Published by Anushree Ramanath, EE Power – Technical Articles: Microgrid Operations and Applications, August 01, 2021.
In this article, we’ll learn about microgrids, their operations, and applications in electrical utilities and various organizations.
Today’s world relies on an uninterrupted electricity supply. A microgrid is a local energy grid with the capability of controlling its components [1]. This translates into the fact that a microgrid can disconnect itself from the traditional grid under disturbances such as faults and operate independently. This is a boon in scenarios including power outages due to severe storms or other environmental conditions, aging infrastructure, or pressure due to increased costs which might otherwise lead to severe uncertainty in power generation and distribution.
A traditional grid helps connect several residential buildings, businesses, and other critical infrastructure with the sources of power. This enables the users to utilize multiple appliances and electronic systems. However, it is evident that all of this is completely interdependent and failure in one of the interconnected system components affects it in its entirety. In such a scenario, a microgrid comes in handy as it can operate as a standalone system, although it is typically connected to the main grid. This is extremely helpful in times of crisis, like power outages or storms. Depending on the resources used to form it, microgrids can run indefinitely.
Microgrids are typically close to the place where there is a need for sustained power. This builds on the proximity and resiliency of its use [2]. The proximity of the microgrid facilitates the reduction of losses incurred during the transmission of energy and the cost of installing power networks. Since redundant distributed energy resources (DERs) are part of the microgrid, improved energy resiliency is delivered. Microgrids can be developed in several topologies and sizes to power a single facility or a vast area. Remote microgrids can provide power to critical services and communities that are housed away from the utility networks.
Understanding the Operation of a Microgrid
A microgrid connects to the main grid at a point of common coupling (PoCC) that maintains the voltage at the same level as the utility grid unless there is some issue with the main grid or any other reason to disconnect from it. The design can also be such that a switch can separate the microgrid from the main grid automatically or manually so that it can function independently as an island. This is illustrated in Figure 1. The core components of a microgrid include a power source, power management system, intelligent controls and energy storage system [3].
Figure 1: Operation of a microgrid [4]
Microgrid control is all about sharing power among multiple energy sources while maintaining stability. The control hierarchy includes primary or inner control embedded in the microgrid along with secondary and tertiary controls designed for interfacing with the main grid and communication purposes, as illustrated in Figure 2. Primary control is local to the microgrid. Secondary and tertiary control aspects form the central control system, requiring communication and limiting flexibility while adding complexity and additional costs. It is a plug-and-play type that enables autonomous primary control with very minimal or no secondary and tertiary control.
Figure 2. Control hierarchies.
he simulation of microgrids can be accomplished using the hardware-in-the-loop technique. The individual units employed for power generation can be modeled adequately. The power or control interface can be simulated using a simulator, while the rest of the system can be simulated in real-time [5]. Physical systems can be simulated with localized controls and additional system-level secondary and tertiary controls to emulate the complete microgrid behavior. This effort helps understand the behavior of the overall system along with the system architecture.
Microgrid Applications
Several organizations are shifting towards hosting microgrids to lower the possible risks while improving operational performance [6]. This is possible as microgrids transfer the control to users and help them achieve energy independence. Traditionally, microgrids have been employed in remote locations that cannot be connected to the central power grid and serve critical infrastructure. However, due to the recent advancements in technology and increased usage of renewable energy sources, microgrids have become more accessible and economically feasible.
Microgrids can be employed in organizations that intend to lower their energy cost, require huge amounts of reliable energy, and for those that pursue sustainability [6]. These are accomplished because when power is generated in-house, the lowest cost fuel sources can be employed for power generation, and costs due to power outages can be effectively eliminated. Connecting to the utility grid versus relying on a microgrid provides economic security while giving a handle to curb the costs incurred due to power quality issues. Also, since microgrids strategically integrate renewable and non-renewable energy sources, variations due to weather conditions and time-of-the-day based availability concerns can be handled effectively.
Author: Anushree Ramanath is a seasoned engineering professional skilled in system-level design, building hardware, coding, firmware, industry-oriented research, software architecture, modeling, and simulations. She received a Ph.D. in Electrical and Computer Engineering from the University of Minnesota Twin Cities with a focus on power and controls. She loves experiencing different cultures through languages, food, or travel while indulging in a variety of fine arts.
Published by Andrzej LANGE1, Marian PASKO2, University of Warmia and Mazury, Department of Electrical, Power, Electronic and Control Engineering (1), Silesian University of Technology, Institute of Electrical Engineering and Computer Science (2)
Abstract. This article presents the consequences of exchanging older metal cutting machines for new ones in terms of electrical energy consumption and quality of consumed electrical energy.
Streszczenie. W artykule przedstawiono konsekwencje w zakresie energochłonności oraz jakości pobieranej energii elektrycznej wymiany starych maszyn do cięcia metalu na nowe. Zmniejszanie zużycia energi w procesie produkcji i parametry opisujące jakość energii
Słowa kluczowe: parametry jakości energii elektrycznej, wyższe harmoniczne napięć i prądów, moc bierna, filtry pasywne. Keywords: electrical energy quality, higher harmonics of voltages and currents, reactive power, passive filters.
Introduction
In industrial plants, devices of various energy consumption levels are used in technological processes. Older devices are often powered with internal elements of machines directly from the network, thus consuming much more energy than newer machines powering internal systems through electronic devices. Not only does it pertain to welding machines [1,2] and lighting [3, 4], but also to machines producing polymer layers. Replacing older devices with new, modern machines significantly reduces the consumption of electrical power but worsens the parameters characterising the quality of electrical power [5,6]. This results from the fact that the power electronics systems of these devices generate higher harmonic powers to the electrical power network and consume capacity reactive power or, alternatively, inductive and capacitive reactive power for the basic harmonics.
Characteristics of the measurement system
To illustrate the influence of modern production devices on the quality of electrical power and energy consumption levels in the production process, machines for cutting reinforcement bars were used for comparison. For analysis of the operation of these devices, measurements for two steel cutting machines were performed:
– Older type steel cutting machines of the METAX GE2 type, year of manufacture 1992; – Modern-type steel cutting machines of the METAX GXN3 type with an energy-saving drive, year of manufacture 2018;
To analyse the energy consumption level and the quality of energy consumed by the machines, measurements with a HIOKI 3196 type power supply quality analyser were performed. During device operation, current, voltages and power were measured to determine energy consumption levels along with higher harmonic currents and voltages in the power supply point. Both machines had the same technological parameters.
Measurements of electrical parameters of the machines
The current function for the current running through the older type METAX GE2 machine is presented in Fig. 1. It deviates little from the classical sinusoid shape. Thus, the percentage value of specific harmonics is very slow, and the total harmonic distortion THDi does not exceed 3% with the coefficient of total interharmonic distortions TiHDi being ca. 1.2% (Fig. 2.) The current consumed by the modern METAX GXN3 machine is highly distorted (Fig. 3), which translates into a significant content of specific higher harmonics and total harmonic distortion of the higher harmonics THDi (Figs. 4 and 5). The new machine consumes, on average, higher harmonics of 25% for the 5th harmonic, 20% for the 7th and 11th harmonics and 15% for the 13th harmonic. The average total harmonic distortion coefficient for higher harmonics THDi is ca. 50% (Fig. 4). The maximum percentage values of higher harmonics in machine supply current are presented in Fig. 5. These values are significantly higher than the average values as the 3rd harmonic reaches 80% and the total harmonic distortion exceeded 120%.
Fig.1. The current function for LV current consumed by the older METAX GE2 machine.
Fig.2. Percentage of higher harmonics and THD for the LV current consumed from the network in respective phases by the older METAX GE2 machine.
Fig.3. The current function for LV current consumed by the newer METAX GXN3 machine with an energy-saving drive.
Fig.4. Percentage of mean values of higher harmonics and THDi for the LV current consumed from the network in respective phases by the newer METAX GXN3 machine.
Fig.5. Percentage of maximum values of higher harmonics and THDi for the LV current consumed from the network in respective phases by a newer METAX GXN3 machine.
During operation, the newer machine consumes on average ca. four times lower current effective value (Fig. 6) and consumes on average ca. four times lower active power (Fig. 7) than the olde rtype machine. This is because the newer machine switches off during idle run and switches on for only a few seconds (Figs. 8 and 9). The engine of the older machine was not switched off during pauses in cutting metal rods, but it kept operating on idle run, thus the machine kept consuming active power and current with high values of the inductive components (Fig.10). The value of reactive power of the basic harmonics is constant for the older machine and is of an inductive character. In the new machine, the reactive power of the harmonic consumed from the network changes dynamically and takes both positive and negative values. During engine operation, when active power reaches high values, the reactive inductive power of the basic harmonic is consumed from the network. When the engine is switched off (no active power is consumed) and only the power electronics converter is operated, the capacitive reactive power of the basic harmonic is then consumed from the network. This results from loading capacitors located between the rectifier and the inverter of the converter circuit. During pauses in the operation of the drive, only a small effective current value is consumed from the network with a small content of specific higher harmonics (Figs. 11 and 12).
Fig.6. Average effective values of currents consumed by older and newer machines
Fig.7. Average value of active power of the basic harmonics consumed by older and newer machines
Fig.8. Variability in effective values of the current consumed by older and newer machines in specific phases.
Fig.9. Variability in active power consumed by both machines in specific phases.
Fig.10. Variability in the reactive power of the basic harmonics consumed by both machines in specific phases.
Fig.11. Variability in content of higher harmonics for the LV current consumed from the network in respective phases by the newer METAX GXN3 machine.
Fig.12. Variability in active power consumed by both machines in specific phases.
Their value does not exceed 20% for the specific harmonics and 40% for THDi. The values of specific harmonics increase significantly when the engine is activated, reaching 70% for the 5th harmonic and 50% for the 7th harmonic. The total value of the higher harmonic in the current powering the machine then reaches 120%. In similar operating conditions with the older machine, the content of higher harmonics in the feeding current did not exceed 8% (Fig. 12).
The use of the newer machine for steel cutting entails higher dynamics of changes in power coefficient (Fig. 13).
Fig.13. Variability of PF power coefficient consumed by both machines in specific phases.
With the older machine, the PF power coefficient kept varying between 0.4 and 0.9 with inductive character. With the new machine, the PF coefficient varies between -1 and 1. With such great variation, not only in the value of the PF coefficient but also in the changes from inductive to capacitive character and the other way round, both being very dynamic, significant problems emerge in compensation of the reactive power of the basic harmonics. For such devices it is not sufficient to use automatically adjusted batteries of condensers controlled by contactors. The contactor reaction time would be too slow for such quick changes in the reactive power of the basic harmonic. The change in the character of the reactive power of the basic harmonic represents yet another problem. For such dynamic changes, either an active filter needs to be used that would not only filter higher harmonics of current, but it would also compensate the reactive power of the basic harmonic [7, 8, 9] or a passive filter could be used that would activate specific elements through thyristor power supply systems of the compensation systems. In the case of a passive filter for compensation, not only should condensers be used for compensation of reactive inductive power, but also chokes for compensation of capacitive reactive power. Two solutions may be used. The first system (Fig. 14) includes a choke activated by a contactor with a parallel battery of condensers activating specific elements through a thyristor switch. The choke should have a value that guarantees the reactive power compensation capacity of the basic harmonic during an idle run of the inverter (the engine is not in operation), as the capacitive reactive power of the basic harmonic is consumed from the network. Batteries of condensers should have such a power that during engine operation they would be capable of compensating not only the reactive power of the basic harmonic sourced by the throttled but also through the engine power system. When the inverter is in idle condition, only the throttle is powered and the condenser batteries are switched off. Once the engine is activated and inductive reactive power of the basic harmonic is sourced from the network and the thyristor switches activate specific elements in follow-up mode to keep the desired power coefficient.
Fig.14. Passive filter compensation system with LC filtration units activated by thyristor switches.
The second system (SVC) consists of a throttle activated by the thyristor switch and condenser activated by the contactor of the switch (Fig. 15). In this solution, it is the condenser that is permanently connected to the network and the throttle controlling the thyristor activation angle adjusts the reactive power of the basic harmonic supplied to the network. The condenser power should be adapted to the maximum values of the inductive reactive power of the basic harmonic consumed by the engine drive system during its operation. The throttle should be selected in such a way so as to compensate only the reactive power of the basic harmonic generated by the condenser.
Because active filters are costly, it is possible to use a hybrid system, i.e. a combination of an active filter with a passive filter [10, 11]. Such a solution decreases the power of the active filter with the power needed for compensation of the reactive power. In such a case, the active filter is used for filtering higher harmonics and the passive filter is used for compensation of reactive power of the basic harmonic. At the same time, the passive filter would also compensate higher harmonics, thus supporting the active filter. In particular, it would filter the lower harmonics, i.e. those to which the passive filter would be tuned. The active filter can also support the passive filter by compensating the reactive power of the basic harmonic.
Fig.15. Passive filter compensation system with LC filtration units activated by a contactor with a throttle activated by thyristor switches (SVC).
