are driving the implementation of electric light trucks at public services. Table 1 identifies the pros and cons, and discusses possible solutions to constraints that were scholarly or empirically identified.
Table 1. Pros and cons of light-duty trucks in public services
.
The novelty of these results brings back solutions to old EVs’ problems, which is the same to say that the eTruck already integrates solutions that addresses issues previously raised in the literature. One of the most relevant finds is related to the improvement of the quality of life, either by improving the breathable air quality or by the releasing of zero-noise emissions. On the other hand, the introduction of EVs improves the Portuguese public administration, since the EVs are cost-efficienct, but it might also incentivise private markets to follow the same ecological practices. However, the author of this paper has doubts regarding the governmental support for the private practices, as better strategies and implementation plans might be needed, similarly to a study conducted in the UK, that revealed ineffectual strategies regarding the EV uptake and infrastructure provision (Heidrich et al., 2017).
Briefly, the paper shows that the implementation of electric light trucks is suitable to improve the efficiency of public services, although there are still some gaps listed in this paper that needed to be solved, among others that were not investigated.
5. Conclusions
The Portuguese government has implemented a comprehensive strategy to push the use of zero-emissions vehicles in some public services. The results suggest that electric light trucks are cost-efficient, zero-noise and eco-friendly (zero-emissions). This study has both theoretical and managerial insights, as we look at the emergence of dominant electric trucks on public services and we provide the most relevant pros and cons of such implementation.
As this study is suitable to build a better understanding of a real-life phenomenon, it also points out some limitations: The ‘eTruck’ term is fictitious, since the author decided not to explicitly reveal the identity of the manufacturer and the respective participants. The reason that led us to undertake an agreement of confidentiality is due the respondents’ reluctance to discuss the disadvantages of the eTruck. A limitation that was mitigated with direct observations and documental analysis, which corroborated the results the author has found from the formal interviews. In the same vein, this case study research does not allow generalisation; however, the author believes the ongoing projects at the aforementioned capital cities might provide some additional and relevant contributions.
The author also suggests avenues for further research, as it would be interesting to carry out a similar study in other countries for corroboration purposes. Thus, an important research area is the development of a better understanding of the topic, since this research is limited to its exploratory nature. Besides yielding a better understanding about the state-of-the-art of new automobile trends, this paper aims at helping guide future policy and planning towards the introduction of electric vehicles in public services.
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Reference to this paper should be made as follows: Reis, J. (2019) ‘Implementing electric vehicles in public services: a case study research’, Int. J. Electric and Hybrid Vehicles, Vol. 11, No. 3, pp.205–216.
Biographical notes: João Reis is an Assistant Professor of Service Management (Aveiro University), Operations Management (ISLA-Santarém) and Supply Chain Management (Portuguese Military Academy). He received a PhD in Management and Industrial Engineering from Aveiro University. During the last 10 years, he has been conducting extensive scientific research in the service industry. He is a research fellow at the research unit on Governance, Competitiveness and Public Policy (GOVCOPP) in the fields of service science, industrial engineering and operations management. During his free time, he frequently performs humanitarian aid in Africa, Asia and around the South-eastern Europe.
Published by Jose Mendi, EE Power – Technical Articles: Power Quality Monitoring Part 1: The Importance of Standards-Compliant Power Quality Measurements, May 17, 2023.
This article discusses the importance of power quality (PQ) measurements in today’s electric infrastructure and reviews areas of application for PQ monitoring. It will cover the IEC standard for power quality and its parameters. Finally, it summarizes the key differences between Class A and Class S power quality meters. Part 2 will illustrate recommended solutions on how to design a standards-compliant power quality meter.
Power quality (PQ) has found a renewed interest due to changing power generation modes and consumption dynamics. The unprecedented growth in renewable sources at different voltage levels has increased the amount of PQ-related issues. Consumption patterns have also seen a wide transformation due to unsynchronized loads added at multiple entry points of the grid and voltage levels.
The Need for Power Quality Measurement in Today’s Electric Infrastructure
Some examples are electric vehicle (EV) chargers that can require hundreds of kilowatts and a great number of data centers and their related equipment, such as heating, ventilation, and air conditioning. In industrial applications, arc furnaces that run by variable frequency drives, switching transformers, etc., not only add a lot of unwanted harmonics to the grid but are responsible for voltage dips, swells, transient brownouts, and flicker
Figure 1. Power quality issues. Image used courtesy of Bodo’s Power Systems [PDF]
Power quality in the utility space refers to the quality of the voltage delivered to the consumer; a series of prescribed regulations for the magnitude, phase, and frequency determine this quality of service. However, by definition, it denotes both voltage and current. While the voltage is easily controlled by the generation side, the current is governed largely by consumer usage. The concept and implications of PQ issues are rather widespread depending on the end users.
The economic impact of bad PQ has been studied and surveyed extensively in the last few years; its effects are estimated to be in the region of billions of dollars worldwide.1 All these studies conclude that monitoring the quality of power has a direct impact on the economic results of many business sectors. Even though it is clear how bad PQ negatively affects the economics of business, monitoring it efficiently and effectively at scale is not an easy task. Monitoring PQ in a facility involves having highly trained personnel and expensive equipment installed on multiple points along the electric system for long or indefinite periods of time.
Power Quality Monitoring Applications
Power quality monitoring is often seen as a cost-saving strategy for some business sectors and a critical activity for others. Power quality issues can arise in a broad range of electric infrastructure, as illustrated in Figure 2. As we’ll discuss later, power quality monitoring is becoming increasingly critical in business sectors such as electric generation and distribution, EV charging, factories, and data centers.
Electricity Utility Companies, Electricity Transmission, and Distribution
Utility companies serve the consumers with distribution systems that include generating stations, which are power substations that supply electricity via transmission lines. The voltage supplied via these transmission lines is stepped down to lower levels by substation transformers, which inject certain harmonics or inter-harmonics into the system. Harmonic currents in distribution systems can cause harmonic distortion, low power factor, and additional losses as well as overheating in the electrical equipment2, leading to a reduction in the lifetime of equipment and increases in cooling costs. Nonlinear single-phase loads served by these substation transformers deform the current’s waveform. The unbalance of nonlinear loads leads to additional losses on power transformers, additional neutral loads, unexpected operation of low power circuit breakers, and incorrect measurement of electricity consumed.3 Figure 3 illustrates the effect of these linear loads.
Figure 2. The dynamics of generation and consumption can lead to power quality issues across electric infrastructure. Image used courtesy of Bodo’s Power Systems [PDF]
Electricity generation by wind and solar photovoltaic (PV) systems injected into the grid cause several power quality problems as well. On the wind generation side, wind intermittency creates harmonics and short-duration voltage variations.4 The inverters in PV solar systems create noise that can produce voltage transients, distorted harmonics, and radio frequency noise because of the high-speed switching commonly used to increase the efficiency of the energy harvested.
EV Chargers
EV chargers can face multiple power quality challenges, both in power sent to and from the grid (see Figure 4). From a power distribution company perspective, power electronics-based converters used in EV chargers inject harmonics and inter harmonics. Chargers with improperly designed power converters can inject direct currents (DC). Additionally, fast EV chargers introduce rapid voltage changes and voltage flicker into the grid. From the EV charger side, faults in transmission or distribution systems lead to voltage dips or interruptions of supply voltage to the charger. Reduction of voltage from the EV charger tolerance limits will lead to activation of undervoltage protection and disconnection from the grid (which leads to a very bad user experience).5
Figure 3. The impact of current harmonics generated by a nonlinear load. Image used courtesy of Bodo’s Power Systems [PDF]
Factories
Power quality problems caused by power supply variations and voltage disturbances cost approximately $119 billion (U.S.) per year for industrial facilities in the United States, as per an Electric Power Research Institute (EPRI) report.6 Additionally, 25 EU states suffer an equivalent of $160 billion (U.S.) in financial losses per year due to different PQ issues, according to the European Copper Institute.7 These figures are linked to subsequent downtime and production losses as well as the equivalent of intellectual productivity losses.8
Figure 4. Power quality issues for EV chargers. Image used courtesy of Bodo’s Power Systems [PDF]
Degradation of power quality is usually caused by intermittent loads and load variations from arc furnaces and industrial motors. Such disturbances give rise to surges, dips, harmonic distortions, interruptions, flicker, and signaling voltages.9 To detect and record these disturbances inside a factory installation, it is necessary to have power quality monitoring equipment at several points throughout the electric installation or, even better, have it at the load level. With the arrival of new Industry 4.0 technologies, power quality monitoring at the load can be addressed by industrial panel meters or submeters to have a comprehensive view of the quality of the power delivered to each load.
Data Centers
Presently, most business activities depend on data centers in one way or another to provide email, data storage, cloud services, etc. Data centers demand a high level of clean, reliable, and uninterrupted electricity supply. PQ monitoring excellence helps managers prevent costly outages and helps manage equipment maintenance, or replacement, required due to issues on the power supply units (PSU). The integration of uninterruptable power supply (UPS) systems into rack power distribution units (PDUs) represents another reason to add PQ monitoring to IT racks inside the data center. This integration can provide visibility to power issues at a power socket level.
UPS system failure, including UPS and batteries, is the primary cause of unplanned data center outages, according to a report made by Emerson Network Power.10 Around a third of all reported outages cost companies nearly $250,000.11 UPS systems are used on every data center to ensure clean and uninterrupted power. These systems isolate and mitigate most of the power problems from the utility side, but they do not protect against issues generated by the PSU of IT equipment itself. IT equipment PSUs are nonlinear loads that can introduce harmonic distortion in addition to other problems caused by equipment, such as those resulting in high-density cooling systems with variable frequency speed-controlled fans. Apart from these issues, PSUs also face interference that comes in multiple forms, such as voltage transients and surges, voltage swells, sags, and spikes, imbalance or fluctuations, frequency variation, and poor facility grounding.
Power Quality Standards Defined
Power quality standards specify measurable limits to the electricity magnitudes as to how far they can deviate from a nominal specified value. Different standards apply to different components of the electricity system. Specifically, the International Electrotechnical Commission (IEC) defines the methods for measurement and the interpretation of results of PQ parameters of alternating current (AC) power systems in the IEC 61000-4-30 standard. The PQ parameters are declared for fundamental frequencies of 50 Hz and 60 Hz. This standard also establishes two classes for measurement devices: Class A and Class S.
Figure 5. IEC power quality standards. Image used courtesy of Bodo’s Power Systems [PDF]
► Class A defines the highest level of accuracy and precision for the measurements of PQ parameters and is used for instruments requiring very precise measurements for contractual matters and dispute resolution. It is also applicable to the devices that need to verify standards compliance.
► Class S is used for power quality assessment, statistical analysis applications, and diagnostics of power quality problems with low uncertainty. The instrument in this class can report a limited subset of the parameters defined by the standard. The measurements made with Class S instruments can be done on several sites on a network, on complete locations, or even on single pieces of equipment.
It is important to note that the standard defines the measurement methods, establishes a guide for the interpretation of the results, and specifies the performance of the power quality meter. It does not give guidelines on the design of the instrument itself.
The IEC 61000-4-30 standard defines the following PQ parameters for Class A and Class S measurement devices.12
Power Frequency
• Magnitude of the supply voltage and current • Flicker • Supply voltage dips and swells • Voltage interruptions • Supply voltage unbalance • Voltage and current harmonics and interharmonics • Rapid voltage change • Underdeviation and overdeviation • Mains signaling voltage on the supply voltage
Figure 6. Classification of power quality parameters in a timescale. Image used courtesy of Bodo’s Power Systems [PDF]
Table 1. IEC 61000-4-30 Class A and Class S Key Differences.
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Differences Between Class A and Class S Defined by the IEC 61000-4-30 Standard
Although Class A defines higher levels of accuracy and precision than Class S, the differences are beyond just levels of accuracy. Instruments must comply with requirements such as time synchronization, quality of probes, calibration period, temperature ranges, etc. Table 1 presents a list of requirements that instruments shall meet to be certified in one or the other class.
Power Quality Summary
Power quality issues are present across the whole electric infrastructure. Having equipment that monitors these PQ issues helps to improve performance, quality of service, and equipment lifetime while reducing economic losses. In Part 2, “How to Design a Standards Compliant Power Quality Meter,” we will introduce an integrated solution and a ready-to-use platform that can significantly accelerate development and reduce costs for developing PQ monitoring products.
References
1. Panuwat Teansri, Worapong Pairindra, Narongkorn Uthathip Pornrapeepat Bhasaputra, and Woraratana Pattaraprakorn. “The Costs of Power Quality Disturbances for Industries Related Fabricated Metal, Machines and Equipment in Thailand.” GMSARN International Journal, Vol. 6, 2012. 2. Sai Kiran Kumar Sivakoti, Y. Naveen Kumar, and D. Archana. “Power Quality Improvement In Distribution System Using DStatcom in Transmission Lines.” International Journal of Engineering Research and Applications (IJERA), Vol. 1, Issue 3. 3. Gabriel N. Popa, Angela Lagar, and Corina M. Diniş. “Some Power Quality Issues in Power Substation from Residential and Educational Buildings.” 10th International Symposium on Advanced Topics in Electrical Engineering (ATEE), IEEE, 2017. 4. Sulaiman A. Almohaimeed and Mamdouh Abdel-Akher. “Power Quality Issues and Mitigation for Electric Grids with Wind Power Penetration.” Applied Sciences, December 2020. 5. George G. Karady, Shahin H. Berisha, Tracy Blake, and Ray Hobbs. “Power Quality Problems at Electric Vehicle’s Charging Station.” SAE Transactions, 1994. 6. David Lineweber and Shawn McNulty. “The Cost of Power Disturbances to Industrial and Digital Economy Companies.” Electric Power Research Institute, Inc., June 2001. 7. Roman Targosz and Jonathan Manson. “Pan-European Power Quality Survey.” 9th International Conference on Electrical Power Quality and Utilisation, IEEE, 2007. 8. Subrat Sahoo. “Recent Trends and Advances in Power Quality.” Power Quality in Modern Power Systems, 2020. 9. A. El Mofty and K. Youssef. “Industrial Power Quality Problems.” 16th International Conference and Exhibition on Electricity Distribution, 2001. Part 1: Contributions. CIRED (IEE Conf. Publ No. 482), IEEE, June 2001. 10. Cost of Data Center Outages.” Ponemon Institute, January 2016. 11. Data Center Outages Are Common, Costly, and Preventable.” Uptime Institute. 12. IEC 61000-4-30:2015: Electromagnetic Compatibility (EMC)-Part 4-30: Testing and Measurement Techniques-Power Quality Measurement Methods.” International Electrotechnical Commission, February 2015.
This article originally appeared in Bodo’s Power Systems [PDF] magazine
Author: Jose Mendia has a B.Sc. in electronics and computer science engineering and joined the Energy and Industrial System Group at Analog Devices in 2016. Currently, he is a senior engineer in product applications at the Edinburgh UK design center.
Published by Dario Pevec *,† , Jurica Babic † and Vedran Podobnik †, Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb 10000, Croatia; jurica.babic@fer.hr (J.B.); vedran.podobnik@fer.hr (V.P.) * Correspondence: dario.pevec@fer.hr † These authors contributed equally to this work.
Abstract: Current trends are showing that the popularity of electric vehicles (EVs) has significantly increased over the last few years, causing changes not only in the transportation industry but generally in business and society. This paper covers one possible angle to the (r) evolution instigated by EVs, i.e., it provides the data science perspective review of the interdisciplinary area at the intersection of green transportation, energy informatics, and economics. Namely, the review summarizes data-driven research in EVs by identifying two main research streams: (i) socio–economic, and (ii) socio–technical. The socio–economic stream includes research in: (i) acceptance of green transportation in countries and among different populations, (ii) current trends in the EV market, and (iii) forecasting future sales for the green transportation. The socio–technical stream includes research in: (i) electric vehicle battery price and capacity and (ii) charging station management. This kind of study is especially important now when the question is no longer whether the transition from internal-combustion engine vehicles to clean-fuel vehicles is going to happen but how fast it will happen and what are going to be implications for society, governmental policies, and industry. Based on the presented literature review, the paper also outlines the most significant open questions and challenges that are yet to be solved: (i) scarcity of trustworthy (open) data, and (ii) designing a generalized methodology for charging station deployment.
Keywords: electric vehicles; green transportation; charging infrastructure; energy informatics; data science; big data
1. Introduction
In the last five years, electric vehicles (EVs) have gained increased popularity [1]. There are multiple reasons behind that fact. Firstly, technology is constantly advancing, and considering research and development trends today, wide acceptance of EVs is a step in the evolution of public and private transport. Secondly, the transportation sector is considered to be one of the main contributors to CO2 emissions, one of the crucial factors behind climate change [2]. Wider acceptance of EVs, and of green transportation in general, is one of the possible solutions to lower those emissions that are part of the greenhouse gases (GHG), as stated by Saber and Venayagamoorthy [3]. Finally, from the economics point of view, EVs are sustainable (i.e., when they are sourced through renewable sources, such as solar energy, wind energy, or biomass energy) and the price of electricity is by order of magnitude lower than the price of fossil fuels (Granovskii et al. [4]) which is an important factor for consumers.
Even with the current growing trend of the EV market share, there are several main obstacles for EVs to release their full potential: battery capacity, battery price, charging time, and availability of charging stations. Nowadays, EV batteries have limited range that they can cover while being fully charged, and as the range increases, so does the price of the battery, which based on our literature review (see Sections 4 and 4.1), is a major influence between potential EV owners. In 2014, the price for an average EV battery, i.e., 30 kWh, was around 12,000 USD and EVs powered by that kind of battery could travel approximately 100 km [5]. The forecast is that in the following years, due to technological advances, the price of batteries will significantly drop [6]. Apart from batteries, a particular focus is placed on charging infrastructure development, i.e., infrastructure used by EV owners to recharge their EVs. The main problem with charging stations is that their infrastructure is scarce, especially in underdeveloped countries [7] (e.g., Croatia has a small number of charging stations, and their placement and control is decentralized and unplanned). Also, before-mentioned factors (i.e., small battery capacity and underdeveloped charging infrastructure) together result in the phenomenon known as range anxiety. Neuber andWood [8] define range anxiety as fear of running out of electricity before reaching an available (i.e., unoccupied) charging station (CS). Despite the increase in the number of EVs on the road, range anxiety is still one of the key negative factors for the potential new EV owners [9].
There are many studies related to EVs. Since the technology related to EVs is relatively new, the majority of those studies are in the field of electrical engineering. The first research papers date to mid-1960s and they are mostly progress reports (Hender [10]) and discussions on recent developments in the field of EVs (e.g., development of EV batteries and engine by Rees et al. [11]). Up until the 90’s, the concept of EVs was not widely accepted and research centered around them was scarce and oriented towards electrical engineering. In early 90’s, the first papers from the field of Information and Communication Technology (ICT) started to appear (e.g., Golob et al. [12], which deals with the problem of forecasting the market penetration of electric and clean-fuel vehicles). Nowadays, there are ever-increasing numbers of EV-related papers from many fields, including:
• social studies (e.g., influence of sustainable transport on society and environment, such as Tanaka et al. [13]);
• economics (e.g., market penetration and economical changes due to increase in electric power consumption, such as [6]);
• informatics (e.g., computational algorithms for managing charging infrastructure, such as Pevec et al. [14], Babic et al. [15]);
• telecommunications (e.g., protocols for communicating with charging stations or for payment, such as Buamod et al. [16] and van Amstel et al. [17]);
• electrical engineering (e.g., development of low cost batteries, power electronics for the chargers, the motor driver, and improving existing technologies, such as Ruiz et al. [18] and Yilmaz et al. [19].)
This paper is a review which explores EVs from the aspect of three interdisciplinary studies—green transportation, energy informatics, and economics—as depicted in Figure 1. That perspective gives us a clear view of the current state and the future development of private transport. Even though green transportation is a generic term for zero-emission vehicles (e.g., cars, trains, and buses), within this work, we use the term green transportation to refer to battery electric vehicles (BEV) and plug-in hybrid electric vehicles (PHEV). Energy informatics is also a generic term that includes a broad field of research with a focus on information in energy systems. This paper only observes energy informatics studies that are strongly associated with EVs (e.g., charging of electric vehicles, the impact of EVs on a power grid). Economics is a highly relevant research domain since the EVs introduce great changes in the petrol industry and vehicle market. Previous three research fields will be observed from the data science point of view. Data science, according to the Van Der Aalst [20] can be defined as a combination of classical disciplines, i.e., statistics, data mining, databases, and distributed systems, for solving various challenges using domain-related datasets. The focus of this paper is an intersection of all four domains and to the best of our knowledge, this is the first review that aims to systematically provide such an overview of this interdisciplinary research field.
