Power Quality Blog

Diagram of Daily Consumption of Electricity, Houses Appliances, Distribution them into Categories by Importance of Electricity Supply

Published by Petr ZÁVADA¹, Karel SOKANSKÝ¹, VSB – Technical University of Ostrava (1)

Abstract. Today, the development of power generation from renewable sources is an effort to maximize the share of total energy consumption. Big source of energy in solar and wind power plants are mostly connected to the distribution network and supply the energy only if it produces (solar power station produced during the day and wind power station produced when is a wind acceptable). For this reason, the energy from these sources are not available for all day and must be taken from other power station (atomic power plant, coal-fired power plant). Our project is designed the source of power which is combined solar power plane, wind power plane and batteries so that electrical energy will be supplied during the all day. The article deals with the study of power consumption of electricity of family house. These houses can be in less occupied areas with their own heating systems. Next step is distribution of household appliances into several categories by the most importance electrical power supplying.

Streszczenie. Aktualny rozwój produkcji energii elektrycznej ze źródeł odnawialnych powoduje starania o mozliwie największy udział tej energii w stosunku do całkowitego jej zużycia. Wielkie źródła energii z elektrowni słonecznych bądź wiatrowych są w większości przyłączone do sieci dystrybucyjnej i dostarczają enegię wtedy, kiedy ją produkują (elektrownie słoneczne w ciągu dnia a wiatrowe przy odpowiedniej sile wiatru). Powoduje to, że energia z tych źródeł nie jest dostępna stale a w czasie, kiedy nie jest produkowana koniecznym jest jej zastąpienie z innych źródeł. Projekt SGS jest propozycją żródła energii, które jest połączeniem baterii słonecznych, elektrowni wiatrowej i akumulatorów tak, aby była możliwa dostawa energii o stałej wartości podczas całego dnia. Artykuł zawiera studium zużycia energii elektrycznej w grupie docelowej (domy na terenie z niższą gęstością zaludnienia z własnym ogrzewaniem za pomocą paliw stałych), czyli w zasadzie przeciętnego domu jednorodyinnego dla sześciu osób a także podział domowych urządzeń elektrycznych na kilka kategorii według priorytetów dostaw energii elektrycznej. (Harmonogram zużycia dziennego energii elektrycznej, urządzenia elektryczne i ich kategoryzacja ze względu na ważność).

Keywords: solar power plane, wind power plane, batteries, power consumption, power supplying.
Słowa kluczowe: elektrownia słoneczna, elektrownia wiatrowa, akumulator, zużycie energii w domu, priorytet dostawy energii.

Introduction

Supply of energy in batteries for immediate consumption is limited and it is the necessary recharge. It can be by using with renewable sources as solar panels and wind turbines. The combination of these two sources are chosen so that the production of sufficient energy to recharge throughout the year. This means that in the summer months primarily relies on solar collectors and in winter months (more windy) production with wind power.

Using batteries as a power supply recharging by solar and wind power

On figure 1 is a block diagram of the proposed project. As a source for batteries recharging, which have a capacity 300Ah and 120 V output voltage, is wind power energy and solar panels. Wind power has 12 kW output power and solar panels has 2 kW output power. For recharging batteries is also possibility used distribution network as is on block diagram (picture 1). Batteries are able to supply the theoretical performance 36 kWh.

Fig. 1 Block diagram with wind power station, solar power station and batteries

Description of the examined family house

To determine the daily diagram of consumption has been selected family house in which permanently 5 people life. Researched family house is built on the outskirts of the village. The house has a solid fuel boiler for heating and hot water. Used home appliances in the house with their consumption are listed in Table 1.

Site build house, the average wind speed at 100 meters above the ground is between 25-50 m/s (figure 3) and average total annual solar radiation is 944-972 KWh/m² (figure 4).

Fig. 3 Map with resulting field of average speed of wind at 100 meters above the ground

Fig. 4 Maps with average total annual solar radiation

Measurement of daily chart selected house

Family home was measured starting from April 17, 2010 11:01:00 April 25 and ending 2010 4:41:00 p.m. Measurement was carried out using apparatus MDS10 and the results were evaluated in program MDS10 and in program Excel. Measurement voltage range was set to 230 V and current range was set to 30 A. Statistical analysis of measurement and evaluation of the individual days (only all day long measurement) are shown in Figures 5 and 6.

Fig. 5. Statistical processing of current and voltage. On the vertical axis shows the percentage frequency of occurrence in the horizontal axis and voltage (230 V) and current (30 A) as a percentage.

In the statistical processing can be seen that the currents move in areas over 130%. These values are due to error in measurement.

Fig. 6. The average consumption of each day for measurement and for each stage separately

The following table there are each household appliances with their average consumption. At next columns there are power consumption per day and per year and average length of working in one day. Appliances are divided into three priorities, where the first priority involves appliances that are permanently in operation and are necessary for safety. The lower rows of the table are given priority for individual consumption and for separately priority consumption.

Table 1: Domestic Appliances and setting priorities:

All appliances are single phase and their total power is 10300 W. If we consider an ideal situation, so the voltage is 230 V to the total consumption of all appliances is about 45 Ah. With a 300 Ah battery power is possible to operate all appliances 6.7 hours. If we consider only the first priority it will be 92 hours.

Conclusion

According to the daily chart, the current consumption in all phases of moving most of the day from 0% to 10% of the set current range (30 A), which is approximately up to 3 A. In this case, the unit lasted about 33 hours. The simplified calculations are not considered losses in individual blocks and are also considered the current charging rechargeable batteries, solar and wind power.

Acknowledgement

This article was created under poject SP/201073, “Využití hybridních obnovitelných zdrojů elektrické energie”

REFERENCES

[1] Murtinger, K., Beranovský, J., Tomeš, M.: Fotovoltaika – elektřina ze Slunce 1. vydání, ERA, 2007
[2] Hradílek, Z. Elektroenergetika distribučních a průmyslových zařízení,VŠB-TU Ostrava, MONTANEX a.s., 2008
[3] Větrná mapa. ČSVE: Česká společnost pro větrnou energii [online]. 19.03.2009, [cit. 30.05.2010]. Dostupný z WWW: <http://www.csve.cz/clanky/detail/35>

Authors: Ing. Petr Závada, E-mail: petr.zavada@vsb.cz; prof. Ing Karel Sokanský, CSc. E-mail: karel.sokansky@vsb.cz. VŠB-TU Ostrava, Department of Electrical Power Engineering – 410, 17. listopadu 15, 708 33, Ostrava – Poruba, Czech Republic,

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 7/2011

General Requirements for a Smart Grid Architecture – Remarks on Standards for Implementation

Published by Anna PAMUŁA, University of Lodz

Abstract. The paper presents importance of standardization and potential standards relevant for Smart Grid implementation mainly focused on ICT standards. Short overview of current work performed by different bodies round the world is presented. The paper presents some recommendation based on the SEESGEN-ICT EU project and reports prepared by JWG (Joint Working Group) and NERC (North American reliability Corporation).

Streszczenie. W artykule omówiono problematykę procesu standaryzacji w architekturach Elektroenergetycznych Sieci Inteligentnych – głównie standardów związanych z obszarem ICT. Zaprezentowano krótki opis efektów prac prowadzonych przez organizacje zajmujące się standaryzacją oraz rekomendacje projektów z tego zakresu. (Problematyka procesu standaryzacji w architekturach Elektroenergetycznych Sieci Inteligentnych)

Keywords: Smart Grids, ICT standards.
Słowa kluczowe: Inteligentne sieci, architektura Smart Grid, standardy, ICT

Introduction

A great number of different projects run all over the world focuses on use cases and general requirements for a Smart Grid reference architecture. The architecture of this new distributed system with the fully interactive infrastructure must be designed and validated with wide implementation of standards. The process of developing standards for Smart Grid is a very big challenge. The great number of different stakeholders (producers, consumers, ICT suppliers), short time before reaching 20-20-20 targets to reduce carbon emissions and to secure the energy supply makes this task more difficult. For Smart Grid, it is not just important to change or establish an individual standard but to adapt the organization and processes for standardization[1].

The ICT has a significant role to play in changing current energy systems and energy consumers habits into a new green economy and lifestyle. Traditional Grid SCADA with some additional automation systems are used for monitoring energy production transmission and distribution of the energy. SCADA system is a crucial component in traditional utility/energy sector. But this hierarchical system is not prepared for massive DER and RES intrusion. Those system are not design and not prepared for supporting new business aggregators, integrators and more active customer behaviors. Future Smart Grid demands new ICT solutions. The future Smart Grids will consists of 3 interacting infrastructures [2]:

− Automation, control and management systems of the physical energy infrastructure management systems;
− Business Management Systems;
− Other Information Management Systems (ICT).

The new information management system should be design implemented and maintained in a way that supports new business opportunities for different stakeholders ensuring [2]:

− Balancing Intermittent production,
− Meeting social goals of Energy Efficiency,
− Meeting customer requirements,
− Meeting economic goals,
− Managing interactions with SCADA systems.

European Smart Grid standardization work

The European Commission Directorate-General (DG) for Energy by the group of mandated Experts Group has created a Smart Grid Task Force that highlighted the importance of standards for as one of the most successful Smart Grids deployment factor. The main idea of European Future Smart Grids provides development of communications, metering and new business systems on each level of energy market as basic element of energy efficiency. To fulfill this vision a set of proven technical solutions, shared technical standards and protocols ready to use in with different type of equipment installed in the power Grid should be established. For the ICT solutions standardization is the most important frame that allows to connect devices from different producers to the Smart Grid with the best quality and proper price. Standards allows to achieve such a elements like: interoperability, defining data models, protocols, communications and information exchange, improving security, safety of the new products and systems.

Report prepared by European Technology Platform SmartGrids: Strategic Deployment Document (SDD), released in April 2010, Deployment Priority #4 IC emphasis the importance of ICT as a instrument of new business and new stakeholders coordination.

The most important for wide Smart Grid deployment is industry support. For that reason European Strategic Energy Technology Plan (SET-Plan) was established in The European Electricity Grid Initiative (EEGI) published detailed roadmap for implementation of Smart Grids 2010-2018. In November 2009 a Smart Grid Task Force was established to advise the Commission on the policy/regulatory directions. It is run by Commission’s Directorate General for Energy Policy (DG ENER) in collaboration of with 6 Directorates and about 25 European associations.

To facilitate the technical level the European Commission issued in 2009 mandated of European standardization organizations CEN, CENELEC and ETSI to cooperate in development of open system architecture. They have formed a Joint Working Group and the result of their work is presented in the report of standardization requirements for European vision.

Smart Grid standardization work

A lot of activities can be noticed around the world. Such a bodies like ISO, IEC, 3GPP and some national comities has done a lot of work that need to be considered because of their influence to the European standardization work. The work done in the area of international standardization can be find in IEC roadmap (the standards from IEC TC 57 Seamless Integration Architecture [3] (IEC TR62357 seems to be the most important), for the European standardization a lot work was published by Smart Grids Task Force (EG1 report on services and functionalities, EG2 report on data handling, security and protection). All standards defined by China are very important and must carefully examined, because it is the main producer of the Grid and home equipment and infrastructure.

The table below presents short survey of some important standards work done by different bodies.

Table 1. The result of standardization bodies work based on [1]

Organisation	Work results
European Standarization Mandate M/441 and Smart Meter Coordination Group	Standards as a voluntary technical specifications and general technical rules for products and systems on the market. 6 aspects of Smart metering considered and examined (support of the functions depends on the country): reading and transmission of measurements, two-way communication between meter and market participant, support of various tariff models and payment systems, remote deactivation and start/finish supply, communication with of he house devices, support of display of data in real time
German Standarization Roadmap E- Energy/Smart Grid	A position paper on the German Smart Grid Standarization that provides recommendation for necessary fields of action, international cooperation and strategy. Research done showed that a lot of standard already exists. Recommendation: existing standards like IEC TC 57 should be used and be the started point for further work mainly for cooperation. System approach should consider also other media and other domains
IEC Strategic Group 3 “Smart Grid Report”	A roadmap for standards and recommendation. Over 100 standards were identified described and prioritized. 12 application areas and 6 general topics were examined. 44 recommendations prepared. IEC TC 57 recommended as a basic. Now working on Mapping Tool to support Smart Grid project managers.
NIST Interoperability Framework	A phase plan intended to accelerate identification of standard. Description of an abstract reference model and identification of 80 essential standards, 14 key areas and gaps for a new standards identified. The work refers to North America standards like ASHRAE and IEEE.
Japanese Industrial Standards Committee	Standards as a fundamental element in the achievement of interoperability. A report published in 2010 established a roadmap to internalization standardization for Smart Grid. 7 main fields of business were identified an 26 priority actions areas assigned. Special core aspects of Japanese economy were identified.
The state Grid Corporation of China- SGCC Framework	Smart Grid standardization roadmap defined Defines 8 domains, 26 technical fields and 92 series of standards.

As it can be seen a lot of work was done, but still there is a strong need for European Standardization Organizations (ESOs) to coordinate their effort to put into effect their work to market.

ICT standardization

Typical Power Grid is a centralized network coordinated by traditional Power utilities.

Power companies are not willing to lose control of the Grid and are not eager to use more communication and open technology. Firstly because of the service level is not yet justified, secondly – the are not enough service guarantee from communication companies. What is more it is still not clarified who is responsible for outages and maintenance. Traditional SCADA systems includes several protocols like: Modbus RTU, RP-570, Profibus and Conitel (vendor specific solutions). In more modern systems they were replaced by open standard protocols like DNP3 (Distributed Network Protocol), mainly applied in North America, and IEC sets of protocols developed under the IEC TC57. It obvious that the industry is now moving to nonproprietary protocols.

In many countries regulatory aspects of Smart Grids are not fully clarified and taking under consideration European Union they differ in members law. The lack of regulations ensuring that investment in new ICT will not lead to losses can be seen as one of the most important factor of slower deployment. It is necessary for the business to work under stable clear rules.