The use of a modern control system for the metal cutting machine also reduces the occurrence of current surges at the moment of connecting the older machine to the network. In a classical activation system, a high start-up current would be generated which would cause voltage drops in the network in the case of large engines. In the new machine, the start-up current is smooth (Fig. 16) with no overcurrents occurring. The drive machine is also switched off in a smooth manner (Fig. 17). The start-up time for such an engine is ca. three periods. It is thus impossible to use a classical (contactor) reactive power compensation system because the time needed for compensation is too short.
Fig.16. The current function for LV current at the start-up of the newer METAX GXN3 machine.
Fig.17. The current function for the LV current at the switch-off of the newer METAX GXN3 machine.
Remarks and conclusions
Replacing the older machines with modern ones with electronic power system causes:
– the consumption of distorted current (Fig. 2) which is connected with generating higher current harmonics and feeding them into the network (Fig. 4).
– the consumption from the feeding network of an average effective current of significantly lower value (Fig. 5),
– reducing the consumption of active power (Fig. 6) and reactive power of the basic harmonic (Fig. 6) and, in consequence, reduces the consumption of electrical power from 13.36 kWh to 3.02 kWh per hour of machine operation.
– a reduction in current effective value during machine idle run (Fig. 8), – the consumption of both inductive reactive power and capacitive power from the network in stand-by mode (Fig. 9),
– an increase in the speed of changes in the value of reactive power of the basic harmonic with a simultaneous change in the character of the circuit from inductive to capacitive (Fig. 9) which results in the necessity of using follow-up inductive reactive power compensation during machine compensation and compensation of capacitive reactive power in standby mode,
– an increase in higher harmonics in the supply voltage as a result of consuming higher harmonic currents and the possibility of occurrence of current resonances.
LITERATURE
[1] Lange A., Pasko M.: The influence of modern welding devices on the quality of electrical power and power consumption levels (in Polish). Przegląd Elektrotechniczny, R. 93 (2017) no 3, 152-155 [2] 2.Orłowicz A. W., Trytek A.: Study of arc and melting efficiency in GTAW process. Archives of Foundry, 2003, Volume 3, Book 8, pp.131-140 [3] Lange A., Pasko M.: The effects of LED light sources on the parameters defining the quality of electricity. ITM Web of Conferences, Volume 19, 01006 (2018) [4] Wandachowicz K., Taisner M.: Diode lamps and modules powered with alternating current (in Polish). Poznan University of Technology Academic Journals. No.92. 2017. pp.117-122 [5] EN 50160: 1998. Parameters of supply voltage in public power distribution networks [6] Power law of 25 September 2012. Journal of Laws, item 1059, vol. 1 [7] Pasko M., Buła D, Dębowski K., Grabowski D., Maciążek M.: Selected methods for improving operating conditions of threephase systems working in the presence of current and voltage deformation Pt. 1.,Pt. 2. Arch. Electr. Eng. 2018 vol. 67 no. 3, pp. 591-602, pp.603-616 [8] Grabowski D., Maciążek M, Pasko M., Piwowar A. Timeinvariant and time-varying filters versus neural approach applied to DC component estimation in control algorithms of active power filters. Appl. Math. Comput. 2018 vol. 319, s. 203-217 [9] Buła D., Pasko M. Stability analysis of hybrid active power filter, Bull. Pol. Acad. Sci., Tech. Sci. 2014 vol. 62 no. 2, s. 279-286 [10] Akagi H.: Active Harmonic Filters. Proceedings of the IEEE, Vol.93, No. 12, 2005, pp.2128-2141 [11] Shitsukane Aggrey Shisiali ,Mathews Ondiek Amuti: Power Quality Improvement using Hybrid Filters. International Journal for Research in Electronics & Communication Engineering, November 2016
Authors: dr inż. Andrzej Lange, University of Warmia and Mazury, Department of Electrotechnology, Power Industry, Electronic and Automation, ul. Oczapowskiego 11, 10-736 Olsztyn, e-mail: andrzej.lange@uwm.edu.pl prof. dr hab. inż. Marian Pasko, Silesian University of Technology, Institute of Electrotechnology and Computer Science, ul. Akademicka 10, 44-100 Gliwice, e-mail: marian.pasko@polsl.pl;
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 96 NR 2/2020. doi:10.15199/48.2020.02.03
Published by Marek PAWŁOWSKI, Piotr BORKOWSKI, Bartosz BALSAM, Lodz University of Technology, Department of Electrical Apparatus
Abstract. This paper presents the concepts of smart electricity grids with particular emphasis on aspects of the metering of customers through the implementation of smart meters. On the basis of the literature study, an analysis of the scope of the functionality of smart meters was performed. The paper presents a model of the smart electricity meter developed by the authors, which has the possibility of working with the energy management system.
Streszczenie. W artykule przedstawiono koncepcje inteligentnych sieci elektroenergetycznych ze szczególnym uwzględnieniem aspektu opomiarowania odbiorców poprzez wdrożenie inteligentnych liczników. Na bazie studium literatury przeprowadzono analizę obszaru funkcjonalności inteligentnych liczników. W artykule zaprezentowano opracowany przez autorów model inteligentnego licznika energii elektrycznej posiadającego możliwość współpracy z systemem zarządzania energią. (Model inteligentnego licznika energii elektrycznej).
Keywords: smart meter, energy management systems, advanced metering infrastructure. Słowa kluczowe: inteligentny licznik, system zarządzania energią, zaawansowana infrastruktura pomiarowa.
Introduction
Technological development and, consequently, the economic development of states determine an increase in demand for energy. The key is not only the amount of energy delivered, but the time of its delivery and its quality. These factors make adaptation and, consequently, development of infrastructure of energy production and transmission necessary. These works are focused on research on the implementation of smart power grids. One of the basic assumptions of these grids is the “activation” of the final user who should be actively involved in the energy market. An intermediate element in this action will be a smart meter, which allows basic communication between the energy supplier and the user. The effectiveness and form of this communication will be an indirect evidence of the validity of the implementation of the solution. In the outlined vision of the development of electricity grids, research on the detailed rules for the operation of smart meters becomes legitimate.
Smart grid
The objective of smart electricity grids is to achieve the most efficient power management. This grid must adapt to changing conditions in real time. The idea of smart grids assumes the implementation of two-way communication between the consumer and the supplier of energy, and the integration of distributed energy sources [2,3].
Achieving these objectives will be possible thanks to a series of sensors, communication systems and control devices. In addition, the intelligent grid management requires ordering local control and large area procedures.
Smart grid should therefore be [2,4]:
• Self-repairing – it should be able to detect the energy disturbances and automatically repair their source. A continuous analysis and monitoring of the status of the grid, affecting their online self-evaluation, will be responsible for the opportunity for self-repairing. This will allow for a quick response, which will significantly mitigate the effects of the interference, or very rapid restoration of power.
• Interactive – it should enable active participation of consumers in meeting their demand. By installing appropriate applications, multidirectional communication between the energy distributor and individual consumers or businesses will be possible. The system designed in such a way will provide the user with a wider range of information that will enable them to manage the energy in a balanced way, both in respect to their needs and current capabilities of the power system.
• Optimized – the intelligent grid is designed to optimize the power system works according to certain criteria and make effective use of the resources. This is aimed to reduce energy losses and improve power grid load level as well as efficiency of management of supply disruptions. When failure occurs, such grid generates additional information for engineers and planners. This will allow for checking exactly what and where is needed, what is the lifespan of the device or which device is damaged. The software also allows for managing the workforce. All this ultimately aims to reduce operating costs, and that will directly contribute to reducing the cost of electricity.
• Protected – in case of natural disasters or attacks, smart grid will ensure a smooth operation. The solutions used reduce physical and information vulnerabilities throughout the system security. They also allow for quick repair of interference. It is important that such a grid should be considered as a whole rather than single units, as, at the time of the attack on the unsecured part of the whole system, the energy from large power plants may be undelivered to the secured part. This grid can be compared to a chain that is only as strong as its weakest link.
• Compatible – the intelligent grid must be compatible, ensuring consistency and compliance of both centralized and distributed generation of energy with the energy storage devices. It must receive all the generated energy and have a tool to store it. It should therefore be adapted to all eventualities of production and reception.
• Integrated – optimized processes, information, management and standardization should be subjected to integration. The grid should contribute to the development of local energy markets and the use of new products. It will link sellers with buyers; among others, it will enable integration with infrastructure of home area network as well as charging control or passing energy from the batteries of electric cars to the network.
An integral part of the smart grid is smart metering creating an integrated computer system comprising of:
• Electronic energy meter dedicated to work in smart metering systems, • Telecommunications infrastructure, • The central database, • Management System.
Smart metering enables real-time two-way communication between the supplier and the consumer of energy. Computer systems in combination with energy meters make it possible to automate both the client side and the supply side of energy. Due to the information given by a smart meter, the electricity grid user may have the ability to manage power consumption with maximum efficiency and cost effectiveness. What is more, the supplier can completely automate the process of settlement with recipients, starting with reading measurement data followed by their processing and analysis, and, finally, issuing the invoice and sending it to the user.
Such a system consists of two parts:
• AMI – Advanced Metering Infrastructure, including: meters, concentrators, modules and communication systems as well as software. • MDM – Meter Data Management, used for data processing and in the process of settlement.
IBM classifies Smart Metering systems according to the most important features [5]. The last one called fourth generation and implemented since 2010, in addition to bidirectional transmission, also works with Home Area Network (HAN).
Advanced Metering Infrastructure
Elements of Advanced Metering Infrastructure (AMI) are designed to enable two-way communication using a variety of media and technology between the central database and individual meters (consumers). This allows remote configuration, receiving data from the user or sending control messages [6].
AMI systems allow for meter reading data from different utility services: water, gas or heat. In addition, they are also able to collect data on all kinds of events that occurred in the network. Specific data should be read at the right intervals, or through direct forcing of both the consumer and the supplier [7].
The use of smart meters has consequences in the form of a dispersion of data sources across the network. Therefore, safe, fast and efficient communication infrastructure is necessary [8].
Because of the extent of the area of communication, AMI can be divided into [4]:
• Home Area Network (HAN) – the network used for control at home, • Local Area Network – the network for automatic meter reading through concentrators, • Extensive Network – the network for the exchange of data between concentrators and specific data acquisition servers.
According to the principles outlined by the President of the ERO [9-12], the smart meter should communicate with the HAN, that is communications infrastructure and equipment (receivers and sources of energy), which react on the information from the meter according to the assumptions and conditions of the end user.
The basic solution which fits into the requirements of the HAN is a display on which the user can only preview the current and archival energy consumption and can receive information from the energy supplier. Nevertheless, this solution has two major drawbacks. First, the cost of installing additional screens is largely unfounded. Taking into account the development of mobile technologies and the fact that practically in most households there is a tablet or smartphone, according to the authors, the solutions, in which the equipment will be able to act as an interface enabling communication with the user should be looked for. The second issue is the type of information and the form of its transmission. It should be noted that the awareness of users in the use of electricity is relatively low [13].
The smart meter
The smart meter is part of the AMI; it contains mainly metering system for measuring energy consumption. However, it is distinguished by the fact that it captures not only the total energy consumption, but the value of the consumed energy and power at specific intervals (usually 15 min.). This allows for getting detailed consumers’ profiles of demand for power. The meter allows real-time transmission of information from the energy supplier to the individual consumer or group of consumers. This information can be the current price for electricity. Currently, in most countries, the electricity market is a regulated market and, for individual consumers, tariffs having one or possibly two rates for electricity, including the fixed peak and off-peak periods, are available. However, this situation needs to be changed because the prices for electricity in the wholesale markets are subject to dynamic change, especially as a result of increase in the share of renewables in the power system. The smart meters will allow for dynamic and diversified over time changes in the prices of electricity for the end users dependent on the current wholesale prices of electricity in the energy markets [14].
In the literature [15-18], it was shown that the most effective and the most desirable information for users is information on the cost of energy consumption, the costs with respect to one day, month and year. Shekara S. et al. [17] point out the elements that the smart meter should include:
• current energy consumption (kWh) • current energy costs expressed in (EUR/kWh or EUR/day), • cumulative daily costs, • energy consumption in the last day, week, month or quarter.
An interesting suggestion is an individual user adjustment of the meter maximum daily energy consumption (the costs), above which an alarm would be signalized [19].
The model of the smart meter working with the energy management system at a communal consumer
As part of the work, the authors developed and made an actual model of the smart meter, which, firstly, has the full functionality of smart meters, as outlined in the position of the President of the ERO and, secondly, has the ability to communicate with the energy management system at a communal consumer [20]. In addition, the smart meter has the ability to change the operation of the software. The model was made for single-phase networks. The model consists of three main systems: acquisition, transmission, and data analysis. Fig. 1 shows a block diagram of the model of the smart meter which was designed and made. The metering system is responsible for the acquisition of metering signals and their transmission to the data transmission system. The data transmission is carried out via Wi-Fi, after which the data is transmitted to the data analysis system.