Figure 1. Thematic positioning of the paper: intersection of interdisciplinary fields.
Figure 2 gives an overview of entities relevant to the interdisciplinary research targeted by this paper, as well as presents relationships among them. An EV owner, who charges her/his EV on a charging station that is connected to a power grid and interacts with other people is in the domain of green transportation. The flow of information from the owner to the power grid can result in an energy efficiency increase, what is the key idea behind energy informatics that studies how to use information and communication technologies to tackle the energy domain challenges. Advanced operations using information flow (e.g., predicting utilization with the goal to optimally allocate charging stations) us characteristic for the data science field of research.
Figure 2. Interaction between entities in research area of interest.
The rest of the paper is organized as follows. Section 2 describes the methodology used to perform the review (e.g., keywords for querying several scientific databases, and filters applied). Section 3 describes methods through which EV-related data can be acquired as well as popular EV data sources, while Section 4 provides outlook of socio–economic factors of green transportation: EV market, together with different forecasts for future of EVs (e.g., market penetration, cost of EVs, or cost of EV batteries). The socio–technological aspect of green transportation is presented in Section 5. Section 6 proposes a research agenda by synthesizing open research questions, while Section 7 concludes this paper.
2. Review Methodology
We now explain the methodology used for the literature review. This review focuses on papers that were published between 2011 and 2018 since, as described in the Introduction Section, studies from earlier years are mainly focused on the electrical engineering aspect of the research area. The next filter is about the subject area: this paper focuses only on computer science and mathematics since the primary focus is placed on data science in the area of EVs and those two broad areas are employing data science relevant methodologies. Lastly, we only consider publications that are either conference papers or articles. The three scientific databases that were used are Scopus, the Elseviers’ database of peer-reviewed literature [21], and IEEE Xplore Digital Library [22].
The keyword that the search was based upon was applied to the title of a paper, the abstract, and keywords of the paper. The core search term was “electric vehicles”, which corresponds to our definition of green transportation (see Figure 1), and with all applied filters as described above, this search resulted with 5612 papers. Both search engines that were used have an option to search within results (i.e., within those 5612 papers), and since area of interest is intersection between four research areas (see Figure 1), search was further refined using three new keywords to cover the remaining three research areas: charging station, data, and market (see Figure 3). Note that the same paper can appear in multiple categories since e.g., one paper can have keywords data and charging station.
Figure 3. Hierarchy of keywords for related work search.
The “data” science part is covered with keywords: analysis (1140 results), prediction (370 results), and big data (81 results). Since the keyword analysis returned 1140 different results, that branch was further extended with keywords: descriptive, context, and behavior so we can differentiate studies that analyze the effect of surroundings (context analysis), and the effect of user behavior on EVs. This group of papers is especially interesting, since this group can cover more topics, including the ones mentioned before (i.e., charging stations and market).
The “market” part covers the area of economics. That branch of related papers is further extended with keywords: forecast and review with 129 and 479 papers with those keywords. Papers in this area are mainly focused on market penetration, battery prices, and the forecast of previously mentioned.
Lastly, the “charging stations” keyword covers the area of energy informatics, after further extending the search for keywords: deployment and location, in this branch of related papers, there were 182 and 264 papers respectively.
The detailed taxonomy of keywords used for the related work is depicted in Figure 3. Each child node is derived from the search results of the parent node (e.g., the keyword prediction returns 370 papers that are all between 1705 papers that were returned by search with the keyword data). After this step, relevant papers were hand-picked after reading their abstract and with regard to the number of citations and relevance for the area of interest.
All papers in this review that are published before 2011 are taken directly from the references of papers found with the previously described method, because of their high relevance and value for the respective research field. The final number of papers that were processed in this review is 96.
3. Role and Sources of Data in the Electric Vehicle Domain
Nowadays, data is one of the most important components in all fields of research which is not surprising as the amount of data generated is constantly growing [23,24]. For example, Kaggle is one of the most popular community-driven data science platforms, that provides numerous interesting datasets and organizes competitions in solving various data science problems [25]. The increase of the data that is being generated is especially significant in the field of transportation, since the transportation sector is responsible for one of its biggest evolutionary steps since the second industrial revolution—electrification of vehicles [26]. The increased flow of data greatly impacts the energy informatics field, as stated byWatson et al. [27,28]: the higher granularity of data the better information system can be developed for optimizing the energy consumption in highly complex systems. The data in this interdisciplinary research field can be obtained through different sources and with different methods. We now describe some of the most popular data sources and methods for data gathering that are used in EV-related studies.
Data repositories have a significant role in enforcing studies in this field since aggregated data they provide can help scientists to conduct the research without the need to perform data collection. Some of the most popular EV-related data repositories besides Kaggle include:
• Alternative fuels data center [29] which contains data about EV sales and charging stations for each state of USA; • Alternative fuel vehicle data [30] which also contains information about alternative fuel vehicles for USA; • EV volumes [31] that contains informations about world sales of EVs; and • data.gov [32] which is a search platform for various datasets.
The valuable data can also be collected through publicly available APIs (application programming interface). Frequently utilized APIs in this research field are:
• Nokia HERE API that is used for routing and calculating distances between geographical coordinates with many advanced parameters, similarly to Google Maps API and Open Street Map API; • Oplaadpalen API that provides information about charging stations around the world same as Open charge map; and • Vehicle API by edmunds that provides the data about vehicles (e.g., manufacturer or engine type).
Besides data repositories and APIs, surveys can also be valuable source of the data. For example, the National Household Travel Survey [33] is an organization that conducts various surveys and provides results via their Web page. If the regulated data is not available, researchers may opt to conduct surveys themselves.
The data can also be obtained through companies. The example of a third-party data provider is ElaadNL, one of the charging infrastructure providers in the Netherlands. It often collaborates with researchers providing them the data about their charging infrastructure transactions [34]. Another example is Renault that is also known to share their data with researches in order to analyze their vehicles’ potential [35].
Some researchers have developed their own methodology for data gathering. For example, the authors in [14,36] conceptualized data gathering of EV-related data by combining data provided by a company as well as several APIs. The other method is to use existing on-board sensors or to install new sensors for collecting and transmitting the data to the cloud for research purposes. Svendsen et al. [37] have developed previously described methodology to derive the EV driving patterns.
In contrast to the above-mentioned research examples, in which the data is available from the deployed system, researches of the energy systems of the future often use simulations to augment the existing data and to tackle interesting research challenges. For example, Babic et al. [38] have developed the agent-based simulation model which as a result provides the data about different business models (in article referred as parking policies) related to charging service. Studies by Ketter et al. [39,40] also employ simulation platforms to obtain data for solving various problems in the field of energy informatics.
One of the common divisions of the data is into primary and secondary data [41]. Primary data is the data collected with methods specifically developed for solving domain-specific problem (i.e., Table 1: Smart ED Platform and manual data collection methods), while secondary data is the data that is collected by someone other than the user (e.g., Table 1: data repositories, or the National Household Travel Survey).
All previously described data gathering methodologies are summarized in Table 1. Category ‘other’, means that the data is initially created to be private but can be shared with others for scientific reasons.
Table 1. Data gathering methodology with examples and tariff model for each data source.
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4. EV Market: Data for Modelling Economic Factors
The EV market is an interesting field of research, because it does not only cover the sales number, but also innovations and current trends in the EV industry from the marketing perspective, (potential) EV owners motivations, constraints, and various forecasts (e.g., sales, battery capacity, etc.). Statistics about the number of EVs and prices are given through various reports on global and local scale.
The number of EVs is growing more and more each year, however the growth is not as steep as expected, as stated by Carty [55], United States, in 2009, invested over 2 billion dollars into development and subsidies for electric cars with goal to increase the number of EVs in US to at least 1 million until the end of 2015. Since at the end of 2016 the number of EVs in US was around 570,000 (Figure 4), one can conclude that the goal was not reached despite the forecasts. One of the main reasons behind that fact is range anxiety and the unfamiliarity of the potential EV owners with the electric vehicles, as we described in Section 4.1. More recent reports [56] suggest that in the 2017 and 2018 cumulative sales of BEVs and PHEVs was around 550,000 which almost doubles the number of EVs in the USA.
Figure 4. The number of electric vehicles (EVs) from 2010 to 2016 in the USA (annually and cumulative), derived from [6,57].
In contrast to well-established car manufacturers of internal combustion engines (ICVs), EV-only manufacturers such as Tesla, become well known in the last decade due to popularity of EVs [58], and they are partially responsible for speeding up the transition to EVs (i.e., competition with other car manufacturers was one of the factors for traditional ICE car manufacturer switching to EVs [59]).
Another fact that supports the claim that EVs are the future of private and public transportation is the end of ICE vehicles (i.e., removing ICE vehicles from the market). Great Britain and France set the year 2040 as the year when ICE vehicles will be removed from the market, and every vehicle that is sold will have electric motor [60–62]. Germany had a similar initiative; the plan was to ban ICE vehicles from the market by 2030, which was proven to be unrealistic and therefore declined [63]. Other countries that have the same initiatives to ban the ICE vehicles are either highly developed and environmentally friendly countries (e.g., Netherlands or Norway) or countries with great air pollution (e.g., India or China) [64].
Figure 5 depicts the popularity of EVs in the global market by the end of 2016. As it can be seen, despite Tesla’s advanced technology, due to the price of the competitors’ vehicles, it is not the most popular option. Instead, Nissan Leaf takes the first spot with nearly 40% market share, although, Tesla plans to change that with the introduction of their Model 3 with the best price-to-range ratio [58].
Figure 5. The global market share of the most popular EVs according to the survey [6].
4.1. EV Acceptance
To increase potential EV owner’s familiarity with electric vehicles, research based on the potential EV owner’s preferences (e.g., range, speed, and comfort) is crucial. The following paragraphs describe studies for parameters that have the highest influence on a decision to buy or not to buy an EV in five regions with the highest EV market penetration. Figure 6 depicts the main findings of those studies. The focus is on the potential EV owners and each circle represents the factor that influences the potential EV owners (i.e., the inner circle is positive, while outer is the most negative).
Figure 6. Factors that influence potential EV owners’ decision to buy EV.
Ko and Hahn 65 stated the importance of knowing the potential EV owner’s preferences about electric vehicles. They further research their preference through the questionnaire among 250 households at the end of June 2009 in Korea. They used six key attributes to asses the willingness to pay for an EV: battery price, holding tax, subsidies type, subsidies level, battery swappability, and availability of recharging infrastructure. As expected, potential EV owners are willing to pay more if EV has a swappable battery and if charging infrastructure is developed and easy to access, since that considerably lowers the range anxiety. The consumers also prefer lump-sum payment over the installment payment of subsidies. This research was of great importance for car manufacturers, governments, and the charging infrastructure providers, because it gives an insight into user preferences for adoption of EVs.
Wee et al. 66 looked into subsides and what effect they have on the EV adoption rate. Authors used rich data set from 50 U.S. states about semi-annual new EV registrations from 2010 to 2015 to develop subsidy-dependent models. Authors conclude that 1000 $ increase in the subsidies for the specific model in a specific state led to around 10% increase in that model registrations number.
Zhang et al. [67] presented a framework used to estimate the elasticity of the demand and supply of EVs. Authors took into consideration the price of EVs, their technology, and incentives (i.e., bus lane access, toll waiver, and charging station density). To test their framework, the data from the organization of actors in the transport sector in Norway was used. The data consists of BEV sales from 2011 to 2013. The authors confirmed their hypothesis that the price is a negative factor, while innovative car technology is a very significant positive factor. Incentives are also positive factors, except access to bus lanes, which in the case of personal consumers can be negative. There is also a significant difference between personal and business potential EV owners—business potential EV owners are less affected by price and technology. However, this work could be further improved by adding the estimated influence of other incentives (e.g., taxes, subsidizing the purchase of EVs) or different data, since Norway has a very specific EV market (i.e., around 25% of vehicles on the road are electric [6]). Authors also stated that higher density of charging stations has a high influence on potential EV owners; since 2013, battery technology has improved and range that EVs can cover has nearly doubled, which means that charging station density should not be critical, but instead smart allocation of charging stations is highly important.
As the studies before, research from Hidrue et al. [68] is based on the data from more than five years ago, collected using on-line survey with the purpose to asses the willingness to pay for electric vehicles. The data was collected in US for 2009. Attributes that were taken into consideration were: price, driving range, time to charge for 50 km driving range, acceleration, pollution, the fuel cost of a preferred gas vehicle. Attributes price and pollution are compared to a preferred gas vehicle. With statistical methods, authors found that driving distance, charging time, performance, and pollution (in that order) have a high impact on potential EV owners. The most important factor is saving (i.e., compared to gas vehicles, since the price of electricity is lower than the price of gas). Authors have explained that behavior with interest to save fuel since long drives consume more fuel. The survey also suggests that younger, educated, and people with a green lifestyle are more likely to buy a EV.
Hoen and Koetse [69] conducted similar research as previous authors. In the Netherlands survey was conducted among 15,221 households with one or more cars (2011). Attributes considered were: car type, price, monthly cost, driving range, recharge/refueling type, additional detour time to reach a fuel or charging station, number of available models, and policy measure. Results show that potential EV owners prefer more conventional technologies (i.e., gas-fueled cars), than alternative fueled vehicles. The main reasons behind that were limited driving range and long refueling time. The novelty of this work is the segmentation of participants into second-hand and new buyers, where second-hand buyers are more sensitive about price than new car buyers. This paper stated that low range and high refueling times are the main factors behind lower acceptance of EVs.
Tanaka et al. [13] explore differences between US and Japanese potential EV owners regarding alternative fueled vehicles. The dataset used was collected over an on-line survey, with around 4000 participants from each state. Attributes used in this model were: purchase price, fuel cost (compared to gas-fueled vehicles), driving range, emission reduction (compared to gas-fueled vehicles), alternative fuel availability (share from all refueling stations), and home plug-in construction fee. Results show that US citizens are more sensitive about price reduction and availability of refueling stations than Japanese, while they are similarly influenced by a driving range and emission reductions. This work also presents an interesting overview for 4 States in US: California, Texas, Michigan, and New York. California has around 50% higher willingness to pay for price reduction than the other three states. The authors concluded, that in the future, due to technology advancement, the share of the alternative-fueled vehicles on the market would be doubled.
Smith et al. [70] conducted similar research as the studies before, but in the year 2017. Using a survey platform, 440 households in Australia were questioned about their preferences in a vehicle choice. As much as 48% answered that electric vehicle is their first choice of vehicle. The most influential negative factor on the potential EV owners is not the low range (i.e., small battery capacity), instead, it is recharging infrastructure availability. As opposed to the previous studies that concentrate assumptions on the social-demographic factor, this research stated that far more important factors are the attitude towards the environment and the technology.
Between newer studies, the notable ones, beside the study by Smith et al. [70] is study by Wang et al. [71] and Anderson et al. [72]. Wang et al. [71] in their paper presents the incentives for the purchase of EVs that are currently active in China and develop a model for the forecast of EV acceptance based on the linear regression. The data used in this research is sales numbers from 41 pilot cities and from the 37 cities with no purchase restriction. For each scenario (i.e., 41 cities and 37 cities), linear regression was performed for BEVs and PHEVs with independent socio–economic variables (e.g., population size, income per capita). The only common factor that was proven to be extremely significant for all cases was the density of charging stations. Other notable factors that influence the decision to purchase the EV in this research are education level and license fee. Anderson et al. [72] applied survey methods to analyze EV owner’s preferences about the charging infrastructure. Authors concluded that more public chargers are needed and that slower chargers are acceptable on more visited locations, while fast chargers are needed on less frequently visited locations.
Previous studies are summarized in Table 2, with factors that were taken into consideration, and the factors that have proven to be the most influential for the (potential) EV owners.
Table 2. Comparison of important factors for purchasing electric vehicles.
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4.2. EV Future Sales
When it comes to exploring future sales of EVs, most of the studies in this field use either agent-based modeling or conjoint analysis methods, very few studies use other methods.
Agent-based modelling is a computational method that observes interaction and evolution of complex objects (i.e., agents) [75]. Agents enable reproduction of complex social interactions, which other methods (e.g., game theory or other equation-based models) cannot as stated by Janssen [76]).
Agent-based modeling was used in by Yang et al. [77], Sullivan et al. [78], and Shafiei et al. [79]. All those studies define multiple agents: consumer population and car population. Studies [77,78] additionally define government and gas supplier agents, while in [77] charger and grid operators are also defined.
Besides the agent-based model, Yang et al. [77] define the system dynamics model that enables authors to analyze the impact of various parameters on the evolution of the defined EV ecosystem. On the case study of China, authors derived results for both models. Firstly according to the results of the system dynamics model, with time, ownership of EVs will grow, while expectedly, ownership of conventional vehicles will drop. Agent-based modeling is used to simulate EV adoption in three types of regions: developed, middle-developed, and underdeveloped. According to the simulation, by 2030, the market share of EVs in developed and middle-developed regions will be between 80% and 90%, while underdeveloped regions will have share of 30%.
Sullivan et al. [78] have used agent-based simulation for the forecast of PHEV adoption rates on the United States market. Complex model, although again without social interactions, provides accurate results for near-future prediction. Market penetration is predicted for 2015 and for 2020. For 2015 results show that sales of PHEVs could reach 2–3% while market penetration would be 1%, which is accurate for US market. The prediction for 2020 is that sales could reach 4–5% while fleet penetration would reach only 2%. This model also explores the role of subsidies, without them, the penetration on the market would be below 1%.
A similar study was conducted using the case study of Iceland by Shafiei et al. [79]. This model does not take into consideration complex dependencies between car manufacturers, energy grid, providers of charging infrastructure, or gas suppliers. Instead, this paper is more focused on the interaction between (potential) EV owners and factors that influence them: marketing, word of mouth, and indirect word of mouth. Predictions developed with this model vary from market share of 70% all to 100% by 2040, dependant on the price of gasoline and the price of EVs.
Another group of studies is about conjoint analysis (i.e., survey-based statistical technique) and choice-based modeling. studies in this field date all to the late 1990s (e.g., Segall [80]), those research results are not applicable today because of different levels of knowledge about EVs. Despite that, those studies have greatly influenced some of the notable studies today.
Glerum et al. [35] have research what influences sales of Renault EV in Switzerland. Their research is based on a survey conducted in 2011. The survey was structured in two phases: stated preferences (i.e., information about vehicles in the respondent’s households) and choice situation (i.e., three different cars similar to their own). To interpret survey results, the author used statistical models: logit and latent variable model. The framework itself is not generated towards annual forecasting, but instead for forecasting market share when certain parameters are changed (e.g., price of EVs, monthly cost, subsidies, etc.). Similar work that does not focus on annual growth rates to 1981, and uses survey where participants ranked 16 cars. Beggs et al. [81] also used logistic model to interpret results.
Using the data from the same year as previous authors, Lebeau et al. [47] analyzed the adoption of BEVs and PHEVs in Belgium based on conjoint choice modeling. The novelty of this research is in the fact that authors modeled the future choice as the weighted function of car utilities (e.g., speed, acceleration, airbags, etc.). The forecast is that the number of PHEVs will be higher than the number of BEVs in the near future (i.e., the prediction was made up to 2030). The baseline is the penetration in the time research was conducted, which was around 4.85% for both PHEVs and BEVs. Prediction for 2020 is 13% while for 2030 it is 45%.
Another work that introduces novelty is studied by Jensen et al. [46]. The authors of this paper created the survey with participants before and after driving the EV. The survey was conducted in Norway, Denmark, and Netherlands since they represent the most developed countries in Europe (EV wise). With basic model assumptions (i.e., assuming EV technology will only improve, which would lower the EV price) model resulted in the prediction of 40% market share for 2020. The problem with this model is the assumption, new technologies do not mean necessarily lower prices. Also, the prediction is consistent with the penetration today, which for Norway is around 30%.
Between notable studies are two papers from 2012, Higgins et al. [44] and Eggers [45]. The first one was conducted based on the survey in Australia. It combines methods of choice modeling, multi-criteria analysis, and Bass diffusion model. The framework is used to analyze adoption patterns in consideration of factors that are important for the potential EV owners. The developed framework estimates the penetration of 45% by 2030. This research also gives insight into the adoption of EVs based on monthly income. The second research is based on the data from Germany, and same as the first research uses a combined method for prediction, choice and diffusion modeling. Predictions from that model are that penetration EVs and PHEVs will be around 55%, which is not the case. The model would have more reliable results if it included human interaction factor [82].
Table 3. Comparison of studies on forecasting future sales of electric vehicles (EVs).