In new intelligent Grid many different stakeholders have to receive and exchange data. Consumers and businesses can make decision only if they receive a proper and actual information offered in Smart Grids by ICT solutions. For this purpose a complex, proven ICT architecture is needed.

So far there is no a clearly defined communication architecture or the transition plan needed that will meet the requirements of the modern Grid to achieve. Vendors who supply sensors, IEDs, DER, and other end-use devices are hesitating to invest in these products until universal standards are adopted [2.] Mature solutions are able to optimize and upgrade the current power systems and reduce CO2 emission. For mature solutions preparation of guide of development presenting standards is a key issue.

Many European project consider the importance of ICT in Smart Grids. One of them is SEESGEN-ICT (Supporting Energy Efficiency in Smart GENeration Grids through ICT 2009 – 2010). Research done by project team confirmed that the IEC 60870-5 and DNP3 based protocols are gradually being replaced by more modern IEC standards based on TCP/IP. New globally accepted IEC standards are IEC 61850 and Common Information Model (IEC 61968 and IEC 61970). It was also highlighted that Common Information Model (IEC International Electrotechnical Model 61970/61968) is recommended standard for the exchange of data between systems. The model consists of two standards IEC 61970 and IEC 61968 with universal elements such as dictionaries, interfaces and data models. UML (Unified Modelling Language) is used for model design. Also new standards for communication with distributed energy resources are based on IEC 61850 and CIM principles. IEC 61850 is based on the latest version of the Manufacturing Message Specification MMS (ISO 9506). IEC 61850 and CIM are also being harmonized [2].

Most presented solutions consider AMI (Advanced Meter Infrastructure) and embedded control devices connected to the local network as a main parts of the infrastructure. In Europe OPENmeter project is addressed to prepare solutions in this area.

Web Technologies [2]

Internet technology based on its protocols and service oriented architecture are nowadays solutions for a new business models.

Future Smart Grid coordination must be considered as management of the resources (action and data for matching supply-demand) and setting up SLA (Service Level Agreements) between all energy market stakeholders. SEESGEN-ICT recommends IEC TC 57 Seamless Integration reference Architecture – IEC TR 62357 for intra grid application management.

It must be noticed that different Smart Grid solutions typically have different IT components. As reports of SEESGEN-ICT Project shows using standard components and platforms in creating new applications in a cost effective way as well as using SLA functionality can make real cloud computing solutions. The SEESGEN-ICT project group has tested Platform as a Service using terminology of Cloud Computing and suggest Infrastructure as a Services solutions towards Smart grid as a Service.

Recommended architecture for models that do not demand real time response is XML web services. It must be noticed that it is not suitable for process control and monitoring. The project experience shows that XML structure[2]:

− leverages several standards to enable data transfer between applications on remote computers allowing,
− technologies and tools (web servers, libraries, APIs, etc) leverage the evolving security standards, user authentication, data transfers, data states and a lot more,
− applications programmers can rapidly build and deploy XML web services using existing tools and frameworks
− XML web services provide that all-important independence from any hardware or software platform.

Following limitations of XML were also considered:

− limitation of transfer OPC data seamlessly from one computer to another using a “standard” interface. Implementations show that the transfer either suffer from extremely high bandwidth or suffer a slow update rate;
− XML messages are very large in comparison to similar DCOM messages that carry the same information, and their sheer size makes them difficult to transport en masse.
− more, the origins of XML as a document format with little to no boundaries in terms of element size or depth that are accessible outside a post-processing / parsing context make it less than optimal for the use as a data exchange format and protocol due in large part to the inability to reliably stream-process, filter and monitor its contents in real-time.

The various standards that are available currently at the distribution Grid level have been explained in many works. The scheme and description of standards can be found in [4]. The scheme presenting use of standards can be also find in [5].

Necessary steps and prioritization of actions recommended by JWG report[1]

Joint Working Group is working on report that provides overview of standards and current activities that are the c step o fulfill the European vision of Smart Grid (based on the Smart Grids Task Force of the European Commission initiatives). The survey was done to prepare a list of existing dictionaries standards sources of definitions according to roadmap structure covering following groups:

− general architecture and concept of the Smart Grid,
− communication,
− information security,
− system aspects and crosscutting issues,
− generation transmission and distribution,
− Smart metering,
− industry energy management,
− in house automation,
− market and actors.

The report shows current status of standardization in cross-cutting and domain-specific topics, gives some detailed recommendations and shows a list of gaps for groups of cross cutting problems (reference architecture, data communication interface, Smart Grid information security, others), domain specific topics (generation, transmission, distribution, Smart metering, industry, home and building) market and actors (roles and responsibilities, recommendation to European Standardization Organizations).

The reports defined 6 major recommendation for further European standardization work:

further works with adequate bodies and groups of stakeholders based on identified gaps with possibilities to consider more topics like energy storage and security supply,
process of standardization must be based on existing international work but also has to support European produces,
Increasing the implementation of currently proven solutions and existing mature domain communication with further standardization of interfaces;
concentration on generic standards that are flexible for new R&D development and market needs,
creating of a single repository for Smart Grid use cases to start detail work on standards,
setting the processes to fit the lack of maturity of many Smart Grid applications.

As the most important further activities JWG report highlights:

− a prioritization of the identified gaps and recommendations based on defined set of criteria,
− developing complete and flexible European reference architecture,
− creating European Smart Grid use cases to start a continuous process o identifying gaps in standards.

Necessary steps and prioritization of standards actions recommended by NERC report

NERC report is devoted to its Reliability Standards. One of the main recommendation is to enhance NERC reliability standards according to continuous Smart Grid evaluation. The necessity to support coordination among relevant SDO (Standard Development Organizations) and activities like NIST Priority Action Plan to harmonize are strongly highlighted. Report shows some area of importance for this work. As one of the example time synchronization of PMU(Phasor Measurement Units) in real time and differences and overlaps between IEEE C37.118 and IEC 61850 is described. Regarding Cyber Security NERC report noticed that the there is number of existing standards for example set of NISTR 7628 documents that work properly while using separately, but when they are using merged together may be in conflict with each other. Therefore recommendations and maps included in NISTR 7628 must be applied. Future work with Canadian Legislative, Regulatory and standards setting bodies for ensuring coordinated and harmonized standards [6].

Summary

The success of rapid Smart Grids deployment rely on proper integration of real-time application and monitoring, advanced communication enabling bidirectional energy and information flow in reliable, efficient and secure way from generation source via aggregators to end users. Smart Grids technologies change the existing distribution systems what demands new tools and new techniques based on proven standards.

As the paper shows a lot of work was done, but further coordination of standard bodies to increase harmonization in standard development is necessary. Fulfilling the gaps and overlaps described in presented reports must be a first step of this work. The solutions for overcoming lack of standardization can be wide use of open standards and non proprietary solutions. Open standard guarantee interoperability between different vendors devices and can be used in provider-independed platforms.

One of the biggest barrier in Smart Grids deployment is insufficient cyber security. To provide to consumer energy in reliable and effective way the integrity of distribution control command is essential. For the safety-critical applications of adaptive protection and reconfiguration integrity of outage data is significant. Must be also noticed that some consumers are not willing to show their energy consumption habits. For that reasons cyber security standards should be included in ICT solutions. To ensure reliability of the system cyber security must be seen as one of the most important Smart Grids deployment factor.

Transformation, replacement, upgrading and transitions of technologies used for SCADA systems towards standard open protocols such as EIC 61850 might take time to be globally adapted. Security, integrity and information protection related to network and information management can be implemented by proper use and embedding of VPN solutions.

REFERENCES

[1] JWG-Smart-Grid-report_V1-0_2010-12-17-all-figures.pdf
[2] SEESGEN-ICT http://seesgen-ict.erse-web.it
[3] IEC Smart Grid Standardization Roadmap, by SMB Smart Grid Strategic Group (SG3) June 2010; Edition 1.0 http://www.iec.ch/SmartGrid/downloads/sg3_roadmap.pdf
[4] De Craemer K. , Deconinck G., Analysis of state-ofthe-art Smart metering communication standards, IEEE Benelux Young Researchers Symposium 2010 in Electrical Power Engineering, Leuven, Belgium, March 29-30, 2010;
[5] Matusiak B.E., Pamuła A., Zieliński J. S., Narzędzia ICT w sterowaniu zachowaniem klienta w inteligentnych sieciach energetycznych, w Komputerowo Zintegrowane Zarządzanie pod red. R. Knosali, Oficyna Wydawnicza Polskiego Towarzystwa Zarządzania Produkcją, Opole 2011, Tom II, str. 88-97
[6] Reliability Consideration from the Integration of Smart Grid

Author: dr Anna Pamuła, Katedra Informatyki, Uniwersytet Łódzki, ul. Matejki 22/26 90-928 Łódź, E-mail: apamula@wzmail.uni.lodz.pl

11^thInternational Scientific Conference FORECASTING IN ELECTRIC POWER ENGINEERING Development Planning, Operation, Maintaining and Management in Power Engineering

PE 2011
Wisła, 14-16 September 2011

Institute of Electric Power Engineering of Czestochowa University of Technology under the patronage of Rector of Czestochowa University of Technology in the last twenty years has organized, in a close cooperation with eminent scientific and technical organizations, a cyclic scientific conference, whose scope are the current issues from a broad field of electric power engineering. The stimulus for setting up this event was the energy crisis, which influenced Poland in Then the first Domestic Symposium “Forecasting in Electric Power Engineering” under the auspices of Commission for Power Engineering, Silesian Branch of Polish Academy of Sciences was organized. Starting with 1996, a small audience symposium has evolved into a biennial domestic conference and since 2008 the conference has gained the status of an international event, where the issues and dilemmas concerning not only Polish electric power engineering, but also those pertaining to EU countries, are discussed. The representatives of EU countries take an active role in the plenary talks. Within the framework of the 11^th International Scientific Conference, the Program Committee of the Conference “Forecasting in Electric Power Engineering PE 2011” has decided to organize the event in the form of a plenary session, where the invited papers shall be presented, as well as in the form of thematic panels, covering following issues:

• Forecasting in electric power engineering
• Planning of power system development
• Selected problems of power engineering (maintenance, measurements and control, management in power engineering)

During the proceedings we take into consideration the forecasting problems for the electricity market, software useful for the power engineering, issues of the integration with the European Union and the role of distribution utilities, power plants and others enterprises in economic transformations

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 9a/2011

Lightning Transient Overvoltage Evaluation

Published by Electrotek Concepts, Inc., PQSoft Case Study: Lightning Transient Overvoltage Evaluation, Document ID: PQS1205, Date: January 26, 2012.

Abstract: This case study presents a wind plant substation lightning transient overvoltage evaluation. A high-frequency transient model was created to simulate the lightning transients and resulting overvoltages and arrester energy duties. A high-frequency model was required to accurately represent the lightning phenomena. MOV surge arresters were evaluated as the power conditioning alternative.

INTRODUCTION

A wind plant substation lightning transient overvoltage evaluation case study was completed for the system shown in Figure 1. The case study investigated the potential for severe high-frequency transient overvoltages on substation transformer primary and secondary buses during lightning strikes on the terminating transmission line and wind collector circuit bus. The power conditioning mitigation alternative of MOV surge arresters was also evaluated.

The simulations for the case study were completed using the PSCAD® program. A high frequency transient model was created to simulate the lightning transients and resulting overvoltages and arrester energy duties. A high-frequency model was required to accurately represent the very high lightning transient frequencies. The lightning surge was assumed to be a current source (e.g., 10 kA) with a very fast rise time (e.g., 8×20 μsec).

Figure 1 – Illustration of Oneline Diagram for Lightning Transient Analysis

SIMULATION RESULTS

For wind plants, the principal risk to equipment insulation is impulsive transients caused by lightning. Lightning transients may enter a substation by various means, including coupling through the substation power transformer from exposed high-voltage transmission lines and direct or indirect strikes to open air equipment.

For the lightning simulations, it was assumed that the wind farm substation is shielded against direct strokes and that a 10kA surge enters the substation due to a lightning flash terminating on the overhead shield wire or structure with a subsequent flashover (a.k.a., back flashover) to a phase conductor or by a lightning flash terminating on the phase conductor due to a shielding failure.

Back flashover is a flashover of insulation resulting from a lightning stroke to part of a network or electrical installation that is normally at ground potential. When back flashover occurs, a portion of the surge current will be transferred to the phase conductors through the arc across the insulator strings. Often, the back flashover causes a temporary phase-to -ground fault that must be cleared by circuit breakers.

The high-frequency transient simulation model included a 138kV wind plant substation and a 6-mile transmission line supplying a 50 MVA, 138/34.5/13.8kV substation transformer. The representation of the system short-circuit equivalent at the 138kV source substation included:

Three-phase (I_3φ) fault current: 17,500 A @ -85.0° (4183 MVA)
Single-line-to-ground (I_φG) fault current: 20,000 A @ -85.0° (4780 MVA)

These values were converted to ohms for the PSCAD representation, which included a three-phase voltage source with positive and zero sequence impedances and a 420Ω damping resistor.

The 6.0 mile, 138kV transmission line was modeled using the following data:

Length: 6.0 mi
Phase Conductor: 795 kcmil (Tern) 45/7 ACSR (OD = 1.063”, R_DC=0.114Ω/mi)
Ground Conductor: 3/8″ EHS (OD = 0.385”, R_DC=5.550Ω/mi)
Tower Configuration: TAN-1
Ground Resistivity: 100 Ω•m

The traveling wave frequency dependent phase model in PSCAD was used to represent the transmission line. The frequency dependent phase model is basically a distributed RLC traveling wave model, which incorporates the frequency dependence of all parameters. This model represents the frequency dependence of internal transformation matrices.