The metering system is responsible for the metering of voltage and current in the circuit under test. The diagram of the metering system is shown in Fig. 2.
An element responsible for the remote transmission of measured values is the cDAQ9191 Wi-Fi module of National Instruments company with NI9215 measurement chart. This module has an Ethernet connection, the Wi-Fi connectivity with antenna, power connector and a slot for popular measurement C Series modules. This solution provides many opportunities through the simple installation and easy replacement of the module with another one of the same series.
The application developed in the LabVIEW software is responsible for data collection and analysis (Fig. 3).
It is an application of National Instruments company fully compatible with all products offered by it. The application includes eight basic groups of blocks:
1. Reading data from cDAQ-9191 measuring unit, 2. The basic waveform analysis, 3. Filtering the results obtained, 4. Reading the phase shift and frequency, 5. The power calculation and creating a graph of its value in the time-domain, 6. Aggregation of energy consumption and its costs, 7. Support for the application, 8. Data Management (writing to a file and/or communication with the energy management system in the building).
Fig.1. A block diagram of the model of the smart meter
Fig.2. A block diagram of the model of the smart meter
TRAN – 230V / 2x15V voltage transformer, BRIDGE – bridge rectifier in the DIP housing, L7915CV – 15V voltage stabilizer, L7815CV – 15V voltage stabilizer, C1 – C4 – capacitors, Rm – measuring resistors, R1 – resistor for the input signal, LA25 – current transformer, LV25 – voltage transformer, kon – assembly connector.
Each group is responsible for a specific task. The end result of groups action is the creation of the model of the smart meter, which allows the management of acquired data.
The basic analysis is designed to visualize the input signals from a transmission system. This allows for verifying the correctness of operation of the system prior to the transfer of signals to filtration systems. The model of the smart meter allows for calculating and recording the vast majority of electrical quantities, including: the instantaneous values, RMS voltage, current, frequency, phase angle, power factor, the value of the instantaneous power, active power, reactive power and apparent power. In addition, the simplified calculations of the cost of energy intake are made. Each of these parameters may be averaged and stored in any time interval.
In addition to the basic elements, the meter was extended by the possibility of communication with power management system at the communal consumer. Such systems are of particular importance in facilities equipped with renewable energy sources [21, 22]. This system was developed based on the CompactRIO controller from National Instruments company [23]. Additionally, a basic user control panel, on which most of the parameters are displayed, was developed. Because the LabVIEW allows for preparing applications for mobile devices, in the future, this application will also be developed for this type of device.
User control panel is divided into three tabs: Parameters, Power and Energy. In the “Parameters” tab, parameters on the voltage and the current flowing in the network at a sampling frequency of 10 kHz and update of the information on the display every second are presented. These are: a graph of voltage, current, RMS supply voltage value, RMS current value, phase shift (φ), sin (φ), cos (φ), frequency and THD voltage coefficient.
In the “Power” tab, the visualization of information on active, reactive and apparent power takes place. The elements of this tab are graphs showing the power values in the time domain. These graphs are drawn in real time for each of the power, which allows for viewing the history within the app by scrolling graphs. In addition, four-column table saving value of each of the power of one second interval is created. After the work of the meter is finished, the values in the table are saved to a text file, which can then be exported to a spreadsheet for analysis or data processing.
The last tab “Energy” displays information about energy consumption and costs. Elements located in this tab are: price per 1 kWh, cost in PLN, the value of the consumed active, reactive and apparent power.
The values of the costs can be used to stimulate the activities of recipients, on the one hand, by an increase in the awareness of users, on the other hand, through the launch of demand-side management programs.
Fig.3. Diagram of data analysis application together with the division into groups
Summary
The developed model of the smart meter includes the fundamental assumptions about smart meters described in the analyzed literature and the position of the President of the ERO. It has the ability to communicate with the power management system and was extended by a range of functionality. It provides information on the current values of many parameters concerning electricity. The application that could be used for the primary display of information on mobile devices was also developed. This solution also provides the ability to easily expand it by functions controlling receivers in the HAN network in the future by using a smartphone or tablet. With the ability to generate a text file with the data, analytical and statistical possibilities also increase. In addition to the summarizing function, the developed model of the smart meter has also some features of the analyzer. It can provide parameters that indicate the quality of electricity.
REFERENCES
[1] The European Technology Platform SmartGrids, http://www.smartgrids.eu/. [2] Gungo r , V.C. ; Sahin, D. ; Kocak, T. ; Ergut, S. ; Bucce l l a , C. ; Cecat i, C. ; Hancke, G.P. Smart Grid Technologies: Communication Technologies and Standards, Industrial Informatics, IEEE Transactions on, 10.1109/TII.2011.2166794 [3] Was iak I . : The Concept of Intelligent Power Microsystems, Przegląd Elektrotechniczny, (2011), nr 6, 35-41. [4] Bi lewi c z K., Smart Metering. Intelligent Metering System, Polish Scientific Publishers PWN, Warsaw 2012 [5] IBM Global Energy & Utilities, Global Trends in Smart Metering, 2010 IBM Corporation [6] Bo rowi k L. , Ku r kows k i M., Energy Control Systems in Lighting Installations, Rynek Energii 2/2013 [7] Kub iak Z. , Urb ania k A., Intelligent Metering System – Development of the Standard, Exemplary Solutions., Rynek Energii 1/2013 [8] Bi lewi c z K., Digital Security of Advanced Metering Infrastructure, Rynek Energii nr 3/2012 [9] The position of the President of the ERO on the necessary requirements to be implemented by the Distribution Network Operators (DNO) smart metering and billing systems, including the objective function and the proposed mechanisms of support for the postulated market model from 31 May 2011. [10] The concept of metering market model in Poland, with particular emphasis on requirements for the Operator of Measurement Information from 9 May 2012. [11] The position of the President of the ERO on detailed regulatory rules concerning stimulation and monitoring of the implementation of the AMI from 11 Jan. 2013. [12] The position of the President of the ERO on the necessary requirements concerning the quality of services using the AMI infrastructure as well as interchangeability and interoperability frameworks of Smart Grid components cooperating with each other and home network elements cooperating with the Smart Grid from 10 July 2013. [13] Borkowski P., Pawłows k i M. : The Potential of Energy Savings in Municipal Recipient. Rynek Energii 1/2012, 101-106. [14] Cen tole l la P., The integration of Price Responsive Demand into Regional Transmission Organization (RTO) wholesale power markets and system operations, Energy 35, 2010, 1568–1574. [15] Darb y, S., Smart metering: what potential for householder engagement?, Building Research & Information 38 (5), 2010, 442–457. [16] Sz k utni k J . , Woytowi c z J . , The Efficiency System In The Distribution Of Electrical Energy in Proc. 18th Intern. Conf. on Electricity Distribution Turin, 2005.. [17] Shekara S. , Reddy Depuru S., Wang L ., De va bhak tuni V. , Smart meters for power grid: Challenges, issues, advantages and status, Renewable and Sustainable Energy Reviews 15, 2010, 2736– 2742. [18] Vassi leva, I . , Wal l i n , F., Dahlqui s t E., Understanding energy consumption behaviour for future demand response strategy development, Energy 46, 2012, 94–100. [19] Laicane I . , Blumberga A. , Rosa M., Blumberga D., Ba r i s sU., The Effect of the Flows of Information on Residential Electricity Consumption: Feasibility Study of Smart Metering Pilot in Latvia., in Proc. Smart SysTech., 2013, 1-9. [20] Bal sam B. , Smart Meter of Electrical Energy with the Use of National Instruments Modules, BSc Thesis, Lodz University of Technology, Department of Electrical Apparatus, 2014. [21] Pawłows k i M. , Bo r kows k i P.: Electrical Energy Management System in Double Unpredictability Objects. Przegląd Elektrotechniczny (2014), nr 9, 191-196. [22] Pawłows k i M. , Bor kows k i P.: Electric Energy Management System in a Building with Energy Storage. Przegląd Elektrotechniczny (2012), nr 12b, 272-274. [23] Bad ows k i W. , Model of Demand Side Management with the Use of CompactRIO Controller, , BSc Thesis, Lodz University of Technology, Department of Electrical Apparatus, 2014.
Authors: dr inż. Marek Pawłowski, e-mail: marek.pawlowski@p.lodz.pl; prof. dr hab. inż. Piotr Borkowski, e-mail: piotr.borkowski@p.lodz.pl; inż. Bartosz Balsam, e-mail: bartosz.balsam@gmail.com Lodz University of Technology, Department of Electrical Apparatus, Stefanowskiego 18/22 Str, 90-924 Łódź, Poland. The correspondence address is: marek.pawlowski@p.lodz.p
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 91 NR 11/2015. doi:10.15199/48.2015.11.56
Published by Ahmad Rizal SULTAN, Mohd Wazir bin MUSTAFA, Technology University of Malaysia, Faculty of Electrical Engineering
Abstract. The type of generator grounding method and the system configuration determine the choice of ground fault protection. Ground fault generator stator can cause serious damage to the generator. Therefore, the total area of the generator stator windings must be protected against hazardous condition. Because the conventional methods are unable to separate ground faults close to the neutral point of view, there should be a method to protect entire stator winding. The paper discusses ways to discern ground faults for the generator stator. Some suggestions are proposed that can help in ensuring the generator stator ground fault more accurate.
Streszczenie. Stojan wirnika generatora powinien być szczególnie chroniony przed zagrożeniami. Konwencjonalne metody nie są w stanie oddzielić błąd uziemienia w pobliżu punktu neutralnego. W artykule opisano metody zabezpieczeń stojana generatora przed błędami uziemienia (Zabezpieczenia stojana generatora przed błędami uziemienia)
Synchronous generators are essential part of the power system. Disruption of the generator stator windings, especially in operation, can stop the generator. The process must be greatly detrimental to the generator companies, as this could result to the termination of plant operations, which means less revenue, as well as very high cost of repairing the damaged generator. Due to the importance of the generator stator winding, a protection system that protects the stator from the ground fault (GF) is a necessity.
The general kinds of the generator fault are the GF [1]. For a single line to ground fault near the neutral, the generated voltage available to drive current to the fault is small. The result is a fault with a low current and also a low neutral voltage displacement. At the most extreme, if the GF happens at a neutral point of its own, where no fault current or voltage displacement.
The importance of detecting GF close to the neutral point of the generator is not dependent on the need to trip because of the fault current magnitude, since it may be negligible and will not, in general, cause immediate damage. If a second ground fault occurs, severe damage may be sustained by the machine because this may result in a short – circuit current not limited by the grounding impedance. This condition may be aggravated if the first GF happens close to, or at the neutral terminal of the generator, because all ground relays operating from the neutral point voltage or current will become inoperative. Furthermore, if the second GF occurs in the same winding, the generator differential relay may also become inoperable since this situation can be categorized as an internal winding fault [2].
This paper reviews the various methods used to discern the GF on the generator stator. The most commonly used protections to discern the occurrences of GF on the generator are overcurrent, overvoltage and undervoltage relay. However, due to the development of protection systems, the protection scheme that utilizes disturbance in the ground, especially for a generator today can be categorized into four main sections, namely the conventional method of stator GF protection, third harmonic protection method, injection protection method and numerical protection method.
Ground Fault Protection of a Generator Stator
An overcurrent / overvoltage generator GF protection should be normally straight forward, safe and reliable. However, it has two insufficiencies. First, it will not detect ground faults close to the generator neutral, and second; it is not self-monitoring. This depends on the open circuit where the platform relay, primary or secondary winding of a current transformer or high resistance cannot be detected before a fault occurs. While grounding faults occur in the systems, every parallel generator has the same voltage, and traditional stator winding grounding fault protections with zero sequence voltage cannot detect which generator was faulted [3, 4].
a. Conventional Method of Stator GF Protection
The overvoltage relay (59G) can detect faults approximately 90 – 95% part of stator winding. These protections are illustrated in Figure 1.
Fig.1. Conventional Stator GF Protection [5]
There is a linear correlation between the voltage identify by the 59G relay and the GF position in the stator winding. For GFs close the neutral (N), the voltage will be detected by the 59G relay. The maximal voltage happens in a GF at the generator terminals (T), where maximum line-to-neutral voltage happens across the neutral grounding transformer. Commonly, the last 5-10% of the winding is not protected by the 59G. GF at the bottom 10% of the stator winding of the generator may not be sensed by the conventional method of stator GF protection (overvoltage or overcurrent relays).
b. Third Harmonic Protection Method
Generally, the output voltage of the generator is not a pure sinusoidal, but distorted by harmonic components. The whole harmonic components generated can be found in triple harmonics as the third harmonic, 9th, 15th and so on. The triple components can be found in all phases and have a large and the same phase angle, which may cause the common point during this phase triple components not to add up to zero for each phase. Therefore, the components appear as triple amount of zero sequences. The third harmonic voltage (THV) is commonly greater than most others triples. Under normal conditions, the characteristics of THV in the stator windings are shown in Figure 2.