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There are two distinguished studies that use none of the methods used above. The first one is a paper by Becker et al. [84], in which the author used simple Bass diffusion that is typically used to describe the process of how new products get adopted. The result is the most interesting part of that research, dated to 2009. It forecast the number of EVs on the US market to approximately 600,000 by 2016, which is accurate according to the global EV outlook for the year 2016 [6]. The reason behind that accuracy is that authors did not only model potential EV owner’s behavior, but oil prices, internal combustion car cost, and other parameters. The article goes further in time, and predicts the 64% of sales and 24% of the fleet (i.e., around 2.8 million) will be EVs by 2030. Other work is Zhang et al. [83]. This research uses multivariate and univariate time-series models for forecast based on the 60-month sales data in China, from January 2011, to December 2015. This work besides the forecast of EV market growth presents the comparison of the two before mentioned models (similar to Du andWitt [85] in the domain of tourism demand). Since the univariate model is used for short term forecast, in contrast to multivariate model (Chayama and Hirata [86]), that methodology is applied in this research too. For the short term forecast (i.e., end of 2017, around 350,000 EVs should be sold). For long term forecast (i.e., 2020) more than 1 million EVs should be sold. Besides from the economic point of view, research from Li et al. [87] forecast the number of EVs with the goal to balance the demand for electricity supply.
The majority of studies in this research area are from developed countries that are focusing their research and development on renewable energy sources. Since the EV industry is not yet fully developed, the market penetration forecast is mainly for the long future (i.e., 15+ years). More details about the main findings are summarized in the Table 3.
5. EV Infrastructure: Data for Modelling Technical Factors
The previous section dealt with challenges in EV market penetration and acceptance (see first three actors in Figure 2). This section summarizes the studies with the main focus on charging infrastructure, vehicle-to-grid technologies, and users’ driving patterns concerning charging and energy balancing.
5.1. Batteries
Batteries are the crucial part of electric vehicles and they are directly connected with EV acceptance rate, as described in previous paragraphs (e.g., range anxiety, charging infrastructure, price, etc.). There are many studies relevant to EV battery, although, not many in the field of data science. The most information about battery capacities and prices are available through global reports and price lists. However, there are some studies about the second use potential of EV batteries like Nauber et al. [88] and [89]. Both works are motivated with restrictions for market penetration growth due to battery cost. The first work is oriented towards defining second-use for retired EV lithium-ion batteries which could partially recover the cost of the battery. Authors concluded that using retired EV batteries as uninterruptible power supply, instead of lead-acid batteries, is more effective and would result in payback through seven years. With various factors in mind (e.g., price of new battery or price of repurposing), authors calculated that the price of the repurposed battery would range from 38–132 $/kWh. The second paper is earlier work of the same authors where they introduce their plan to research the second-use of EV batteries.
Ahmadian et al. [90] reviewed the various studies on battery degradation models and compared them with each other. Ahmadian et al. concluded that degradation of batteries is primarily caused by two factors: (i) time degradation and (ii) cycle degradation. Time degradation is dependant on temperature and the age of the battery, while cycle degradation is dependant on the number of charging cycles and the depth of discharge. The main contribution in research by Ahmadian et al. is a conceptual framework that enables the use of batteries degradation models for smart grid studies.
From the market perspective, the best situation of current trends is given in the report [6]. Figure 7 depicts the prices of battery in from 2010 to 2015. As can be seen, the prices stagnate from 2013 to 2015, those prices are relevant even today. Prices stay the same because of physical restrictions (e.g., materials used and dimensions) and because of the lack of mass battery production. Tesla plans to change that with its Gigafactory that would mass-produce the batteries [91]. To produce battery with higher capacity, one of the options is to build a larger battery. The problem with large batteries is safety, the larger the battery is, the greater the chances are that it will break. Ruiz et al. [18] extensively reviewed the standards for safety testing of batteries.
The rest of the studies that do not belong in the electrical engineering field are closely related to the prediction of the state of charge (SOC) and prediction of available range in the future based on various factors and past development.
Figure 7. Battery prices, derived form [6,92].
5.2. Charging Stations
Charging stations are in this state of development, underdeveloped [93,94]. They are an important factor in the acceptance of EVs as a primary transport solution, since the problem of range anxiety is closely dependent on the number of charging stations [9]. Charging stations can be categorized based on the speed of charging and ownership. Based on the charging speed chargers are divided into four types. Level-1 charging is a synonym for charging a car via the household outlet of 120 volts. Level-2 charging chargers at the 240 volts and provides five times faster charging than Level-1. Level-3 and Level-4 charging is also known as fast-charging since it provides energy for approximately 125 miles per hour, depending on a type of vehicle. Based on the ownership, the charging stations can be divided as private chargers and public chargers. Private chargers are considered those that are installed in someone’s home or as private ownership of someone (e.g., private firm parking). Public chargers are available to anyone, and they are the main focus of the majority of researchers, since, data related to public charging stations are more accessible than for private chargers [95]. The future of charging stations is in the wireless chargers that can be placed under the road and ensure charging even while driving [96].
5.2.1. Deployment
Charging station deployment is one of the most challenging tasks, since it is not enough to simply place charging station somewhere, it is important to strategically place charging station on the right location. This subsection will provide survey of studies and their methods towards achieving that goal. Most of them can be divided into two categories, weather they use real-world data or simulation data, majority of studies in this field are either optimization problems or simulation, as can be seen in Table 4.
Table 4. Categorization of studies about charging station (CS) deployment based on data and methodology.
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He et al. [104] proposed a mathematical framework for the macroscopic deployment of charging stations taking into account the equilibrium between demand and supply of energy. User’s desire to choose a destination was formulated based on: time, price, and availability of chargers. Supply-side was formulated as the price of providing electricity. This paper focuses on a large scale charging station (CS) deployment and this framework is able to answer only how many CSs should be deployed in a certain region—specific location of CS cannot be determined.
Ip et al. [52] implements a two-step approach to decide optimal location for new CS. Although research methodology is similar to the one authors of the previous paper used, this one provides a more accurate location for CS. The first step is to determine pieces of roads that are utilized the most and to divide them into x–y grid. The second step is to cluster those squares in the grid based on the intensity of road utilization and to apply an optimization algorithm to decide the most suitable cluster for CS deployment. This method uses the data generated by various sensors on the road (assuming there are sensors) and the limitation of this study is that collecting the data needed for calculations is impossible out of specifically developed areas. However, this work proposes the framework that itself is general and can be applied whenever there is a need for deciding the optimal location for something (e.g., train station or restaurant).
Frade et al. [105] on the case study of Lisbon, Portugal, implements an optimization model (i.e., maximize coverage) for CS deployment taking into account coverage of a single charging station between 400 and 600 m walking distance and the demand for CS. To estimate the number of EVs, regression was used with parameters: the size of household, building type, age, education, and employment. With those parameters, an accurate model for the number of cars can be derived, but the number of EVs was further estimated with information about EV penetration. The demand for charging stations was calculated independently for day and night time, since those two time intervals have completely different patterns. This work, however, does not account for increasing EV penetration, and for factors that influence utilization of charging stations (e.g., places of interest), therefore, charging stations could be underutilized.
Chen et al. [100] deals with the charging station deployment problem from the perspective of car parking. Firstly, based on the data from Washington state, parking space and duration were determined. This information was used to build a regression model for zone-level parking demands and trip-level parking demands. The last step is using mixed linear integer programming to chose the optimal place for charging stations based on minimization of price and distance between zones that have great parking demand. This model has proven to be fast and reliable, but it does not include data only on electric cars—for parking location and duration. The location of existing charging stations has great influence on EV owner parking behavior. As opposed to previous studies, this one besides mathematical programming uses regression for forecasting demand for zone and trip level parking, which is valuable information for different fields of research.
Xi et al. [106] have developed a model for deploying charging stations in a way that maximizes their use by private EV owners. The model does not use real-world data concerning charging stations, EVs, or driving patterns, instead, based on the number of population and households, authors have estimated the number of cars, and with the 1% EV penetration—number of EVs. The trip data was artificially generated by Mid-Ohio Regional Planning Commission. Using the integer programming optimization technique, the authors calculated an optimal number of charging stations in each traffic analysis zone. Another finding of this study is that a combination of level 1 (i.e., 1.4 kWh) and 2 (i.e., 4 kWh) chargers is the most efficient, but with not enough funds, only level 1 chargers should be deployed.
Yan et al. [53] tested their optimization method on the case study based on the 30-day taxi trace with 315 taxis and 4638 landmarks in Rome. Optimization methods goal is to maximize the flow of vehicles, with constraints to budget, charging availability, EV battery capacity, and energy consumption. With their algorithm, under different budget scenarios authors calculated the optimal number of charging stations at each landmark. This work has a simplified environment, where authors assumed that the cost of deploying charging stations is the same for all charging stations, and that cars and drivers are homogeneous, which is not the case in reality. There are many social factors that influence the driving patterns, charging stations are only one of many.
The following studies, while also using optimization methods, base their optimization techniques on genetic and greedy algorithms.
Research by Hess et al. [101] aims to decide the optimal location of a charging station based on the genetic optimization algorithm. The only data that is used in this research is the map of Vienna, parameters of electric cars, and the location of gas stations – this research as initial location of charging stations assumes the location of gas stations. The optimization function used is to minimize the whole trip time of an electric vehicle owner. This research extended the well-known traffic simulation tool SUMO with electric vehicle behavior. This work could be further improved by taking into account positions of current charging stations instead of gas stations.
Mehar and Senouci [102] are proposing a genetic algorithm that takes into consideration area traffic density, land cost, infrastructure cost, investment cost, transportation cost toward the CS, charging station capacity and, energy grid capability. To optimize the placement of charging stations, authors propose to minimize two objective functions: minimize the objective cost and minimize the transportation cost. The algorithm was tested on a simulation that describes the traffic in Cologne (Germany) from 6 a.m. to 8 a.m., since that time window is considered to be peak hour. The algorithm is fast but lacks some context information. It does not take into consideration the proximity of charging stations to public transport, or shops. Even if traffic is dense in a certain area, the population of cars in that area does not have to be comprised of EVs (i.e., authors assumed EV rate).
As opposed to previous studies, research by Sadeghi et al. [103] has a goal to optimally place fast chargers in the urban area. Fast chargers have the capability to fully charge EV battery in 20–30 min [110]. The approach is based on genetic optimization algorithm, with no EV related dataset. Authors have defined six test scenarios: minimize all cost, ignore land cost, ignore the cost for EV owners, ignore the electric grid loss, no electricity charge to CS owners, private sector invest in CSes. Authors decided to set the minimal distance between charging stations to 3 km, and considering previous scenarios they proposed optimal positioning of fast-charging stations. This work is greatly significant considering the amount of research about deploying fast-charging stations. Xie et al. [107] are also dealing with the challenge of fast charger deployment. They tackled the challenge in three phases: (i) 2015–2019, (ii) 2020–2024, and (iii) 2025–2029. Authors developed optimization-based model that serves as a decision support system for policymakers for where, when, and how many fast chargers should be deployed.
A study by Vaziveh et al. [99] is using real-world data collected through the cell phone data over the Boston area, and with that, whole trip of a user was known. The goal of that research was to minimize the aggregate distance all drivers have to drive, from the end of their intended trip to the nearest charging station. Methods used to achieve previously described goal were: greedy and genetic algorithm. With those heuristic algorithms, near-optimal locations of charging stations can be found. Although the algorithm used in this paper includes the parameter charging station coverage, which limits the number of charging stations, it does not include the cost of new charging stations, or contextual information if a user really needs to charge on the end of the trip, which makes this model currently not reliable. While this work uses a genetic algorithm with the same goal as the previous two studies, this one builds the model with real-world data.
The next three studies are based on machine learning techniques. First, two uses only clustering, enhancing it with mathematics. The second research uses out-of-the-box machine learning algorithms to forecast utilization of charging stations and decide where another one could be deployed. Naturally, both studies use real-world data.
Andrenacci et al. [97], used the demand-side approach to decide the best placement for new CSes. Data used in this work is real traffic flow (i.e., GPS data) from 6% of privately owned cars in Rome. The assumption is that all of those are electric (i.e., switch to electric transportation). All destinations that ended in Rome’s urban area are further clustered in sub-areas where charging infrastructure is associated with the center of a cluster. The next step is to mathematically calculate the demand for energy, the sum of all energy spent to arrive at the goal, and that is the number of CSs needed in that area. This method has high-quality data, and valuable division of Rome urban area into sub-areas. However, the number of CSes is not reliable, since the assumption is that all vehicles are electric (i.e., full conversion to electric transportation) and that all vehicles can satisfy their energy needs without queuing. This work does not provide an exact location where CS should be deployed, rather the number of CSs in a specific sub-area.
Momtazpour et al. [98] used a synthetic dataset because of the lack of real-world data. Authors take into consideration the duration of charging and decided to place chargers in locations that people visit for an extended period of time. The region of Portland was divided into three clusters: high electricity load-low charging need-low stay duration, low electricity load-high charging need-high stay duration, and low electricity load-low charging need-low stay duration. Based on the cluster description, the second cluster is ideal for deployment of charging stations: it can handle electricity load since it is low, there is a need for more chargers, and people stay there for an extended period of time. This work included places of interest in their research and the energy load making it significant and highly valuable.
Pevec et al. [36] has developed a real-world, data-driven, generic framework for extending EV charging infrastructure. The data used in that framework is from ELaadNL, one of the biggest charging infrastructure providers in the Netherlands. The data consist of all transactions for four consecutive years (i.e., 2013–2016). The first part of the framework clusters existing charging stations in clusters based on the distance between them with the hierarchical clustering method. After charging stations have been clustered into zones, in each zone utilization of charging stations was calculated and used as the dependent variable in the machine learning algorithm. The framework uses machine learning algorithm XGBoost to predict utilization when certain parameters are changed. Parameters taken into consideration were: places of interest, EV penetration, time of day, number of charging stations in the defined zone, number of competitors charging stations, and is it weekend or weekday, since it has a drastic effect on charging pattern. The third part of the framework based on the optimization function provided decides the best zone to place another charging station. The precision of the framework is (i.e., the place where another charging station should be deployed) is dependant on the distance that clusters are based on.
The last category in research in this field is simulation-based research. Those research do not use real-world data, only some information to tune the simulation. All the relevant data is generated by simulation itself.
Sweda and Klabjan [108] have developed and described an agent-based decision support system for the placement of charging stations. Although, they use real-world data for prices and sales numbers of electric vehicles, most of the parameters are artificially tuned (e.g., driving patterns, state of charge, etc.) with randomness. This study manages to implement social interactions between car owners and with that it is possible to simulate the decision to buy EV and increase the EV population in the system. Another feature of the model is to compare sales of alternative fuelled cars with a dependency to fossil fuel prices. This work is based on the area of Chicagoland. The model is tested against two different proposed charging station placements. When comparing results with the current state in that area, improvement can be noticed. The major downside of this approach is that it does not offer a possible location for CS, it analyses the placement provided to it. An updated version of the research is provided in a full report by Sweda and Klabjan [109].
Authors Lu and Hua [51] developed a location-sizing model for the charging station. The goal is to optimize the location and the size (i.e., number of plugs) of a charging station, based on the demand. Their model is based on queuing theory and it is a continuation of earlier work by Capar et al. [111].
As we mentioned before, range anxiety is one of the greatest challenges left to overcome in order to raise EV acceptance, and it is closely related to the development of charging station infrastructure. Even though the capacity of the EV battery is nominally enough for intra-city traversal, the familiarity with the existing gas station infrastructure greatly influences potential EV owners in the decision not to buy an EV. Pevec et al. assessed the range preferences of potential EV owners considering the settlement hierarchy based on the settlement population [112]. As for the inter-city traversal, most of the EVs do not have sufficient battery capacity and this is the prime example of the range anxiety. One of the solutions considered by the researchers is the deployment of charging stations along the highway near existing gas stations, since they have necessary infrastructure [113,114].
Another major challenge in the field of the charging station deployment is the capacity of electric power distribution networks capacity. As mentioned in Section 4 the number of EVs is expected to grow, and that could cause major issues since the demand for electricity could be higher than the supply. Wange et al. [115], Abdalrahman and Zhuang [116], and Masoum et al. [117] took an approach to the charging station development considering previous limitations, i.e., ensuring reduced power loss of distribution systems.
In this Section the problem of charging station infrastructure development was investigated, and one of the conclusions is that the behavior of EV owners is extremely important for strategical planning of the charging infrastructure. Therefore, the next Section will explore user charging behavior.
5.2.2. User Charging Behavior
Section 4 explored the behavior of potential EV owners, and assumed the behavior of the EV owners based on the behavior of the owners of traditional fossil-fueled vehicles. This section explores the user behavior in more detail, since it is not only important for the charging infrastructure providers and the EV manufacturers, it is also important for the power grid management.
Qian et al. described [118] four different scenarios of user charging patterns with the goal of modeling the load demand of the energy grid. The first presented scenario was uncontrolled domestic charging which is characterized with no incentive for owners to charge off the peak hours. The second scenario is uncontrolled off-peak domestic charging where incentives to charge the EV in off-peak hours have been introduced. Smart domestic charging is defined as charging accordingly to the real-time electricity rate to decrease the cost for EV owners and to decrease the load on the energy grid. The last scenario is presented as uncontrolled public charging throughout the day where a certain share of EVs charge at the working place on the public chargers. Besides describing the charging patterns of the EV owners, this research compares that behavior with the load of energy grid.
Koroleva et al. [119] have introduced their research in progress about exploring the demand response of EV owners in response to the price of the electricity. Factors that authors considered in their model are range anxiety, uncertainty about the travel, risk attitude, and social influence. The model uses a simulated EV environment to observe the driving and charging behavior of EV owners. In the future authors plan to implement the mobile application that would use that model to visually describe patterns when certain factors change.
To determine a load on the energy grid, researchers Taylor et al. [120], in the scope of a larger project, have developed a framework that is based on the data acquired by the National Household Travel Survey [121]. Based on the traveled distance, the battery state of charge is estimated and assuming that PHEV owner charges the vehicle to the full capacity, load on the energy grid can be calculated. An interesting observation in this work is about the traveled distances and the times of home departures/arrivals. The longer the traveling time is, the earlier is the time of departure. The energy grid is under heavier load around 5 PM which correspondent with the times of PHEV owners arriving at home from work—this leads to the conclusion that PHEV owners are likely to charge their vehicle when they arrive to home.
Like the previous study, Kelly et al. [122] are basing their research on the data provided by the National Household Travel Survey and also describes users charging behavior at home based on different parameters. The peak in energy grid load is highest around 8 p.m., and noticeably higher on weekdays than on the weekends. Load on the energy grid caused by EV charging is never zero, since at all times cars are charging. After analyzing the impact of battery capacity on the load, authors concluded that increased battery capacity does not only increase the magnitude of the load on the energy grid, but also shifts it in time (i.e., the peak will occur later than with the batteries with smaller capacity). From the demographic aspect, the authors concluded that the households with the highest income generate peaks in the energy grid load 41% higher than the households with lower income and the households with lower income have earlier peaks. Regardless of the driver sex, based on the sample provided by NHTS, the older population generate a peak in the load earlier than the younger people.
Dealing with the same problem as previous studies (i.e., energy grid integration), Shao and Rahman [123] also derived conclusions about the EV owners charging behaviors. Using the same data (i.e., NHTS) that indicates that cars are parked for more than 90% of the time and that arrivals to home from work are in different times of the day, authors calculated (again based on the distance traveled and battery state of charge) that the peak occurs at 6 p.m. with one hour variance.
As opposed to the previous research, the next studies do not describe patterns of EV owners charging and driving behavior as a consequence of solving a different problem, but as a problem on its own.
Develder et al. [48] conducted research that is based on determining EV owners’ charging patterns. Two different real-world datasets were used, each one belonging to the different EV charging infrastructure providers (ElaadNL and iMove). Based on clustering the arrival and departure times of EV to the charging station, charging session has been classified as park to charge when charging times are scattered through the day and the duration of charging session is not much longer than the time needed to charge the EV, charging near home sessions are characterized with departure times in the morning, and with the arrivals in the evening. Lastly, charging sessions have been also classified as charging near work where departure times are in the evening and the arrival times are in the morning. Besides this conclusion, with simple statistics, authors also concluded the pattern differences between weekdays and weekends. The contribution of this work is not only in the previously stated conclusions, but in the fact that previously stated conclusions were drawn for two infrastructure providers and compared between them.