The program calculates the line constants for the transmission line before each simulation begins. The 60 Hz impedance values from the line constants output were compared with the transmission line impedances to assure that the line was modeled correctly. The calculated surge impedance of the transmission line was approximately 420Ω. A portion of the line constants output includes:

A 110kV (88kVMCOV) station class MOV surge arrester was modeled at the transmission line termination point. The ratings for the arrester included:

Rated Voltage (Duty Cycle): 110 kV
Maximum Continuous Operating Voltage (MCOV): 88 kV
Maximum Energy Discharge Capability: 9.8 kJ/kV_{rated MCOV}
Maximum Energy Discharge Capability: 862.4 kJ
10 kA, 8×20 μsec Discharge Voltage: 274 kV (2.43 per-unit)

A 27kV (22kV_MCOV) station class MOV surge arrester was modeled on the secondary winding of the substation transformer. The ratings for the arrester included:

Rated Voltage (Duty Cycle): 27 kV
Maximum Continuous Operating Voltage (MCOV): 22 kV
Maximum Energy Discharge Capability: 9.8 kJ/kV_{rated MCOV}
Maximum Energy Discharge Capability: 215.6 kJ
10 kA, 8×20 μsec Discharge Voltage: 64.8 kV (2.30 per-unit)

A traditional inductive transformer model generally looks like an open circuit to the very high frequency lightning transient. The 60 Hz transformer model can be improved by adding capacitances between windings and from the windings to ground. This type of model will act as a capacitive voltage divider to transfer a portion of the surge from the primary to the secondary windings. The bushing and winding capacitance values included in the model were C_hg = 8ηF, C_lg= 8ηF, and C_hl = 12ηF.

Other substation equipment, such as circuit breakers and instrument transformers, were represented by their stray capacitances to ground. Typical stray capacitance values of substation equipment are provided in Annex B of IEEE Std. C37.011. The values used in the simulation model included:

Effective Capacitance (High-Side of Transformer) 15,000 ρF
Effective Capacitance (Low-Side of Transformer) 3,000 ρF

The high-frequency transient simulation model was based on the substation oneline diagram and other information, such as the 138kV transmission line specifications. The steady-state voltage at the 138kV substation bus was 1.05 per-unit prior to the transient event. For the worst-case analysis, it was assumed that all of the 34.5kV collector circuit breakers would be open during the simulations. Two lightning surges were simulated; one on the terminals of the 138 kV transmission line entering the substation and the other on the 34.5kV bus on the transformer secondary.

Case 1 involved a lightning strike to one of the terminals of the 138kV transmission line entering the substation. The specification of the current waveform was a 10kA magnitude, with an 8×20μsec characteristic (Phase A). The lightning surge current waveform is shown in Figure 2.

Figure 3 shows the simulated transformer primary voltage for Case 1. The peak transient voltage was 272.323kV (2.42 per-unit). Figure 4 shows the corresponding transformer secondary voltage. The peak transient voltage was 56.437kV.

The peak current for 88kV_MCOV arrester on the transmission line terminal (transformer primary) was 9.3 kA and the maximum arrester energy was 51.8kJ, which is approximately 6.0% of the assumed arrester energy capability of 862.4kJ. The simulated protective margin for the 550kV BIL rating for the transformer primary winding was determined using:

The protective ratio for the transformer primary winding was 2.02 (550kV BIL / 272.323kV). An adequate margin has a ratio greater than 1.20 (see IEEE Std. 1313.2).

Figure 2 – Illustration of the Simulated Lighting Surge Current Waveform

Figure 3 – Simulated Transformer Primary Voltage for Case 1

Figure 4 – Simulated Transformer Secondary Voltage for Case 1

Case 2 involved a lightning strike to the 34.5kV bus (transformer secondary winding). The specification of the lighting surge current waveform (see Figure 2) was a 10kA magnitude, with an 8×20μsec characteristic.

Figure 5 shows the simulated transformer secondary voltage for Case 2. The peak transient voltage was 64.719kV (2.29 per-unit). Figure 6 shows the corresponding transformer primary voltage. The peak transient voltage was 31.042kV.

The peak current for 22kV_MCOV arrester was 9.9kA and the maximum arrester energy was 13.6kJ, which was approximately 6.3% of the assumed arrester energy capability of 215.6 kJ.

The simulated protective margin for the assumed 200kV BIL rating of the transformer secondary winding was determined using:

The protective ratio for the transformer primary winding was 3.09 (200kV BIL / 64.719kV). An adequate margin has a ratio greater than 1.20 (see IEEE Std. 1313.2).

Figure 5 – Simulated Transformer Secondary Voltage for Case 2

Figure 6 – Simulated Transformer Primary Voltage for Case 2

SUMMARY

This case study summarized a wind plant substation lightning transient overvoltage evaluation. A high-frequency transient model was created to simulate the lightning transients and resulting overvoltages and arrester energy duties. A high-frequency model was required to accurately represent the lightning phenomena. MOV surge arresters were evaluated as the power conditioning alternative.

REFERENCES

IEEE Guide for the Application of Insulation Coordination, IEEE Std. 1313.2-1999, IEEE, October 1999, ISBN: 0-7381-1761-7.
IEEE Application Guide for Transient Recovery Voltage for AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis, IEEE Std. C37.011-1994, IEEE, ISBN: 1-55937-467-5.
Electrical Transients in Power Systems, Allan Greenwood, Wiley-Interscience; Second Edition, April 18, 1991, ISBN: 0471620580.
R.C. Dugan, M.F. McGranaghan, S. Santoso, H.W. Beaty, Electrical Power Systems Quality, McGraw-Hill Companies, Inc., November 2002, ISBN 0-07-138622-X.

RELATED STANDARDS
IEEE Std. 1313.2

GLOSSARY AND ACRONYMS
CF: Crest Factor
DPF: Displacement Power Factor
PF: Power Factor
PWM: Pulse Width Modulation
THD: Total Harmonic Distortion
TPF: True Power Factor

The Trouble With Capacitors Part 2

Published by R. Fehr, P.E., Engineering Consultant, Jan 1, 2004

Here’s how to reduce the effect of capacitor switching on your power system

Last month’s article discussed exactly what happens when electric utilities switch shunt capacitors: power system components are exposed to transient voltages and currents produced by energizing and de-energizing these devices. Those transients may be short-lived, but they have high peak values and frequencies much greater than the power system fundamental frequency. Now it’s time to investigate the effects these transients have on your power system.

Negative effects aplenty. When an electric utility or end-user energizes a shunt capacitor, the high-frequency switching transients produce overvoltages on the capacitor bus. These overvoltages may be significant enough to cause arrester operation or even equipment failure. However, these overvoltages aren’t always localized. Since the transients have a frequency much higher than the power system frequency, the system behaves much differently when subjected to these high frequencies than it does with the normal power system frequency.

Transient high voltages can often show up a considerable distance from the capacitor bus due to resonance-like conditions caused by the switching transients. Fast transient overvoltages, which occur as the transients pass through transformers and are magnified due to capacitive coupling, are symptomatic of this condition. Open-circuited lines also contribute to overvoltage conditions by reflecting the traveling transient voltage waveform back to the source, where it can add to the standing voltage waveforms and produce high-voltage surges.

De-energizing a shunt capacitor can also cause power quality problems due to the transient overvoltages produced by re-ignitions and restrikes during the current interruption process. Because of the increased probability of excessive arcing in the switching device during current interruption, the likelihood of switching device failure increases significantly during the de-energization process.

In both the energizing and de-energizing cases, equipment very near or at significant distances from the capacitor location will be subject to voltage stresses that could result in insulation failure. Surge arrester operation and failure are also more likely to occur during capacitor switching than under steady-state conditions.

High-frequency transients at the capacitor location often introduce electrical noise into control circuits, which can lead to equipment misoperation. Even nuisance tripping or damage to devices is possible. A ground potential rise during the transient period can cause system protection misoperation and even pose a safety hazard to personnel working nearby if grounding is inadequate. These effects are potentially serious and must be counteracted in some way.

Double trouble. To further complicate the situation, two or more capacitor banks located close to one another make the situation worse. These negative effects of the transient become extremely severe in the vicinity of the capacitors for several reasons.

The transients produced when a shunt capacitor is energized in the vicinity of an already-energized shunt capacitor are much more severe than those produced when a single isolated capacitor is switched. It’s very important to understand this phenomenon, which is called back-to-back switching, to avoid the negative consequences.

The zero voltage that occurs at the moment of contact closure when the second capacitor is energized makes it appear to the system as a short circuit. This apparent momentary short circuit will cause any energized capacitor nearby to discharge into the second capacitor. In addition, the two capacitors in parallel appear as a larger equivalent capacitance rather than one capacitor alone, making the inrush current magnitude much larger than for a single capacitor. The inductance of the system between the two capacitors, L_eq, is the quantity that limits the inrush current. Back-to-back induction can be found with the following equation:

This higher inrush current has a frequency much higher than the inrush current for a single capacitor. Both the magnitude and the frequency of the inrush current during back-to-back switching are typically an order of magnitude larger than those seen when energizing a single capacitor. The frequency of the inrush current can be hundreds of times greater than the power system frequency, f_s, as shown in the equation below:

Reducing the effects of capacitor switching. You can use one of several methods to reduce the problems associated with capacitor switching, including equipping circuit breakers with pre-insertion resistors, outfitting circuit switchers with pre-insertion reactors, or tightly controlling the point on the waveform when the capacitors are actually switched.

Pre-insertion resistors. One technique involves breaking the switching operation into a multi-step process and inserting a temporary impedance into the circuit during one of the steps. This approach breaks one large transient into two or more smaller ones. Circuit breakers can be built with internal pre-insertion resistors to reduce the magnitude of switching transients. These resistors, typically in the 100-ohm to 400-ohm range for single, extra-high voltage (EHV) capacitors, are in series with the interrupter when the contacts close, but remain in the circuit just long enough to damp the transients, usually for ½ to 1 cycle. After that time, they’re shorted out with a make-before-break connection between the resistor contacts and the main contacts, and remain out of the circuit until the next time the breaker is closed. This process is shown schematically in Fig. 1.

Pre-insertion resistors add both complexity and cost to a breaker. They’re also another item that requires scheduled maintenance. While pre-insertion resistors reduce transients during capacitor energization, they’re not used when the capacitor is de-energized, so those transients go unabated during that form of switching.

Pre-insertion reactors. Circuit switchers can be outfitted with pre-insertion reactors to reduce transients when closing into capacitive loads. These reactors, which are small air-core inductors typically in the 10-millihenry to 40-millihenry range, are mounted externally on the switcher. When the switcher closes, a spring-loaded contact blade swipes over a reactor contact, energizing the reactor in series with the capacitor when it touches the reactor contact. The reactor remains in the circuit while the blade swipes across the reactor contact, usually for about seven to 12 cycles, depending on the system voltage. The blade keeps moving past the end of the reactor contact, at which point a make-before-break connection is made with the main contact. The blade comes to rest on the main contacts, which keeps the capacitor in the circuit, but bypasses the reactor. The brief period while the reactor is in series with the capacitor is sufficient to dampen the energization transients.

Schematically, this process is identical to the pre-insertion resistor switching process shown in Fig. 1. While less expensive and probably more reliable than pre-insertion resistors, pre-insertion reactors are complicated, require careful alignment and adjustments, and are rather exciting to watch as they arc and spark when operated.

Point-on-wave switching. In the ’80s, a different approach to managing capacitor switching transients began to emerge. Instead of electrically modifying the system with resistors or reactors to reduce the effect of the switching transients, engineers left the system as is but addressed the transients themselves. By precisely controlling where on the voltage waveform the contacts touch, it’s possible to greatly reduce the magnitudes of the switching transients. This approach, called controlled switching or point-on-wave switching, is used extensively throughout the world not only for switching capacitors, but also for switching shunt reactors, energizing transformers, and even switching transmission lines.

Here’s how it works. As the voltage difference across an open circuit breaker increases during closing, the dielectric strength of the gap between the open contacts decays. It’s possible to measure a breaker’s pre-strike voltage in relation to the time of contact touch. Graphing these measurements approximates a line whose slope represents the rate of decay of dielectric strength (RDDS). For a breaker to be a suitable capacitor-switching device, its RDDS must exceed the rate of the system voltage change. Fig. 2 shows the acceptable RDDS region.

Ideally, the capacitor should be energized when the voltage across the breaker contacts is zero to minimize switching transients. Consider the zero crossing on the voltage waveform, which is our point-on-wave “target” for contact touch. Fig. 3 on page 22 shows a green shaded region that depicts the range of mechanical scatter for a particular circuit breaker. The purple shaded region shows the range of dielectric scatter for the same circuit breaker (Sidebar below). Since the scatters are additive, contact touch can occur anywhere along the black portion of the voltage waveform.

Fig. 3 – Point-on-wave “target” for contact touch

This possible contact touch time window is skewed considerably to the left of the target, indicating a high probability of circuit completion prior to the zero voltage crossing. Early completion of the circuit will result in pre-striking, which will produce severe transients. This situation can be avoided by slightly retarding the point-on-wave target to just after the zero crossing, as shown in Fig. 4. This delay increases the probability of contact touch at the zero crossing.

Fig. 4 – Point-on-wave target to just after the zero crossing

Another benefit of controlled switching is, unlike the transient mitigation methods discussed earlier, you can use it for capacitor de-energization. The precise timing of each pole is controlled by a microprocessor. The software controlling the timing can be adaptive to adjust for physical and environmental variables, thereby reducing some of the uncertainties represented by scatter. Over the years that controlled switching has been used, the control methods have become much more sophisticated than those used by the first controllers.

Controlled switching is complicated greatly by the mechanical limitations of the circuit breaker. Even with contact speeds in excess of 10 m/sec, transient recovery voltage characteristics of some circuit breakers aren’t adequate for some capacitor switching applications. Mechanical and dielectric scatter can lead to a switching performance that’s considerably less than optimum performance. One way to avoid these problems is to replace the mechanical circuit breaker with a solid-state switching device.

Solid-state electronics don’t solve all the challenges involved with controlled switching. In fact, they tend to exchange one set of problems for another. They do, however, appear to be a viable technology for capacitor switching, particularly at low and medium voltages. Solid-state switching eliminates mechanical scatter, reduces dielectric scatter, and can almost do away with inrush and the associated harmonics during capacitor switching.