Fig.2. Third harmonic voltage at normal condition [6]
When the GF occurs close to neutral at the generator, the THV at the terminal point will be equivalent to the total third harmonic generator yield. While the voltage of harmonics is in the neutral point, the sum reaches down to zero. The model of THV during GF at a neutral point is shown in Figure 3.
Fig.3. Third harmonic voltage during ground fault at neutral [6]
The opposite occurs when a GF develops in terminal point on the generator. The THV in the terminals will drop to zero, while the THV in neutral point increases to a total of all the THV of the generator produced. These characteristics are shown in Figure 4.
Fig.4. Third harmonic voltage during ground fault at a terminal [6]
Based on the above characteristics, when a GF occurs close to the terminal point, the THV at the neutral will decrease, while the terminal will be enlarged. Similarly, the opposite occurs when it crashes near the terminal point.
The following sections describe the five stators of GF protection using the principle of THV method.
b.1 Third-Harmonic Neutral Undervoltage Relay [4]
This protection method is the combination of the conventional (59G & 27TH) and third harmonic neutral voltage (see Figure 5). The third harmonic was measured across the generator neutral grounding resistor. The basic concept of the scheme is that, when the generator stator GF happens close to the generator neutral, the THV reaches to zero. If the third harmonic generator is a sufficiently neutral voltage as long as normal condition, to avert false operation of the relay from energizing, then such generators are candidates for 100% schemes using third-harmonic neutral detection.
The third harmonic undervoltage relay can detect an absence of THV at the generator neutral resulting from a GF close to the neutral. The 27TH and 59G relays must be filtered to prevent fundamental or third harmonic voltages respectively from affecting the operation. The 27TH relay should, if not self-protected, include circuitry to protect its coil from sustained overvoltage. This scheme offers the advantage of not requiring any additional high-voltage equipment, other than those needed for conventional ground-fault detection schemes for single stator generators.
The scheme can also be used for cross-compound and split-winding machines by adding a second VT and third harmonic relay to monitor the voltage at the neutral of the ungrounded stator winding. The scheme provides protection when the main breaker is open, provided that the terminal voltage is above the pickup of the supervisory relay 59.
Supervision is required during the start-up and shutdown either by using a breaker contact or an undervoltage relay so that the relay is disabled when the generator is off-line. Some generators provide very low levels of THV when the generators are lightly loaded. In order to improve the security of this scheme, an underpower relay (device 32) can be used to control the undervoltage relay of third harmonic neutral. The disadvantage with this scheme is the absence of 100% coverage until a relay 59 picks up.
Fig.5. Undervoltage relay scheme of third-harmonic neutral [7]
b.2 Voltage Relay of Third Harmonic at The Generator Terminal [2]
This protection method is supplied by a wye-grounded broken-delta transformer, which can be wye-wye for digital relay. This scheme is shown in Figure 6. Upon the occurrence of a generator neutral ground, the THV available at the line generator terminals becomes elevated. The accompanying overvoltage is used to operate a relay used for this application and must be set so that it will be unresponsive to the maximum THV appearing at this point during normal system operation.
An advantage in this scheme is that it will also detect GF in the bus or in the delta winding when the generator de-connector is open. However, it also has shortcomings due to the need for a three-phase VT on the machine terminals.
b.3 Third Harmonic Voltage Comparator Relay
This method distinguishes the third harmonic in the terminal and a neutral at the generator. This scheme, shown in Figure 7, utilizes the fact that the third harmonic residual voltage in terminals of a machine increase, while the THV at the neutral decreases, for a fault nearby the neutral. The comparison of the third harmonic residual voltage to the neutral third harmonic content may be nearly constant under all load conditions in many un-faulted machines.
Fig. 6. Third harmonic voltage Relay at a generator terminal [2]
Small changes in this ratio may require the reduced sensitivity parameters. The coincidence between the functions of the equipment 59GN and 59D may exist. The settings for both relay should be determined during field testing in conjunction with commissioning. The third harmonic differentials relay 59D detects GFs close to the neutral as well as at the terminal. Relay 59GN, which is used to measure the fundamental frequency neutral voltage, can detect a fault in the upper section of the winding as well as overlapping much of the winding covered by 59D. The (comparator) relay sensitivity is least for a fault in the middle of the windings. At some point in the winding, the difference between the neutral and terminal THVs is equal to the relay setting. Double GFs tend to reduce the sensitivity for the differential relay, and multi-winding machines to offer application difficulties that require careful consideration.
Fig. 7. Third harmonic ratio comparator [2]
b.4 Adaptive third harmonic level detector
The two voltages are applied to derive the third harmonic source voltage at the generator by using the vector combination of the signals (see Figure 8). The THV in the neutral and that of the residual voltage in terminal are continuously compared with the derived source voltage to detect a grounding the first 15% of the windings close to the neutral. The detection scheme then indicates a fault in its zone of coverage if the THV at the neutral is less than 15% of the source voltage and if one-third of the residual third harmonic at the terminals exceed 85%. Ideally, in comparison, the two are equivalent, but in practice, it has been verified that the two are indeed different in a number of ways according to the MW and MVAR typical generator installations.
This approach, in effect, utilizes adaptive undervoltage and overvoltage level detectors, where the setting level adapts to the level available relaying a signal, i.e., the magnitude of source voltage. It is imperative that the detectors will be blocked when the third harmonic source voltage is less than some minimum values, below which the voltage signals are considered unreliable for relaying. A level of 0.75% of nominal phase to neutral voltage on the generator is considered safe.
Fig.8. Adaptive third harmonic level detector [2]
b.5 A Recent Third Harmonic Protection Method
In the protection method, the scheme of THV phasor at the terminals or in the generator neutral are periodically evaluated and stored. Every currently estimated third harmonic magnitude is identified with the saved magnitude before the time of approximately one second. If the difference between magnitudes being compared is greater than the fixed, the verge magnitude as the trigger signal is produced. To date, the application of an existent algorithm using the microcontroller in the system, and preparatory tests using synthesized by voltage signals of GF have been confirmed effective.
Other new methods of fault protection in assuming the differential phase angle of delta THV [8] does not contribute to the THV neutral of the generator, and the generator GF of a stator is equivalent to the terminal, and can be used to discern the GF of the stator of generators.
c. Sub-harmonic Injection Protection Method
The sub-harmonic injection scheme has two main methods, 12.5 Hz and 20 Hz. The signal source of 12.5 Hz is in series with neutral grounding resistance, and the 20 Hz signal source is parallel with the neutral grounding resistance [8]. While a GF happens, the current increases due to the less resistant faulty path, and thus will be detected. The main advantage of the sub-harmonic 64- 100% relays is that, they are quite sensible, regardless of the machine reactive loading.
The scheme of sub-harmonic injection is another main method for unit-connected generator using third-harmonic. The following will explain the principle of stator GF protection by using subharmonic injection method.
c.1 Neutral or Residual Sub-harmonic Voltage Injection (comparator injection and measurement voltages)
This scheme, using a voltage injection at the neutral or residually in the broken-delta VT secondary, can detect GFs anywhere in the part of stator winding, including the neutral point. Full GF protection is available, even when the generator is starting up and during turning gear if the injected voltage source does not originate from the generator. Certain schemes inject a coded signal into a sub-harmonic frequency that can be synchronized to the frequency of a system. When compared with other injection schemes, this coding improves the security within the relay system without sacrificing dependability. For proper relay performance, the scheme is dependent on a reliable subharmonic source.
The use of sub-harmonic frequencies may offer improved sensitivities in relation to the higher-level impedance path of the generator capacitances at these frequencies. Such frequencies are not normally present at the generator’s neutral. This comes with a disadvantage of the economic penalty associated with providing and maintaining a reliable sub-harmonic source and injection equipment.
The major advantage of neutral injection schemes is that they provide 100 GF protections independent of the 95% GF protection schemes [2].
c.2 Neutral sub-harmonic voltage injection (measurement of voltage and current)
This scheme is shown in Figure 9. This scheme uses the sub-harmonic current injection at the generator neutral that can detect the GF’s entire generator stator winding, including the delta windings in a generator step-up transformer (GSU). Full GF protection can be provided without the field being energized, such as during the initial start-up of the generator and turning gear with the independent sub-harmonic voltage supply. Certain schemes inject a coded signal into a sub-harmonic frequency that can be synchronized to the frequency on the system such as for a 50 Hz system used a 12.5 Hz. This coding improves the security within the relay system without sacrificing dependability.
The scheme uses voltage and current measurements as the secondary circuit of grounding transformer of the generator. The voltage and current measurements are derived from the injected signal as they are placed across the generator grounding transformer secondary. In this manner, the reflected impedance of delta winding of the GSU and the generator are measured. If a GF is not present anywhere in the generator zone, the impedance measured is the natural capacitive coupling to be ground of the entire generator zone. If a GF develops, the impedance becomes less than natural capacitive coupling values, and alarm and/or trip set points will be applied.
Fig.9. Subharmonic Voltage Injection Scheme [5]
The major advantage of neutral sub-harmonic injection schemes is that they provide 100 % GF protection even when the generator is not in service and during start-up prior to application within the field [2].
c.3 A Recent Compensated Injection Scheme
In the protection scheme, an additional reactor that connects the compound with the resistance load in the distribution of transformers secondary side is applied to compensate the leakage capacitance from the stator windings of the generator to be grounded. The injection signal frequency is tuned to adapt to the variation of the capacitance caused by temperature varying, insulation aging, etc. Thus, the influence of the capacitance is cleared and high-impedance of GF protection for the entire generator stator winding is taken directly as adjustment measurement the grounding fault resistance and comparison of the session threshold [7].
d. Numerical Protection Method d.1 Using wavelet transforms.
The THV at a generator neutral and terminals will vary simultaneously when a ground fault occurring in the generator stator, even if it is grounded with a high resistance. Therefore, the signals which be measured by the protective device contain significant transient components. They have much more fault information than the steady component. As wavelets are well suited for the analysis of the non-stationary signals, one will have the ability to extract important information from the noised signals. This information can be used to discern the GF [9].
The scheme (Figure 10) considers the overall produced voltage in the machine, and the use of Wavelet Multi Resolution Analysis (MRA). MRA is an ideal method to the analysis of the transients of the power system [10] and the protection of generators [11]. THV terminal and neutral signals are analyzed using of Wavelet MRA to discern GFs. In this scheme, maximum polarities coefficients are compared to other discrimination grounds transient faults. In this method, the maximum coefficient decreases for the increase in fault impedance. The scheme offers inherent discrimination of sudden load imbalance [12].
Fig.10. Wavelet based Multi Resolution Analysis [12]
Problem of Stator Ground Fault Protection
The most important thing to consider in the use of various stator GF protections is the type of the stator winding of generators, which has an expressive effect upon the efficiency of the GF protection and units of supply, thus should be considered in evaluating the performance of the protection system and analysis for optimization [13]. The generator active load affects the extent of the supply voltage protection element, and is satisfied with the THV [14].
Conclusion
Implementation of GF protection, especially for the 100% coverage scheme, depends on the generator’s neutral, based on the type of stator winding and generator connection. To get the best performance, the characteristics of stator GF protection must be evaluated with pre-fault conditions, fault conditions and various load condition.
REFERENCES
[1] Gilany, M, Malik OP, Generator Stator Winding Protection with 100% Enhanced Sensitivity, Electrical Power and Energy Systems, 2002 [2] IEEE Std C37.101™-2006, IEEE Guide for Generator Ground Protection. [3] Wang Yuanyuan, A Novel Protection for Stator Winding Grounding Faults in Multi-Generator-Systems, Tencon-IEEE Region 10 Conferences, 2006 [4] C. H. Griffin, Generator Ground Fault Protection using Overcurrent, Overvoltage and Undervoltage Relay, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-101, 1982 [5] Charles J. Mozina, 15 Years of Experience with 100% Generator Stator Ground Fault Protection (What Works, What Doesn’t and Why), 62nd Annual Conference for Protective Relay Engineer, 2009 [6] Blackburn J.Lewis, Protective Relaying: Principles and Application, Marcel Dekker, New York, 1998 [7] IEEE Std C37.101™-1993, IEEE Guide for Generator Ground Protection [8] NengLing T, Yan D, Stator Ground Fault Protection Based on Phase Angle Differential of Delta Third Harmonic Voltages, Electric Power System Research, 2005 [9] Nengling T, et al, Wavelet-based Ground Fault Protection Scheme for Generator Stator Winding, Electric Power Systems Research, 2002. [10] Xiangjun Z, et al, Improvement of Subharmonic Injection Schemes for Huge Hydro-generator Stator Ground Protection, International Conference on Power System Technology PowerCon Volume 2, 2002 [11] A.W.Galli, G.T.Heydt, P.F.Ribeiro, Exploring the Power of Wavelet Analysis, IEEE Computer Applications in Power, Oct. 1996, pp. 37-41. [12] S A Gafoor, P.V. Ramana R., Wavelet-ANN Based Ground Fault Protection Scheme for Turbo Generators, Electric Power Components and Systems, 35:5, 575-590, 2007 [13] M. Zielichowski, Third Harmonic Ground-Fault Protection System of Unit-Connected Generator with Two Parallel Branches Per Phase, Electric Power Systems Research, 78-2008 [14] Fulczyk M, Influence of Generator Load Conditions on Third- Harmonic Voltages in Generator Stator Winding, IEEE Transactions On Energy Conversion, Vol.20, 1, 2005
Authors: Ahmad Rizal Sultan, Faculty of Electrical Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Malaysia 81300, E-mail: rizal.sultan@fkegraduate.utm.my Mohd Wazir Mustafa, Faculty of Electrical Engineering, Universiti Teknologi Malaysia(UTM), Skudai, Malaysia 81300, E-mail: wazir@fke.utm.my
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 10/2013
Published by Alex Roderick, EE Power – Technical Articles: Understanding Solar Photovoltaic (PV) Power Generation, August 05, 2021.