Frenkie and Krems [124] investigated the EV owner driving and charging behavior using the data collected from travel and charging diaries from EV owners provided by EV and private charging station. The dataset contains only information from Monday to Friday, since weekends have atypical patterns. The average distance per user for a day is 38 km, while the maximum distance traveled without recharging is 124.9 km. The charging patterns are different than in most studies, since this study uses private charging stations that are available to the EV owners, and they can charge their car when needed, not when the opportunity arrives. On average, users charged 3.1 times per week, while the charging event occurred when the remaining capacity is around 30% or below 15%, which is also when the car system notifies the owner about the state of charge.
Bingham et al. [54] used the data collected from the Smart ED platform (i.e., platform for collecting the data from pure electric driven two-seat passenger car). Based on the data it was calculated that battery consumption is equivalent to 1.275% of the battery state of charge, which leads to the conclusion that, on average, the EV in this case study can travel 78.4 km on full battery (i.e., from 71 km to 88 km). Authors concluded that reducing the amount of accelerating and decelerating, a significant amount of energy can be saved, which would extend the driving distance of EV.
Pevec et al. [14] have reported as a part of their contributions the statistics which depicts EV owners charging behavior on the case of Netherlands, based on the dataset provided by one of the charging infrastructure providers in the Netherlands (i.e., ElaadNL). This research describes utilization of EV charging stations through different time intervals (i.e., hourly, daily, and yearly), on the hourly basis there are two peaks in the utilization levels, around 8 a.m. and 5 p.m., which corresponds with the time of EV owners arrival to work and to home from work, also, the utilization of parking spaces follow that pattern with the drop in utilization right before the peaks in the charging stations utilization-EV owners and on the road, thus parking space is unoccupied. On the daily basis, authors concluded that there is no difference in utilization patterns on weekdays, but the weekdays greatly differ from weekends where utilization has only one peak midday. On the yearly basis, utilization has a significant drop during the summer, when people usually go on a vacation. Besides the user charging behavior, this research also describes utilization from the charging station perspective (e.g., is charger located near home, or near the workplace, how specific chargers are utilized, etc.). Figure 8 depicts a comparison of charging station and parking spot utilization per hour of the day where previously described behaviors can be observed.
Figure 8. Comparison of hourly charging station and parking spot utilization, taken from [14].
Babic et al. [43,125] in their research have modeled the willingness to pay for charging service. The model used three control variables: charging speed, referent electricity price, and state of charge. Based on the randomize values for control variables, users answered the survey (deployed via Qualitrics) with the price they are willing to pay for the charging service (i.e., answers were collected using Mechanical Turk, crowd-sourcing platform). After collecting the data, multiple linear regression model was developed with the goal to analyze the influence of certain variable and the combination of variables on the willingness to pay for charging service. As a continuation of this research, Dorcec et al. [126] extended this methodology with the information about the time-of-the-day when EV is being charged. This research, as well as previous research, confirmed the hypothesis that referent price and state of charge have a great role in EV owners willing to pay for a charging service.
One of the most common conclusions in this research area is about user charging times, i.e., when are they charging their car, and for how long, which is important for managing the electricity demand and supply. Besides the demand and supply, this information can also be used for smart charging station placement [14]. More interesting observations related to user charging behavior are represented in Table 5.
Table 5. EV owners’ charging behavior and patterns.
.
5.3. Vehicle-To-Grid
Vehicle-to-grid (V2G) is a concept of a process in which electric vehicles provide power to the energy grid while parked and connected to a charger, since most of the time, the car is parked and thus, battery unused (Clement et al. [127]). With this method, owners of EVs can return some of the cost, since providing electricity to the grid would be compensated (e.g., free charging, money) (see Figure 2—bidirectional energy exchange between EV-CS and CS-energy grid). A simple scenario of V2G technology is as follows when there is a high demand for electricity, electric vehicles that are parked and connected to the charger would discharge and when overall energy consumption is low, they would charge. The vast majority of work in this area is focused on the implementation of V2G technology. However, some researchers are focused on scheduling and the impact of the realization of that technology.
He et al. [128] have developed an optimization framework for scheduling EV charging and discharging times. First, they solve the problem of minimization of the cost on a global scale. This approach has proven to be inefficient, since, it assumes that the arrival times and load during the day is known in advance. The second problem was defined on a local scale (i.e., EVs that perform charging and discharging in one parking lot). This approach is applicable on a larger scale, and is resilient to dynamic EV arrival. The authors tested their framework on a case study involving the data Toronto on 21 August 2009. The simulation results indicated that the local scheduling can achieve results close to those on a global scale.
Wang et al. [74] have defined V2G EV as an electric vehicle that has low driving time and high parking time, which ideally describes personal vehicles. The goal of this study was to analyze the impact of EV charging on energy grid load. Authors propose three models: uncontrolled charging where user randomly charges EV, controlled charging by tariff structure (charge during off-peak hours), and controlled charging/discharging (charge during off-peak, discharge during on-peak hours). The first model as expected has proven to be the worst during peak hours, while the second and third models improved the load of the power grid during peak hours. The third model was able to efficiently exchange energy with the power grid and further flatten the load curve.
Soares et al. [129] utilize Particle Swarm Optimization (population-based stochastic optimization, similar to the genetic algorithm, Kennedy [130]) to tackle the problem of energy management with a high number of V2G capable EVs. This paper introduces a method that is for the order of magnitude faster than standard non-linear programming, and can find an optimal solution in a matter of seconds, which is of great importance for the day-ahead planning.
In this area of research, there are some studies that focus on energy grid load balancing with agent-based modeling: Kahlen et al. [131], Vytelingum et al. [132], Kamboj et al. [133], Valogianni et al. [134], and Ramchurn et al. [135]. All those studies have defined their own models with agents (e.g., car, electricity provider) with different behavior (e.g., electricity storage provider has a goal to maximize the cost, EV owners charge randomly).
More extensive research on vehicle-to-grid EV integration is provided in research by Mwasilu et al. [136].
Currently, vehicle-to-grid technologies are tested in Netherlands with the collaboration with Stedin, GE, Renault, and ELaadNL [137], and in USA, PG&E are converting company-owned Prius to V2G PHEVs at Google campus, while Xcel Energy is converting six Ford Escape Hybrids into V2G capable vehicles as described by Fang et al. [138].
6. Discussion
Throughout the paper, EV-related studies from fields of green transportation, energy informatics, and economics are reviewed and summarized in a systematic way by using the data science perspective (see Section 2), as explained in Figure 1. The described research area is gaining an increase in popularity with the growing trend of EVs on the market [6]. Up until now, the data science approaches, methods, and tools in the domain of EVs were present only in a small number of studies, since the research focus was mainly on the electrical engineering aspect (i.e., the number of EVs was not large enough for implementing solutions based on the data science and there was no enough data). However, the situation is changing what can be noted from a growing number of EV-related data science research papers. Consequently, data science is becoming a highly relevant approach for green transportation, energy informatics, and EV-related economics studies. Researchers are actively cooperating with the industry since there is no conventional way to gather the EV-related data and the private, i.e., company-owned, data is the most used source in various studies (e.g., [48,139]). Following paragraphs will consolidate main scientific observations for research problems covered in the paper: EV acceptance, EV market penetration, charging station deployment, and EV owner charging behavior.
Based on the insights in Section 4, EV acceptance is usually tackled with conjoint analysis with different factors considered, e.g., range anxiety, education, age, and income. The most important factors in EV adoption are proven to be government incentives and high availability of charging stations which consequentially lowers range anxiety. Negative influence on the EV adoption rate is mostly long recharging times and low range with a fully charged battery. The second part of Section 4 deals with the research problem of predicting EV future sales. Researchers in the sales forecast field mostly use analysis based on the historical data and well established statistical approaches or simulations that mimic potential owners’ adoption rate and other complex EV environment interactions. Some of the studies analyzed in this paper, i.e., those that are dated before 2015, have accurate predictions for the near future and very optimistic predictions for the period of the next 10 years (i.e., growth around 30–40%).
Section 5 deals with the charging station deployment and user charging behavior, which has proven to be valuable information for deciding the location for new charging stations. Both research problems employ similar methods to tackle their respective challenges: data analysis, machine learning, mathematical programming, and simulations, with the emphasis on the latter two. Majority of studies about EV owners charging patterns have similar conclusions: EV owners are most likely to charge their car when they arrive to work and to home from work, i.e., peaks in the charging station utilization are around 8 a.m. and 5 p.m. Besides the charging station deployment, charging behavior is an important aspect in research related to energy grid load demand optimization. The next observed challenge, the one dealing with the deployment of charging stations, is nowadays the most important since it directly impacts EV adoption and consequentially the development of EVs. While being extremely important, the EV charging infrastructure is generally underdeveloped due to short existence. Lack of data in this research area is the reason why researches are mainly employing methods of mathematical programming and simulations. For now there is no generally applicable method for deployment of charging stations, since, to the best of the author’s knowledge, the existing studies are specific and cover either specific area, i.e., due to simulation restrictions, or specific case, e.g., macro/micro deployment or deployment along the highways. Finally, one of the greatest challenges in this domain is the adaptation of existing energy infrastructure to accommodate the EV charging needs. This challenge is being tackled by the smart charging research, partially discussed in the Section 5.3. In order to offload the energy grid, it is important to determine in which time intervals the electric vehicle should be a charge, should it be used as energy storage during the peak load times, and how to manage the EV battery to satisfy both the owner’s needs and the energy grid.
7. Conclusions
This paper is a data science perspective review of the multi-disciplinary research area centered around electric vehicles. The review was systematically performed using specific keywords, as explained in Section 2, which ensured a detailed overview of data-driven research performed in the field of EVs in the period from 2011 to 2018.
Based on the presented review, we conclude that data science should be today widely used to solve various EV-related challenges. The EV-related data is nowadays generated from numerous sources such as road sensors, vehicles, and EV charging stations. Furthermore, industry more and more provides researchers with otherwise private data and catalyzes the development of high-quality data-driven research. Of course, both researchers and industry need to be careful about what and how data can be shared and analyzed not to compromise data and end-user privacy, where data (pseudo-)anonymization methods will play an important role. However, it is not only that a data-driven approach is nowadays possible for the EV-related research, but such an approach is sometimes necessary and very often it generates beneficial added value. There are various emerging research problems that cannot be tackled using traditional methods, such as mathematical programming. An example is the smart charging station management, i.e., deploying, removing, and re-allocating charging stations. There are numerous research initiatives that aim to solve this problem by not using real-world data that requires setting many assumptions, making them less accurate and consequentially lowering their applicability in real-world scenarios.
Based on the analysis in the paper, we identified two most important unsolved challenges in the research field of EVs, when observed from the perspective of data science: data acquisition and methodology for charging station deployment.
Sources of EV-related data nowadays exist, but are still scarce. The most common way of acquiring the data is either through the cooperation with private companies or through proprietary devices developed for the research purpose, what presents a major obstacle for producing high-quality data science research in the EV area even though the amount of data being generated from EVs and charging stations is growing every day.
The importance of the generalized methodology for charging station deployment is already elaborated in this paper. A potential solution lies in using open access to charging station data, which will enable designing and fine-tuning of advanced machine learning algorithms, and other data science approaches, for charging station deployment. In their future work, the authors plan to propose the data-driven computational framework for charging station deployment.
Author Contributions: Conceptualization of this research was performed by J.B., V.P. and D.P. Methodology, as well as the investigation was done by D.P. D.P. prepared the initial draft of the paper, while J.B. and V.P. performed reviewing and editing. Visualization and analysis was preformed by D.P., while J.B. and V.P. supervised the work and acquired funding.
Funding: This research received no external funding.
Acknowledgments: The authors acknowledge the support of the Croatian Science Foundation under the grant DOK-2015-10-1777. This research has also been partly supported by the European Regional Development Fund under grants KK.01.1.1.01.0009 (DATACROSS), KK.01.2.1.01.0020 (RASCO-FER-SMART-EV) and KK.01.2.1.01.0077 (bigEVdata). Conflicts of Interest: The authors declare no conflict of interest.
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Source & Publisher Item Identifier: Electronics 2019, 8(10), 1190; https://doi.org/10.3390/electronics8101190 Received: 2 September 2019 / Revised: 28 September 2019 / Accepted: 13 October 2019 / Published: 18 October 2019 (This article belongs to the Special Issue Electric Vehicles in Smart Grids). URL: https://www.mdpi.com/2079-9292/8/10/1190
Published by Milan ŠIMKO1, Daniel KORENČIAK1, Miroslav GUTTEN1, Richard JANURA1, University of Zilina, Slovakia (1)
Abstract. The first part of paper deals with the base information about diagnostics and analysis of transformer insulating system in time domain. The second part of paper deals with proposal measuring system of moisture analysis by return voltage method (RVM) for power oil transformers. RVM method is wide state specifying method which is not set in standards but in many cases is a method which is determining a clear and exact result. The results have mainly shows moisture content, content of conductive impurities in oil and degree of aging of paper insulation impact.
Streszczenie. Przedstawiono metody diagnostyki stanu izolacji transformatorów w czasie rzeczywistym. Zaproponowano nowy system diagnostyki bazujący na analizie wilgotności na podstawie napięcia powrotnego. Badania potwierdziły przydatność metody. (System diagnostyczny do analizy stanu izolacji transformatora bazujący na pomiarze napięcia powrotnego)
Keywords: Transformer, diagnostics, measuring system, insulation Słowa kluczowe: diagnostyka, izolacja w transformatorze, napięcie powrotne
1. Introduction
Influence of operating conditions leads to aging of individual parts of transformer, and also to changes of the major electrical and mechanical properties. To the check of the condition greatly contributes electro-technical diagnosis, whose main task is to find a clear relation between the change in functional characteristics of the machine and some measurable values. The assessment of these measured values must be visible not only the rate of change, but also whether it is a permanent or reversible state. The aim of diagnostics of transformers is to verify that the machine complies with the determined conditions in accordance with standards [1].
Economically reliable and effective power delivery always is the primary concern to utilities all over the world. Insulation diagnostics is one of the requirements for safe operation of transformers. Conventional methods to assessment of insulation condition are its loss factor, insulation resistance and partial discharge measurement, etc. These methods, however, provide only partial picture about the polarization processes in insulating material.
Deregulation of power market has increased the competition and also emphasized on the search for the new, efficient and effective methods for diagnosing the insulating system. The use of the return voltage method is significant way to detect ageing of the insulation of operating power transformer in a non-destructive manner [2].
To prevent a damage state of transformers, we perform different types of the measurements that should illustrate an actual condition of the measured equipment. It is therefore important to choose a suitable diagnostics for the right prediction of such conditions. [3, 4]
2. The Diagnostic Insulating Methods in Time Domain
The most often methods use measurements of winding resistance and impedance, voltage ratio, insulation resistance, winding capacities are also measured in some cases. If it is possible in terms of machine dimension partial discharges are measured or by means of acoustic sensors implemented directly on the machine. The thermal camera can capture the distribution of the temperature fields of machines in their surface under load, etc.
In last few years several diagnostic techniques have been developed and used to determine the power transformer insulation. That means this techniques must determine insulator composed from transformer oil and paper in main. Named techniques are DGA (Dissolved Gas Analysis), DP (degree of polymerization) and Furan analysis by HPLC (High Performance Liquid Chromatography). In nowadays is possible to capture very low current involved in dielectric relaxation process. This is door open to technique like RVM (Return Voltage Measurement) or PDC (Polarization Depolarization Current). Those techniques have been introduced in 90’s. This measurements technique has gained popularity for its ability to assess the condition of oil and paper separately without opening the transformer tank [5].
For PDC analysis is DC voltage step (amplitude U0) of some 100 V is applied between HW (high voltage) and LV (low voltage) windings during a certain time, the so-called polarization duration. Thus a charging current of the transformer capacitance, i.e. insulation system, the so called polarization current, flows. It is a pulse-like current during the instant of voltage application which decreases during the polarization duration to a certain value given by the conductivity of the insulation system. After elapsing the polarization duration, the switch goes into the other position and the dielectric is short circuited via the ammeter. Thus, a discharging current jumps to a negative value, which goes gradually towards zero. The simple measurement system of RVM method is shown in Fig.1 [6].
Fig.1. Principal scheme of RVM measurement system.
The RVM method consists of plotting the measured maximum response times with respect to the charging time, from which it is possible to determine the moisture content of the insulation in high-voltage oil equipment. In general, this method is intended for non-destructive, off-line determination of the state of the isolation system of transformers, cables or other devices that are comprised of the conductor and the insulator [7].
If the method is applied to an oil transformer, it determines the moisture content at the oil-paper dividing line. Measured values determine the time constant and the slope of the voltage response rise.
Based on the relationships listed in [8], paper moisture and conductivity in oil can be calculated with sufficient precision.
3. Introduced Measuring System for Determining Insulating State
The RVM method itself – measuring the voltage response of an insulating system, does not belong to Slovakia among the used methods. The proposal is based on the need to measure this method and because of the high cost of a narrowly specified commercial device. According to available information, this method is used in Slovakia only for measuring the insulation of the cables. Although this method was originally designed for cable diagnostics, its simplicity and precision in determining humidity is also applicable to high voltage electric machines.
The proposed system can therefore be used to analyze the paper moisture state of power transformers. Parameters of the proposed measurement system for diagnostics of transformers are presented in Table 1.
The source section of the device consists of two separate transducers. The main source for powering the ARDUINO platform is the rectifier from 230 VAC to 9 VDC / 2A. Despite the maximum supply voltage of 12 VDC, the output voltage of the stabilized voltage of 9 V is sufficient. The control voltage of the switching modules is from a separate inverter for protection. This voltage is at 5 VDC.
Fig. 2 shows a block diagram for the ARDUINO platform. This design also uses a back-up and at the same time a smoother source consisting of two series of connected 3.7 V batteries of type SJ 18650, each of which has its own protective charging circuit type TD1836.
TABLE I. PARAMETERS OF MEASURING SYSTEM BY RVM METHOD
.
Fig.2. Connecting of power supply to the measurement system platform.
The control voltage of the switching modules is powered by a separate rectifier to protect the control panel. This voltage is at 5 VDC. In the Fig. 3 are a block diagram and a signaling of switch point source points and a voltage divider that supplies a switching module K4.
Fig.3. Power supply connecting of switching modules.
In order to protect the platform’s sensitive circuits and switch module control, both inverters are secured by a fuse. The high-voltage part of the measuring instrument is made of a single-phase transformer with an output voltage of 2100 V. This voltage is then guided by a bridge rectifier composed of four PRHVP2A-20 high-voltage diodes and a series-parallel connection of capacitors of the MKPI 337 type with a resulting capacity of 667 nF for reliable smoothing of the given run.
Since the proposed measuring instrument used to measure the voltage response of the transformer isolation system, that is to say, is connected between the coupled HV (high-voltage) and LV (low-voltage) windings, the load resistance is at the GΩ level. This high value is almost empty. Therefore, the output voltage when measuring 50 MΩ load with high voltage resistor can reach 2000 VDC. The diagram of the connection of the high voltage part is in Fig.4.
Fig.4. Connecting of high-voltage and rectifier part of the system.
Output voltage control, measurement and short-circuit connection are realized by ARDUINO switching modules controlled by 5 V voltage. Fig. 5 shows the connection and marking of the terminals.
Fig.5. Connecting of control and measured part for transformer 22/0.4 kV.
The switching module K1 consists of a series connection of SRD-05VDC-SL-C switching relays on a four channel switching module to achieve the required electrical strength. The left section of Fig.5 represents the normal no voltage state of the connection, therefore the terminals T1 and T2 are connected via the discharging resistance Rv.
To achieve a higher degree of safety, the switching modules K1, K2 and K3 are switched by the K4 module. This and switching module K3 ensures that, when switched on, in which no measurement is made, there is no dangerous voltage between terminals G1 and G2.
The measuring part of the instrument consists of the voltage divider shown in Fig. A serial connection of ten 1 GΩ precision measuring resistors is used to provide input 10 GΩ. This divider is a voltage at the platform input with a 100 MΩ input at a maximum of 4.95 V. If the input voltage between the M1 and M2 terminals exceeds 500 V (5 V between AAI and GND), the two Zener diodes Z1 and Z2 provide a secure upper the voltage limit.
Fig.6. Connecting of measuring input of platform ARDUINO.
4. Experimental Measurement and Diagnostics
The evaluation of the measurement and therefore the determination of the moisture content in the paper part of the isolation system of the oil transformer 22/0.4 kV can be determined from the analysis of the charging time and the maximum Umax voltage response according to Fig. 7 till Fig. 9.
For this evaluation, it is sufficient to write real time and measured voltage to the SD card. From this stored text document, time and voltage values are evaluated on a separate PC in one of the available computational programs. These text documents are named as rvmx, where x is the serial number of the measurement. For better orientation, the creation time is also indicated.
