Algorithm for Management of Energy in the Microgrid Bus Kristjan Peterson Tallinn University of Technology (Estonia) kristjan.pt@mail.ee Abstract This paper presents an algorithm for energy management of the microgrid bus and a flowchart for realization of the algorithm. I. INTRODUCTION Main goals of microgrid development are increased energy consumption, need for higher reliability of electric power supply and reduction of environmental problems. A microgrid frequently consists of both AC and buses. A bus is useful if the microgrid contains power supplies (e.g., batteries or solar cells) or consumers (batteries, ultracapacitors, industrial applications). Energy management in an AC bus is described in [1]. Energy management of the microgrid bus is described in [2], but that energy management method cannot work together with a BMS (battery management system). The aim of this paper is to describe principles of the bus energy management and present an algorithm for control of an experimental microgrid. In the second section, most important terms that help to understand the following parts are described. Third section describes the topology of both AC and buses of a microgrid. The fourth section covers the energy management principles of the bus and the fifth section presents the bus energy control algorithm. II. IMPORTANT TERMS Commonly the word prosumer has two meanings: a union of words of a producer with a consumer or a professional consumer [3]. Producing consumer type of a prosumer either generates energy or consumes energy. A professional consumer is a well educated, skilled consumer who commonly makes smart purchasing or selling decisions using additional information [3], [4]. It is important to know that a prosumer is a person who controls the system but not a machine. An electric car is here also regarded as a prosumer, because energy can move not only from a grid (or from a microgrid) to an electric car, but also vice versa. Also, a microgrid is regarded here as a prosumer, because every microgrid consists of electric energy sources and can supply energy to the main grid. III. COMPOSITION OF THE MICROGRID Structural schema of a microgrid with AC and buses is presented in Fig 1. As depicted in the figure, the microgrid is comprised of a substation, AC bus and bus circuit s, an AC/, AC/AC, /, AC// s, and a control system. The substation of the microgrid is connected to different prosumers with a high voltage grid. As shown, the high voltage side of the substation includes a power transformer and a power switch. The static switch between substation bus bars and the AC bus is meant for automated synchronization and switching and is commonly realized with symistors. Static switches are installed in places where synchronous switching is important and where other devices (s) are not able to realize this function itself. Static switches are under control of the microgrid automated control system. Every consumer or prosumer is provided energy through a separate circuit. s are installed in places where synchronous switching is not necessary or where s before or after these circuit s are capable of autonomous synchronizing with the bus. s are realizing protection and control functions. The AC load(s), AC/ and AC// (s) are supplied energy from the AC bus. The AC// is bidirectional and with adjustable voltage level in the AC and side. The power transformer and the bidirectional AC/ are between the AC and bus. The power transformer before the AC/ is guaranteeing galvanic isolation between the AC and buses and helps to stop harmonics spreading in the microgrid (power transformer windings are connected to delta/star connection). Other AC/ s need to be bidirectional only if there is a prosumer after the. The bus is connected with the AC/ s through circuit s. prosumers or consumers are connected with the AC/ s through circuit s. The automated control system consists of a microcontroller (or microcontrollers, or a microcontroller with extension modules and server computer. Data from the AC/, the, from other AC/AC and AC// s and from current and voltage s is directed to the microcontroller(s) and is then stored in the server. and voltage s are needed in feeders where there is no or smart energy meter installed. A server computer is needed for data collection and for providing an interface to the microgrid owner (prosumer). 170
Substation of microgrid Substation bus bars Microgrid 24/0,4 kv Power switch Static switch Integrated AC& bus AC bus Automated control system Microgrid control devices and software (Computer, devices), microgrid subject (Ownwer of microgrid) AC// Electric car () AC/AC AC device Consumer or generator Static switch AC/ bus Citcuit () / (E.g. ultracapasitor) / (E.g. solar power station) Fig. 1. Structural schema of the microgrid (E. Pettai). 171
IV. GENERAL ENERGY MANAGEMENT PRINCIPLES IN AC PART OF THE MICROGRID The main principle of the energy management of the microgrid that is temporarily in an islanded mode is: if there is too much energy, then it should be shared with others (prosumers, consumers); if there is not enough energy, then it should be taken from others (prosumers); if others do not share, then consumption should be cut down [5]. In the islanded mode, when there is not enough energy in the microgrid, the microcontroller of the microgrid control system can switch off some consumers; take extra energy from the battery of the microgrid or from the battery of an electric vehicle (vehicle to grid mode (V2G)) if the electric vehicle BMS allows energy transfer in this direction. Energy management is realized by a microcontroller that is the main part of the automated control system of a microgrid. The microcontroller obtains information about circuit positions, static switch positions, current and energy flow directions of each feeder, active and reactive power, s working modes, microgrid battery and ultracapacitor state. If an ultracapacitor or a battery of the microgrid is charged, the microcontroller sends charging signals to the corresponding. If the sends a ready signal back to the microcontroller, the microcontroller closes the circuit after the. Information about battery the SOC (state of charge) is also transmitted from BMS to the microcontroller. If an electric vehicle with an empty battery is connected with the microgrid, it will communicate with the control system of the microgrid. The control system will adjust the AC//, followed by switching on the corresponding circuit after the and charging the battery of the vehicle. The control system also counts the consumed energy and sends the counted value to the server computer. The algorithm of connection is analogous when some other consumer connects to the microgrid. V. BUS ENERGY MANAGEMENT Energy management principles in the bus are: AC/ and / adjustable voltage level