Learn about grid-connected and off-grid PV system configurations and the basic components involved in each kind.
Solar photovoltaic (PV) power generation is the process of converting energy from the sun into electricity using solar panels. Solar panels, also called PV panels, are combined into arrays in a PV system. PV systems can also be installed in grid-connected or off-grid (stand-alone) configurations. The basic components of these two configurations of PV systems include solar panels, combiner boxes, inverters, optimizers, and disconnects. Grid-connected PV systems also may include meters, batteries, charge controllers, and battery disconnects. There are several advantages and disadvantages to solar PV power generation (see Table 1).
Table.1 Solar Photovoltaic (PV) Power Generation
Advantages
Disadvantages
• Sunlight is free and readily available in many areas of the country.
• PV systems have a high initial investment.
• PV systems do not produce toxic gas emissions, greenhouse gases, or noise.
• PV systems require large surface areas for electricity generation.
• PV systems do not have moving parts.
• The amount of sunlight can vary.
• PV systems reduce dependence on oil.
• PV systems require excess storage of energy or access to other sources, like the utility grid, when systems cannot provide full capacity.
• PV systems have the ability to generate electricity in remote locations that are not linked to a grid.
• Grid-connected PV systems can reduce electric bills.
Table 1. There are advantages and disadvantages to solar PV power generation.
Grid-Connected PV Systems
PV systems are most commonly in the grid-connected configuration because it is easier to design and typically less expensive compared to off-grid PV systems, which rely on batteries. Grid-connected PV systems allow homeowners to consume less power from the grid and supply unused or excess power back to the utility grid (see Figure 2). The application of the system will determine the system configuration and size. For example, residential grid-connected PV systems are rated less than 20 kW, commercial systems are rated from 20 kW to 1MW, and utility energy-storage systems are rated at more than 1MW.
Figure 2. A common configuration for a PV system is a grid-connected PV system without battery backup.
Off-Grid (Stand-Alone) PV Systems
Off-grid (stand-alone) PV systems use arrays of solar panels to charge banks of rechargeable batteries during the day for use at night when energy from the sun is not available. The reasons for using an off-grid PV system include reduced energy costs and power outages, production of clean energy, and energy independence. Off-grid PV systems include battery banks, inverters, charge controllers, battery disconnects, and optional generators.
Solar Panels
Solar panels used in PV systems are assemblies of solar cells, typically composed of silicon and commonly mounted in a rigid flat frame. Solar panels are wired together in series to form strings, and strings of solar panels are wired in parallel to form arrays. Solar panels are rated by the amount of DC that they produce. Solar panels should be inspected periodically to remove dirt, debris, or snow, as well as to check electrical connections.
Since photovoltaics are adversely affected by shade, any shadow can significantly reduce the power output of a solar panel. The performance of a solar panel will vary, but in most cases, guaranteed power output life expectancy is between 10 years and 25 years. Solar panel power output is measured in watts. Power output ratings range from 200 W to 350 W under ideal sunlight and temperature conditions.
Solar Arrays Construction and Mounting
When solar arrays are installed on a property, they must be mounted at an angle to best receive sunlight. Typical solar array mounts include roof, freestanding, and directional tracking mounts (see Figure 4). Roof-mounted solar arrays can blend in with the architecture of a dwelling and will save yard space.
Figure 4. Typical solar array mounts include roof, freestanding, and directional tracking mounts on the roof or on the ground. Image courtesy of Greensarawak
Roof-mounted solar arrays attach to the roof rafters and are engineered to handle the same forces and climate conditions as the rooftop. Composition shingles are considered the easiest roofing on which to mount solar arrays, while slate and tile roofing materials are often considered the most difficult. The main drawback of roof-mounted solar arrays is that they require access for maintenance.
Freestanding solar arrays can be set at heights that allow convenient maintenance. However, freestanding solar arrays usually require a lot of space. Also, freestanding solar arrays should not be mounted on the ground in areas that receive a lot of snow.
Solar array mounts can also be either fixed or tracking. Fixed solar arrays, which are often roof-mounted or freestanding, are preset for height and angle and do not move with the sun. Directional tracking solar arrays move with the sun from east to west and adjust their angle to maintain the maximum exposure as the sun moves. Directional tracking solar arrays can increase the daily energy output of a PV system from 25% to 40%. However, despite the increased power output, directional tracking arrays may not justify the increased cost due to the complexity of the mounting system.
PV Combiner Boxes
A PV combiner box receives the output of several solar panel strings and consolidates this output into one main power feed that connects to an inverter. PV combiner boxes are normally installed close to solar panels and before inverters. PV combiner boxes can include overcurrent protection, surge protection, pre-wired fuse holders, and preconfigured connectors for ease of installation to the inverter. The use of pre-wired connectors saves running wires to the inverter. PV combiner boxes should be inspected periodically for leaks or loose connections.
PV combiner boxes are not required for every PV system installation. For example, when there are only two or three strings of solar panels, a combiner box may not be required. In these cases, the strings of solar panels are connected directly to the inverter.
PV Inverters
An inverter is a device that receives DC power and converts it to AC power. PV inverters serve three basic functions: they convert DC power from the PV panels to AC power, they ensure that the AC frequency produced remains at 60 cycles per second, and they minimize voltage fluctuations. The most common PV inverters are micro-inverters, string inverters, and power optimizers (See Figure 5).
Figure 5. Microinverters are connected to each solar panel, which are connected in parallel, and convert DC directly to AC. String inverters are used with multiple solar panels connected in series. Power optimizers are installed on each solar panel, which are connected in parallel. Image courtesy of Letsgosolar
A microinverter is a device that converts DC power to AC power and is mounted directly to individual solar panels. Because the DC to AC conversion happens at each solar panel, the microinverters maximize the potential output of a system. For example, if one solar panel is shaded by a tree, it will not affect the output of any other solar panels. Microinverters also eliminate the need for potentially hazardous high-voltage DC wiring.
A string inverter is a device that converts DC power to AC power from several solar panels that are connected in series. However, in a series configuration, if one of the solar panels stops producing electricity, even due to temporary shading, it can decrease the performance of the whole system. String inverters are in the high-voltage range (600 V to 1000 V) and are used with large PV systems with no shading concerns. Usually, only one string inverter is needed for a residential application.
A power optimizer (maximizer) is a hybrid microinverter system that conditions the DC power before sending it to a centralized inverter instead of converting the DC power from the solar panels directly into AC power. Power optimizers, like microinverters, still perform well when one or more panels are shaded or when panels are installed facing different directions. Power optimizer systems tend to cost more than string inverter systems but less than microinverter systems.
PV Disconnects
Automatic and manual safety disconnects protect the wiring and components of PV systems from power surges and other equipment malfunctions. Disconnects ensure that the PV system can be safely shut down and system components can be removed for maintenance or repair. With grid-connected PV systems, safety disconnects ensure that the generating equipment is isolated from the grid for the safety of utility personnel. A disconnect is needed for each source of power or energy storage device in the PV system. An AC disconnect is typically installed inside the home before the main electrical panel. Utilities commonly require an exterior AC disconnect that is lockable and mounted next to the utility meter so that it is accessible to utility personnel.
Author: Alex earned a master’s degree in electrical engineering with major emphasis in Power Systems from California State University, Sacramento, USA, with distinction. He is a seasoned Power Systems expert specializing in system protection, wide-area monitoring, and system stability. Currently, he is working as a Senior Electrical Engineer at a leading power transmission company.
Published by Marek MALINOWSKI, Jacek CHMIELOWIEC, Grzegorz PAŚCIAK, Tymoteusz ŚWIEBODA, Electrotechnical Institute, Division of Electrotechnology and Materials Science
Abstract. Evaluation of FC/SC-based UPS has been done in terms of energy containers usability in the power emergency generation system. The Uninterruptible Power Supply delivers electric support for connected loads basing on hydrogen energy direct conversion into electricity with aid of low-temperature PEM fuel cell and additional energy carriers – supercapacitors. The paper contains description of design and rules of operation, and measurement results that have been determined under various conditions to examine usefulness of FC/SC implementation.
Streszczenie. Na podstawie badań skonstruowanego zasilacza awaryjnego typu UPS bazującego na ogniwie paliwowym typu PEM-FC oraz dodatkowych magazynach energii – superkondensatorach, dokonano oceny sensowności wykorzystania tych zasobników energii w systemie awaryjnej generacji energii elektrycznej. Praca zawiera opis projektu i zasady działania oraz wyniki pomiarowe, otrzymane w różnych warunkach działania zasilacza. (Ocena przydatności zastosowania ogniwa paliwowego typu PEM-FC oraz superkondensatorów w układzie zasilania awaryjnego).
Fossil fuels are the best resources intended for energy production discovered by the human race. The advantages such as independence of weather conditions, latitude or seasons of the year, high conversion efficiency and availability of effective technology explain why fossil fuels are still widely used around the world. The supremacy of carbon based fuel resources is due to physical, economic and even historical reasons. The chemical accumulated energy can be oxidize according to the current utilization. The production costs are still fully acceptable. Looking at the history and duration of past energy transitions – from wood to coal and from coal to gas and oil may be predicted that the next transition will be also long, expensive and doubtful process. Advantages of “old” recourses and long path of development the new ones show that it will be severe task to change human habits in terms of hydrocarbon combustion. However, there is no way out [1].
There are various alternatives to traditional carbon fuels. The huge power generation linked with fissions of radioactive atoms delivers CO2-free energy in hundreds of nuclear power plants. However, similarly to fossil fuels the resources will become depleted. The common word properly describing the other alternatives is renewable. The energy sources which are renewable seem to be the only way to keep and cover the growth of energetic utilization. Solar radiation, winds, biomass, tides and hydroelectricity are widespread utilized for energy generation. Unlike fossil fuels the renewables can have a sustainable yield. They are suited both to large scale and to remote or off-grid applications. Due to intermittent character, especially for solar and wind resources, the excess of obtained energy should to be stored in order to utilize in case of the temporary lack of renewables. One of the efficient way in terms of storage is production of the hydrogen by using various method such as electrolysis, photolysis, or thermochemical splitting. After these processes the energy can by restored at any time. For these purposes fuel cells have been designing – the devices that allow high-efficient direct conversion from chemical energy of source fuel to the electricity and even the heat. The water appears as a result of the conversion. Hence, the connection between the hydrogen storage and renewables utilization creates great opportunity to implement inexhaustible, ecologic energy generation processes regardless of scales, areas or applications.
Plenty of applications for fuel cells supplied by the hydrogen can be found [4÷8]. Proton exchange membranes fuel cells (PEMFC) due to their low operation temperature, high electrical efficiency and flexibility have potential to substitute sources of electric energy such as batteries or accumulators. Moreover they are non-polluting low mass and volume power generators which can be used in various application. Typical examples are small-scale power plants, portable devices and electric vehicles.
The other prospective applications are emergency backup systems such as Uninterruptible Power Supplies (UPS) which supply external loads in case of failure of the power network. According to how typical configuration of UPS looks like, above devices generate electricity basing on built-in batteries or various power engines such as internal combustion ones. This configuration involves various well-known disadvantages lowering thereby reliability and lifetime of UPS. The fuel cells (FC) as an excellent power sources can substitute popular sources of the electricity. The aim of this paper was technical description and usability evaluation of a new designed and constructed UPS intended for lab, domestic or office utilities. The FC/SC UPS has been designed as a hybrid device supplied by supercapacitors (SC) and PEM-FC stack with hydrogen as a fuel.
Overall system design
The design and construction of the FC/SC hybrid UPS were done in multistage process. The main tasks have been divided as follow: electrical part, control system, gaseous part. As far as fuel cell plays significant role in terms of electricity generation it’s crucial to show internal structure of the supply which is adjusted mainly due to the stack principle of operation. From this point of view usability can also be evaluated.