Measurement of the voltage response depends largely on the temperature difference between the object and the surroundings. Since the measured transformer is unconnected to the grid and is located in the laboratory, the temperature difference is zero [9]. This is confirmed by measuring the winding temperature on the transformer by incorporating the Neoptix temperature probes and measuring the outside temperature with a 22 ° C by thermometer.
Measurement of return voltage by RVM consists of the four steps of Fig. 7 [10]. In the first step, the LV and HV transformer terminals are connected to the test voltage for the charging time tN. This step is called charging. In the second step, there is a discharge for tV = tN / 2, where the LV and HV terminals are short-circuited. In the third step, the measurement of the voltage response and the time itself is carried out until the maximum voltage is reached. The last fourth step of measuring the voltage response consists of a recovery before another cycle for a time at least equal to tN.
The time behaviour and the individual measurement steps are shown in the Fig. 7 and the graphical representation with the maximum value of the measured voltage response values, depending on the charging time, is shown in the Fig. 8.
The measurement of the voltage response of the insulation system consists of determining the moisture content of the paper part. This determination is derived from the characteristics of Fig. 9, which are obtained by actual measurements on samples of different humidity at different temperatures. These evaluation curves in another version are also reported in the literature [11].
Fig.7. The shape of the test voltage by RVM method.
Fig.8. The time behaviour of voltage response of the insulation system.
Fig.9. Evaluation curves for the voltage response measurement method.
In Fig. 9 is shown the intercept point for moisture content of 3.5% corresponding to the highest possible moisture value in the paper section of the transformer insulation. Since the moisture content was also controlled by the dielectric spectroscopy with method of frequency response and the result of the evaluation was the same, it is obvious that no significant amount of sludge is deposited on the paper. The suspension itself in the transformer oil does not have a more serious effect on the result of this measurement.
Conclusion
This experimental analysis with designed system can be used as new system platform for determination of moisture in power oil transformer.
The proposed system, in comparison with other commercial devices, can evaluate the moisture status of paper part in the transformer insulation.
The measuring method RVM are unique in terms of analysis of insulating system of oil power transformers. In comparison with other methods, the RVM method can evaluate the moisture condition of the insulation paper of the power transformer with high accurate. This high reliability in determining moisture in paper was shown by determining the same result (3.5%) on the same measured distribution transformer as in case other method by frequency dielectric spectroscopy.
Precise determination of moisture content is very complex, because in the measured object in which the maximum voltage response is reached at shorter charging times, the moisture content of the insulation is higher. The maximum voltage response of the new transformers at lower moisture is achieved with longer charging times, what is the problem. From high accurate measurement it has been found a necessary to attach a sufficiently large high voltage (minimal from 2 kV).
Workers from individual test centres formulated their proposals and suggestions during the preparation phase as well as realization phase of the system development. After the system was brought into life, measurements of transformers became easier, safer and more accurate – in accordance with requirements from valid IEC standards.
This work was partially supported by the Grant Agency VEGA from the Ministry of Education of Slovak Republic under contract 1/0471/20.
REFERENCES
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Authors: Assoc. Prof. Milan Šimko, PhD.; Assoc. Prof. Daniel Korenčiak, PhD.; Prof. Miroslav Gutten, PhD.; Ing. Richard Janura, PhD.; Faculty of Electrical Engineering and Information Technology of the University of Žilina Department of Measurement and Applied Electrical Engineering, Univerzitná 1, 010 26 Žilina, Slovak Republic, E-mail:gutten@fel.uniza.sk.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 96 NR 12/2020. doi:10.15199/48.2020.12.12
Published by Lorenzo Mari, EE Power – Technical Articles: How to Reduce the Soil’s Resistivity, September 28, 2020.
Lowering the resistance of a grounding electrode is not always enough to reduce the soil’s resistivity. Reducing the resistivity of the soil around the electrodes can help achieve adequate resistance to the ground.
The elements that most affect the soil resistivity are moisture content, ionizable salts, and porosity. Water and ionizable salts combine to form an electrolyte, which is a conductor of electricity. Porosity is an indicator of the ability of the soil to retain the electrolyte.
Some methods to reduce the soil resistivity include:
• Water retention • Chemical salts • Bentonite • Chemical-type electrodes • Ground enhancement materials
Water Retention
Most soils lose moisture when they receive direct sunlight. The sun heats the ground and causes the water contained in it to rise to the surface and vaporize, dispersing in the atmosphere. The longer the heating process, the drier the soil.
Excessive drainage can also quickly leach away the salts in the soil and dry out the deeper layers.
The water molecules ionize the minerals in the soil and cause them to become conductive. Without moisture, an electrical connection to earth is not possible. Figure 1 shows how the resistivity varies as a function of moisture content for various types of soil.
Figure 1. Soil resistivity and moisture content
As observed, there is a strong association between the water content and the resistivity of soils. The best soil types require a minimum of 4% water (by weight), while the poorest require at least 14%.
Some areas of the world have soils with more than enough moisture. Others, however, have none. Deserts are mainly arid and have little to no soil moisture. A simple standard copper rod will not serve as a ground connection in these places unless water is added to the soil.
The soil may or may not retain an appropriate amount of moisture, according to its degree of porosity. This condition directly impacts both the distribution of the content of ionizable salts and the formation of the electrolyte. Improving the soil’s ability to retain moisture is an effective way to decrease its resistivity.
Where there is no moisture, providing it will achieve reasonable grounding. It is only required to moisten the soil near the grounding electrode. For the best results, the electrode/soil interface should be wet.
An infrequent but effective way to maintain soil moisture is to plant vegetables around the grounding electrodes. Vegetables retard runoff and retain irrigation water and salts in the soil, helping to keep the area moist and the salts dissolved.
An irrigation system is also helpful in keeping the soil moist. The installation of an automatic moisture sensing and water supply system, in combination with a conventional water supply system, can precisely control the moisture content of a given soil.
Chemical Treatments
The use of ion-producing chemical compounds like sodium chloride, magnesium sulfate (epsom salt), copper sulfate (blue vitriol), and calcium chloride around the grounding electrodes, decreases the soil’s resistivity and the electrode’s resistance to ground.
The most widely-used chemical is magnesium sulfate. It is low-cost, has strong electrical conductivity, and has little corrosive effect.
Ordinary rock salt is cheap. Common salt (sodium chloride) is highly corrosive. This corrosive effect may cause nearby metal objects to deteriorate. Despite being an excellent conductor of electricity, its adverse effects remove it from the list of preferred chemicals.
The chemical treatment indirectly increases the diameter of the electrode by modifying its surrounding soil. When the soil is porous, the solution permeates quickly into a large volume of earth, making a large equivalent diameter, with quick results. In contrast, when the soil is compact, the chemicals take time to spread, and results are produced more slowly.
A practical way of applying these compounds is through a circular trench excavated around the ground rod, preventing direct contact with the electrode (Figure 2).
Figure 2. Soil treatment with a circular trench. Image based on a drawing from IAEI.
It can be beneficial to supply a little water through a pipe to accelerate the effect of the salts. The amount of water should be sufficient to keep the area moist, but without washing away the salts.
The chemicals are gradually washed away by natural drainage through the soil and rainfall, requiring periodic replacements. The period for replacement varies depending on site conditions, but it may be years. An adequate maintenance scheme will ensure that chemicals will have long-lasting effects.
A particular characteristic of the chemical treatment is the reduction of seasonal variations of the resistance to ground. These variations come from the periodic drying and wetting of the soil.
Use caution, as local authorities may prohibit the use of chemicals if they are not considered environmentally friendly.
Bentonite
Adding bentonite to the soil reduces its resistivity and the ground resistance of the electrodes.
Bentonite is a fine-grained, highly plastic clay, formed by volcanic action. It may be used as soil replacement and filler material for electrical grounding in places with high resistivity. The conductive Bentonite clay is a sodium activated montmorillonite. Bentonite is chemically hydrated, innately stable, and retains its properties over time.
Bentonite absorbs moisture from the surrounding soil and swells up to several times its dry volume. It adheres to the surface of the grounding rods and cables laid in trenches, reducing the contact resistance and increasing their diameter artificially.
The resistivity of bentonite depends on the water content. The water inside the pores allows the electrical currents to move through the bentonite. The resistivity value is lower in the liquid state than in the plastic or solid state and is on the order of 250 Ω∙cm at 300% moisture.
In addition to reducing the resistance to ground of rods and cables, the moisture retention process of the bentonite compound protects against corrosion.
Bentonite performance is highly dependent on the amount of rainfall, soil moisture, and temperature at the site. In hot climates, the soil dries out, and the bentonite does not work as desired. It may separate from the electrodes, increasing the resistance to the ground.
Chemical Rods
Chemical rods are suitable for high resistivity soils — rock, mountain tops, sandy soil — and places with excessively high or low temperatures.
This type of rod is a tube filled with mineral salts distributed evenly. It has holes along its length, allowing the entry of soil moisture. The moisture combines with the salts and dissolves them. The saline solution then seeps out through the holes and soaks into the surrounding soil, continuously conditioning a large volume around it.
The materials available are copper, stainless steel, and hot-dipped galvanized iron. Its length choices are the same as conventional rods: 240 cm (8 ft) and 300 cm (10 ft).
They may be installed by drilling holes in the ground, and, for rocky soils, manufacturers offer horizontal rods. It is customary to place a grounding enhancement fill around the rod to improve the interface with soil.
These rods also require maintenance. For this, they have a removable cap for inspection and chemical supply (Figure 3).
Figure 3. Chemical rod. Image courtesy of Lightning Eliminators
Grounding Enhancement Fill
Replacing all or part of the soil around an electrode with a low resistivity filler will facilitate the achievement of low ground resistance. The higher the percentage of earth swapped, the lower the ground resistance.
A grounding enhancement fill may have resistivities as low as 50 Ω∙cm (much lower than bentonite). It works in a trench, around a ground rod or substation grounding conductors, either dry or in a slurry.
The main properties are: constant resistance, low resistivity, maintain moisture, stability, low freezing point, resistance to leaching, non-corrosive, and maintenance-free.
Reviewing How to Reduce the Soil’s Resistivity
When the grounding electrode does not achieve a low enough resistance, another option is to reduce the resistivity of the soil. There are several methods to accomplish this: water retention, chemical salts, bentonite, chemical-type electrodes, and ground enhancement materials.
While all methods are effective, the selection will depend on the site’s particular conditions and the ability to carry out proper maintenance when required.
Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.
Published by Rafael ALIPIO, Renan SEGANTINI, Federal Centre of Technological Education of Minas Gerais
Abstract. This paper assesses the transient distribution of potentials along a grounding grid subjected to currents representative of first and subsequent strokes. It is shown that the transient non-uniform distribution of potential along the grounding system may lead to the flow of impulsive current between pieces of equipment grounded at distinct points. The methodology presented in this paper is useful in determining engineering actions to reduce the risks of electromagnetic disturbances propagation due to uneven potential distribution along grounding grids.
Streszczenie. Obliczono chwilowe rozkłady potencjałów w uziomie kratowym podczas odprowadzania prądów piorunowych pierwszego i kolejnych wyładowań głównych. Nierównomierny rozkład potencjału może prowadzić do przepływu prądów impulsowych pomiędzy urządzeniami uziemionymi w różnych punktach. Metoda jest użyteczna do ustalenia środków redukujących zagrożenie związane z nierównomiernym rozkładem potencjału w systemie uziomowym. (Propagacja zaburzeń elektromagnetycznych w uziomie kratowym podczas odprowadzania prądów piorunowych).
Keywords: lightning response of grounding, transient analysis of grounding, transient potential distribution, multiport wideband model. Słowa kluczowe: odpowiedź uziomu na udar piorunowy, analiza uziomu w stanie nieustalonym, chwilowy rozkład potencjału, szerokopasmowy model wielowejściowy.
Introduction
Extended meshed earthing systems, called grounding grids, are commonly used in large installations, such as substations, to protect and safeguard personnel and equipment against the hazards and devastation that may be caused by the flow of fault currents [1]. They also provide reference voltages for electrical and electronic systems.
The grounding grids are usually designed considering only low-frequency occurrences (50/60-Hz ground-fault currents) [2]. However, the transient response of grounding grids may be also important, for instance, when they are fed by lightning currents [3]. This can occur when lightning directly strikes the substation components or when it strikes spans of power lines near the substation. In both cases, a noticeable portion of the current is driven to the ground.
When subjected to lightning currents, the grounding grid response presents certain complexities that make its behaviour quite different from that presented at low frequency [4]. Due to the impulse nature of lightning currents, they present a wideband frequency content ranging from dc to several MHz. In this frequency range, the grounding system shows different behaviour at different frequency intervals. Among other aspects, this frequency dependent behaviour of grounding leads to an uneven potential distribution along the grounding grid [3].
The non-uniform distribution of potentials along the grounding grid may be source of electromagnetic disturbances. For instance, it is common in modern substations or industrial plants the existence of electrical panels in the control room that are responsible for remote commanding the operation of some equipment installed in the substations yard. In many cases, the equipment and the panel are grounded at distinct points of the grounding system (see Fig. 1 of reference [3]). Hence, when the grounding is subjected to lightning currents, the resulting non-uniform distribution of potentials may cause the flow of impulsive currents through the closed path between the equipment at the substation yard and the electric panel at the control room. Such loop currents are source of electromagnetic disturbances, causing equipment malfunctions, failures and damage.
The objective of this work is to make a sensitivity analysis of the potential distribution in a grounding grid subjected to impulsive currents. The present paper is an extension of the previous analysis developed by the first author in [3], considering two main new aspects. First, in order to simulate the wideband behaviour of the grounding grid, an accurate multiport model is developed, which can be promptly included in widespread time-domain electromagnetic transient tools, such as ATP-EMTP, EMTPRV, and PSCAD. This multiport model allows simulating the grounding system in conjunction with the substation components, and is suitable for developing accurate electromagnetic transient studies using time-domain tools. Secondly, realistic lightning current pulse waveforms are used, which reproduce the observed concave rising portion of typical measured lightning currents.
Modelling of grounding systems
As mentioned, lightning currents present a wideband frequency content ranging from dc to several MHz. Therefore, to develop accurate analysis of the transient response of grounding systems, their frequency-dependent behaviour should be considered. To this aim, a wideband model of the grounding grid is obtained as briefly described in next paragraphs.
The wideband response of the grounding grid is determined using the accurate Hybrid Electromagnetic Model (HEM) [5], in a frequency range from dc to several megahertz. In particular, HEM is used to determine the grounding admittance matrix Yg(s) over the frequency range of interest [6]. The grounding admittance matrix physically relates the vector of nodal voltages of grounding system and the vector of injected current into each grounding node. The Hybrid Electromagnetic Model solves Maxwell’s equations numerically via the vector and scalar potentials using the thin wire approximations [5]. The calculations are performed in frequency domain and, if required, time domain results can be obtained by means of inverse Fourier or Laplace transform. The accuracy of the results provided by this model in terms of the impulse response of grounding was proved by comparison with experimental results, considering different grounding arrangements (for instance, horizontal electrodes and rods in [7] and large grids in [8]).
After calculating the frequency response of the grounding grid, a pole-residue model of the calculated nodal admittance matrix Yg(s) is obtained. The objective is to calculate a pole-residue model (1) which approximates (“fits”) the original data as close as possible.
.
In case of a physical system, the admittance matrix Yg(s) is symmetrical. Hence, Rm, D and E are also symmetric, being D and E real matrices. In this work, E is set equal to zero, D is related with the low-frequency response of grounding and the sum of rational functions represents the frequency response of grounding. The approximated model Yfit fits the results calculated using the accurate electromagnetic model.
To obtain a pole-residue model (1), the Vector Fitting (VF) technique is used [9]. First, the pole-residue model of the grounding system admittance matrix is obtained. Then, in order to obtain stable time-domain simulations, the passivity is enforced by perturbation of model parameters. Further details regarding the VF and the passivity enforcement by perturbation can be found in [9, 10].
Finally, once the passive pole-residue model of the grounding system admittance matrix is obtained, it can be represented in the form of an electrical network, which can be promptly included in time-domain simulations. Considering this approach, the rational functions can be easily converted into basic network elements (R, L, C). The network has branches between all nodes and ground, representing the diagonal elements of Yfit, and between all nodes, representing the off-diagonal elements of Yfit. Once determined the equivalent electrical network, it can be imported directly into time-domain electromagnetic transient tools.
Developments
We consider a square grounding grid of 60 m x 60 m, composed of square meshes with space between conductors of 5 m, as depicted in Fig 1. The conductors are constructed from copper with 7-mm radius and the grid is buried at a depth of 0.8 m in a uniform soil. There different values of soil resistivity ρ are considered, 300, 1000 and 3000 Ωm, comprising low, moderate and high values of resistivity. The relative permittivity is assumed εr=10 and the relative permeability is assumed μr=1. In a conservative approach, the frequency dependence of the electrical parameters of soil is neglected [7].
In this study we have used two lightning current waveforms corresponding to the typical first and subsequent return strokes, based on observations of Berger et al. [11], according to [12], Fig. 2. The current waveforms are chosen by Rachidi et al. [12] to fit typical experimental data and are reproduced by means of a sum of Heidler’s functions [13]. It should be stressed that subsequent stroke, which has larger rate of rise of the front, has higher frequency content in comparison with the first stroke, as mentioned in [14]. On the other hand, first stroke currents have larger energy content, due to their higher amplitude and longer duration, in comparison with subsequent strokes.
It is assumed that the discharge directly strikes the lightning protection system of the substation and the current is distributed by down-conductors through the four corners of the grid. The resultant Grounding Potential Rise (GPR) developed in points A and B of the grid, see Fig. 1, are then calculated.
The multiport wideband model of the grounding system was obtained according to Section II. It is worth mentioning that both the pole-residue model and the electrical network were obtained using the public domain calculation package for rational approximation of frequency dependent admittance matrices available in [15]. All time-domain simulations presented in the next sections were developed in the Alternative Transients Program (ATP) [16].
Fig.2.
Fig.2. The first return-stroke current pulse is characterized by a peak value of 30 kA, zero-to-peak time of about 8-μs and a maximum steepness of 12 kA/μs, whereas the subsequent return stroke current has a peak value of 12 kA, zero-to-peak time of about 0.8-μs and a maximum steepness of 40 kA/μs
Results of Grounding Potential Rise (GPR)
Before analyzing the results, it is important to state some basic aspects concerning the propagation of current and voltage waves along buried bare conductors in soil. The wave propagation is dictated by the medium propagation constant, which is given approximately by
.
for a given angular frequency ω. In particular, the attenuation of the wave is related with the real part of the propagation constant, called attenuation constant (α). It increases with frequency and with medium conductivity. Thus, larger attenuation of voltage and current waves propagating along bare conductors buried in soils of higher conductivity (lower resistivity) is expected. Similarly, current and voltage pulses of shorter front times are expected to suffer stronger attenuation, due to their higher frequency content.
Figs. 3, 4 and 5 illustrates the GPRs developed in points A and B respectively for soil resistivity of 300, 1000 and 3000 Ωm, in response to current pulses representative of (a) first and (b) subsequent strokes. Based on the results, two main periods can be distinguished in the transient behavior of grounding grid: 1) a fast transient period and 2) a slow transient period.
In the fast transient period, the propagation and inductive effects are pronounced. In this period, the distribution of potentials along the grounding grid is not uniform, since the voltage wave experiences a strong attenuation as it propagates from the current impression points. In the analysed case, the non-uniform potential distribution is related with the transient potential difference between earth terminations A and B, vAB(t). In order to state a criterion to judge whether the potential distribution is more or less uniform, the ratio between the peak value of vAB(t) and the peak value of the transient potential developed in point A, vA(t), is calculated. The larger this ratio, the more non-uniform the potential distribution. Considering the results of Figs. 3, 4 and 5, for soils of 300, 1000 and 3000 Ωm, the ratios between the peaks of vAB(t) and vA(t) are around 37% and 99%, 13% and 92%, 4% and 45%, respectively for first and subsequent strokes. Thus, the more conductive the soil is, the more the potential distribution is non-uniform. This is due to the fact that the attenuation effects are much more significant in soils of higher conductivity. Furthermore, note that the differences between the curves of GPR along the fast transient period are more pronounced in case of subsequent strokes, due to their higher frequency content in comparison with first stroke currents.
Fig.3. GPRs developed in points A and B for a soil resistivity of 300 Ωm in response of currents representative of (a) first and (b) subsequent strokes
Fig.4. Same of Fig. 3, but for a soil resistivity of 1000 Ωm
Fig.5. Same of Fig. 3, but for a soil resistivity of 3000 Ωm
In the slow transient period, the GPR curves of points A and B present a similar behaviour and are basically coincident, indicating that all the points of the grounding grid are at the same potential. This behaviour is associated with the tail of the impressed current waves, which contain the low-frequency components of the current. Thus, during this period the propagation and inductive effects are negligible and the grounding grid presents a uniform potential distribution and can be assumed to be equipotential across its area.