Because the current leads the voltage by 90°, the anode-to-cathode voltage of the switching device is reverse-biased for ¼ of the cycle while the current direction is from anode to cathode. This means that self-commutating devices like thyristors must be provided with a gating signal through the full 360° to ensure full conduction. Semiconductor switches used in capacitive switching applications must also endure high peak inverse voltages (PIVs) in excess of 3.5 times the line-to-line voltage. This is because the charge trapped in the capacitor when the switch commutates off holds the absolute value of the capacitor voltage at peak while the system voltage continues to oscillate. This condition exposes the switch to full peak-to-peak voltage, which is 2.83 times the rms voltage. When you allow for tolerances in system voltage and a reasonable safety factor, this PIV requirement can exceed 3.5 times the line-to-line voltage. These requirements lead to high cost components and implementation challenges at the higher voltages. However, as power electronics continue to evolve, these challenges are sure to be conquered.

Fehr is an independent engineering consultant located in Clearwater, Fla.

Sidebar: Circuit Breaker Mechanics

The stored energy system that moves the movable contact when the breaker’s trip or close coil is energized is made up of springs or hydraulic components. The spring has a spring “constant” that determines the contact velocity that will be attained upon opening. However, this spring constant tends to vary slightly with the spring’s temperature, age, and the amount of time it was in its compressed state. The performance of the hydraulic system also varies somewhat with temperature, pressure, and the condition of the hydraulic fluid. These variables lead to slightly different operating characteristics each time the breaker operates. Upon statistical analysis of these variables, it’s possible to determine a probabilistic distribution of operating speed. This range of mechanical performance is called mechanical scatter.

Likewise, the insulating medium that surrounds the contacts, usually sulfur hexafluoride gas (SF6) or a vacuum chamber for modern medium- and high-voltage breakers, tends to have slightly different electrical properties under different operating conditions. The number of operations performed by the interrupter, the purity of the dielectric medium, the pressure in the interrupter, and temperature of the dielectric medium are the major variables that affect the electrical properties of the dielectric. More importantly, the condition, namely the surface roughness, of the interrupter contacts and the surrounding dielectric materials, such as gas porting nozzles, influence the electric field distribution within the interrupter. It’s also possible to statistically analyze these variables and develop a dielectric scatter. Smooth contact surfaces and clean porting nozzles are vital for successful capacitor switching.

The Trouble With Capacitors Part 1

Published by R. Fehr, P.E., Engineering Consultant, Dec 1, 2003

Although shunt capacitors offer several advantages at all voltage levels, those advantages come at a price. Not only must you purchase, install, and maintain capacitor equipment, you must be able to switch it in and out of service to get the most from the system. When load levels are high, a shunt capacitor system is beneficial. When the load drops off, however, the capacitor can do more harm than good. An excess of capacitance in service can lead to higher than desired voltages, excessively leading power factors, and resonance phenomena.

This is why many capacitor banks are designed with switching mechanisms that allow you to connect them to and disconnect them from the system as needed, sometimes even as often as several times a day. While this may seem like a simple proposition, it can lead to problems because switching a capacitor bank is different than switching a normal load. To understand this difference, we first need to understand how an electric circuit is energized and interrupted.

Switching a ‘normal’ load. Energizing a load begins with a switching device in the open position. When the switch is closed, the load is connected to the rest of the energized system and whatever voltage magnitude exists on the switch’s source side will be applied across the load at the instant of contact touch. If the load is a pure resistance, this isn’t a problem. The voltage across the resistance (shown in blue in Fig. 1 above) will give rise to a current flow (shown in red) according to Ohm’s Law.

Fig. 1 also shows the switch closing just as the voltage waveform crosses zero. This is the preferred time of closing. If the switch closes at any other time, the voltage and current waveforms on the load side of the switch will experience a sudden “jump” or discontinuity. The rapidly rising voltage and current during this transient period adversely affects the system’s power quality. (Part 2 of this article, which will appear in next month’s issue, will explore this phenomenon in detail.)

Now let’s look at how a resistive load is de-energized. Prior to de-energization, the voltage and current waveforms are in phase. At some point, the switch contacts will begin to separate. It’s not physically possible to fully separate the contacts instantaneously because of the inertia associated with the contact masses that must be accelerated. One electrical cycle at 60 Hz has a period of about 16.7 msec. Most mechanical and hydraulic operators will take several electrical cycles to transition the contacts from closed to fully open. This operation, which is slow in electrical terms, will be the root cause of a serious problem.

When the switch contacts begin to open, the dielectric strength of the gap between the contacts is low, since the separation distance of the contacts is small. As the separation distance grows, so does the dielectric strength of the gap. After the voltage waveform crosses zero, its magnitude begins to grow. This voltage that builds across the separating contacts is called the recovery voltage. It may grow more quickly than the dielectric strength across the parting contacts, so current will re-establish through an arc between the contacts.

As the switch contacts continue to separate, the dielectric strength of the gap will quickly exceed the recovery voltage. When the dielectric recovery of the switching device grows more quickly than the system recovery voltage, the arc will extinguish when the current waveform next crosses zero, the current will stop flowing, and the circuit will be successfully interrupted.

Fig. 2 – De-energizing process graphically

Most switching devices are designed to safely dissipate the energy of the arc. A small transient disturbance will occur to the voltage waveform during the arcing, but this short-lived perturbation is usually insignificant. Fig. 2 shows the de-energizing process graphically.

Switching a capacitive load

If the load contains inductance or capacitance, the situation is quite different. The laws of
physics state that neither the current flow through an inductor nor the voltage across a capacitor can change instantaneously. In reality, some inductance and some capacitance are present in all circuits, although the values may be very small.

Fig. 3 – Graphically, energizing a capacitor: expected response and actual response

So, what happens when the switch is closed to energize an inductive or capacitive load? In the case of a capacitive load, the current waveform leads the voltage waveform by 90°. If the switch contacts close as the voltage waveform crosses zero, the current would have to instantaneously jump to its maximum value at that time, giving what’s referred to as the “expected response” (Fig. 3). But an instantaneous change in current isn’t physically possible, so instead a very fast — but not instantaneous — change in current that overshoots the maximum value takes place. The peak current inrush magnitude, as defined in the following equation, is a function of the rated capacitor current and the strength of the system to which the capacitor is connected, quantified by the available short-circuit current.

The differential equation that describes this case has a solution that contains an exponentially damped sinusoid. This transient decay occurs at a frequency much higher than the power system frequency, typically in the kilohertz range. This frequency is determined by the same parameters that defined the peak inrush current and is described by the equation below.

As the current rapidly increases, the voltage rapidly decreases, following Ohm’s Law. The voltage and current waveforms oscillate, or ring, at a frequency much higher than the power system frequency. After a short period of time, the waveforms then settle down to their steady-state values, as expected. Graphically, energizing a capacitor looks like the “actual response” labeled in Fig. 3.

Fig. 4 – De-energizing a capacitive load

De-energizing a capacitive load poses even more challenges. Because the current waveform leads the voltage waveform by 90°, the current is interrupted very close to its zero crossing when the voltage is at its maximum absolute value. Looking at Fig. 4 on page 20, the initial current interruption occurs at the y-axis. At that time, both the system voltage and the voltage on the capacitor are at their maximum negative values. As the contacts open, the charge that maintains the capacitor voltage is trapped in the capacitor, thus keeping the capacitor voltage constant at its maximum negative value. The capacitor voltage is shown as a dashed black line in Fig. 4. According to IEEE Standards 18-2002 and 1036-1992, the trapped charge in a power capacitor must dissipate such that the voltage on the capacitor is no more than 50V 5 min. after de-energization. This voltage decay is very slow compared to the timeframe discussed in this article, so it’s necessary to consider capacitor voltage while de-energized to be constant, as shown by the horizontal dashed black line in Fig. 4.

As the contacts in this example continue to separate — a process that will take about three electrical cycles or 50 msec at 60 Hz — the dielectric strength of the gap between them increases in a fairly linear fashion, as shown by the solid black line in Fig. 4. But the voltage difference across the contacts, which is the difference between the sinusoidal system voltage and the constant capacitor voltage, increases more quickly. At the restrike point the voltage across the parting contacts exceeds the dielectric strength of the gap between the contacts. This will cause an insulation breakdown, which will result in an arc that re-establishes current flow. This re-establishment of current flow occurs after a quarter of a cycle of initial interruption in the example. Thus, this re-establishment of current flow is called a “restrike.” Had the restrike occurred in less than a quarter of a cycle after initial interruption, it would have been called a “re-ignition.”

When the current is re-established, it becomes a high-frequency, exponentially decaying sinusoid. The high-frequency current oscillations give rise to high-frequency voltage fluctuations, similar to that of the capacitor energization case. Resistance present in the system quickly damps these oscillations.

At the next current zero, the arc will be interrupted again, but this time the contacts will be farther apart than during the first interruption attempt, thereby providing a greater dielectric strength between the parting contacts. At the second interruption attempt in the example, the dielectric strength between the parting contacts, which are still at less than half of their ultimate separation distance apart, will slightly exceed the voltage difference across the opening contacts. This will allow a successful current interruption. In some cases, a second restrike would occur at this point, and successful interruption would have to wait until the third attempt.

A capacitive switching device must be designed to endure the thermal stresses caused by the re-ignitions and restrikes. Some circuit breakers fail to meet this level of performance. This is why switching devices used for capacitor switching must be designed specifically for that application. In many cases, such devices have a higher transient recovery voltage rating than general-use circuit breakers. This makes the slope of the solid black lines labeled “dielectric strength between contacts” in Fig. 4 steeper, which reduces the probability of re-ignitions and restrikes.

Part 2 of this article will examine the detrimental effects capacitor switching has on the rest of the system and discuss the methods of minimizing them.

Fehr is an independent engineering consultant located in Clearwater, Fla.

Sidebar: Why We Use Capacitors

Capacitors have many uses in electric power systems. When used as sources of reactive power they’re connected line-to-neutral, or in shunt. These shunt capacitors, which are often called “power factor correction capacitors,” are used at all voltage levels.

At the transmission and subtransmission levels (above 34.5kV), shunt capacitors increase the power transfer capability of a transmission system without requiring new lines. Due to the high cost, long lead-time, and problems associated with transmission line construction, utilities use high-voltage capacitors today more frequently than ever.

High-voltage shunt capacitors also support the transmission system voltage, which is often necessary when the transmission grid is pushed to and perhaps beyond its design limits as a result of open access to the grid and decreased capital spending on network upgrades. Since the capacitors produce reactive power (VARs), generators no longer need to produce as much, enabling them to operate at higher power factors and produce more real power (watts). Also, fewer VARs transported through the transmission system not only frees additional capacity on the lines for watts, but also reduces system losses by reducing the total current flowing on the lines.

Shunt capacitors also slightly increase transmission bus operating voltages. As the transmission voltage increases, less current is necessary to supply a typical load, so transmission losses decrease again.

Utilities use shunt capacitors at distribution and utilization voltages to provide reactive power near the inductive loads that require it. This reduces the total current flowing on the distribution feeder, which improves the voltage profile along the feeder, frees additional feeder capacity, and reduces losses. In fact, substation transformers experience lower loadings when utilities install sufficient capacitors on the distribution system. The reduced loadings not only improve contingency switching options on the distribution system, but also extend equipment life and defer expensive additions to the system.

Voltage Magnification and Nuisance Tripping during Capacitor Bank Switching

Published by Electrotek Concepts, Inc., PQSoft Case Study: Voltage Magnification and Nuisance Tripping during Capacitor Bank Switching, Document ID: PQS0902, Date: October 15, 2009.

Abstract: The application of utility capacitor banks has long been accepted as a necessary step in the efficient design of utility power systems. In addition, capacitor switching is generally considered a normal operation for a utility system and the transients associated with these operations are generally not a problem for utility equipment. These low frequency transients, however, can cause problems for low voltage power electronic-based loads.

Adjustable-speed drives are susceptible to dc link overvoltage trips caused by utility capacitor switching. In general, an increase in input inductance (choke or isolation transformer) will reduce the possibility of nuisance tripping. However, if the customer has power factor correction capacitors on the same bus, it may be necessary to take additional remedial actions. This case study investigates the potential for voltage magnification and nuisance tripping during utility capacitor bank switching on a 24kV distribution system.

INTRODUCTION AND MODEL DEVELOPMENT

The potential for voltage magnification and nuisance tripping during utility capacitor bank switching was studied for the system shown in Figure 1. The accuracy of the system model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities, such as capacitor bank inrush and rated current and voltage rise.

Figure 1 – Oneline Diagram for the Capacitor Bank Switching Case Study

Voltage magnification occurs when the transient oscillation initiated by the energization of a utility capacitor bank excites a series resonance formed by a step-down transformer and power factor correction capacitor bank on the utility’s or customer’s lower voltage system. The result is a higher overvoltage magnitude at the lower voltage bus. Previous research has indicated that the worst magnified transient occurs when the following conditions are met:

The rating of the switched capacitor bank is significantly larger (>10) than the lower voltage power factor correction bank (e.g., 16.2 MVAr vs. 300 kVAr).
The energizing frequency of the utility capacitor bank is close to the series resonant frequency formed by the step-down transformer and the lower voltage capacitor bank.
There is relatively little damping (resistive load) provided by the lower voltage load (typical industrial plant configuration – primarily motor load).

Nuisance tripping refers to the undesired shutdown of a customer’s adjustable-speed drive or other power-electronic-based process device due to a transient overvoltage on the device’s dc bus. Very often, this overvoltage is caused by utility transmission or distribution capacitor bank energization. Considering the fact that many distribution banks are time clock controlled, it is easy to see how this event can occur on a regular basis, thereby causing numerous process interruptions for the customer.

The nuisance tripping event consists of an overvoltage trip due to a dc bus overvoltage on voltage-source inverter drives. Typically, for the protection of the dc capacitor and inverter components, the dc bus voltage is monitored and the drive tripped when it exceeds a preset level. This level is typically around 780 volts (for 480 volt applications), which is only 120% of the nominal dc voltage. It is important to note that nuisance tripping can occur even if the customer does not have power factor correction capacitor banks.