Electrical part
Electrical part (fig. 1.) contains double conversion system: AC/DC and DC/AC. An additional converter has been used because of fuel cell and supercapacitors presence. The converter is important for the stack which works under wide voltage range. FC/SC Supply was designed according to technical parameters of commercial 600 W fuel cell stack. This power can not be transfer in whole to the output of UPS due to its different electric equipment’s such as air compressor, electromagnetic valves, control systems or measurement instruments. They require the total power of app. 180 W. Taking into account the conversion efficiency of built-in converters, the rated power of the Supply drops to 400 W which is the first seen limitation of fuel cell application. This loss can be significantly scaled-down by implementation of air pressure vessel instead of electrical compressor that spends considerable amount of the energy or by implementation of the stack which has the blowers both for air compression and cooling.
Fig.1. Fuel cell and supercap – based UPS: schematic diagram of the electric part
Control system
Basically, the control system designed for UPS plays the most important role, so particular attention has been paid to develop that system. During development process three different drivers were tested to meet all requirements according do project assumptions. For the first time, ATmega8-based circuit was implemented [2] to control each particular component built-in the supply. That system, due to small number of peripherals and low frequency working with, had considerable limitations thus DSP-based driver was proposed and then constructed [3]. However, the problems being appeared, that resulted mainly from principle of DSP operation, excluded also this driver. Eventually, control system basing on ATmega128 was designed. The circuit was constructed according to the first control system driver project. It fulfills all of the assumptions mostly as a consequence of control algorithm adjustment. The system allows to work with 11.05911 MHz frequency as well as external quartz-crystal resonator can provide. Various measurements and control components were implemented, thus all-in 8 ADC channels, 20 I/O ports and two PWM channels of the microcontroller with additional communication interfaces are being used to deliver necessary functionality for the UPS. As plenty of parameters have to be captured during back-up system operation, three different noise canceller techniques have been implemented for embedded analog-to-digital controller.
This includes short analog signal path, LC network between analog and digital supplying and usage of ADC channels only as analog ones. By virtue of this the following conclusion can be found. Accurate measurement and management is not only important due to proper operation of the supply but it also plays significant role in terms of scientific investigation. In the other words, measurement of important parameters with high accuracy facilitates evaluation of internal energy sources such as fuel cell stack and supercapacitors in specific condition of UPS operation.
Gaseous part
Figure 2 presents the diagram of gaseous part. The 500 sl metal hydride tank supplies the fuel cell through the pressure regulator to obtain max. 1.5 bar on the anode side. There are electromagnetic valves (V1, V2), mass flow controller and pressure transducers (T1, T2) installed due to managing of hydrogen utilization.
As was proven during investigations the UPS basing on the fuel from the tank is able to supply connected load for 15 minutes. These investigations were performed at the conditions of open V2 valve (0.4 bar initial hydrogen pressure) so the fuel was only spent partially. In order to increase the efficiency of conversion, fuel cell’s dead-end capable system has been implemented. The solution provides great opportunity to spend as much fuel as possible.
Fig.2. Fuel cell – based UPS: gas subsystem connections
Fig.3. Comparison graphs between dead-end-capable configuration (upper) and open H2 outlet of built-in fuel cell (lower), performed at the same level of refilling for hydrogen storage canister
The difference between time of operation is shown in figure 3. At a constant power of 400 W and in case when the hydrogen outlet is open, UPS is able to work for about 15 minutes. During the research hydrogen pressure has been corrected twice to extend the time of operation, possibly in maximal-way. The upper graph shows automatic managing of hydrogen utilization. In this case control system closes V2 valve (dead-end-capability) to increase conversion efficiency thereby the time of working is doubled – approximately to 35 minutes. In order to use dead end capability, outlet valve is periodically being opened for a few seconds mainly due to water steam which mitigates the level of fuel conversion. This is indicated by temporary rise of fuel cell’s voltage resulting from hydrogen purification.
As a supplement of design description, figure 4 and 5 depict configuration of particular components in the UPS.
Fig.4 – Front side of the UPS: 1 – electrical part and: 2 – Converter AC/DC, 3 – Converter DC/DC, 4 – Supercapacitors (SC)
Fig.5 – Rear side of the UPS: 1 – fuel cell (FC), 2 – metal hydride tank, 3 – inverter, 4 – compressor, 5 – electronic μC – based driver
Usability evaluation
Constructions of various applications of fuel cells is not only due to better characteristics and parameters in comparison to the existing products – because it could be often controversial, but by reason of necessity for environmental-friendly technologies’ implementation. Such an opportunity give fuel cells, therefore different reports predict enormous growth in this market. Looking at the advantages of fuel cells two issues appear. The first is efficiency the stack can convert chemical energy with, which is as high as 40% for electricity generation in case of PEM fuel cells. The other is size of the energy that can be produced which is up to the amount of hydrogen being stored. That’s theory. However, some limitations of fuel cell application in the Supply during investigation are being seen. First of all is the principle of fuel cell operation which involved the UPS to be constructed in sophisticated way. So, apart from the fuel cell stack, various devices have to be engaged to conduct chemical conversion of the hydrogen into the electricity. This includes but is not limited to hydrogen containers, air compressors, blowers, electromagnetic valves, flow controller and temperature, pressure, and current sensors. During fuel cell operation the problem may occur with one of the above devices, leads to the failure of the entire system. That sounds particularly in comparison to the classic batteries’ principle of working.
The other issue is the amount of energy that has to be spent to keep the stack on. This results, as above, from implementation of particular elements which require electricity to work. For instance, the power spent for own purposes during UPS operation is as high as 25% of the stack rated power.
Looking at the structure of the typical fuel cell stack various materials can be found that are susceptible to ageing processes. For example typical membrane electrode assembly (MEA) of the fuel cell is made from Nafion-based electrolyte film with carbon/platinum electrodes on each side. It’s common situation the efficiency is partially decreased when fuel cell stack works or is stored in severe conditions which rely on low humidity, over-range temperature or presence of carbon oxide in the fuel. Such a phenomena has been observed since fuel cell stack purchase for the UPS. There are results of voltage-current measurements depicted in figure 6. At the beginning the stack worked as well as fuel cell’s manufacturer declaration. However, after four years we observed approximately 50% reduction of the rated power. The ageing process we predict is connected with inappropriate storage place such as low humidity what permanently scaled-down the ionic conductivity of membrane electrolytes.
Fig.6. The influence of ageing processes on fuel cell stack performance
Fig.7. 3 kF supercapacitors/Pb-acid battery comparison characterized by comparable volume
The second devices built in the UPS intended to store the energy are supercapacitors. Although state-of-the-art supercapacitors have enormous capacitance, typically they can not successfully substitute the batteries in terms of capability of energy storage. As is shown in figure 7 there are two curves made due to comparison of built-in supercapacitors and Pb-acid battery. In spite of comparable volume they have, the battery obtained about 28 Ah of electric charge transferred, while the supercapacitors with capacitance of 3 kF only 2.3 Ah. Thus, it seems that supercapacitors application in stationary systems may be senseless. However, advantages such as huge number of charging-discharging cycles, low internal resistance, long life time and wide range of operating temperature, convince that these energy carriers may find various applications, especially mobile and for transportation ones.
Conclusions
Various tests have been made to evaluate fuel cell and supercapacitors based UPS. The principles of the project excludes typical disadvantages of the classic UPS, such as limited time of operation or long period of charging. This is provided respectively with fuel cell implementation which generates as much energy as amount of hydrogen is stored and with supercapacitors that are characterized by low internal resistance which leads to short time of charging.
Metal hydride tank containing about 500 sl of hydrogen gives an opportunity to supply 400 W load for 15 minutes, however, this value was significantly increased up to 35 minutes by controlling of the fuel utilization using dead-end-capable system. By mounting of the high pressure tank which would have an ability to store more hydrogen or even by replacing of high energy consumption air compressor with a pressure oxygen vessel this value can by further elongated.
The other way is to substitute assembled fuel cell stack by so-called open-cathode stack which is adjusted to use the blowers both for cooling and oxidant delivering at the same time. That solution would eliminate air compressor, bringing additive power to external loads.
As fuel cell stack was used in this project the following issues appeared that have negative influence on usability evaluation: the quantity of the equipment which is required by the stack to operate and which in case of problem with one of them, may lead to the failure of the entire system; the ageing processes that lower the efficiency of fuel cells; capability of supercapacitors to store the energy comparing to the other sources such as Pb-acid battery. In spite of up-to-date disadvantages, the applications of fuel cells in solutions such as uninterruptible power supply will find confirmation due to current enormous development of these energy converters and general hydrogen technology.
Additional question is unavoidable transformation to the hydrogen economy which seems to be closer than we can think about.
The research was supported by Wroclaw Research Centre EIT+ under the project “The Application of Nanotechnology in Advanced Materials” – NanoMat (POIG.01.01.02-002/08) financed from the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2)
REFERENCES
[1] Kerr R. A.: Do We Have the Energy For the Next Transition?, Science, 329, (2010), 780-781. [2] Malinowski M,. Paściak G., Bujło P., Chmielowiec J.: Uninterruptible power supply (UPS) as an example of the fuel cell and ultracapacitors application. Proceedings of the Electrotechnical Institute, Pl, 248, (2010). [3] Malinowski M,. Pasciak G., Debowski L.: Uninterruptible Power Supply Unit with Fuel-Cell as a Backup Energy Source and DSP-based Control System. PCIM Europe 2011, 17-19 May 2011, Nuremberg, Germany, 1181 – 1186 [4] Hwang J. J., Wang D. Y., Shih N. C.: “Development of a Lightweight Fuel Cell Vehicle”, J. Power Sources, 141, (2005), 108-115 [5] Bujło P., Sikora A., Paściak G., Chmielowiec J.: Energy flow monitoring unit for Hy-IEL hybrid (PEM fuel cell-supercapacitor) electric scooter. Electrical Review, 86 (2010), No. 3, 271-273 [6] [2] Weigl J., Saidi H.: “Pios Hydrogen Fuel Cell Tricycle”, J. Hydrogen Energy, 30, (2005), 1035-1036 [7] Bujło P., Bieniecki S., Pasciak G., Chmielowiec J., Mazurek B., Perz J.: “Hybrid Fuel Cell Supercapacitor System for HY-IEL Electric Scooter Drive”, Proceedings of 17-th World Hydrogen Energy Conference, Brisbane, Australia 15 – 19 June 2008 [8] Varkaraki E., Lymberopoulos N., Zachariou A.: Hydrogen based emergency back-up system for telecommunication applications, J. Power Sources, 118, (2003), 14–22
Authors: mgr inż. Marek Malinowski, E-mail: m.mal@iel.wroc.pl, dr inż. Jacek Chmielowiec, E-mail: chmielna@iel.wroc.pl, dr inż. Grzegorz Paściak, E-mail: g.pasciak@iel.wroc.pl, mgr inż. Tymoteusz Świeboda, E-mail: t.swieboda@iel.wroc.pl, Electrotechnical Institute, Renewable Energy Sources Section, M. Sklodowskiej-Curie 55/61, 50-369 Wroclaw, Poland.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 8/2013
Published by Lorenzo Mari, Master of Science degree in Electric Power Engineering, Rensselaer Polytechnic Institute (RPI), EE Power – Technical Articles: Grounding for Noise Reduction in Electronic Systems, April 06, 2021.
Learn about everyday grounding systems to reduce common-mode noise.
Grounding is the primary method of reducing noise pickup. A good grounding and bonding design can solve a considerable percentage of noise problems.
Isolated Ground (IG) Systems
When disturbances like EMI, RFI, or electrical impulses caused by welders, variable speed drives, appliances, and others, are present in the grounding system, they create common-mode noise between neutral and ground that may affect electronic equipment.
The National Electrical Code (NEC) permits the installation of isolated ground receptacles (IG), i.e., no connection between the yoke and the grounding terminal. Yoke is the metal frame behind the receptacle that is used to fix the device to the outlet box.
An IG system’s ideal objective is to provide low neutral-to-ground voltages in the electronic equipment’s AC input This prevents the noise from passing through the logic circuitry and corrupting the processed data.
When properly installed, the neutral to the isolated ground voltage (noise) should be lower than from neutral to the metallic conduit.
But there is nothing isolated in this grounding arrangement. The isolated equipment grounding conductor (EGC) provides a low impedance path for the ground-fault current flow between the receptacle grounding terminal and the neutral in the service equipment or the secondary of a separately derived system (e.g., an isolation transformer). The isolated EGC, and not the conduit, is the safety ground for the electronic equipment – the only route for the fault current to return to the source. Green insulation with a longitudinal yellow stripe identifies the isolated EGC.
The receptacle’s metal frame must be grounded. The receptacle’s frame has separate ground connections bonded to the general ground system through the metallic conduit, an insulated – the green wire – or bare equipment grounding conductor run with the circuit conductors, or another wiring method that serves as an EGC.
The NEC allows the isolated EGC to pass through several subpanels without connection to the grounding bus. As a practical matter, the isolated EGC may terminate at the subpanel where the noise attains an acceptable level.
Figure 1 shows an isolated ground system supplying data processing (DP) electronic equipment.
Figure 1. IG system supplying DP equipment.