Results of Impulsive Loop Currents and Energy Dissipated
Figs. 6-8 illustrate the transient potential difference between earth terminations A and B, vAB(t), respectively for soil resistivity of 300, 1000 and 3000 Ωm, considering both (a) first and (b) subsequent strokes.
Fig.6.
Fig.6. Transient potential difference between earth terminations A and B for a soil resistivity of 300 Ωm, considering the impression into the grounding grid of current pulses representative of (a) first and (b) subsequent strokes
Fig.7. Same of Fig. 6, but for a soil resistivity of 1000 Ωm
Fig.8. Same of Fig. 6, but for a soil resistivity of 3000 Ωm
It can be seen that, along the fast transient period, the potential differences between the two earth terminations are significant and present short rise-time, mainly in case of subsequent strokes. Such potential differences may lead to the flow of impulsive loop currents between equipment grounded at distinct earth terminations and connected among each other, for instance, by control or communication cables. Along the slow transient period, there is no current flowing between the equipment, since, there is no potential difference within the grid area (the grid is at a constant potential).
The heating resulting from the energy dissipated while the loop current flows into and through a “victim” circuit is the source of damage. The lightning parameter that is most closely related to this effect is the specific energy or action integral [17]. The response of a “victim” is represented by its equivalent resistance. The dissipated energy, and therefore associated damage, can be roughly estimated as the product of the specific energy by this resistance [17].
In order to make a first assessment of the damage caused by the flow of loop currents, Fig. 9 illustrates the energy dissipated considering the application of the voltages depicted in Figs. 6-8 to a normalized equivalent resistance of 1 Ω. The figure also includes results of further simulations developed for the same grounding system buried in a soil of 100 Ωm.
It can be seen from Fig. 9 that the trend of higher energy dissipation in case where the grid is fed by currents of first strokes is inverted with increasing the soil resistivity. This interesting finding can be explained as follows. Due to the propagation characteristics in high-resistivity soils (lower attenuation and higher propagation velocity), the potential distribution is more uniform along the grounding grid in case of first stroke currents, which present lower frequency content in comparison with subsequent strokes. Thus, in spite of the higher energy content of first stroke currents, in case of grounding systems buried in soils of high resistivity, the energy dissipated by impulsive loop currents tends to be more pronounced considering subsequent currents striking the substation.
Fig.9.
Fig.9. Energy dissipated considering the application of the voltages depicted in Figs. 6-8 to a normalized equivalent resistance of 1 Ω, including additional simulations for the same grid buried in a soil of 100 Ωm
Summary and conclusions
This paper assessed the transient distribution of potentials along a grounding grid subjected to currents representative of first and subsequent strokes. The methodology presented in this paper is useful in determining engineering actions to reduce the risks relative to the non-uniform potential distribution along grounding grids when subjected to lightning currents. Considering the grid analyzed in this paper, one practical measure consists of connecting the two distinct earth grounding points with an aerial conductor to a metal bar, preferably located at the midpoint between the two points. Then, this bar is connected to the earth grounding grid by means of a proper conductor. Depending on the configuration of the power plant this solution is not always feasible due to physical limitations, or even due to cost constraints. In such cases, it is essential to know the distribution of potentials along the earth grounding grid, especially when it is subjected to lightning currents, in order to define alternative solutions. In particular, the proper installation of surge protective devices at the terminal of sensitive equipment can be done based on the accurate knowledge of the transient potential distribution along the grounding grid.
This work was supported in part by The State of Minas Gerais Research Foundation (FAPEMIG), under grant TEC – APQ-02017-16.
REFERENCES
[1] L. Grcev, Transient Electromagnetic Fields Near Large Earthing Systems, IEEE Trans. Magnetics, 32 (1996), No. 3, 1525–1528. [2] IEEE Guide for Safety in AC Substation Grounding, IEEE Std.80 (2013). [3] R. Alipio, M. A. O. Schroeder, and M. M. Afonso, Voltage distribution along earth grounding grids subjected to lightning currents, IEEE Trans. Industry Applications, 51 (2015), No. 6, 4912–4916. [4] S. Visacro, A comprehensive approach to the grounding response to lightning currents, IEEE Trans. Power Delivery, 22 (2007), No. 1, 381–386. [5] S. Visacro and A. Soares Jr., HEM: a model for simulation of lightning-related engineering problems, IEEE Trans. Power Delivery, 20 (2005), No. 2, 1026–1208. [6] R. Alipio and Fellipe M. S. Borges, Multiport wideband modeling of large substation grounding grids for transient analysis, in Proc. 10th Asia-Pacific International Conference on Lightning (2017), 313–317. [7] R. Alipio and S. Visacro, Impulse efficiency of grounding electrodes: effect of frequency dependent soil parameters, IEEE Trans. Power Delivery, 29 (2014), No. 2, 716–723. [8] S. Visacro, R. Alipio, C. Pereira, M. Guimarães, and M. A. O. Schroeder, Lightning response of grounding grids: simulated and experimental results, IEEE Trans. Electromagnetic Compatibility, 57 (2015), No. 1, 121–127. [9] B. Gustavsen and A. Semlyen, Rational approximation of frequency domain responses by vector fitting, IEEE Trans. Power Delivery, 14 (1999), 1052–1061. [10] B. Gustavsen, Fast passivity enforcement for pole-residue models by perturbation of residue matrix eigenvalues, IEEE Trans. Power Delivery, 23 (2008), No. 4, 2278–2285. [11] K. Berger, R. B. Anderson, and H. Kroninger, “Parameters of lightning flashes,” Electra, no. 41, pp. 23–37, 1975. [12] F. Rachidi, W. Janischewskyj, A. M. Hussein, C. A. Nucci, S. Guerrieri, B. Kordi, and J.-S. Chang, “Current and electromagnetic field associated with lightning-return strokes to tall towers,” IEEE Trans. Electromagn. Compat., vol. 43, no.3, pp. 356–367, Aug. 2001. [13] F. Heidler, “Analytische blitzstromfunktion zur LEMPberechnung,” in Proc. 18th Int. Conf. Lightning Protection, Munich, Germany, 1985, pp. 63–66. [14] L. Grcev, “Impulse efficiency of ground electrodes,” IEEE Trans. Power Del., vol. 24, no. 1, pp. 441–451, Jan. 2009. [15] B. Gustavsen, Matrix Fitting Toolbox [Online]. Available: https://www.sintef.no/projectweb/vectfit/, 2009. [16] L. Prikler, H.K. Hoidalen, ATPDraw Manual, Version 5.6, 2009. [17] CIGRE Lightning Parameters for Engineering Applications, Working Group C4.407, Aug. 2013.
Authors: prof. Rafael Alipio, Department of Electrical Engineering, Federal Center of Technological Education of Minas Gerais, Av Amazonas, 7675, postal code: 30510000, Belo Horizonte, Brazil , E-mail: Rafael.Alipio@des.cefetmg.br; Renan Segantini, E-mail: renan_nikel@hotmail.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 94 NR 2/2018. doi:10.15199/48.2018.02.02
Published by Wojciech BĄCHOREK1, Mariusz BENESZ1, AGH University of Science and Technology (1)
Abstract. This paper presents the influence of distributed generation DG and reclosers placement on the reliability of distribution network. The brute force method and evolutionary algorithm were used to solve the optimization task. The placement of the switches was determined independently using two criteria. The first criterion is the reserve factor, while the second criterion is the SAIDI index. The proposed methods were tested on a real medium voltage distribution system in two variants of operation: with and without DG.
Streszczenie. W artykule przedstawiono wpływ generacji rozproszonej i lokalizacji reklozerów na niezawodność sieci dystrybucyjnej. Zastosowano algorytm przeglądu zupełnego oraz algorytm ewolucyjny. Lokalizację reklozerów ustalono stosując dwa kryteria: współczynnik rezerwowania oraz wskaźnik SAIDI. Obliczenia zrealizowano dla rzeczywistej sieci średniego napięcia. (Wpływ generacji rozproszonej na rozmieszczenie łączników sekcjonujących w sieci średniego napięcia).
Keywords: distribution networks reliability, reclosers, SAIDI, distributed generation. Słowa kluczowe:niezawodność sieci dystrybucyjnych, łączniki sekcjonujące, SAIDI, generacja rozproszona.
Introduction
One of many important optimization problems solved for distribution networks is the sectionalizing switches placement problem (SPP). This problem is still very important due to the growing popularity of distributed generation sources (DG). Distributed generation improves voltage levels, reduces power losses and improves reliability of the network [1, 2, 3]. Sectionalizing switches, that allow network reconfiguration, should be placed taking into account DG sources. Reconfiguration efficiency is ensured by reclosers [4, 5] and remote-controlled switches [6, 7].
Different methods and objective functions are used to solve the SPP problem. Calculation methods of solving SPP problem may be divided into two groups: based on the classical mathematical methods (for example the mixed integer linear programming [1, 8] or the fuzzy method [9]) and based on the heuristic methods (for example the ant colony optimization algorithm [3, 4, 10], the genetic algorithm [5, 10, 11], the particle swarm optimization method [12, 13] or the taboo search algorithm [14]). Regardless of the calculation method applied, objective functions based on different criteria are assumed. The most applied objective functions are cost criterions and reliability indices. The cost of the switches is taken into account in the articles [3, 4, 12, 14]. The authors of the papers [8, 15] took into account the maintenance costs of switches. In [11], the impact of the cost of DG sources on the placement of sectionalizing switches was considered. SAIDI, SAIFI indices are considered in [4, 9]. The main task of the article is to analyze the placement of sectionalizing switches in the real distribution network with DG. This analysis was carried out independently using two criteria: reserve factor ρ [16, 17] and SAIDI index [4, 9].
The solution sought should correspond to the highest or smallest value of the objective function, respectively for the first and the second criterion. Regardless of the criterion applied, a brute force method and an evolutionary algorithm were used.
First of all, the optimization problem was formulated – the benefits from the installation of sectionalizing switches and the impact of distributed generation on reliability were presented. Next, a description of solution method based on the evolutionary algorithm and a computational example are presented. The calculations were carried out separately for the two selected criteria. In each case, three levels of DG were taken into account.
Problem formulation
A. Idea of sectionalizing switches installation
An example of a simple distribution network is shown in Fig. 1. This is a typical medium voltage power line. Two sectioning switches divide the network into three sections called X, Y and Z. An additional power supply (DG) is connected to the Z section.
Fig.1. The idea of installing sectionalizing switches.
In case of a failure, different scenarios of shutdowns in the network are possible. The first scenario concerns the situation when it is not possible to provide back-up power to the network by closing a switch normally open at the tie point (power supply from MFP B). The second scenario, unlike the first one, assumes the possibility of supplying backup power to the line. In both scenarios, the possibility of a back-up supply using DGs could be considered. This may mean supplying a part of the network only from an additional source (island operation in first scenario) if the load demand of the section to which the DG is connected does not exceed the DG capacity.
In case of a failure of e.g. Y section, the recloser separating X and Y sections is opened and disconnects from the power supply of Y and Z sections. Then, after opening the switch between the Y and Z sections, it is possible to restore power to the Z section. This is possible in the second scenario or in the first scenario with adequate DG capacity.
The proper placement of switches is therefore an important optimisation problem for distribution networks. Sectioning switches ensure reduction of energy not supplied (ENS) [15] to customers and improvement of network reliability indices, which are of interest to Distribution System Operators (DSO).
B. Reserve factor
One of the many criteria for selecting optimal sectionalizing switch placements is the reserve factor ρ (1). This factor was defined and applied in the calculations described e.g. in works [16, 17].
.
where: E – sum of all customers’ energy in the network, Ei – energy of customers who have been disconnected as a result of a failure in i-th section, NS – number of all sections in the network, NBi– number of branches in the i-th section, λij – average failure rate of j-th branch in i-th section, rij– average outage time of j-th branch in section i.
The minimum value of the reserve factor is 0. The maximum value of the factor is 1. The solutions with the highest reserve factor shall be selected from among all the solutions of the switch placement.
C. SAIDI
The objective function of placement of switches in the distribution network may also be SAIDI index. SAIDI is one of the basic reliability indices used by DSO. This index is calculated and published by DSO on the basis of registered power interruption incidents. SAIDI is an index of an average system duration outages in the supply of electricity expressed in minutes per customer per year. This is a sum of the interruption duration multiplied by number of customers exposed to the effects of the interruption during the year, divided by the number of customers connected to the network (2). Among all the solutions of the switch placement, a solution with the lowest SAIDI value is selected.
.
where: NK – number of power delivery points, Nk – number of customers in k-th power delivery point, Uk – annual duration of unscheduled interruptions in k-th point.
The annual duration of unscheduled interruptions Ukis given by (3):
.
where: NC – total number of possible fault locations, λi – average failure rate of distribution elements grouped together (section), ri – average outage time of distribution elements grouped together.
Index (2) was calculated with a statistical approach based on combinatorial reliability analysis. Dedicated software developed by the authors takes into consideration: the types of sectionalizing switches, their locations and the possibilities of the alternative supply of the line.
Calculation method
In order to determine optimal switch placements, two calculation algorithms were used. If a small number of switches (1 or 2 switches) were assumed, a brute force method (complete overview of the solutions) was carried out. An evolutionary algorithm was used to locate at least 3 sectionalizing switches. This has resulted in a reduction of the calculation time. The solution of a task is written in a form that is “elgible” to the evolutionary algorithm. This is realized with the use of the real coding method.
The solution is expressed in a sequence of numbers called a chromosome (Fig. 2). The number of string elements is equal to the number of placed switches (SN). The location of the switch (SL) in the network is determined at each position of the string in the form of a suitable index.
Fig.2. Chromosome structure
Their initial population (specified in number) is random. The evolutionary operators are elaborated for creating new solutions. The selection, crossover and mutation operators were used. Stochastic sampling with replacement mechanism was used in the selection procedure. The procedure of a one point crossover of individuals was applied.
The calculations were made using the computer program written in the C ++ programming language.
Case studies
A. The examined MV distribution network
The proposed method is tested on 15 kV modified real-life distribution network located in Poland [17]. The system with 87 MV/LV substations is shown in Fig. 3. This system has 348 possible sectionalizing switches locations. The installation of reclosers is assumed. The total length of the power line is 74.73 km while the lateral branches are 51.98 km. The total peak load 4134 kW. There are 1152 customers connected to the network. The failure rate of MV branches is λ = 0.08 f/yr.km and the mean time to repair is r = 3.83 h/f.
The calculations were carried out in two variants. The first variant assumes that it is not possible to provide backup power to the network by closing the normally open switch. The second one allows such a possibility. Additionally, in a selected point of the network, the connection of a dispersed generation was allowed. The following three power sources were considered: PG1 = 630 kW, PG2 = 1105 kW, PG3 = 2066 kW. These variants of the DG were named respectively G1, G2 and G3. The variant without DG was named as G0.
There are two tie points in the network. They provide the possibility of back-up power supply for the analysed network by closing the normally open switch (tie point). The size of the reserved network depends on the type and place of failure and the number and location of sectionalizing switches. In the power line (Fig. 3) it is not possible to provide backup power to the entire network via a tie points. For more information on backup power limitations, see [17]. In order to solve the task of sectionalizing switches placement, two criteria were applied independently: the maximizing the value of the reserve factor and the minimization of SAIDI. Two scenarios related to the possibility of back-up power supply are being considered:
• scenario I: it is not possible to provide back-up power to the network by closing a switch normally open at the tie point (with and without DG),
• scenario II: backup power supply is possible by closing the switch at the tie point (with and without DG).
The analysed MV power line meets the requirements of the DSO with respect to current load capacity and voltage levels. These requirements are described in the work [17]. The following evolutionary algorithm parameters were chosen: size of population = 24, crossover probability = 0.8, mutation probability = 0.03, number of generations = 3000.
Fig.3. Diagram of analyzed MV power line
B. Analysis of calculation results
In this chapter, the results of the placement of the reclosers are presented. Calculations were made for two previously described scenarios, in each of them for different number of reclosers. The placement of 1 to 8 switches was assumed. The optimal placement of the switches was determined independently for both criteria ρ (1) and SAIDI (2). The location of the DG source in the medium voltage power line is shown in Fig. 3.
For the analysed network without sectionalizing switches, the reserve factor ρ = 0, SAIDI = 1329.2 min/cus.y. and ENS = 575.5 MWh.
The optimum sectionalizing switch placements according to scenario I for networks without and with DG source have been presented in Table 1. The values of SAIDI have been also shown in Fig. 4.
Table 1. Results of the sectionalizing switch placement (scenario I)
.
Table 2. Results of the sectionalizing switch placement (scenario II)
.
The optimum placements of reclosers according to scenario II for networks without and with distributed generation (DG) source have been presented in Table 2. The values of SAIDI have been also shown in Fig. 5.
Fig.4. SAIDI for different number of switches (scenario I)
The presented results confirm three directions of improvement of reliability of distribution networks. They rely on: possibility of reconfiguration and back-up supply of the network from other sources than during its normal operation (compare scenario I and scenario II), connection of DG sources to the network (compare variants G0 to G3) and installation of sectioning switches (note the differences in location from 1 to 8 switches) – Fig. 4 and Fig. 5.
Fig.5. SAIDI for different number of switches (scenario II)
In this paper the influence of the number of reclozers and their placements on the value of SAIDI index was analysed. Though reclosers are not always justified economically [17], they significantly reduce the unreliability of the distribution networks. The results obtained for the presented real distribution network confirm the above observations. In this paper, apart from SAIDI results, the results of the reserve factor were presented. For all cases, energy not delivered to consumers was also calculated. The results obtained for both criteria are almost identical in all cases. However, it is not possible to conclude on this basis that both criteria are interchangeable. These indices depend in part on different parameters (see (1) and (2)). This is due to the assumption that there is only one type of customer in the network under analysis, with the same average power demand.
The installation of a DG source improves the reliability indices but the change in the value of the indices depends on the numbers of sections and customers and also the load demand in each section. It can be seen that the calculation algorithm intends to determine the sections in such a way that it would make the best use of the capacity of the source. In other words, in the case of island operation (DG is the only power source), the DG source supplies only those sections whose load demand does not exceed the capacity of the source.
Conclusion
The article solved the problem of the placement of sectionalizing switches (reclosers) in the real medium voltage network. An evolutionary algorithm was used to solve this problem. In addition, the DG source was taken into account. The computational example illustrates tile effectiveness of the proposed method. For the analysed network, almost identical results were obtained by performing independent calculations for two different criteria (reserve factor ρ, SAIDI index). Future research will focus on calculations for many sources of distributed generation in the distribution network.