An adjustable-speed drive system consists of three basic components and a control system as illustrated in Figure 2. The rectifier converts the three-phase ac input to a dc voltage, and an inverter circuit utilizes the dc signal to produce a variable magnitude, variable frequency ac voltage, that is used to control the speed of an ac motor.

Figure 2 – Adjustable-speed Drive Simulation Model

SIMULATION RESULTS

Energizing a shunt capacitor bank from a predominantly inductive source creates an oscillatory transient that can approach twice the normal system peak voltage (V_pk). The characteristic frequency (f_s) of this transient is given by the following expression:

f_s = 1 / 2π√(L_s ∗ C) ≈ f_system ∗ √(X_c / X_s) = f_system ∗ √(MVA_sc / MVAr_3φ) = f_system ∗ √(1 / ΔV)

where:
f_s = characteristic frequency (Hz)
L_s = positive sequence source inductance (H)
C = capacitance of bank (F)
f_system = system frequency (50 or 60 Hz)
X_s = positive sequence source impedance (Ω)
X_c = capacitive reactance of bank (Ω)
MVA_sc = three-phase short circuit capacity (MVA)
MVAr_3φ = three-phase capacitor bank rating (MVAr)
ΔV = steady-state voltage rise (per-unit)

The energizing frequency for the 16.2 MVAr, 24kV (74.60μF) distribution capacitor bank with a source strength (I_3φ) of 16.85 kA (2.18mH) may be approximated using the following expression:

f_s = f_system ∗ √(MVA_sc / MVAr_3φ) = 60 ∗ √(700.44 / 16.2) =394.5Hz

where:

MVA_sc = √3 ∗ 24kV*16.85kA = 700.44MVA

The steady-state voltage rise for this case may be approximated using the following expression:

ΔV = (MVAr_3φ / MVA_sc) ∗ 100 = (16.2 / 700.44) ∗ 100 = 2.3%

Finally, the peak inrush current (I_pk) (refer to Figure 3) may be approximated using the following expression:

I_pk = V_pk / √(L_s / C) = 24kV * (√2 / √3) / √(2.18mH / 74.60μF) = 3625A

where:
V_pk = peak system voltage (line-to-ground)
L_s = positive sequence source inductance (H)
C = capacitance of bank (F)

Figure 3 – Inrush Current during Capacitor Bank Energization

It is important to note that the peak inrush current is estimated without including resistance in the calculation, and in general, actual and simulated values are somewhat lower. The peak simulated inrush current for the 16.2 MVAr capacitor bank was 3273 amps (90% of calculated value).

The maximum transient overvoltage (refer to Figure 4) at the 24kV substation bus when energizing the 16.2 MVAr capacitor bank was 1.66 per-unit. Typical overvoltage magnitude levels range from 1.3 to 1.8 per-unit for larger substation capacitor banks. The maximum transient overvoltage (refer to Figure 5) at the 4.16kV bus was 1.34 per-unit so voltage magnification did not occur for this system.

Figure 4 – Substation Bus Voltage during Capacitor Bank Energization

Figure 5 – 4.16kV Bus Voltage during Capacitor Bank Energization

Figure 6 shows the resulting dc voltage on the 10 hp adjustable-speed drive in the customer facility. The peak transient voltage is 808 volts, which is somewhat higher than the assumed trip level of 780 volts, so it is assumed that the drive will trip for this case.

Figure 6 – ASD dc Link Voltage during Capacitor Bank Energization

The effectiveness of synchronous closing control on the substation capacitor bank switch was evaluated in a series of cases that varied the timing error from an ideal voltage zero closing. Synchronous closing is independent contact closing of each phase near a voltage zero. Previous analysis has indicated that a closing consistency of ±1.0msec provides overvoltage control comparable to properly rated pre-insertion resistors.

Figure 7 shows the resulting 24kV bus voltage for the worst-case synchronous closing control case with a +1.0msec error. The maximum transient overvoltage is reduced from 1.66 per-unit to 1.09 per-unit.

Figure 7 – Substation Bus Voltage with Synchronous Closing Control

Figure 8 shows the resulting dc link voltage for the adjustable-speed drive for the synchronous closing control case with a +1.0msec error. The dc overvoltage is reduced from 808 volts to 748 volts, so it is assumed that the drive will not trip for this case.

Figure 8 – ASD dc Link Voltage with Synchronous Closing Control

A pre-insertion resistance provides a means for reducing the transient currents and voltages associated with the energization of a shunt capacitor bank. The impedance is shorted-out (bypassed) shortly after the initial transient dissipates, thereby causing a second transient event. The insertion transient typically lasts for less than one cycle of the system frequency. The performance of pre-insertion impedance is evaluated using both the insertion and bypass transient magnitudes, as well as the capability to dissipate the energy associated with the event, and repeat the event on a regular basis.

Pre-insertion resistors and high-loss pre-insertion inductors are one of the most effective means for controlling capacitor bank energizing transients. The optimum resistor value for controlling capacitor bank energizing transients depends primarily on the capacitor bank rating and the source strength. It should be approximately equal to the surge impedance (Z_s) formed by the capacitor bank and source:

R_optimum ≈ √(L_s / C)

where:
L_s = positive sequence source inductance (H)
C = capacitance of bank (F)

The optimum resistor rating for the 16.2 MVAr, 24kV (74.60μF) substation capacitor bank with a source strength (I_3φ) of 16.85 kA (2.18mH) may be approximated using the following expression:

R_optimum ≈ √(2.18mH / 74.60μF) = 5.4Ω

A 6.4Ω resistor was chosen for the simulation because it is available commercially. Figure 9 shows the resulting 24kV bus voltage for the 6.4Ω pre-insertion resistor case. The maximum transient overvoltage is reduced from 1.66 per-unit to 1.12 per-unit.

Figure 9 – Substation Bus Voltage with Pre-insertion Resistor

Figure 10 shows the resulting dc link voltage for the adjustable-speed drive for the 6.4Ω pre-insertion resistor case. The dc overvoltage is reduced from 808 volts to 710 volts, so it is assumed that the drive will not trip for this case.

Figure 10 – ASD dc Link Voltage with Pre-insertion Resistor

The most effective methods for eliminating nuisance tripping are to reduce the energizing transient overvoltage, or to isolate the drives from the system with series inductors, often referred to as chokes. The additional series inductance of the choke will reduce the transient magnitude at the input to the drive and the associated current surge into the dc link filter capacitor, thereby limiting the dc overvoltage.

While determining the precise inductor rating for a particular application may require a detailed computer simulation study, a more common approach involves the widespread application of a standard 3% value. The 3% rating is based upon the drive kVA rating and is usually sufficient for most applications where voltage magnification is not also a concern. Generally, the choke is specified in %X and hp. However, the inductance of the choke may be approximated using the following relationship. A 3% choke for the customer’s 10 hp drive would have the following inductance:

where:
f_system = system fundamental frequency (50 or 60 Hz)
X = inductive reactance of ac choke (%)
kV_φφ= system rms phase-to-phase voltage (kV)
hp = Horsepower rating of the drive (hp)

Figure 11 shows the resulting dc link voltage for the adjustable-speed drive with a 3% choke applied to the ac terminals. The dc overvoltage is reduced from 808 volts to 736 volts, so it is assumed that the drive will not trip for this case.

Figure 11 – ASD dc Link Voltage with a 3% Choke

SUMMARY

Observations and conclusions for this case study include:

1. The devices and equipment being applied on the power system are more sensitive to power quality variations than equipment applied in the past. New equipment includes microprocessor-based controls and power-electronic devices that are sensitive to many types of disturbances. Controls can be affected, resulting in nuisance tripping or misoperation as part of an important process, or actual device failure can occur.

2.Capacitor bank switch selection and configuration will generally depend on switch capabilities (e.g., short circuit interrupting and capacitance switching ratings), mitigation device selection (e.g., pre-insertion vs. synchronous closing), site considerations, and an economic evaluation.

3.Inrush currents during energization should be below rated breaker/switch capabilities.

4.Transient overvoltages related to voltage magnification at lower voltage buses were found to be below arrester protective levels for the simulated system. However, these transients may exceed levels that could cause nuisance tripping of adjustable-speed drives.

5.Transient overvoltages associated with energization of the 24kV capacitor bank can be significantly reduced with the application of synchronous closing control or pre-insertion resistors. In addition, the resulting overvoltages at distribution capacitor banks and lower voltage customer locations were also reduced, thereby significantly reducing the probability of localized customer problems due to sensitive equipment or low voltage power factor correction.

REFERENCES

G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and T. Grebe, Impact of Utility Switched Capacitors on Customer Systems Part II – Adjustable Speed Drive Concerns, IEEE Transactions PWRD, pp. 1623-1628, October, 1991.

G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and R. Zavadil, Impact of Utility Switched Capacitors on Customer Systems – Magnification at Low Voltage Capacitors, IEEE Transactions PWRD, pp. 862-868, April, 1992.

Electrotek Concepts, Inc., Evaluation of Distribution Capacitor Switching Concerns, Final Report, EPRI TR-107332, October 1997.

RELATED STANDARDS
IEEE Std. 1036-1992

GLOSSARY AND ACRONYMS
ASD: Adjustable-Speed Drive
PWM: Pulse Width Modulation
MOV: Metal Oxide Varistor
TVSS: Transient Voltage Surge Suppressors

Overhead Line Fault Section Positioning System Based on Wireless Sensor Network

Published by Ai-hua Dong, Xinlin Geng, Yi Yang, Ying Su, Mengyao Li, Henan Polytechnic University

Abstract. This paper introduces overhead line fault detection and location system as the core of industrial control computer. The combination of software and hardware, the methods of current rate of change and zero current detection are used, and so the accuracy of short circuit fault detection is improved. The features of ground phase voltage drop and the 5th harmonic current are integrated. Zigbee wireless chips are used to make into independent signal transmission system. The system has been put into operation, running in good condition.

Streszczenie. W artykule przedstawiono system detekcji i lokalizacji awarii w sieci napowietrznej do implementacji w komputerze przemysłowym. W metodzie brana jest pod uwagę m. in. wielkość zmian prądu oraz obecność prądu kolejności zerowej, co poprawiło dokładność wykrywania awarii. Zastosowano także bezprzewodowe łącze Zigbee w celu zwiększenia niezależności przesyłu sygnałów w systemie. Badania potwierdziły poprawność działania. (System lokalizacji awarii w sieciach napowietrznych z zastosowaniem sieci czujników bezprzewodowych).

Keywords: overhead lines; short-circuit fault; ground fault; online detection.
Słowa kluczowe: linie napowietrzne, zwarcie, zwarcie doziemne, detekcja online.

Introduction

As an important part of the transmission and distribution, high-voltage overhead lines often produce grounding, short circuit fault due to various reasons [1], brought great risks to the user’s safety of production. In addition, the electric power generated devastating by the strong short-circuit current, so early detection and cut off short circuit promptly, the protection equipment and switch itself can be avoided to withstand huge thermal shock and electric power , it has great significant for stable operation of the power system[2]. Currently, the line fault indicator is the main equipment for the point of failure detection and location, but the fault indicator only has fault flop function, does not has the functions of launch automatically and transfer. Also some of the existing line fault location system [3], line fault segmentation positions through the fault indicator, and the transmission using the GSM/GPRS network communication [4], multi-launch mode. Each emission point is equivalent to a mobile phone running, high operating costs, not suitable for long-distance power lines. To this end, we use the new self into an independent transmission system, ZigBee wireless transmitter module, based on the existing line fault detection method, and fault detection and signal transmission circuit as a whole, in order to achieve reliable transmission of the fault signal detection, signal, while at the same time to reduce system operating costs. The system has been installed in a supply line in our country, has good application prospects.

Basically composition of the system

The composition of the system is shown in Fig.1. Detection and launchers mounted on the transmission line towers, including the fault detection circuit and signal transmission circuit[5], signal detection and short-circuit fault, ground fault discrimination done by the fault detection circuit and the results of the distinguish is sent to the wireless transmitter circuit. In order to save the launch of nodes, we choose the model of JN5139-Z01-M01, high-power ZigBee wireless communication module. Its basic performance: operating frequency of 2.4GHz, compatible with IEEE802.15.4 and ZigBee protocol, in the broad environment, up to 1 km communication distance; transmit power: +2.5 dBm, the emission current is less than 37mA. Each detection and launching devices set the address code, the failure of segmentation positioning can be achieved according to the address code. ZigBee wireless communication module in the device, to form an independent wireless transmission network by itself. The fault signal is transported to the substation transceiver, step by step and relay, and then the signals are received through a wired transceiver and send to the substation Industrial Control Computer (ICC), ICC be used to analyze and deal with these fault information. When failure occurs, it can achieve the audible alarm, and display, save or print the fault type and fault location, and provide the basis for the investigation of line fault and analyze the cause of the malfunction. Substation ICC will send fault information to the transceiver, through the GPRS network; the line maintenance will get the fault information by personnel phone and be notified to carry out line maintenance. In this way, taking less communication network resources communication network resources, savings in operating costs, while achieving a rapid troubleshooting and timely maintenance on the line to reduce outage time and improve the reliability of power supply system. And if necessary, the ICC of the substation can be Networked with the monitoring host of the production scheduling system through the network cable and the production scheduling system may at any time to display, print and store test data, so that the production scheduling staff can keep abreast of the operation of the line. In addition, the system also has the self-test function of the signal transmission. The ICC periodically inspects each point through response mode by the software program. It can timely comprehend the status of each device in such a way, in order to detect problems in time to ensure reliable operation of the system of signal transmission.

The principle of fault detection

A. Short-circuit fault detection principle

In order to improve the reliability of the short circuit fault detection, the system selects the short-circuit fault detection technology, based on current rate of change [6]. Its hardware circuit is shown in Fig.2. In which, the CT is a small current transformer, used to implement the sampling of the line current. Here selection of the open-type current transformer in order to facilitate installation.