The metal raceway is grounded, employing a connection to the service equipment enclosure. The isolated EGC, connected to the service equipment’s neutral, passes through the downstream panelboard and terminates in the electronic equipment cabinet. The NEC permits the equipment cabinet’s isolation from the raceway containing the supply conductors, using nonmetallic raceway fittings.
When the electronic equipment manufacturer specifies an isolated ground for the equipment, they usually do not provide an isolated ground terminal for that purpose. There is only one ground terminal attached to the AC power supply, chassis, cabinet, and zero reference. In this case, the safety ground – green or bare wire – should not be connected to that ground terminal. The equipment cabinet should be isolated from the ground and other metals in contact with the safety ground to keep the isolated EGC as the only path for ground-fault current.
Power to electronic equipment should not share a common branch circuit supplying noise-generating devices. For example, feeding a copier – with motors and heaters – from the same circuit will inject high-frequency noise into the grounding system, disrupting the electronic equipment operation. If the copier requires an IG system because it has microprocessor-based circuitry, it should be connected to separate branches. Not to mention powering coffee makers, clocks, radios, vacuum cleaners, electric drills, and other noise-generating devices.
An important fact is that IGs do not always enhance the equipment’s performance.
Shielded Isolation Transformers
Sometimes there is a long separation between the main service equipment and the electronic equipment, resulting in a long EGC between the electronic equipment and the connection to the ground in the power source. The long wire will have a relatively high impedance, reducing the fault current – needed to open circuit breakers and melt fuses – and increasing the time for fuses and circuit breakers to clear faults. Also, recalling that noise currents circulate through the EGC, the augmented impedance will develop larger noise voltages.
One way of shortening the EGC and reducing the above effects is by installing a shielded isolation transformer near the electronic equipment and its panelboard. The shielded isolation transformer has excellent insulation between its primary and secondary windings, taking the main service equipment grounded neutral out of the picture and restoring the ground at the secondary winding.
Bonding the EGC to the new, closer ground will make a better return path for fault currents and reduce common-mode noise.
Figure 2 shows a diagram of a shielded isolation transformer.
Figure 2. Shielded isolation transformer for electronic equipment.
A Practical Arrangement to Reduce Common-mode Noise
Figure 3 shows a practical arrangement to supply power to plug in electronic equipment, like a set of personal computers and peripherals, using the IG principle. This arrangement should provide an acceptable common-mode noise rejection level.
Figure 3. Grounding using an IG receptacle.
The power source is a shielded isolation transformer close to the panelboard reducing the wire length and noise. The transformer’s shield rejects a lot of high-frequency noise.
According to the NEC, the isolation transformer qualifies as a separately derived system – the neutral is not carried through from the input to the output – requiring a neutral-to-ground bond at the low voltage side. This bond provides a zero-volt reference to the electronic equipment.
The isolated EGC runs from the isolated-ground-type receptacles – powering the electronic equipment – directly to the isolation transformer’s neutral-to-ground bond through the conduit. The EGC may pass through subpanels without connection to the equipment metal frame grounding bus. Bond the metal conduit carrying the isolated EGC to the enclosures at both ends. The NEC requires all wires originating at the secondary winding – phase, neutral, isolated EGC, and green wire when used – to run in the same conduit.
This arrangement reduces troubles when there is a connection between the peripheral and the computers’ grounding system through the data wire’s shield.
Figure 4. Typical SRG using the raised floor.
The room’s cabinets may be grounded to a single point, typically the grounding bar in the AC panel supplying power to the cabinets. The AC grounding bar is the single point ground for the power wiring and the cabinets.
An excellent option is placing the power source inside the room – e.g., an isolation transformer – grounded within or at the room’s periphery. Computer power centers are complete assemblies to supply branch circuits to data processing equipment, with control, monitoring, and alarm functions.
Short leads connect the isolation transformer’s secondary neutral, all signal grounds, and cabinets to the signal reference grid.
Figure 5 shows typical ground connections in a data processing room.
Figure 5. DP equipment connected to AC power source ground and SRG.
An Overview of Grounding for Noise Reduction in Electronic Systems
The NEC allows an isolated equipment grounding conductor to provide a noise-free, zero-volt reference in electronic systems having microprocessors.
The isolated EGC runs from the neutral/ground junction point (main bonding jumper) on the service equipment, or a separately derived source, to the electronic equipment or isolated receptacles.
The isolated EGC – green wire with yellow stripes – must be installed with the phase, neutral, and safety ground – green wire – conductors in the same conduit and can pass through subpanels without connection to the grounding bus. Any of the approved wiring methods must ground the IG receptacle’s metal enclosure.
Advantageously, an electronic equipment room receives power from a dedicated transformer, like an isolation transformer. The isolation transformer may be shielded and is useful for common-mode noise attenuation. Its primary function is to provide a separate energy source at the closest point to the electronic equipment and isolate it from other energy sources on the premises.
The transformer’s neutral-ground bond serves as a single point for all grounds in the electronic equipment room.
A signal reference grid supplies an equipotential plane for a broad frequency band, providing multiple paths between its parts. If one direction is high impedance due to resonance, other ways of different lengths provide a low impedance route with negligible potential differences between any two points on the grid. The result is a practical equipotential reference for signals from DC to the megahertz range.
About the Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.
Published by Martin SLIVKA, Radomír GOŇO, Stanislav RUSEK, VŠB-Technical University of Ostrava
Abstract. The paper deals with statistical analysis of data on faults and failures in electrical power distribution. We used the statistical analysis of both MV overhead and cable lines, and electrical power stations of one distribution company. The data were collected from 2000 to 2009 with focus on the duration of failure with respect to different types of equipment. To compare and analyze the data, we used confidence intervals and also statistical distribution of data-sets.
Streszczenie. W artykule analizowano dane statystyczne błędów i defektów w przesyle energii elektrycznej. Wykorzystano dane linii średniego napięcia kablowych i napowietrznych. Dane były zebrane w latach 2000 do 2009. (Analiza okresu trwania uszkodzeń w sieciach przesyłu energii)
Keywords: Mean failure time, confidence interval, exploratory data, distribution analysis. Słowa kluczowe: uszkodzenia, przesył energy elektrycznej, analiza statystyczna
Introduction
Monitoring the durations of failures is valuable because it gives information on how quickly a distribution company is able to repair a failure. These data can be used as a basis for the maintenance optimisation of distribution network equipment and for the breakdown service operation at distribution companies. Hereby stated data is anonymous and confidential and the selection criteria may not allow a wholly objective assessment. All input data are at the minimum of 3-minute duration [1]. The dataset comprises approximately 7200 values read from 1 January 2000 to 31 December 2009 [4]. The failure data comprises the date of event, its duration and the type of failed equipment. Mean failure time τ for individual months and years was calculated [3]:
.
where: N – number of failures of one equipment type, t – duration of failure (h).
Fig. 1 Box-and-whisker plot (whiskers representing duration in minutes)
Graphical Comparison of Data
The box-and-whisker plot represents the distribution of variables in different datasets. The minimum, the lower and upper quartiles, and the means are not too far from each other; however, the maximum is the furthest. Most failures have low duration as Fig. 1 illustrates.
The bar chart in Fig. 2 represents the trend in failure duration within 2000-2009. Concerning overhead lines, it shows considerable decrease in the failure duration, similarly to the electrical power stations.
Fig. 2 Comparison of different datasets – failure duration
Exploratory Data Analysis (EDA)
First, outliers were excluded by means of z-coordinate. In this case, outliers are those with absolute value of z-coordinate greater than 3. After eliminating outliers, exploratory data analysis was conducted [2].
.
Table 1 – Descriptive data statistics
.
Fig. 3 Histogram of failure duration – cables
Fig. 4 Histogram of failure duration – overhead lines
Fig. 5 Histogram of failure duration – electrical power stations
The EDA shows that all data have pointed distribution. Skewness reflects asymmetry in the distribution of values surrounding the mean – evidently the values above the mean prevail. The mode of electrical power stations is 71 minutes, while cables and overhead lines have 3 minutes. The median of cables is distinctively higher, probably due to the demanding character of repairs.
Frequency histogram
Histograms graphically represent the frequency of occurrence of assessed quantity, in our case failure duration with respect to selected classification. The number of classes is given by the Sturges’ rule. The bar chart depicts the rate in different classes. The chart shows that the failures with the shortest interval are proportionally prevalent. In longer duration the chart shows a steep drop in the rate. The equipment of electrical power stations shows the highest percentage with 92 % value of the first class. The line chart depicts proportional distribution of cumulative frequencies which correspond with proportional data distribution from the shortest failure duration up to a given class. These histograms were devised in MS Office 2010. Figures 3, 4, and 5 illustrate failure duration rate distribution for different datasets.
Distribution Analysis
Distribution analysis of several samples tests hypothesis (H0) which assumes the same original set of the basic probability distribution in comparison with alternative hypothesis (HA) which assumes inequality of mean values of samples (HA: does not hold H0). Distribution analysis can be conducted in the ANOVA table, or with Kruskal-Wallis one-way analysis of variance. The ANOVA table assumes normality of analyzed data. When this assumption is not supported, Kruskal-Wallis one-way analysis of variance can be used, however, at the cost of lower sensitivity compared to the ANOVA table.
Table 2 – Chi-Square a Kolmogorov-Smirnov test results
.
Normality Testing
Out of many methods there are to be used for testing normality, we used chi-square goodness-of-fit and Kolmogorov-Smirnov tests for reasons of good availability in STAGRAPHICS Plus 5.0.
Chi-square test tests the number of frequencies in selected classes from analyzed data. It compares them with the number of frequencies that would occur in case of selected distribution. Only the test results for cable are shown, as the results of the other datasets were similar – also not with the character of normal distribution.
To illustrate normal distribution, we used a Q-Q plot Fig. 6. The blue line in the plot shows congruity of the empirical and the theoretical distribution functions, the latter of which originates in the normal distribution. The empirical distribution function from the distribution we analyzed is represented by the points. The points are not aligned with the blue line, but they more or less deflect from it. The analyzed data do not have the distribution function of normal distribution, therefore they do not originate from it.
Fig. 6 Q-Q Graph
Table 3 – Kruskal-Wallis test results
.
Distribution Analysis
The abnormality of data does not allow distribution analysis by means of table ANOVA, therefore we used Kruskal-Wallis test. The test compares medians of individual samples and tests the null hypothesis that the medians are equal.
The Kruskal-Wallis test
The P-value in the test is zero and therefore there is 95% certainty that the datasets are statistically different. As every dataset is specific in its own way, intuitively we can assume the correctness of such conclusion.
To analyse in more detail differences in datasets we conducted the post – hoc analysis. As the data distribution is abnormal, the Tukey HSD test which is used to find means that are significantly different from each other was used. The test was conducted for 99% confidence interval of data reliability.
Fig. 7 Mean values and intervals of 99% reliability
It is evident that these three independent homogeneous groups have completely different character. The values from the Fig. 7 are in the Tab. 4.
Table 4 – Table of mean values form Tukey HSD test and confidence intervals with 99% reliability
.
Mean failure time
Mean failure time is a significant value that speaks for the condition of given equipment and the demands for its repair. It can be useful to know what the range of interval of mean failure time is.
Table 5 – Confidence intervals for selected reliability intervals
.
Conclusion
The paper deals with statistical representation of reliability data. Altogether, 7186 were analyzed in the monitored period of time. The variables have pointed and abnormal distribution with the prevalence of values above average. The analyzed variables are statistically different, which confirms the intuitive assumption. The mean failure times and reliability confidence intervals are to be found in Tab. 5. It is clear that the distribution of more accurate reliability intervals is closer to the mean value.
This work was supported by the Czech Science Foundation (No. 102/09/1842), by the Grant of SGS VŠB – Technical University of Ostrava (No. SP2013/137) and by the project ENET (No. CZ.1.05/2.1.00/03.0069).
REFERENCES
[1] Provozovatelé d i s t r i bučních soustav : PPDS, Příloha 2: Metodika určování nepřetržitosti distribuce elektřiny a spolehlivosti prvků distribučních sítí. (2011) [2] Briš R., L i tschmannová M., Statistika I., Sylaby k předmětu VŠB-TU Ostrava (2004) [3] Martínek, Z., Královacová, V., The Solution for Re-pairable Units. Proceedings of the 11th international Scientific Conference Electric Power Engineering 2010, University of Technology Brno (2010) [4] Krejčí, P., Santarius, P., Hájovský, R., Velička, R., Čumpelík, R., PQ Monitoring in Selected Networks of Czech Republic. Przeglad Elektrotechniczny, vol. 88, nr. 7b/2012, 183-185
Authors: Ing. Martin Slivka, VŠB–TU Ostrava, Department of Electrical Power Engineering, 17. listopadu 15, Ostrava, martin.slivka.st1@vsb.cz; doc. Ing. Radomír Goňo, Ph.D., radomir.gono@vsb.cz; prof. Ing. Stanislav Rusek, CSc, stanislav.rusek @vsb.cz
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 11/2013
Power quality monitoring doesn’t often become an issue until incidents such as system malfunctions, equipment failures, process interruptions, data loss, IT disruptions or even power failure have occurred. As a rule, incidents of this sort are very time-consuming because they cause can’t always pinpointed right away. Beyond this, failures are always associated with costs which could have been avoided.