REFERENCES
[1] Heidari A., Agelidis V.G., Kia M., Considerations of sectionalizing switches in distribution networks with distributed generation, IEEE Trans. Power Del., 30 (2015), no. 3, 1401-1409 [2] Mao Y., Miu K.N., Switch placement to improve system reliability for radial distribution systems with distributed generation, IEEE Trans. Power Syst., 18 (2003), no. 4, 1346-1352 [3] Falaghi H., Haghifam M.R., Singh C., Ant colony optimization-Based method for placement of sectionalizing switches in distribution networks using a fuzzy multiobjective approach, IEEE Trans. Power Del., 24(2009), no. 1, 268-276 [4] Tippachon W, Rerkpreedapong D., Multiobjective optimal placement of switches and protective devices in electric power distribution systems using ant colony optimization, Elect. Power Syst. Res., 79 (2009), 1171-1178 [5] da Silva L.G.W., Pereira R.A.F., Mantovani J.R.S., Optimized allocation of sectionalizing switches and control and protection devices for reliability indices improvement in distribution systems, IEEE/PES Transmission and Distribution Conference and Exposition: Latin America, 2004, 51-56 [6] Xu Y., Liu C.C., Schneider K.P., Ton D.T., Placement of remote-controlled switches to enhance distribution system restoration capability, IEEE Trans. Power Syst., 31 (2016), no.2, 1139-1150 [7] Carvalho P.M.S., Ferreira L.A.F.M., Cerejo da Silva A.J., A decomposition approach to optimal remote controlled switch allocation in distribution systems, IEEE Trans. Power Del., 20 (2005), no.2, 1031-1036 [8] Siirto O.K., Safdarian A., Lehtonen M., Fotuhi-Firuzabad M., Optimal distribution network automation considering earth fault events, IEEE Trans. Smart Grid, 6 (20015), no.2, 1010-1018 [9] Bernardon D.P., Sperandio M., Garcia V.J., Russi J., Canha L.N., Abaide A.R., Daza E.F.B., Methodology for allocation of remotely controlled switches in distribution networks based on a fuzzy multi-criteria decision making algorithm, Elect. Power Syst. Res., 81 (2011), 414-420 [10] Teng J.H., Liu Y.H., A novel ACS-based optimum switch relocation method, IEEE Trans. Power Syst., 18 (2003), no. 1, 113-120 [11]Nematollahi M., Tadayon M., Optimal sectionalizing switches and DG placement considering critical system condition, 21st Iranian Conference on Electrical Engineering (ICEE), 2013, 1-6 [12] Golestani S., Tadayon M., Optimal switch placement in distribution power system using linear fragmented particle swarm optimization algorithm preprocessed by GA, 8th International Conference on the European Energy Market (EEM), Zagreb, Croatia, 25-27 May 2011, pp. 537–542 [13] Bezerra J.R., Barroso G.C., Leão R.P.S., Sampaio R.F., Multiobjective optimization algorithm for switch placement in radial power distribution networks, IEEE Trans. Power Del., 30 (2015), no. 2, 545-552 [14] da Silva L.G.W., R.A. Pereira R.A.F., Rivier Abbad J., Sanches Mantovani J.R., Optimized placement of control and protective devices in electric distribution systems through reactive tabu search algorithm, Elect. Power Syst. Res., 78 (2008), 372-381 [15] Billinton R., Jonnavithula S., Optimal switching device placement in radial distribution system, IEEE Trans. Power Del., 11 (1996), no. 3, 1646-1651 [16] Bąchorek W., Optimal arrangement of sectionalizing switches in medium voltage distribution network, Przegląd Elektrotechniczny, 90 (2014), no. 4, 24-27 [17] Bąchorek W., Benesz M., Influence of Sectionalizing Switches Placement on the Continuity of Customers Power Supply, Progress in Applied Electrical Engineering (PAEE), Koscielisko, Poland, 18-22 June 2018
Authors: dr inż. Wojciech Bąchorek, AGH University of Science and Technology, Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering, Department of Electrical Engineering and Power Engineering, 30 Mickiewicza Av., 30-059 Krakow, Poland, E-mail: wojbach@agh.edu.pl; dr inż. Mariusz Benesz, AGH University of Science and Technology, Faculty of Electrical Engineering, Automatics, Computer Science and Biomedical Engineering, Department of Electrical Engineering and Power Engineering, 30 Mickiewicza Av., 30-059 Krakow, Poland, E-mail: mariusz.benesz@agh.edu.pl.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 96 NR 4/2020. doi:10.15199/48.2020.04.24
Published by Lorenzo Mari, EE Power – Technical Articles: Power System Grounding: Understanding Lightning Strikes, November 13, 2020.
Learn about the fundamentals of lightning strikes and the risk they pose for electric power systems and operator safety.
Lightning is an electrical discharge of the accumulation of electrostatic electricity from cloud to cloud, within a cloud, or from cloud to Earth.
Lightning poses a stark danger, leading electric supply industries to systematically study atmospheric discharges and their impact on electrical power systems. This article emphasizes the lightning that takes place between the cloud and the electric power system.
The Lightning Problem
Due to its extraordinary manifestation and the hazards for both lives and structures, lightning is a phenomenon that impacts society. Research around lightning develops valuable tools and procedures to recognize severe thunderstorms and protect people and equipment.
Figure 1. Lightning poses a hazard to both living creatures and structures it comes in contact with.
Earlier lightning investigations gathered an understanding of the discharge process. Nowadays, many researchers worldwide continue to successfully resolve the many remaining questions. However, the complexity of the phenomena challenges quick and complete explanation.
A lightning strike is a high current discharge that lasts only several millionths of a second.
A widely accepted theory of lightning holds that clouds acquire charge or are at least polarized. They contain separate negative and positive charges that attract opposite polarity charges within the cloud and between it and neighboring masses such as the Earth and other clouds, creating strong electric fields.
The potential gradient in the air between charge centers within a cloud or between a cloud and Earth is not uniform but is greatest where the charge concentration is highest. The highest charge concentration and most significant voltage gradient in cloud-to-Earth discharges generally occur in the cloud.
Whenever the voltage gradient reaches the dielectric limit for air, the air in the high-stress concentration region ionizes. A breakdown or lightning flash occurs — this current discharge is frequently of high magnitude.
While intracloud and cloud-to-cloud lightning do not create a direct hazard to structures or persons on the ground, the induced voltages in long cables present a risk to control and signal equipment employing electronic or semiconductor devices.
Around 90% of the discharges logged by the National Lightning Detection Network in the USA are cloud-to-cloud. Cloud-to-cloud lightning constitutes dangerous electromagnetic interference (EMI) threats to aircraft, prompting avionics designers to study them.
Cloud-to-ground lightning discharges are of utmost interest because of their hazards to life and property. The high current flowing during the lightning flash can melt conductors, ignite fires, damage equipment, trip power circuits, and produce deadly voltages for living creatures.
Despite its short duration, lightning is the most significant single cause of power outages, as concluded in many operating companies’ reports worldwide. Lightning strikes that produce problems for the power engineer happen on or near a power system.
The Accumulation of Electricity in Clouds
There are several theories to explain the accumulation of electricity in clouds. Two of them are:
• Wilson’s Ionization Theory • Simpson and Scrase’s Breaking-drop Theory
Wilson’s Ionization Theory
C.T.R Wilson (1920) explains his theory by following the progress of water droplets through the rising air currents of a thunderstorm, attributing the droplets’ electrification to contact with air ions. The atmosphere commonly presents many small positive or negative ions with mobilities of about 1 cm/s under the action of a 1 V/cm field. There are also many large ions of much smaller mobility.
According to Wilson, the number of these ions increases in thunderclouds due to the substantial electrical fields. The raindrops falling or rising in the air currents of the thunderstorm shall meet the ions. These drops are polarized with a positive charge on their lower surface and a negative charge on their upper surface, which is the field’s normal direction. Later, they attract negative charges to themselves.
The atmosphere contains clusters of ions of both signs at all times and the raindrops capture them selectively, acquiring a negative charge and leaving a preponderance of positive charge in the air. Updrafts carry the positive air and lighter drops to the top of the cloud. The larger raindrops bring a negative charge to the base of the cloud. Thus, according to Wilson, the cloud’s upper region becomes positively charged and the lower area becomes negatively charged. The polarization of most thunderclouds happens this way.
Simpson and Scrase’s Breaking-Drop Theory
Simpson and Scrase (1937) made investigations in clouds with instruments sent up in balloons, measuring the potential gradient’s magnitude and its polarity throughout its height and at different portions of the cloud. They also measured the electric field at the Earth’s surface beneath a thundercloud as it passed overhead.
According to this theory, when water droplets break-up they get a positive charge and the surrounding air obtains a negative charge.
Figure 2 shows what Simpson and Scrase believe happens in the cloud. Positive and negative signs indicate the charges within the cloud. The cloud’s progress is right to left, and the solid lines represent streamlines of air, with their separation proportional to the wind velocity. This separation shows the high winds that appear as the storm approaches.
Figure 2. A generalized diagram showing air currents and distribution of electricity in a typical heat thunderstorm. Simpson and Scrase, 1937.
The air enters the storm from the left and passes under the cloud’s front, where it takes an upward direction. This upward current prevents raindrops from falling through it. Drops falling in this region are broken up, and the charges separate.
The lower region of positive charge is associated with a strong upward current. To the rear of this region, the vertical wind is weaker and the resulting heavy rain is positively charged. Apart from this local area of positive charge, the lower half of the cloud is negative and the top is positive.
The region of separation between the negative charge and the upper positive charge occurs at levels with temperatures between 0°C and -20°C. These temperatures are below the freezing point. For this reason, the deduction is that the generation of the upper charge depends on the presence of ice crystals and not on the existence of water drops.
The air near the top of the cloud tends to become positively charged, while the negatively charged ice crystals move slowly down to melt and recycle or fall as rain. The position of the lower positive charge supports the idea that the breaking-drop process generates it.
Simpson and Robinson confirmed these conclusions in 1941. These intricate and active charge patterns create conditions favorable for a lightning strike.
The Mechanism of a Lightning Strike
Schondland et al. gave an excellent description of lightning in a series of papers published from 1934 to 1938. A lightning strike to Earth starts when the charge along the cloud base produces a concentration of opposite charge on the Earth (Figure 3).
Figure 3. The cloud leads to the accumulation of opposite charges on the Earth.
Whenever the voltage gradient reaches the limit for air, the air in the region of high-stress concentration ionizes or breaks down, producing an ionized channel to Earth. The electric field intensity to cause breakdown at atmospheric pressure is approximately 30 kV/cm. In the cloud, considering the moisture content and lower pressure, the voltage gradient is lower, on the order of 10 kV/cm.
Observations made with the “Boys camera” – developed by Charles V. Boys in 1926 to produce a time-resolved image of the phenomenon – indicate that the breakdown creates a stepped leader strike.
Figure 4. Charles V. Boys with his camera developed specifically to take pictures of lightning.
The stepped leader is a discharge that progresses somewhat unexpectedly by short steps from the cloud to the Earth. Figure 5 shows a schematic diagram of a Boys camera.
Figure 5. Schematic diagram of a Boys camera. C.V. Boys, 1926.
The cloud’s charge flows through the ionized channel, sustaining the high voltage gradient at the channel’s tip, keeping the breakdown process ongoing. The establishment of a lightning strike is a gradual breakdown of the arc path instead of the air’s instantaneous breakdown for the total channel’s length.
Figure 6 shows downward leaders spreading from a cloud to Earth.
Figure 6. Stepped leaders propagate toward Earth.
A leader step is about 50 m long, completed in approximately 1 µs. The leader takes at least several microseconds to reach the Earth’s surface due to the irregular path and pauses between pushes.
The leader’s direction is toward Earth, but every step’s specific angle of departure is random. Each step approaches Earth at a different angle, giving the overall lightning flash its typical zigzag appearance.
The reason for the step leader recesses seems to be a depletion of the charge centers, reducing the electric gradient at the tip below the critical value for ionization at that position. The leader progresses quickly when receiving a new charge from the cloud.
In 1958, Griscom proposed the prestrike theory as a stepping mechanism. This theory states that a discharge similar to the leader rises from Earth to meet the leader before it reaches the ground. As the stepped leaders approach the Earth, the electric field at the surface grows until it exceeds the critical magnitude to originate upward connecting strikes. Then, upward strikes, usually from high points in the vicinity, intercept the downcoming leaders (Figure 7).
Figure 7. A lightning strike to Earth, showing upward strikes.
The launch of an upward strike from Earth starts the attachment process. When downward and upward discharges meet, they complete the connection.
A high-current power return strike moves quickly up the leader´s ionized channel after connection to Earth. This strike is more intense and faster than the leader. The result is the neutralization of the charge in the leader’s channel or the channel’s gradual discharge to Earth (Figure 8). The leader and the return strike contribute to transport charge from cloud to ground.
Figure 8. Power return strikes from Earth to cloud.
The leader originating the first return strike takes what looks like an optically intermittent course. Frequently, there will be several strikes to Earth down the initial channel. What looks like a single flash of lightning is the effect of several high-amplitude, short-duration current impulses or strikes — as many as 30 or 40.
The leaders triggering the return strikes that follow move continuously as a downward dart through the preceding return strike path and are called dart leaders.
The Empire State Building Study
What happens when the ground is a tall object, like a building, tree, or electric power line?
Between 1935 – 1941, McEachron and colleagues photographed strikes on top of the Empire State Building in New York City employing the Boys camera. The study was discontinued during the war and resumed in 1948.
The Empire State Building is a steel-frame structure topped by a tower reaching a height of 380m. An elaborate procedure using a set of instrumentation was employed to record as much data as possible. A fundamental discovery made was that in virtually all cases, the first stepped leader advanced upward from the top of the building to the cloud, rather than downward from the cloud as found in flat land. Only in a few cases did they find the original stepped leaders were downward.
There wasn’t a return streamer from the cloud after the upward stepped leader. But succeeding discharges consisted of a continuous downward leader and an upward return streamer.
Another discovery was that a small current, perhaps of a few hundred amperes, continued to flow between current peaks. The researchers concluded that it was a direct-current arc likely to persist for the strike’s entire duration with superimposed current peaks of several magnitudes.
More recent research determined that upward lightning discharges occur only from entities taller than about 100m or bodies of lesser height stationed on mountain tops.
A Review of Lightning Research and Characteristics
Lightning surges and strikes can be very destructive to life and power system equipment. They are a frequent cause of power outages and damage to property.
The buildup of electricity in clouds is associated with ionized air, moisture in the atmosphere, and upward winds.
The impact of ice on ice in the cloud’s upper regions may produce a separation of electric charge, similar to raindrops breaking.
Usually, the lower portion of the cloud is mostly negative, and the upper part mainly positive, with a region of mixed charge at levels with temperatures between 0°C and -20°C.
Another mechanism in the accumulation of charges is the water to ice transition in the cloud.
Photographs of lightning strikes taken with the Boys camera led to the following conclusions regarding the mechanism of the lightning strike to relatively flat terrain or low structures:
• Most strikes recorded originated from negative polarity clouds. • The process opens with a stepped leader flowing from cloud to Earth. • Each leader approaching Earth instigates upward connecting strikes. • After connection to Earth, a high-current power return strike flows rapidly up the leader´s ionized channel. • Successive strikes have a continuous or dart leader proceeding downward from the cloud. • The strikes consist of many separate discharges.
A study on the Empire State Building discovered a difference in the strike mechanism: most of the original stepped leaders proceed upward from the top of the building to the cloud, rather than downward from the cloud as is the case with flat terrain and lower structures, and no return streamers followed. The following discharges’ stepped leaders were downward from cloud to Earth, and all the return strikes were upward from Earth to cloud.
Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.
Published by Łukasz TOPOLSKI, Jurij WARECKI, Zbigniew HANZELKA, AGH University of Science and Technology
Abstract. Harmonic currents in power cables cause additional power losses associated with phenomena that increase the temperature of the cable insulation and make its service life shorter. For these reasons, it is important to choose methods for determining active power losses, which ensure adequate computational accuracy. This paper compares the methods for determining active power losses on the example of a low voltage cable line supplying a non-linear load.
Streszczenie. Przepływ wyższych harmonicznych prądu przez linie kablowe skutkuje powstawaniem dodatkowych strat mocy czynnej związanych z ujawnianiem się niekorzystnych zjawisk, które prowadzą do wzrostu temperatury izolacji oraz skrócenia czasu jej życia. Z powyższych względów ważną kwestią staje się wybór metod wyznaczania strat mocy czynnej zapewniających odpowiednią dokładność obliczeń. W artykule przeprowadzono porównanie metod wyznaczania strat mocy czynnej na przykładzie linii kablowej niskiego napięcia zasilającej nieliniowe obciążenie. (Metody wyznaczania strat w liniach kablowych z obciążeniem nieliniowym).
Keywords: higher harmonics, skin effect, proximity effect, additional power losses. Słowa kluczowe: wyższe harmoniczne, zjawisko naskórkowości, efekt zbliżenia, dodatkowe straty mocy.
Introduction
Polyethylene (PE) has been used as electrical insulation in underground distribution and transmission class power cables for over three decades. The polyethylene in power cables is a special grade, which has cross-linked molecules to allow it to deal with extremely high temperatures without melting or flowing under load. The operating temperature of XLPE insulated power cables is 90oC and its service life is estimated at approximately 30 years under purely sinusoidal currents. The flow of harmonic currents through power cable causes additional active power losses associated with the higher rms value of current and the appearance of adverse phenomena, which include the skin effect, the proximity effect and the impact of the metallic cable screen (shield), which, in turn, lead to increased insulation temperature and shorter cable service life.
Failures of commonly used power cable are a great nuisance for consumers and a cause of considerable financial losses for companies. Due to the above, it is becoming increasingly more important to define accurate, fast and simple methods for determining additional power losses, in order to reduce power cable line load with the fundamental current harmonic, to prevent it from overheating.
The aim of this paper is to analyse methods for determining active power losses in cable lines with single-strand and multi-strand conductors, operating in environment rich in current harmonics. By way of example, authors performed calculations of the active power losses for an actual low voltage power cable line supplying an electrostatic precipitator assembly in an industrial facility. The results obtained with different methods were compared and discussed.
Nature of cable losses under a non-linear load
Power losses in cable lines are Joule’s losses, caused by the current flowing through a conductor. The basic term describing Joule’s losses is defined as a product of the value of current square and conductor resistance (I2 * R). In case of supplying a non-linear load, the current of that load contains higher harmonics, and the resistance become dependent on the frequency. That dependency of resistance on current frequency is one of the causes for the additional power losses in power cable lines.
The value of equivalent resistance of a conductor for alternating current is impacted by the following physical phenomena. The first phenomenon is the skin effects, which reveal that the current in a conductor does not flow evenly throughout the entire cross-section but only a part of it, depending on the current frequency (fig. 1). The higher the frequency, the closer the current flows to the outer surface of the conductor (current density decreases from the surface towards the inside).
Fig. 1. Skin effect visualization
The skin effect is mainly caused by eddy currents originating from the electromotive force induced in a conductor by the electromagnetic field generated by the primary current flowing through the conductor. Eddy currents cause the primary current to fade in the center of the conductor and strengthen the flow in its upper layers.
Another physical phenomenon impacting the resistance of a conductor is the proximity effect. The effect appears in sets of two or more conductors located close to each other. If the current in the conductors flows in the same direction, the biggest current density appears in the conductor parts most remote from each other (fig. 2). In contrast, when the current flows in opposite directions, the biggest current density can be found in conductor parts closest to each other (fig. 3).
The proximity effect, similar to the skin effect, is also caused by eddy currents. Primary current flowing through one of the conductors generates a time varying magnetic field, which then induces electromotive force in the second conductor, which, in turn, forces the flow of eddy currents, causing primary current densification in a part of the conductor depending on its flow direction.
Both the skin effect and the proximity effect cause the current density to be nonuniform in the cross-section of the conductor and cause higher cable losses.
Fig.2. Proximity effect visualization for coherent current flow directions
Fig.3. Proximity effect visualization for opposite current flow directions
The increase in active power losses in a cable line is also influenced by a phenomenon caused by the impact of the metallic cable screen (shield) (if a given cable has it). Primary current flowing through an operating conductor induces electromagnetic force in a metallic cable screen (shield), forcing the flow of eddy currents in this part of the cable, which results in additional active power losses and increased temperature, thus limiting the maximum capacity of the cable line.
Resistance determination methods
Cable conductor resistance for harmonic current h, with respect to the skin effect, the proximity effect and the impact of the metallic cable screen (shield), is expressed by the relationship [1,3,5,7]
.
where: RDC– DC conductor resistance [Ω], xs– resistance increment in a cable conductor caused by the skin effect, xp – resistance increment in a cable conductor caused by the proximity effect, xa– resistance increment in a cable conductor caused by the impact of the metallic cable screen (shield).
In order to determine the resistance increment coefficients due to the skin effect, the following methods are applied for practical computations.
S1 Method. The cable conductor resistance increment coefficient, with respect to the skin effect, is determined with the use of the Bessel functions [1, 3, 5, 7]
.
where: μ – magnetic permeability of a cable conductor material [H/m], γ – conductor conductivity [m/Ω*m2], kS – correction coefficient depending on a cable conductor design (kS = 1 – single-strand conductor and kS = 0,4 – multi-strand conductor), s – cable conductor cross-section [m2], f – grid rated frequency [Hz], h – harmonics order, J0, J1 – Bessel functions of the first kind of zero and one order, respectively.
S2 Method. According to this method, the cable conductor resistance increment coefficient, with respect to the skin effect is determined by the dependence [8]
The relationship (4) holds true, if the condition d ≫ δ is met.
S3 Method. The cable conductor resistance increment coefficient caused by the skin effect is determined based on the relationship [4]
.
where: p – conductor circumference [m].
S4 Method. The international standard IEC-60287-1-1 [6] recommends determining the resistance increment coefficient with respect to the skin effect, based on the relationship
.
The cable conductor resistance increment coefficients, due to the proximity effect, may be determined with the use of the following methods.
P1 Method. Calculating the resistance increment coefficient based on the Bessel functions [1, 3, 5, 7]
.
where
.
where: kP– correction coefficient depending on the cable conductor design (kp = 1 – single-strand conductor kp= 0,3 – multi-strand conductor), D – distance between axes of conductors [m].
P2 Method. The international standard IEC-60287-1-1 [6] recommends determining the cable conductor resistance increment coefficient, with respect to the proximity effect, based on the relationship
.
The cable conductor resistance increment coefficient, due to the impact of a metallic cable screen (shield), is determined by the dependence [1]
.