Fig.2. Short circuit fault detection circuit schematic diagram

The resistance R of the resistor in parallel across the current transformer used to transform the current signal into a voltage signal. Through the diode VD rectifier and filtering capacitor C1, the AC voltage signal Change into a DC voltage signal, and then the resistance of R1, R2 partial pressure are then given to the LM393 comparator inverting input and the inverting input, the comparator is used to achieve short-circuit fault discrimination. Short-circuit fault discrimination as follows: As resistor R1 and R2 are equal, the voltage of noninverting input V+=2V-, the voltage of noninverting input is higher than the inverting input, according to the principle of the comparator the output of the comparator is high potential. When the measured line produce short-circuit fault, the current will suddenly increase, corresponding to the DC voltage signal will also suddenly increase, However, due to the termination of the noninverting input is connected a capacitor and the inverting input is not connected capacitor. Thus, when the voltage mutation occurs, the potential of inverting input terminal rise faster than the noninverting input terminal potential, causing the inverting input of the potential is higher than the noninverting terminal potential. According to the principle known of the comparator, the voltage output UOUT is low.

B. Ground fault detection principle

At present, the main method of the existing single-phase ground fault detection are [7]: zero sequence current method, capacitive current method, the first half-wave method, the fifth harmonic method and the signal injection method. When the lines of a phase to ground fault, the phase voltage will be reduced, so that the three-phase voltage will be asymmetric, usually there will be inductive load grid, line current will be distorted, produce large amounts of high-order harmonic current, appears 3, 5, 7 …… harmonics. However, since the 6 ~ 66kV distribution network belongs to the neutral point non-effectively grounded system [8]. Therefore, the third harmonic current can not be through the grid, other harmonic components accounted for a small proportion, so the 5th harmonic is the most obvious.

Fig.3. 5th harmonic current detection schematic diagram

Because of this, we use the fifth harmonic current method, by detecting the line current; extract the 5th harmonic component, depending on the size of the 5th harmonic current to determine the ground fault. Detection circuit is shown in Fig.3. First, the line current is measured with a special open-type current transformer, separated the 5 harmonic by selective circuit, and then changed into a DC voltage signal by the rectifier circuit, the final outputted by the comparator circuit. There may be some higher harmonic current in the normal circuit due to the presence of nonlinear load in the circuit line. Therefore, the comparative output is used in here. According to the predicted values of the 5 harmonic current in the normal, the baseline value of the comparator is to be determined. When 5 harmonic current is greater than or equal to the reference value of the comparator, the output is high potential. It can be judged to be ground fault; otherwise, the output is low, it is nonground fault. In addition, to further improve the reliability of the ground fault detection [9], we also detect circuit phase voltage, the same as one of the necessary conditions to determine the ground fault. When the line voltage decreases, while the 5th harmonic current detection circuit outputs a high potential at this time determined to ground fault, the other cases are non-ground fault.

Installation and application

After multiple simulations detection circuit to meet the requirements, then to product the PCB, weld the component, assemble. The system is installed in a line site in Henan, run in November 2009. The total length of the installation of line about three kilometers, the line voltage rating of 6kV load current of 300A. The power supply system is the neutral point ungrounded system. According to the situation of the distance range, were selected seven detection points. The whole line to install a total of 21 fault detection devices, each test point (A, B, C) to install three detection devices. The average distance between the two detection points is about 450 meters. Detection devices onsite installation and application picture is shown in Fig.4. The host part is placed in the control room, which mainly include: industrial control micro-computers, wireless receiver, monitors, etc…The main part of the wireless receiver device is a ZigBee wireless communication module, whose role is to receive the detection signal through the wireless transmitter step by step .

Fig.4. Detection device installing picture

The IPC gets the data wired, analyzes and processes them. Taking Kingview 6.51 software as development platform [10], the monitoring program and display with many functions are designed, such as communication, fault display, the database stores, SMS messages, etc. Home page of the picture is shown in Fig. 5.

It contains seven conversion interfaces. It contains seven conversion interfaces. Where the first three are the screens of fault display, the remaining four screens are the alarm of history, database, SMS sending and exit button. The main function of the control host as follows: (1) Fault display function. Fault display is divided into graphical display and the report shows. (2) Database function. When the system detects a failure, the relevant fault information is stored in the corresponding database. Classification to query in a database, according to the time or the type of the fault, can also generate reports, print at any time or at the setting-time. Database is also directly connected with Excel to save the information in the form of Excel tables, user-friendly; (3) SMS sending. When a fault occurs, first generate the corresponding fault information, there are time, location, fault type, and then the fault information will be sent directly to maintenance personnel mobile phones with the TC35T SMS sending module connected to the computer interface, in order to deal with failure, shorten the time of failure blackout.

Conclusions

The results of the system operation show that, The results of the system operation show that the accuracy rate of the short circuit fault detection close to 100% and accuracy rate of the ground fault detection up to 80%, and can achieve reliable transmission by relay and step, and the running costs of the system is significantly reduced, to achieve the desired goal. The safety and reliability of the power supply system becoming more demanding, urgent need for reliable fault detection and location devices. The system can be timely detection of line faults, quickly find the failures, fast processing, and rapid restoration of electricity, to reduce the outage frequency and outage time, it has great significance to improve the safety and reliability of power supply. Therefore, this technology has good prospects for promotion and application.

Acknowledgements

The work described in this paper is financially supported by science and technology innovation project of Henan Coal chemical industry group. Also, the corresponding author wishes to thank Reviewers for their useful comments and suggestions.

REFERENCES

[1] LI X, YI Z, WANG H. Study on Fault Locating for 10kV Distribution System [J]. Hubei Electric Power, 2011(35):26-28.
[2] LI Y,LU J P, LI J. Fault Location Based on On-line Computation of the Transmission LineDistributedParameters[J]. High Voltage Engineering, 2001,33(11):185-189.
[3] LIU S D,ZHENG Y,YANG F M. Line fault fast localization based on GIS fault indicator[J]. Distribution & Utilization, 2006, 23(6):33-35.
[4] ZHANG P Z, ZHAO W B. Design of Short Circuit and Earthling Fault Indicator Based on GPRS[J]. Low Voltage Apparatus, 2008(10):38-41.
[5] XU R M. An Automation Location System of Distribution Line Fault Based on Fault Indicator [J]. Electrical Equipment, 2005, 6(10):66-67.
[6] FENG R H, FENG M L, KONG J S. Short Circuit Protection Method Based on the Current Rate of Change [J]. Coal Mine Machinery, 2008, 29(5):171-173
[7] SUN Z H, HOU Y M. Analysis on the Detection Principle of Single-phase Ground Fault Indicator [J]. Distribution & Utilization, 2004, 21(5):30-32.
[8] GUO J H, TAN W P. Summary on fault location principle in power system [J]. Relay, 2006(3):76-81.
[9] YE D.A Practical Improvement of Faulted Circuit Indicator in Distribution Network [J]. Electrotechnical Application,2008,27(5):62-65.
[10] XIE M. Design of Remote Monitoring System of Parallel Connection and Collective Selective Control Elevators [J]. Techniques of Automation and Applications, 2010, 29(11):42-44

Authors: prof. Ai-hua Dong, Henan Polytechnic University, School of Electrical Engineering and Automation, Jiaozuo, E-mail: dah@hpu.edu.cn; Xinlin Geng, Henan Polytechnic University, School of Electrical Engineering and Automation, Jiaozuo, Email: 13523187211@163.com;Yi Yang,Henan Polytechnic University, School of Electrical Engineering and Automation, Jiaozuo, E-mail: yangyi@hpu.edu.cn; Ying Su, Henan Polytechnic University, School of Electrical Engineering and Automation, Jiaozuo, E-mail: 815360759@qq.com; Mengyao Li, Henan Polytechnic University, School of Electrical Engineering and Automation, Jiaozuo, E-mail: 136681953@qq.com.

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 3b/2013

Rapid Voltage Changes – Definition and Minimum Requirements

Published by Karstein Brekke, Norwegian Water Resources and Energy Directorate, Norway
Conference: CIRED – The 20th International Conference and Exhibition on Electricity Distribution, Prague, 8-11 June 2009

Renewable Energy Sources Intrusion into Smart Grids – Selected Problems

Published by Bożena E. MATUSIAK, Jerzy S. ZIELIŃSKI, University of Lodz

Abstract. The aim of the paper are selected problems resulting from Renewable Energy Sources (RES) intrusion into Smart Grids (SG) presentation. Upon considerations there are: wind turbines influence on power system operation, RES and microgrids operation with a Power System, new Energy Market for islanded and connected to power system microgrid, Energy Management System and ICT after intrusion RES. Conclusion presents perspective of RES development.

Streszczenie. Celem artykułu jest przedstawienie wybranych problemów spowodowanych dołączeniem odnawialnych źródeł energii (OZE) do sieci inteligentnej. Rozważono: wpływ turbin wiatrowych ma eksploatację systemu elektroenergetycznego, Współpracę OZE I mikrosieci z tym systemem, nowy rynek energii dla pracy wyspowej i mikrosieci współpracującej z systemem, Energy Management System i ICT po dołączeniu OZE. W zakończeniu przedstawiono perspektywy rozwoju OZE. (Wybrane problemy spowodowane dołączeniem odnawialnych źródeł energii (OZE) do sieci inteligentnej)

Keywords: Renewable Energy Resources (RES), Smart Grid (SG), Energy Market, Microgrid.
Słowa kluczowe: Odnawialne Źródła Energii (OZE), Rynek Energii, Sieci inteligentne.

1.Introduction

According to the fifth property of Smart Grid (SG) [1]: “It accommodates all generation and storage options” all existing renewable energy sources (RES) have to be connected to the SG. Before consideration problems resulting from different RES intrusion into SG it is necessary to pay some attention to more general issues.

Distributed generation can positively or negatively affect the Smart Distribution Grids. For example [2], in radial distribution networks, distributed generators can cause bidirectional power flows, alter the existing voltage profiles, affect the operation and coordination of the existing protection devices such as relays, reclosers etc. and compromise safety. In particular they can cause energized electric islands within the host networks which can form due either to faults causing an upstream feeder breaker to open automatically, or to inadvertent opening of the breaker for maintenance operations.¹

V. Hamidi et al. [3] composed the following list of barriers when implementing the SG technologies:

Technology readiness and development of technologies still under development.
Cost of SG technologies.
Need of re-structuring the industry.
Compatibility with current standards and technologies.
Lack of market power (energy market [1]) – for smaller utilities.

In addition to the above list it has to remember that connection RES to weak distribution grid (MV or LV) sometimes it is not possible in result aging lines, too low thermal limitation and voltage rise effect [2]; the problem is especially important for wind farms.

¹ The IEEE and other organizations have identified anti-islanding standards. As such number of anti islanding techniques have already been adopted commercially.

2.Wind Generation

Significant wind generation growth in coming decade assure it’s dominating position among RES and implies challenges in power system operation due to intermittent nature of wind. Wind generators are increasingly connected to an electrical grid through power electronic based converters and differ significantly from conventional synchronous generators particularly in terms of their impact on the electromechanical stability of the grid. The inertia of synchronous machines is stabilizing the grid frequency during transients and it is important to design technologies improving the stabilizing effect of the wind generators [4]. In that paper the Authors consider addition of a control loop that would feed (draw) active power in response to a decline (rise) in the time derivative frequency, thereby seeking to mimic the inherent inertial response of conventional synchronous generator.

The idea of inertial emulation neglects two important limitations in bandwidths and in the magnitude of power change that can be applied. In order to overcome these limitations, a work being actively considered is to supplement the control available from energy storage devices such as batteries. Typically energy storage devices offer fast control action (high bandwidth) while having narrow limits on the magnitude of power and energy that can be absorbed or supplied. On the other hand, varying the mechanical input power to a wind turbine through changes in its blade pitch is a relatively slow control action (low bandwidth) but one which can have broad limits before saturation reached [4].

Another type of challenges arise with the offshore wind farms located a long distance from the coast. For decreasing high losses accompanying AC transmission, High Voltage Direct Current (HVDC) cables with Voltage Source Converters (VSC) are to be applied. Multi terminal VSC-HVDC consisting of more than two converters has following benefits: bulk power transmission, AC network interconnection over a long or medium distance, and economical advantages² [5].

Great capacity of wind farms results that the traditional “fit and forget” approach where all technical limitations are satisfied in many credible operational scenarios has significantly reduced the ability of certain networks to integrate more generation capacity as the extra costs are not viable for most wind farm developers. In this context, it is envisaged that the true potential of distribution networks to accommodate large renewable generation capacities will only be realized by applying active management schemes. In [7] it is proposed the innovative use of synchrophasor technology to actively manage wind power generation output in congested distribution networks, resulting in the connection of more capacity and hence, the delivery of more energy as opposed to the “fit and forget” approach. This is achieved by applying an angle-based constraint that is determined according to the network characteristics (i.e. a proxy the thermal limits, voltage limits etc.) and using minimal communication. Results from a radial test feeder considering two wind farms demonstrate the effectiveness of the technique in exporting more energy, although at expense of smaller capacity factors, whilst keeping the system secure.

² The idea of the VSC-HVDC application has been used also in [6].

3.RES and Microgrids connected to the Smart Distribution Grid

According to [1] development of RES (except of wind farms) in Poland is a result of private activity what results in number of generators dispersed (DGs) in region. Each of these DGs ought to be connected to distribution network and realize its private schedule what means that DGs belong to Virtual Power Plant (VPP).

Individual DGs, partners in virtual power plants, with different technical characteristics connected to the distribution network implies serious technical – as well as organizational problems depending on their number, capacity, schedules etc. Introduction of controlling interface between DGs and distribution network will be valuable solution simplifying a distribution network operation. Of course the interface between separate DG and distribution network is too expensive and it is necessary to group together number of DGs located not far one from the another. When we can collect several DGs with suitable location we can compose them in Microgrid³.

Microgrid – it is interconnection of small modular generation⁴ to Low- or Medium- voltage distribution systems. Microgrids can be connected to the main power network or be operating islanded, in a coordinated, controlled way⁵. Microgrid connected to the distribution network (in future smart distribution grid) needs creation of Active Distribution Network (ADN) passing following stages:

remote monitoring and control of DG and RES,
determination of great number of DG and RES management,
full active power management together with real time communication and remote control.