enables energy transport in both directions and realizes that sway energy management between prosumers and microgrid. Energy transfer from the bus to the AC side is not allowed if prosumers themselves need this energy or will be overloaded. Protection functions are the highest priority, feeders with short circuit or overload are switched off immediately. Protection functions are realized in the automated control system. Figure 2 shows a detailed view of the bus and its feeders. Each feeder is equipped with a circuit, a / and contactors before and after the / (Fig. 2). If some prosumer is demanding constant voltage, and the / is not able to measure voltage or is not able to send measured data to the automated control system, a voltage measuring is also installed in this prosumer feeder. There are three different current flow directions: from the AC/ to the prosumer, from the prosumer to the AC grid and through the bus from one prosumer to another (other) prosumer (prosumers). Based on these direction possibilities, a flowchart of energy management in the bus was composed (Fig. 3). In the flowchart AC power ok check means that the control system checks the sufficiency and quality of the AC power. During the battery charge time, the control system checks the open circuit voltage (OCV) of the battery, battery s state of charge (SOC) and battery charging current that must be smaller than maximum allowed current of a battery (Imax). In the flowchart in some check cases action directions are not shown in alternative answer cases. In these alternative answer cases further action will be interrupted and turned to the start position. When the automated control system receives a signal from the prosumer that this prosumer needs energy from the grid, the automated control system switches on to the circuit before the /, adjusts / voltage level according to the prosumer demand and after that switches the prosumer to the grid with the contactor after the /. If the circuit is in the off position, the automated control system sends the error message to the developer. Microgrid bus AC/ () AC (E. g. ultracapacitor) Producer (E. g. solar power cell) Fig. 2. Detailed structural schema of the bus of the microgrid. 172
Start (Microcontroller power on) Microcontroller self check. Send error signal voltage level voltage level YE Connections between microcontroller and other devices after / after / BMS to charge mode? of battery feeder after / AC power is All energy transfer directions are possible possible to transfer energy only from to AC side and between feeders Start kwh measurement Start kwh measurement OCV>min Slow charge after AC/ power Receive Determine Imax value Imax value Voltages and positions in side is OCV>CU Fast charge softstart (ultracapacitor) Data check ultracapacitor (battery) BMS data check battery Producer (solar cell) Data check solar cell producing energy? ultracapacitor YE SOC 100% required? Switch off circuit after / Constant voltage mode SOC target reached? Charge? Charge? Charging completed softstop energy need in AC side? Switch solar cell circuit off enough energy to charge ultracapacitor and battery? after / Adjust AC/ power and switch this over to boost mode Adjust / voltage Fig. 3. Flowchart of bus power management. 173
When the automated control system receives a signal from the prosumer that this prosumer is ready to transmit energy to the AC grid, the automated control system switches on to the contactor after the /, adjusts the / voltage level according to the data of the prosumer and the voltage level in the common bus. After voltage level adjusting the automated control system switches the prosumer with the contactor before the / to the grid. If one prosumer (or producer) is ready to feed another prosumer or other prosumers through a common bus, automated control system will switch to the feeding prosumer contactor after the / on, adjusts the - voltage level according to the data of the prosumer and the nominal voltage level in the common bus. The automated control system will After voltage level adjusting, switch on to feeding prosumer contactor before -. Also, the automated control system will switch on other(s) consuming prosumer(s) contactor(s) before the - (s) and adjust this (these) (s) voltage level according to the voltage level in the common bus, and after that it switches on contactor(s) after the - (s) to switch consuming prosumers to the common bus. VI. CONCLUSIONS Principles of energy management in the bus were described and the flowchart of bus energy management is presented. The principles described are implementable for prosumer energy management and are integrated with the BMS system, but current s are needed on each feeder and bidirectional / s with adjustable voltage level in the feeder side. VII. FUTURE STUDIES The algorithm provided here will be developed further and tested in practice under the experimental microgrid in Tallinn University of Technology. The experimental microgrid will enable studies of energy flow and communication movement during EV charging. The experimental microgrid is described in detail in papers [6], [7]. ACKWLEDGMENT This research has been supported by Estonian Ministry of Education and Research (Project SF0140016s11), Estonian Science Foundation (Grant ETF757) and European Social Fund (projects Doctoral School of Energy and Geotechnology II and VA431 Support of Research and Development in Energy Technology). REFERENCES [1] T. Korõtko, M. Mägi, K. Peterson, E. Pettai, R. Teemets, Analysis and Development of Protection and Control Functions for Li-Ion Based s Provided by Low Voltage Part of Distribution Substation (appear in print) [2] G. Iwansky, P. Staniak W. Koczara Power management in a Microgrid Supported by Energy Storage in Industrial Electronics(ISIE) Gdansk 2011 pp. 347-352. [3] M. Mägi, K.Peterson, and E.Pettai, Analysis of Protection and Control Functions of Low Voltage Part of Substation for Smart Grid Applications, in Proceedings of 8 th International Conference 2012 Electric Power Quality and Supply Reliability: 2012 Electric Power Quality and Supply Reliability Tartu 2012, 2012, IEEE, pp. 297-304. [4] G. Ritzer, N. Jurgenson, Production, Consumption, Prosumption: The Nature of Capitalism in the Age of the Digital, Journal of Consumer Culture (10:1), pp. 13-36, 2010. [5] K. Peterson Microgrid for TUT Power Engineering Building, Tallinn: 7th International IEEE Conference-Workshop Compatibility and Power Electronics, CPE 2011, pp54-57. [6] K. Peterson Development of Experimental Microgrid in Tallinn University of Technology, 12th International Symposium "Topical Problems in the Field of Electrical and Power Engineering, Kuressaare 2012, pp101-102. [7] K. Peterson Effectiveness Analysis of Microgrid Modules, Doctoral School of Energy and Geotechnology II, Pärnu 2012, pp146-148. 174