Determination of power losses in cable lines
Active power losses under the flow of distorted current by a three-phase four-conductor cable line with the same cross-section area are a sum of losses in the phase conductors ΔPL and the neutral conductor ΔPN [4]
.
Power losses in the phase conductors and the neutral conductor can be divided into loss components for the fundamental (h = 1) and higher harmonics current (h = 2,3,…) in the form of
.
where
.
and
.
where: IA1, IB1, IC1– rms values of fundamental harmonic phase currents [A], IN1 – rms value of fundamental harmonic neutral conductor current [A], ΔPLh, ΔPNh – additional power losses in the phase conductors and the neutral conductor [W].
When distorted currents flow through a cable line, the rms values of currents are determined with the relationships
.
where: IF – rms value of distorted phase current (F∈A, B, C) [A], IN – rms value of distorted neutral conductor current [A].
Active power loss increase caused by the flow of distorted current in proportion to the losses caused by the flow of the fundamental harmonic current is determined by the relationship [1]
.
According to [2] a derating factor for a cable line with flowing symmetrical distorted currents is calculated based on the assumption that IF1 is an rms value of fundamental harmonic current
.
hence
.
Comparison of loss determination methods on a selected example
Description of the analysed object
The selected example is a four-conductor low voltage cable line supplying an electrostatic precipitator assembly operating in a cogeneration industrial plant. Apart from the electrostatic precipitator assembly, the cable line also supplies such load as rappers, electrostatic precipitator substation lighting, welding sockets, monitoring, control cabinets and external lighting [9].
The calculations involved a comparison of the supply systems with a YAKXS 4x185mm2 cable line (without a metallic screen), with single-strand sectoral SE and multi-strand RMC aluminum conductors. The length of the cable line in the analysed system is 300 metres. Table 1 shows the specifications of the cable line supplying the electronic precipitator assembly.
Table 1. Specifications of a YAKXS 4x185mm2 cable
.
Figure 4 shows a cross-section of the analysed cable line.
Fig.4. Cross-section of a YAKXS 4×185 mm2 cable developed based on the specifications [10]
Figure 5 shows a schematic diagram of the electrostatic precipitator assembly supply system.
Fig.5. Electrostatic precipitator power supply system diagram [9]
In table 2 are given rms values of higher harmonics current and the THDI coefficient of current recorded at peak load in low voltage switchgear, supplying the electrostatic precipitator assembly.
Table 2. RMS values of higher harmonic currents recorded in an electrostatic precipitator assembly power supply system [9]
.
Comparison of resistance increments determination methods
Using the methods S1-S4, the resistance increments of a single-strand sectoral SE cable conductor were determined. The results obtained are shown in figure 7.
Fig.7. Resistance increment xs(h) in a YAKXS 4x185mm2 cable single-strand sectoral SE conductor
Based on figure 7, it can be seen that resistance increments determined with methods S1-S3 provide very similar results for the entire spectrum of higher harmonics present in the system. The biggest discrepancies were obtained when using the S4 method, which become even bigger with increasing harmonic order.
Fig.8. Resistance increment xs(h) in a YAKXS 4x185mm2 cable mutli-strand RMC conductor
Figure 8 also shows resistance increments due to skin effect, but for multi-strand RMC conductor. It can be noted that results of resistance increment most convergent with the S1 method were obtained using the S4 method. This is due to the fact that methods S1 and S4 include correction coefficients, which depend on the design of a cable conductor. Whereas methods S2 and S3 do not have such coefficients.
Fig.9. Resistance increment xp(h) in a YAKXS 4x185mm2 cable single-strand sectoral SE conductor
Figure 9 shows resistance increments for a single-strand sectoral SE conductor due to the proximity effect. As can be noted, resistance increment determined with the P2 method using the relationships recommended by the international standard IEC-60287-1-1 is very similar to the resistance increment determined with the P1 method. A minor increase of the discrepancies between the two methods can be seen from about the 20th harmonic.
Fig.10. Resistance increment xp(h) in a YAKXS 4x185mm2 cable mutli-strand RMC conductor
Figure 10 also presents resistance increments due to the proximity effect, but in a multi-strand RMC conductor. In this case, regardless of the method selected, the results obtained are identical for the entire harmonics spectrum. Resistance increments for a single-strand sectoral SE cable conductor, with respect to the skin effect and the proximity effect are shown in figure 11.
Fig. 11. Total resistance increment xs(h)+xp(h) in a YAKXS 4x185mm2 cable single-strand sectoral SE conductor due to the impact of higher harmonics
As can be seen in figure 11, the most divergent resistance increments values were obtained when using the relationships recommended by the international standard IEC-60287-1-1 (a combination of methods S4 and P2).
Fig.12. Total resistance increment xs(h)+xp(h) in a YAKXS 4x185mm2 cable multi-strand RMC conductor due to the impact of higher harmonics
In contrast, for a multi-strand conductor cable (fig. 12), the situation is different. In this case, resistance increments values most convergent with the increments obtained with the Bessel functions (a combination of methods S1 and P1) were obtained using the relationships recommended by the international standard IEC-60287-1-1 (a combination of methods S4 and P2).
Power losses in a cable line
Using the determined total resistance increments for a cable conductor (fig. 11 and fig. 12), and based on the relationships (15) – (26), the power losses for the cable line in question, a percentage loss increments and a derating factors were determined. The loss computations were conducted for two cases – assuming the absence of higher harmonics in the system and assuming their presence.
The computations assumed that the electrostatic precipitator assembly is a symmetrical load; hence, the fundamental current harmonic does not flow through the neutral conductor. Calculation results are shown in tables 3 and 4.
Table 3. Power losses, percentage loss increments and derating factors in a YAKXS 4x185mm2 cable line with single-strand sectoral SE conductors
.
Table 4. Power losses, percentage loss increments and derating factors in a YAKXS 4x185mm2 cable line with multi-strand RMC conductors
.
Conclusions
The paper discusses the causes of additional active power losses in cable lines supplying non-linear loads, and presents and compares the methods used to determine them. Based on the analysis conducted, the following conclusions can be formulated:
1) The design of cable conductors impacts loss size. Multi-strand conductors are characterized by lower resistance increments, which results in smaller losses compared to single-strand sectoral conductors.
2) Power losses, both for a single-strand and a multi-strand cable conductors, are characterized by small discrepancies, regardless of the selected resistance increment determination method. This is associated with the fact that the electrostatic precipitator power supply system contained current harmonics of up to the 25th order, and harmonics of up to the 15th order had significant amplitudes. All of the presented methods for the determination of increment coefficients for resistances up to the 15th harmonic are characterized by small discrepancies between the values obtained; hence, the differences in the resulting losses are minor.
3) When determining losses in cable lines with single-strand conductors, operating in an environment with current harmonics with the majority of amplitudes up to circa 20th order, any resistance increment coefficient determination method can be applied. Whereas, in the presence of current harmonics above the 20th order, methods S1 – S3 and P1 – P2 are recommended to be used for the determination of resistance increments. The relationships defined in method S4 in this range of harmonics lower the resistance increment, which will lead to lower power loss results.
4) When determining the losses in cable lines with multi-strand conductors, the relationships recommended by the international standards IEC-60287-1-1 can be used as a method for the determination of resistance increment coefficients alternative to the methods based on Bessel functions (combination of the S1 and P1 methods).
REFERENCES
[1] Degeneff R.C., Halleran T.M., McKernan T.M., Palmer J.A., Pipe – type cable ampacities in the presence of Harmonics. IEEE Transactions on Power Delivery, 8 (1993), No. 4, 1689 – 1695 [2] Demoulias C., Labridis D. P., Dokopoulos P. S., Gouramanis K. Ampacity of Low-Voltage Power Cables Under Nonsinusoidal Currents, Power Delivery IEEE Transactions on, 22 (2007), 584-594 [3] Desmet J. et al., Simulations of losses in LV cables due to nonlinear loads, Power Electronics Specialists Conference, PESC 2008, IEEE Conference, 2008, 785 – 790 [4] Ducluzaux A., Cahier technique no.83 – Extra losses caused in high current conductors by skin and proximity effects, Schneider Electric, (2002), 7 [5] Hiranandani A., Calculation of cable ampacities including the effects of harmonics, IEE Industry Applications Magazine, 4 (1998), No. 2, 42-51 [6] IEC 60287-1-1, Electric cables – Calculation of current rating – Part 1: Current rating equations (100% load factor) and calculation of losses – Section 1: General, 2006. [7] Kot A., Nowak W., Szpyra W., Tarko R., Analysis of impact of nonlinear loads on losses in power network element, Przegląd Elektrotechniczny, 88 (2012), nr 8, 327 – 328 [8] Popovic Z., Popovic D., Chapter 20 The Skin Effect, Introductory Elektromagnetics, Prentice – Hall, ISBN 978 – 0201326789, 1999, 387 [9] Warecki J., Hanzelka Z., Gajdzica M., Wskaźniki jakości dostawy energii elektrycznej w sieci zasilającej elektrofiltry przemysłowe – analiza przypadku, Przegląd Elektrotechniczny, 90 (2014), nr 4, 86 – 87 [10] Tele-Fonika Kable S.A., Kable i przewody elektroenergetyczne, Katalog 2015, 157
Authors: mgr inż. Łukasz Topolski, Akademia Górniczo-Hutnicza w Krakowie, Wydział Elektrotechniki, Automatyki, Informatyki i Inżynierii Biomedycznej, Katedra Energoelektroniki i Automatyki Systemów Przetwarzania Energii, Al. Mickiewicza 30, 30-059 Kraków, E-mail: lukas.topolski@gmail.com; prof. dr hab. inż. Jurij Warecki, Akademia Górniczo-Hutnicza w Krakowie, Wydział Energetyki i Paliw, Katedra Podstawowych Problemów Energetyki, Al. Mickiewicza 30, 30-059 Kraków, E-mail: jwarecki@agh.edu.pl; prof. dr hab. inż. Zbigniew Hanzelka, Akademia Górniczo-Hutnicza w Krakowie, Wydział Elektrotechniki, Automatyki, Informatyki i Inżynierii Biomedycznej, Katedra Energoelektroniki i Automatyki Systemów Przetwarzania Energii, Al. Mickiewicza 30, 30-059 Kraków, E-mail: hanzel@agh.edu.pl
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 94 NR 9/2018. doi:10.15199/48.2018.09.21
Published by Lorenzo Mari, EE Power – Technical Articles: AC Equipment Grounding: How Dangerous are Electric Shocks?, July 31, 2020.
This article looks at the effects of AC electric current on the human body.
An electric current passing through the human body can cause death. On many occasions, the cause of death is in the heart, which, subjected to intense and irregular activity, is exhausted and stops. Even a small amount of current that enters through the hand and exits through one or both feet could pass through the heart.
Effects of Electrical Shock
Many people think high voltage causes fatal shocks. However, numerous accidents occur by manipulating low voltage systems. The effects of a shock depend on the magnitude and duration of the current, frequency, physical attributes of the individual, gender, and trajectory through the body. The higher the voltage, the higher the current through the body, under any given set of circumstances.
Sources on the physiological effects of electrical currents are relatively abundant, but they quote broadly disagreeing figures. Table 1 shows commonly accepted values and the consequences when applying 60Hz, AC electrical currents to the body. It shows a 500Ω, 70kg, adult, who is grasping a live conductor with both hands and closing the circuit by standing with both feet in the water.
Table 1 Current range and effect of 60 Hz AC on a 70 Kg, 500 Ω, adult
Current (60 Hz)
Physiological phenomenon
Feeling or lethal
< 1 mA
None
Imperceptible
1 mA
Perception threshold
1-3 mA
Mild sensation. Let-go
3-10 mA
Painful sensation. Let-go
> 10 mA
Paralysis threshold of arms
“No let-go” or freezing. Cannot release handgrip; if no grip, the victim may be thrown clear (may progress to higher current and be fatal)
30 mA
Respiratory paralysis (asphyxiation)
Stoppage of breathing (frequently fatal)
75 mA
Fibrillation threshold percentile 0.5%
Heart action uncoordinated (probably fatal)
250 mA
Fibrillation threshold Percentile 99.5%
4 A
Heart paralysis threshold (no fibrillation)
The heart stops for the duration of the current passage. For short shocks, may restart on the interruption of current (usually not fatal from heart dysfunction)
≥ 5 A
Tissue burning
Fatal when burning vital organs
.
Most data, especially the data concerning the current levels required to cause fibrillation, are extrapolated from experiments with animals. Most of these experiments are often fatal and not suitable for human beings.
The response to electrical current is approximately proportional to 1/√t. However, there is wide variability among individuals. A more massive subject requires more current for the same physiological effect.
The “no let go” or freezing figure of 10mA causes a temporary paralysis of the extensor or flexor muscles, rendering the shock victim incapable of releasing the current source. The paralysis may also cause tensing of the muscles, pushing the victim away from the source, and maybe saving his or her life.
The respiratory paralysis at the 30mA level may cause death, but it is also reversible if the current is removed promptly. The muscular paralysis will cease, and breathing will resume.
The 75mA figure for ventricular fibrillation is the value that will cause that effect in approximately 0.5% of the population, and the remainder 95.5% will require contact with more significant currents. Ventricular fibrillation is a medical condition where the heart ends its blood pumping role and beats at a fast rate, with eventual brain damage and death due to insufficient oxygen.
A person with ventricular fibrillation may recover without intervention, but this event is extremely unusual. The basis for restoring the heart to regular activity is to stop it by applying a high current. Hopefully, the heart will resume its regular pumping action after disconnecting the current.
The criteria in many international standards to design grounding mats is to keep the magnitude and duration of the current applied to the human body to values below those that can cause ventricular fibrillation of the heart.
The Electrical Resistance of the Body
The electrical resistance of a human being depends on the following factors:
Physical condition. Nature of the points where the current enters and leaves.
• The dry skin has a high resistance, approximately 100kΩ at low voltage. The epidermis, which is the outermost thin layer of the skin, has high resistance because it is nonvascular, i.e., lacks the blood supply. In the range of 500V – 1 000V, the resistance drops to about 1kΩ.
• The dermis is a thick layer of skin beneath the epidermis that has little resistance because it is interlaced with blood vessels, to provide nourishment and waste removal for both dermal and epidermal cells. And the blood contains mineral ions, which increase its electrical conductivity.
• A scratch in the epidermis or something else that breaks the skin will expose the dermis, and the value of resistance will drop. It is reasonable to estimate that, under this condition, an arm or a leg has a resistance of about 500Ω. The “500Ω man,” frequently found in literature, holds a live conductor with both hands and stands with both feet in the water.
• Some researchers state that the average or reasonable resistance of human beings is from 1 000Ω to 2 000Ω, foot-to-foot, and 500Ω to 1 000Ω arm-to-foot, depending on the diverse factors involved.
Voltage of the line or electrical device.
• The body’s electrical resistance decreases as the voltage increases because the higher the voltage, the more numerous the points of the skin that are damaged, with increased access to the dermis.
The Body as a Circuit Parameter
A person can only be affected by electricity when it becomes part of the electrical circuit. That is why the best way to avoid an electric shock is to prevent contact with energized parts. But a large number of electrical devices handled daily increases the exposure to electricity and the possibility of unwanted contact.
Fig. 1 Birds are immune to electric shock as long as they are not part of the electrical circuit. Image courtesy of Pixabay
Like any other electrical parameter, there are two ways in which a person can become part of a circuit: series and parallel. When connected in series, the person is in the only path for current flow, and when connected in parallel, other channels share the current flow.
Figure 2 shows the nature of the problem. A person is touching an appliance that operates on electricity, such as a drill. The resistance Ri is the factory-installed appliance insulation, Reg represents the resistance of a conductor connected from the appliance housing to the power supply ground, and Rb is the sum of three resistances: the person’s body, the contact of the hands with the appliance and the touch of the feet with the floor.
Fig. 2 Simple electrical model with the body as a circuit parameter
In the diagram above, Ri is in series with the parallel paths of Reg and Rb. When the insulation is excellent, Ri is essentially infinite, and no current will flow through Reg and Rb. But, if the insulation fails (ground fault), Ri decreases, and current can flow through Reg and Rb.
We can now analyze three circumstances when there is a ground fault in the appliance. In the first case, the device does not have the conductor that connects it to the power supply ground, which is equivalent to Reg = infinity, and all the fault current will circulate through the person. Here, the person is in series with the fault circuit. In the second case, Reg = 0 and no current will flow through the person. In the third case, Rb = infinity, and no current will flow through the person either.
In real life, Rb will have finite values, and the correct grounding must ensure that, if there is a ground fault, the fault current that passes through the body is not enough to affect it for the duration of the fault.
Reg must be low enough to carry most of the fault current, with a magnitude adequate to clear the fault in a timely fashion. A low-impedance equipment grounding conductor connected effectively to the source ground will help to attain this goal.
Rb should be kept as high as possible avoiding wet earth and simultaneous contact with metallic objects. Usually, electrical workers are required to wear insulated gloves and shoes to increase resistance. It is usual practice in substations to spread a layer of high resistivity material on the earth’s surface above the ground grid. Standard materials are gravel and asphalt, and the effect is to increase the contact resistance between the soil and the feet, reducing the current through the body.
Appliance manufacturers make Ri very high using techniques like double insulation. This sort of equipment does not require an equipment grounding conductor given the unlikelihood of the user contacting energized parts. However, double insulation is not flawless, and there have been electrocutions when immersing the appliance in water.
The use of sensitive, fast ground protection is also helpful.
Current Exposure Time and Ventricular Fibrillation
The prevention of ventricular fibrillation is the objective that guides the recommendations of international standards regarding the design and implementation of grounding mats.
As indicated above, there are many published works about the effect of electric current on the human body, especially at the 50Hz and 60Hz frequencies that are the standards for power systems worldwide. Particularly noteworthy are the experiments conducted by C.F. Dalziel and W.R. Lee with animals (dogs, sheep, pigs, and cows) in a range of 10kg to 80kg. The results of these studies apply to humans. There are also findings from electrocution accidents.
Dalziel, Lee, and other researchers concluded that the amount of current that the human body can withstand in a range of 0.03s to 3s, is related to the energy absorbed by the body through the equation:
Sb = Ib² · ts, where:
Ib = nonfibrillating shock current in Ampere
ts = exposure time (duration) in seconds
Sb = empirical constant related to the shock energy tolerated by 99.5 % of the population = 0.0135 for 50kg body weight, and 0.0246 for 70kg body weight.
Then, Ib = √( Sb/ts) = 0.116 · ts-1/2 for 50 kg, and Ib = 0.157 · ts-1/2 for 70 kg
The exposure voltage V = Rb · Ib
Figure 3 shows the fibrillation threshold for an adult. It is a log-log time–current-voltage plot of the shock current (Ib) and the exposure voltage (V) vs. the exposure time (ts), for the range 0.03s to 3s. It assumes an arm-to-arm or arm-to-leg resistance (Rb) of 500Ω and includes body weights of 50kg and 70kg.
Fig. 3 Fibrillation threshold for 70kg and 50kg adult. Voltage based on Rb = 500Ω.
Important conclusions, derived from Figure 2:
• Straight lines fit the pairs (Ib, ts) and (V, ts) • The lower the exposure time, the higher the current tolerated • Magnitude and duration are a function of body weight, i.e., people with higher body weight undergo the same currents for longer • The tests are only valid for the range 0.03s – 3.0s
Relationship to Power Systems
Most power system voltages have a high risk of electrocution, especially in wet locations. Taking a simple household appliance like a hairdryer rated at 120V and a body resistance of 500Ω, one calculates a current of 240mA, which is likely to cause fibrillation — a fatal effect.
Even in cases where the current is not enough to cause fibrillation, it could cause a painful surprise, and the person could have an accident as a consequence of the involuntary reaction to the shock, such as a fall.
The electrical power circuits, as well as the devices connected to them, must be treated with extreme caution and in compliance with all applicable regulations in such a way to preserve life. The leading standard to follow for safety is the National Electrical Code (the NEC), whose purpose is “the practical safeguarding of persons and property from hazards arising from the use of electricity.”
A Review of Electrical Shock and its Effects
High voltage and low voltage can cause fatalities. The effects of the current on the body depend on the magnitude, duration, frequency, physical condition, gender, and path of the current.
The most dangerous effect caused by an electric current is ventricular fibrillation. During this condition, the heart stops pumping blood. The benchmark in the design of grounding systems is the prevention of fibrillation.
The electrical resistance of a person depends on the physical condition, the nature of the contact points, and the system voltage.
To avoid an electric shock, do not form part of the electrical circuit.
Safety standards, like the NEC, protect people from the improper use of electricity.
Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.