ADN operation implies necessity of application one of two different strategy: microgrids or virtual consumers. Concept of virtual consumer [1,12] is adaptation of a model similar to information and business ability of Internet. Electrical energy bought from conventional generators, RES or storage devices, according to demand is delivered to agreed nodes. The system would use new ICT technologies as well as advance power electronics and storing devices.

Diversity of RES and storage devices as well as architecture and collaboration with power system implies necessity to define control strategy in operation.

“Building Network “ strategy emulate “vacillatory source” in islanded network. DER unit realizing this strategy controls voltage in the connection with the system node setting up the system frequency.

Power and energy management strategy is very important in islanded microgrid and it is more critical than in power system because of specific characteristics of the microgrid. It is worth of mention that according chapter 1 microgrid is one of important part of smart grids.

Microgrid being consortium of private owners is more convenient for distribution grid nevertheless it is still part of virtual power plant.

³ The same opinion concerning role of the microgrid in RES intrusion into the grid one can find in [8]
⁴ It is worth of mention that to RES are included also all types of storage devices including also Electric Vehicles [9] and optimal integration of the energy storage systems in smart distribution grids has been considered in [10]
⁵ Process of reconnection from Islanded state to connection with distribution network is considered in [11].

4.New Energy Market for Islanded and Connected to Power System Microgrid

The main question for a new shape of EM with microgrids is: What kind of business models would support the implementation of Energy Efficiency (EE) and dissemination of implementation RES and creation DER? The business models on which the European thematic projects works last year were focused are those which concern the aggregator of Demand Response (DR), Distributed Generation (DG) and Distributed energy Storages (DS), which have been together called – Distributed Energy Resources (DER). Aggregation functions can be taken care of by an independent organization or an existing market participant, e.g. an electricity supplier (retailer) other forms of selling activities or virtual Power Plants (VPP). In each case, these organizations have been called: the aggregators.

The aggregator is defined in the following brief way (according to the definition from SEESGEN_ICT European Project): The aggregator is a company who acts as a mediator between electricity end-users, who provide distributed energy resources, and those power system participants who wish to exploit these services [12,13] Nowadays, the aggregators for complex business models integration for DER management on the Energy Market have been needed.

The main aggregator’s job is to provide a link between the end-users, i.e., the providers of demand response, prosumers, DGs and the buyers. Where the consumer is alone to provide demand response, he should have a direct relationship with buyers of demand response services. Without an intermediary, this would lead to very many bilateral relationships between market participants. Their management is not in the interest of buyers of demand response services, such as TSO’s. For example, small consumers do not have access to electricity exchange and arranging the access could be expensive.

Not all consumers on the new energy market with islanded and connected microgrids can also provide DER in a cost-efficient way. For example, their load flexibility may be too small or cause too much inconvenience. Alternatively the consumer may offer flexibility at a time of day or year when it is of low value. The aggregator must evaluate the above-mentioned parameters based on information of what kind of appliances the consumer has, and what is their usage pattern.

ICT tools specifically for this purpose have been developed e.g. in the EU-DEEP project and in other similar European projects. Their usage constraints are sometimes born from the physical characteristics of the appliance, their costs and sometimes from the consumer’s desire of convenience. The aggregator will develop an understanding of the common usage constraints and time patterns of flexibility of different types of appliances over time and may agree about them individually with each consumer.

Signals in information area, like “above the market and energy grid” must be received, appliances controlled, and measurements sent in an automated manner using the new technology and ICT tools. The aggregator can take care of installing the proper control and communication equipment for whole system integration.

Smart meters along with their bidirectional communication and load control features can be exploited in mentioned functions of integration. However, these features have not been standardized. Also the measurement resolution may not be high enough and time delay of load control calls may not be low enough for the aggregator’s purposes. The aggregator collects together different realized and forecasted requests for distributed energy services, and evaluates his contractual position, taken into account forecast of consumption based on existing retail contracts and forecast of variable-output generation. He combines the different requests and identifies their whole synergies. He then calculates how to best respond to these requests by load control. The aggregator can take advantage of economies of scale in controlling a large group of consumers and acquire sophisticated optimization software to support the load control decisions.

The aggregator also makes sure that the load control decisions do not cause problems for the electrical network. One possibility is that he does this validation by consulting system operators (DSO’s and TSO). The aggregator sends his planned schedules for DER control to concerned DSO’s with information about the involved network nodes. The DSO’s then evaluate if power quality constraints will be violated by the load control actions, and send the validation result back to the aggregator.

Finally the aggregator must provide financial incentives to the consumers to participate in demand response provision. These could take many forms and there are many ways to set up the business. The consumers could be rewarded by being offered an availability payment, call payment (payment for flexibility energy provided), or percentage of the aggregator’s profits. The aggregator monitors the consumer’s performance and rewards him accordingly.

The idea of VPP is also useful for realization of Demand Response (DR) aggregator functionalities [14] (see also [15]).

The VPP can be defined as “an information and communication system with centralized control over an aggregation of distributed generation, controllable loads and storage devices”. Its main function is to control the supply and manage the electrical energy flow not only within the cluster (chosen local DER) , but also in exchange with the main grid. “It represents a single entity to the system operator and electricity markets and enables visibility and control over a cluster of distributed generation”. A VPP at a high-development stage can also offer ancillary system and power quality services. The VPP is thus a controlled operation of aggregated DG units. In such a VPP an active control is obtained through an ICT infrastructure which consists of intelligent devices and smart meters, wireless and cable connections, central control computer management system (CCCMS) and software applications see [13].

In present situation most DGs, controllable loads and storage devices are invisible to network and system operators. Their aggregation into a VPP will enable their visibility to the VPP operator (VPPO) in first place and finally to the network and system operator. At distribution level, the VPPO can be an independent system operator (ISO) or the distribution system operator (DSO). When more VPPs are developed in a service area of the transmission system operator (TSO), once again they can be aggregated by this TSO into a large scale virtual power plant (LSVPP) with central control computer system for coordination and management.

The control system of VPP of course involves huge data transfers between smart meters, agents and central ICT system in order to manage the available DG and deliver the contracted energy and services.

Recent developments like the broadband cable (glass fibre) or wireless (WMAX) communications can provide connections with enough speed and capacity to transfer the required data. With such an ICT system the VPP can be presented to the system operator as a single technical entity which is able to offer ancillary system services for all other market participants. In Poland, nowadays the Independent Operator of Measurement (IOM) has been established – it involves with high quality of data acquisition and data metering supported services for all market participants

The VPP represents all contracted DG units in the wholesale electricity markets as a single commercial entity. In order to participate in these markets, the VPPO needs to develop or make use of software applications that are able to forecast the power generation of the VPP.

In general, the VPP facilitates the visibility of the aggregated DG units and their impact on the distribution network to the VPPO as well as the DSO. In addition, the ICT infrastructure of VPP, which provides active control, can be employed to introduce active control to the passive distribution network.

In conclusion: mentioned new models of business – such can be the aggregator role on the energy market and special ICT for realization and work supporting is the main goal of planned shape construction of future, open and full competition energy market together with Smartgrid.

5.Energy Management System and ICT after Intrusion RES

For ISO or TSO and Reliability Coordinators, the following areas of focus are the most representative of new functional needs related to the integration of RES [8]:

• Provide the operator a central repository with advanced data processing and alarming for renewable prediction conditions and forecast management (RES Forecast Plan).

• Provide an estimate of non-telemetered production (frequent in Distributed Generation) using flexible and field-proven up-scaling algorithms (RES Estimation).

• Enhanced Generation Control & Dispatch to automatically counteract renewable production power balance disturbances, optimize reserve calculations and provide curtailment facilities (RES Generation Control to Dispatch).

• Enhanced Security and Simulation: fed by RES production forecast inputs and taking into account forecast accuracies to support dispatcher training and asses multiple renewable production penetration scenarios (RES Network Security and Simulator).

• Advanced User Interface: data at user’s finger tips to help the operator efficiently assessing current and future renewable production and impacts (UI).

• Extended Historian: to support reporting and data archiving (Historian). These functional requirements match SCADA/EMS functions.

Several factors affect the viability of Active Management (AM) schemes in distribution systems with distributed generation. One of the critical factors is the control system reliability. While a lot of work has been done on the technical and economic aspects of active distribution network management, almost no attention has been paid to the impact of the reliability of control and communication systems on the expected benefits of active management strategies. Investigation the impact of control system reliability on the benefits of AM are presented in [14] (see also [16]).

Utilization of RES has been receiving considerable attention in recent years what implies increasing information system requirements. Specially the reliability information system is more important for implementing the smart grid. In [18] a Web Based Online Real-time Reliability Integrated Information System WORRIS Version 1.0 has been presented. This system yields the chance for customer to choose the electrical energy resource under environment of variety kind of resources in future.

Data delivery in the power grid today is, in the most part, hard-coded, tedious to implement and change, and does not provide any real end-to-end guarantees. Application have started to emerge that require real-time delivery in order to provide a wide-area assessment of the health of the power grid. In [18] two novel communication infrastructures that facilitate the delivery of power data to intended recipients has been presented.

6.Conclusion

Complexity of the SG development implies necessity to investigate new solutions enabling to limit some of the barriers.

For example, to support a high penetration of intermittent solar and wind power generation, many regions are planning to add new high capacity transmission lines strengthing grid synchronization but also increasing the grid’s short circuit capacity, and furthermore will be very costly. With a highly interconnected grid and variable RES, a small grid failure can easily star cascading outages resulting in large scale blackout.

In [19] has been presented “Digital Grid” where large synchronous grids are divided into smaller segmented grid which are connected asynchronously via multi-leg IP addressed ACs/DC/ACs converters called Digital Grid Routers. These routers communicate with each other and send power among the segmented grid through existing transmission lines which have been re-purposed to digital transmission lines. The Digital Grid can accept high penetration of renewable power, prevent cascading outages, accommodate identifiable tagged electricity flows, record these transaction and trade electricity as a commodity (see also [15]).

Interesting proposal of measures to integrate a city district with a high share of building integrated photovoltaic system into the electric grid has been presented in [20]. And the next issue: new business models of energy market activities with RES and islanded Microgrids – how to integrate and increase new functionalities of Smartgrids have been considered.

The above considerations enable a general conclusion that the SG development needs a very wide researches necessary for decreasing costs and efforts necessary for reaching success [21].

REFERENCES

Abbreviations: 2046363- number of paper presented the time of IEEE PES Conference on Innovative Smart Grid Technologies Europe, October 11-13, 2010, Gothenburg, Sweden, PWRS – IEEE Trans. on Power Systems

[1] Matusiak B.E., Zieliński J.S.: Renewable energy Resources – Partners in Virtual Energy Market. Rynek Energii, no. 1, 2011, 133-137.
[2] Chaitanya A., B., DeMarco Ch. L.: Observer-Based Distributed Control Design to Coordinate Wind Generation and Energy Storage. 2045144
[3] Hamidi V., Smith K.S.,Wilson R.C.: Smart Grid Technology Review within the Transmission and Distribution Sector. 2048005
[4] Kulmala A., Mutanen A., Koto A., Repo S. Järventausta P.: RTDS Verification of a Coordinated Voltage Control Implementation for Distribution Networks with Distributed Generation. 2018783
[5] da Silva R., Teodorescu R., Rodriguez P.: Power Delivery in Multiterminal VSC-HVDC Transmission System for Offshore Wind Power Applications. 2047365
[6] Zhang L., Harmefors L., Nee H.P.:Interconnection of To Very Weak AC Systems by VSC-HVDC Links Using Power-Synchronization Control. PWRS, vol. 26, No.1, 2011, 344-355.
[7] Kulmala A., Mutanen A., Koto A., Repo S. Järventausta P.: RTDS Verification of a Coordinated Voltage Control Implementation for Distribution Networks with Distributed Generation. 2018783
[8] Williams B., Gahagan M.: Using Microgrids to Integrate Distributed Renewables Into the Grid. 2048121
[9] Jabłońska M. R., Zieliński J.S.: Electric Vehicles’ Influence on Smart Grids, Aktualne Problemy w Elektroenergetyce, APE ’11, Jurata 8-10 czerwca 2011,t.II,137-142.
[10] Ma K., Mutale J.: Incorporating Control System Reliability in Active Management Distribution Systems with Dispersed Generation. 1986771.
[11] Laaksonen H., Kauhaniemi K.: Synchronized Re-Connection of Island Operated LV Microgrid Back to utility Grid. 2042721
[12] Matusiak B.E. , Pamuła A., Zieliński J.S.: Technologiczne i inne bariery dla wdrażania OZE i tworzenia nowych modeli biznesowych na krajowym rynku energii. Rynek Energii, no. 4, 2010, 31-35.
[13] Matusiak B, Pamula A; Barriers to DER Aggregation Business Related to Different ICT Tools –Two European Countries Review; ISIM Warszawa 2010; Information Systems in Management, Part VIII: Information and Communication, Technologies for e-Business; WULS Press Warsaw 2011; ISBN 978-83-7583-263-1 p. 70-80.
[14] Kucęba R, Struktura inteligentnego wspomagania zarządzania wirtualną elektrownią; Rynek energii nr1 luty 2011, 80-85 ISSN 1425-5960.
[15] Malko J; Sieci inteligentne jako czynnik kształtowania sektora energii elektrycznej; Rynek energii nr 2 kwiecień 2010 str 80-87 ISSN 1425-5960.
[16] El Bakari K., Kling W. L.; Virtual Power Plants: an Answer to Distributed Generation. 2047460
[16-6] Goutard E.: Renewable Energy Resources in Energy Management. 2046839.
[17] Choi J., Park J., Cho K., Song T., Cha J.: Web Based Online Realtime Information System for Reliability of Composite Power system including Wind Turbine Generators. 2047573.
[18] [Germanus D. Dionysiou I., Gjermundrod H., Suri N., Bakken D., Hauser C.: Leveraging the Next – Generation Power Grid: Data Sharing and Associated Partnerships. 2046994.
[19] Rikiya A., Hisso T.,., McQuilkin D.: Digital Grid: Communicative Electrical Grids of the Future. 2046363.
[20] Stifter M., Kathan J.: Sun^PowerCity – Innovative Measures to increase the Demand Coverage with Photovoltaics. 2048196.
[21] Matusiak B.E., Pamuła A., Zieliński J.S.: New Idea in Power Networks Development. Selected Problems. Przegląd Elektrotechniczny (Electrical Review), R. 87, 2/2011, 148-150.
[22] Darvishi A., Alimardani A., Hosseinian S.R.: Optimal Integration of Energy Storage System in Smart Distribution Grid. 2048297
[23] El-Khattam W., Yazdani A., Sidhu T.S., Seethapathy R.: Investigation of the Local Passive Anti-Islanding Scheme in a Distribution System Embedding a PMSG-Based Wind Farm. PWRD, vol. 26, No. 1, Jan. 2011, 42-52.
[24] [Germanus D. Dionysiou I., Gjermundrod H., Suri N., Bakken D., Hauser C.: Leveraging the Next – Generation Power Grid: Data Sharing and Associated Partnerships. 2046994
[26] http://seesgen-ict.rse-web.it/ European Project Proceedings
[27] Ochoa L.F., Wilson D.H.: Angle Constraint Active Management of Distribution Networks with Wind Power. 2047855

dr Bożena E. Matusiak, Uniwersytet Łódzki, Wydział Zarządzania, Katedra Informatyki. mail: bmatusiak@wzmail.uni.lodz.pl Prof. dr hab. inż. Jerzy S. Zieliński, kierownik Katedry Informatyki na Wydziale Zarządzania Uniwersytetu Łódzkiego, uczestnik projektów europejskich: EU DEEP, SYNERGY+, MORE MICROGRIDS, SEESGEN-ICT. mail: jzielinski@wzmail.uni.lodz.pl