Power quality is defined in EN 50160, which describes the characteristics of the voltage in electrical power supply networks. However, edition 3 of IEC 61000-4-30 specifies the degree of accuracy required for the measurement of the quality of electrical networks. The standard differentiates amongst different device classes. The measured values obtained from different devices manufactured by various suppliers are rendered comparable in the case of a class A (A = advanced), and class A devices are always used when accurate measurements are required. By means of this standard, reliable, reproducible and comparable results are obtained which can be used for billing purposes.
The following measurements are standardized by edition 3 of IEC 61000-4-30, and are mandatory for class A devices:
• Power frequency • Magnitude of the supply voltage • Voltage unbalance • Voltage dips/swells/interruptions • Rapid voltage change • Harmonics/interharmonics, THD • Flicker • Mains signaling on the supply voltage
The Difference Between Class A and Class S
Whereas measuring accuracy is very high in the case of class A devices, measuring accuracy requirements specified for class S devices (S = survey) are much lower – data and events are only logged qualitatively and fewer demands are placed upon measuring accuracy. Furthermore, class S devices don’t have to measure as many quantities. The following measurements are mandatory:
• Power frequency • Magnitude of the supply voltage • Voltage unbalance • Voltage dips/swells/interruptions • Rapid voltage changes
Overview Table – Class A versus Class S
.
Normative Power Quality Monitoring: Voltage Events
Published by Burak AKIN, Yıldız Technical University
Abstract. In this paper, efficient PV to PEV’s li–ion battery power transfer is investigated. Dual-interleaved boost topology is applied to the PV to PEV direct power transfer system as DC-DC converter. Proposed dual interleaved boost topology is reached 97 % total efficiency with inversely coupled input boost inductances.
Streszczenie. W artykule opisano metodę transferu energii zed źródła fotowoltaicznego do baterii pojazdu elektrycznego. Zastosowano przekształtnik DC-DC. Osiągnięto 97% sprawności. (Transmisja energii zew źródła fotowoltaicznego do pojazdu elektrycznego)
Keywords: PV – photovoltaic , PEV – plug-in electric vehicle Słowa kluczowe: bateria słoneczna, pojazd elektryczny
Introduction
It can be difficult to find out grid connected energy, or it is expensive due to first initial costs in rural areas. So, solar power can be a solution as a renewable energy source. For this reason, photovoltaic cell can be used for energy producing from the sun. PV modules can be produced for solar farms by using PV cells. Although PV modules are expensive and relatively low efficient, in near future with the developing technologies it is predicted that the price will be lower and efficiency will be higher. In this paper reliable and high efficient power transfer is investigated from solar power used PV modules to PEV systems.
PEV systems normally demand the related energy from the grid connected energy sources by AC-DC converter. Because the PEV uses high DC voltage li-ion batteries, it is important to use boost converter topology to produce high DC voltage from AC grid with power factor correction (PFC) circuits [1, 2]. However, PV to PEV power transfer has the advantages of direct DC-DC conversion system with boost converter topology without PFC, THDi, reactive power and AC grid interface concerns. Before the efficient power transfer to the PEV system, maximum available power should be consumed from the PV solar energy system.
Solar energy system is renewable but for a limited time effective power source. So, to get high efficiency from the PV to PEV system first maximum power has to be demanded from the PV modules. Maximum power production is possible by controlling the PV modules with maximum power point tracking (MPPT) control systems [3, 4, 5]. This kind of control of the modules can produce the maximum available power instantly. Every switching cycle, efficient DC-DC power transfer charges the li-ion batteries.
MPP is the maximum power of current and voltage rates at one point. MPPT control gets the benefits of the maximum power while daylight condition rapidly changing. There are many cheap and easy MPPT system developed in literature [3, 4, 5]. Some MPPT algorithms are such as perturb and observe, constant voltage, incremental conductance, short circuit pulse, open circuit voltage and temperature method [5].
In this paper the main concern is to get the highest produced power from the PV solar module and transferring it with the highest efficiency to the PEV li-ion battery in standalone solar power system for further energy demand. Also, system complexity and cost problem and dimensions are the other concerns. So after PV cell architecture, PEV li-ion battery should be investigated.
PEV li-ion battery has generally 400 V DC input voltage with approximately 30 kWh energy capacity, so the battery needs efficient and high power charge system. For this reason, CCM working dual interleaved boost converter is investigated. For the calculation, Ii and Vi input current and voltage, Pi and Po input and output power, ηmin predicted minimum efficiency, λ duty rate, ∆IL input current surge, fsw switching frequency, L boost inductance, Vo output voltage and Co output capacitor is represented respectively. The formulas from 1-7 are taken from [2].
.
For DC-DC power conversion, there is no need for input diode bridge. Furthermore the boost converter has the advantage of direct DC-DC distribution system without PFC, THDi, reactive power and AC grid interface concerns.
To decide which boost topology should be used in DC-DC power conversion, conventional, dual and interleaved boost topologies are investigated. Conventional boost has the advantage of simply control and few components with efficiency disadvantage. Dual boost topology has the advantage of high efficiency with high THDi and current stresses disadvantage. Interleaved boost has the advantage of high efficiency and low current stress with more components disadvantage.
The proposed converter has dual interleaved boost topology to get the benefits of the two converters. To increase the efficiency, inversely coupled inductances are added as boost inductances of the DC-DC dual interleaved boost converter. In Eq. 8 and 9, inversely coupled inductance voltage calculation is represented. In here, L1 and L2 are the main boost inductances and M is the mutual inductance. The proposed converter topology is shown in Fig. 1.
.
In the proposed topology, L1 and L2 are main boost inductances, S1 and S2 are the main power switches and D1 and D2 are the main power diodes. Solar module and li-ion battery are added as input and output power sources. Ci and Co are the input and output capacitors respectively, D3 and D4 are power switches reverse diodes.
In the proposed converter, because solar module has limited power source maximum efficiency of the system is important. And also, for economical, power density and dimension problems, the converter has to be simple and cheap design with high switching frequency. Dual interleaved boost topology has two branches with 180o phase delay. Steady state waveforms are shown in Fig. 2. Because solar module output voltage is small and li-ion battery input voltage is high enough, duty ratio is generally bigger than 0.5. Efficiency can be improved by adding inversely coupled inductances mutual effect. Also, current stress is lowered by equally shared input power by two boost converter. This proposed converter can be easily adjusted for high power appliances by adding more boost branches and coupled inductances to the system. In this situation control strategy is important for efficient power transfer.
Fig.2. Steady state waveforms of dual interleaved converter
Control strategy
Generally dual interleaved boost controller senses each boost inductances currents with input and output voltage to generate switching signals. However, proposed converter senses input current, input voltage and output voltage to calculate switching signals. Easy and simply control strategy is developed to control the power switches.
Input current and voltage control is important to use PV modules at maximum power point (MPP). In this paper constant voltage MPP is used to control PV modules at MPPT. At a constant input voltage of PV module, input current is controlled to convert maximum power to the PEV li-ion battery with DC-DC dual interleaved converter. Short circuit protection is added to the system to control PV module at MPP.
Output voltage is sensed with input voltage to calculate duty ratio for the dual interleaved boost converter. Also output voltage regulation is added to the control system to prevent excessive voltage to the PEV li-ion battery. To control the system, calculated duty ratio, input current short circuit protection and output voltage regulator are all generates the S1 switching signal. S2 is controlled with 180o phase delay to S1. However to prevent in rush currents to the output capacitor time delay is added to the control system. Also, for steady state conditions input capacitor Ci and output capacitor Co is added to the system with initial values. For high power density and also fast response of the system high switching frequency is important for the control.
Simulation results
Proposed dual interleaved boost topology POWERSIM circuit schema is shown in Fig. 3 with control circuit. Both interleaved branches working as parallel with 180o phase delay. Control circuit first senses input voltage, input current and output voltage then generates switching signals. First generated signal applied to the first branch power switch S1, afterwards second signal is applied to the second branch power switch S2 with 180o phase delay. As a result both branches shares input current with lower current stress on power switches. Vg1 and Vg2 gating signals is shown in Fig.4 Input current is applied to the inversely coupled inductance to increase the efficiency of the converter.
For steady state response, input capacitor Ci is set to 60V DC and output capacitor Co is set to 400 V DC initial values. All efficiency and other calculations are done according to the steady state condition.
Stand alone solar power system is designed for 3 kW power from beginning to the end resistive load. So, for PV module MPP is set to 60V and 50 A which is for 3 kW maximum power, dual interleaved boost converter is designed for 3 kW output resistive load with 53,34Ω , input and output capacitors are set to 1μ for each watt so 3000μF for total power. PEV li-ion battery has 400V input voltage approximately 30 kWh energy, which is enough at least one week for 3 kWh energy consumption a day.
For better performance, silicone carbide (SIC) semiconductors are used with real specific values in the simulation. S1 and S2 IGBT (IXGH 60N60B2) have lower than 1.8 V saturation voltage with 100 kHz switching capability. Power diodes D1-D2 and reverse power switch diodes D3-D4 (STPSC2006CW) have lower than 1.4 V saturation voltages with better reverse recovery performance.
Fig.4. Switching signals of the converter for 100 kHz
The proposed dual interleaved DC-DC boost converter has input current short circuit protection and output over voltage protection to provide safety regulations. PV module is controlled by constant voltage MPPT control, so input current short circuit protection is important for the PV system. Also, output over voltage protection is important for the PEV Li-ion battery. PV module output or DC-DC dual interleaved boost converter input current and voltage waveforms are shown in Fig 5 with average measured values.
Fig.5. PV module output current and voltage waveforms
Output voltage waveform is shown in Fig. 6 for PEV Li-ion battery with measured average values. In here, voltage regulation is 0.8% is calculated.
Fig.6. PEV Li-ion battery voltage and current waveforms.
Steady state waveforms of dual interleaved converter in Fig. 2 are also observed in Fig. 7 from the simulation results.
Fig.7. Steady state waveforms of dual interleaved converter POWERSIM simulation
DC-DC dual interleaved boost converter components S1, S2, L1, L2, D1 and D2 power losses are calculated and it is shown in Fig. 8. Dual interleaved boost converter output power is measured 2911 W and input power is measures 3000 W. As a result, maximum efficiency of the PV to PEV power transfer system is calculated and it is reached 97% value at full load. In Fig 9, efficiency of the converter is shown from 10% to 100% load condition.
Fig.8. S1, S2, L1, L2, D1 and D2 power losses
Fig.9. Efficiency of the converter from 10% to 100%
Conclusion
PV to PEV efficient power transfer is investigated in this paper with dual interleaved boost converter. Inversely coupled inductances are added as input boost inductance. Each power switch work with 180o phase delay and shares input current with lower stress. Input current, voltage and output voltage are sensed to generate power switch’s gating signals. High performance DC-DC dual interleaved boost converter is applied to the PV to PEV energy transfer system. The proposed converter has 97% efficiency at full load of 3 kW power at 100 kHz switching frequency. Stand alone PV modules can transfer the maximum energy efficiently to PEV systems by proposed converter. This system can be easily improved to upper power levels. After, end user can use PEV li-ion battery as a power source for a house in rural areas.
REFERENCES
[1] Beltrame, F.; Roggia, L.; Schuch, L.; Pinheiro, J.R.; ,“A comparison of high power single-phase power factor correction pre-regulators” 2010 IEEE International Conference on Industrial Technology (ICIT), pp 625-629, May 2010 [2] Akın, B.; Bodur, H.; , “A New Single-Phase Soft-Switching Power Factor Correction Converter,” Power Electronics, IEEE Transactions on , vol.26, no.2, pp.436-443, Feb. 2011 [3] Jaw-Kuen Shiau; Der-Ming Ma; Pin-Ying Yang; Geng-Feng Wang; Jhij Hua Gong; “Design of a Solar Power Management System for an Experimental UAV,” Aerospace and Electronic Systems, IEEE Transactions on , vol.45, no.4, pp.1350-1360, Oct. 2009 [4] Pastre, M.; Krummenacher, F.; Kazanc, O.; Pour, N.K.; Pace, C.; Rigert, S.; Kayal, M.; , “A solar battery charger with maximum power point tracking,” Electronics, Circuits and Systems (ICECS), 2011 18th IEEE International Conference on , vol., no., pp.394-397, 11-14 Dec. 2011 [5] Faranda R., Leva S., “Energy comparison of MPPT techniques for PV Systems”, WSEAS TRANSACTIONS on POWER SYSTEMS, pp 446-455, Issue 6, Volume 3, June 2008
The correspondence e-mail: bakin@yildiz.edu.tr
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 9/2013
Power Quality Blog 