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 9a/2011

Voltage Harmonics Transfer through Medium Voltage Instrument Transformers

Published by Jarosław ŁUSZCZ, Politechnika Gdańska, Wydział Elektrotechniki I Automatyki

Abstract. Voltage transformers are widely used in power quality monitoring systems in medium and high voltage grids. This paper presents accuracy problems related to voltage harmonics transfer through instrument transformers. A simplified lumped-parameters wideband circuit model of the voltage transformer is proposed and verified by simulation and experimental investigations. A number of voltage transformers have been tested in the frequency range up to 30 MHz. The obtained results prove that broadband voltage transfer characteristic of the voltage transformer usually exhibits various irregularities, especially in high frequency range, which are related to windings’ parasitic capacitances and cannot be neglected in accuracy analysis.

Streszczenie. Przekładniki napięciowe są powszechnie wykorzystywane do pomiarów parametrów jakości energii elektrycznej w sieciach średnich i wysokich napięć. W artykule przedstawiono problemy związane z dokładnością przenoszenia zniekształconych przebiegów napięć poprzez przekładniki napięciowe. Zaproponowano oraz zweryfikowano symulacyjnie i eksperymentalnie uproszczony szerokopasmowy model obwodowy przekładnika napięciowego. Weryfikację eksperymentalną przeprowadzono dla kilku typowych przekładników napięciowych stosowanych w sieciach średnich napięć w zakresie częstotliwości do 30 MHz. Przeprowadzone badania potwierdziły występowanie istotnych nieregularności charakterystyki częstotliwościowej przenoszenia związanych z wpływem pojemności pasożytniczych uzwojeń, które nie mogą być pomijane przy określaniu dokładności pomiarów. (Transformacja zniekształceń harmonicznych napięcia w przekładnikach napięciowych średnich napięć)

Keywords: voltage transformers, power quality, voltage harmonic distortions, overvoltages.
Słowa kluczowe: przekładniki napięciowe, jakość energii, zniekształcenia harmoniczne napięcia, przepięcia.

Introduction

Voltage harmonic distortion level is one of the significant parameters of power quality in power system. Voltage distortion assessments, especially in medium voltage (MV) and high voltage (HV) grids, are usually based on measurements in which voltage transformers are commonly used. The transfer ratio of a voltage transformer fed by distorted primary voltage with harmonic components can be significantly different for frequencies higher then fundamental. During the last decades major problems related to voltage distortions have been usually encountered in frequency range up to 40th harmonic, mostly in LV grids. Nowadays, due to the evident increase of the overall power of nonlinear loads connected to grid and higher modulation frequencies widely used, distorted voltage propagates deeply into MV grids and goes evidently above frequency band of 2 kHz.

Classic circuit model of voltage transformer

VTs are mostly used in MV and HV systems for separation of the measuring and protecting circuits from high voltage hazard. Rated primary voltages of VTs, typically used in power system, have to correspond to rated voltages of MV and HV transmission. Secondary rated voltage levels usually used in typical measuring and protection systems are: 100 V, 100/3 V, 100/√3 V which results with transformation ratios of the order from a few tenth up to few hundreds for MV VT and more than a thousand for HV VT. Such a high transformation ratio and low rated power of VT has significant influence on its specific parameters, especially related to performance in wide frequency range.

The classic equivalent circuit model of two windings transformer usually used for modelling VT for power frequency is presented in Fig.1. This model consists of leakage inductances of primary winding L_p and secondary winding Ls and magnetizing inductance L_m. Corresponding resistances represent VT losses in magnetic core R_m and windings R_p, R_s. Based on these parameters frequency dependant transfer characteristic for frequencies higher than the nominal (50 or 60 Hz) can be estimated. Theoretical wideband transfer characteristic of VT modelled by using classic circuit model is presented in Fig.1 where low corner frequency of pass band f_low and high corner frequency of pass band f_high can be defined based on 3 dB transfer ratio decrease margin assumption. Low and high frequency response of VT can be determined analytically based on VT classic circuit model parameters according to formula (1) and (2).

Fig. 1. Transfer ratio frequency characteristic of VT determined using classical circuit model

Concluding, low frequency response of VT is mostly dependant on ratio of leakage to magnetizing impedance which limits transfer characteristic in low frequency range, while high frequency response depends mainly on sum of leakage and load impedances.

Modelling of VT in high frequency range

Modelling of VT in a HF range using classic circuit model is usually not adequate enough because of existence of parasitic capacitances of windings and frequency dependant grid impedance and VT load impedance. Parasitic capacitances of VT windings are usually unwelcome and unluckily unavoidable; there are only various techniques used to reduce its values and distribution. Consequences of parasitic capacitances are especially significant for multilayer windings with high number of turns which is characteristic for high voltage and low power transformers like VT.

Identification of distributed partial parasitic capacitances for particular VT requires detailed specification of winding arrangement is extremely elaborative and usually does not provide adequate enough results. Difficulties of parasitic capacitances identification can be reduced by defining lumped equivalent capacitances which represent groups of partial capacitances related to entire winding or part of windings; for example single layer of winding. Lumped representation of parasitic capacitances allows reducing winding model complexity and consequently simplifies noticeably its parameters identification process. Winding model simplification level, which is possible to apply, should be closely correlated with the expected adequacy in a given frequency range and depends evidently on particular winding arrangement complexity. Commonly, three methods of winding parasitic capacitances circuit representations are used to model transformer windings:

• winding terminals related – where all defined lumped equivalent capacitances are connected to windings’ terminals only,
• partially distributed – lumped parasitic capacitances are specified for most representative internal parts of winding, like for example windings layers, winding shields,
• fully distributed – windings are modelled as a series and parallel combination of inductances and capacitances which form ladder circuit with irregular parameter distribution.

Generally, more detailed parasitic capacitance representation allows obtaining higher accuracy in wider frequency range. Nevertheless, the model complexity should be kept within reasonable limits to allow achieving higher usefulness because of parameters identification process simplification.

Fig. 2. Broadband circuit model of the VT with lumped parasitic capacitances referenced to windings terminals: C_p – primary winding, C_s – secondary winding, C_ps – interwinding

Identification of VT parameters in wide frequency range

Voltage transformation ratio of VT in wide frequency rage is closely related to impedance – frequency characteristics of primary and secondary windings. Therefore, measurement results of VT magnetizing and leakage impedances within the investigated frequency range are the fundamental data resources for analysis its broadband behaviour and allow estimating circuit model parameters. Measurement of VT impedances can be done similarly to a typical no load and short circuit tests recommended for power frequency with use of sweep frequency excitation.

Distributed parasitic capacitances of VT windings are modelled by the lumped capacitances related to windings terminals only (Fig. 3). This assumption reduces noticeable model complexity and allows determining parasitic capacitances based on the measured windings impedances. In the analysed case primary and secondary windings of the investigated VT are one side grounded which limits furthermore the number of lumped capacitances necessary to be determined.

Detailed analysis of VT magnetizing and leakage impedance-frequency characteristics and identification of specific resonance frequencies allows estimating parameters of the VT circuit model presented in Fig. 3. The method of determination parasitic lumped capacitances is based on identification of resonance frequencies which are usually possible to determine by using the measured impedance characteristics [5].

Simulation examination of VT broadband circuit model

The investigated circuit model of a VT can be examined by simulation in any PSpice compatible environment in the conducted disturbance propagation frequency range up to 30 MHz. The essential verification of VT model adequacy has been done by determining magnetizing and leakage impedance characteristics which allows verifying model representation adequacy of magnetic coupling between windings.

The developed VT circuit model can be used for simulation analysis of the influence of the VT parameters and its load on the voltage transfer ratio frequency characteristic. The exemplary simulation results of VT voltage transfer ratio characteristics calculated for different resistive loads are presented in Fig. 3. It can be noticed that the VT voltage transfer characteristic change essentially for frequencies higher than the main resonance frequency observed on the leakage impedance, which is about 100 kHz for the evaluated case. Above this frequency VT voltage transfer ratio depends mainly on winding parasitic capacitances and magnetic coupling between windings becomes less meaningful.

Simulation results demonstrate that in frequency range close to leakage impedance resonance VT load has the major influence on the VT transfer characteristic. Increase of resistive VT load reduces significantly VT voltage transfer ratio around this frequency. Obtained simulation results confirm that VT load has significant influence on VT performance in HF range.

Fig. 3. Simulation results of the influence of resistive load of VT on voltage transfer ratio frequency characteristic

Experimental tests of VT transfer characteristic

Experimental investigations have been done for voltage transformers typically used in MV power system with primary and secondary windings grounded. Presented exemplary measurement results have been obtained for VT of 50 VA rated power and 20 kV/0.1 kV nominal transformation ratio. Parameters of the proposed VT circuit model for simulation have been identified by analysis of secondary windings impedance-frequency characteristics measured for no load and short circuit configuration. Measurements have been done in frequency range from 10 Hz up to 30 MHz, which is a range typically used for the analysis of conducted disturbances in power system. Particular attention has been paid to the frequency range below 10 kHz which is obligatory for power quality analysis, especially for analysis of power system voltage harmonics related phenomena.

Accurateness of voltage transfer characteristics (magnitude and phase) of VT is a fundamental aspect for identification and measurement of power quality related events in power system. For the investigated VT the voltage transfer ratio and voltage phase shift characteristics have been measured to reveal measurement accuracy problems of power quality assessment in MV systems. Magnitudes versus phase transfer characteristic of VT measured for frequency range typically used in power quality measurement systems (up to 9 kHz) are presented in Fig. 4.

Fig. 4. Normalized VT voltage ratio vs. phase shift angle for frequency band up to 9 kHz

Experimental investigations prove that magnitude and phase errors increase noticeably with frequency. In frequency range up to 2 kHz, the highest magnitude error of about 11 % and phase shift error almost 8^o, have been obtained for frequency of 2 kHz. These results confirm that voltage harmonics measurement in MV grids by using VT can be not accurate enough in applications with noticeable harmonic content above approximately 1 kHz.

Magnitudes and phase inaccuracy of VT obtained in frequency range from 2 kHz up to 9 kHz are evidently larger and its frequency dependence is more complex, therefore more difficult to model using simplified circuit models. Magnitude errors in this frequency range reach almost 180% and phase shift error almost 80^o, which cannot be accepted in power quality measurement applications.

Conclusions

The main problems with accurate modelling using circuit models are related to windings’ parasitic capacitances and especially identification of its unequal distribution along windings. In order to model the influence of parasitic capacitive couplings existing in a typical VT several simplifications should be considered. The method of VT parasitic capacitances analysis based on the lumped representation is often used and particularly rational, nevertheless, it limits the frequency range within which acceptable accuracy can be obtained.

Parameters of simplified circuit model can be determined based on wideband measurement of leakage and magnetizing impedances. Unfortunately such model can be successfully used only in the limited frequency range. For typical VT used in MV grids the uniform part of transfer characteristic can be obtained usually only up to a few kHz. Above this frequency VT usually exhibits a number of resonances which change evidently its transfer characteristic and cannot be expressed adequately by simplified circuit models. Wideband performance of VT in a particular application is also noticeably related to its load level and character (inductive or capacitive).

The use of VT in power quality monitoring systems in MV grids influences essentially measurement accuracy finally obtained. In power quality measurement applications where dominating harmonics emission is expected only in frequency range below 2 kHz VTs can provide sufficient accuracy in many applications, nevertheless its voltage transfer characteristic should be carefully verified with taking into account particular operating conditions. In the contemporary power grids, harmonics emission spectrum injected to the power system can be much wider than up to 2 kHz, especially by contemporary high power electronic applications. In the frequency range from 2 kHz up to 9 kHz, which is already well specified by harmonic emission standards, use of typical VT is not reliable enough. Measurement errors in frequency range up to 9 kHz are usually not acceptable, because of resonance effects which commonly appear and are difficult to predict.

REFERENCES

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Authors: dr inż. Jarosław Łuszcz, Gdansk University of Technology, Faculty of Electrical and Control Engineering ul. Sobieskiego 7, 80-216 Gdańsk, E-mail: jlusz@ely.pg.gda.pl

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 8/2012