Overview of microgrid technologies



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Proceedings of 2015 IEEEInternational Conference on Mechatronics and Automation August 2 - 5, Beijing, China An Overview on Microgrid Technology Xuesong Zhou, Tie Guo and Youjie Ma Key Research Laboratory for Control Theory & Applications in Complicated Systems Tianjin University of Technology 391, Binshui Xidao, Xiqing District, Tianjin, 300384, China [email protected] energy, and convert these clean energy into electrical energy, which can not only increase the diversity of energy use, but is also an inevitable choice to solve the problems of increasing depletion of fossil energy, serious environmental pollution and other issues. The main features of traditional power system is that the whole grid are linked together into a large grid, the most significant advantages of centralized power grid is the ability to fully enhance the efficiency of energy use, while there are some drawbacks in centralized power grid: high costs, operational difficulties, it is difficult to meet users’ increasing requirements for safety and reliability [1]. Especially in recent years, after several series of large area blackout accident, the vulnerability of power grid has fully been exposed. Therefore, people began to look for other ways for the development model of the power system. After the blackout in North America in 2003, the experts have come to the conclusion-it is much easier and faster to develop distributed power than reform the power grid to enhance security. Distributed generation is a technology that use a variety of dispersed energy to generate power, including renewable energy and local fossil fuels which can be easily got [2]. Distributed generation has the advantages of less pollution, higher energy efficiency, flexible installation location, and compared with centralized power generation, it can save transmission and distribution resources and operating costs, and reduce line losses of concentrated transmission. Distributed generation can reduce the total capacity of power grid, improve peak and valley performance and the reliability of power supply. It is a strong complement and effective support of power grid [3]. In the past 30 years, most countries have put on the agenda of distributed generation, people began to carefully study the potential benefits of distributed generation system. Undoubtedly, distributed generation is one of the development trend of power system. With the penetration of distributed generation increasing, its inherent problems are apparent. It has high costs of single access and control difficulties. On the one hand, distributed generator is an uncontrollable source, so large system tends to take ways of restriction, isolation to deal with it, in order to reduce the impact on the power grid. On the other hand, the distribution system has some characteristics, which make distributed generation have to connect and operate in the form of load, this results in the structure of the distributed generation would be extremely limited [4]. Abstract - Microgrid technology can effectively integrate the advantages of distributed generation, and also provide a new technical way for large scale application of grid-connected generation of new energy and renewable energy. Microgrid can not only enhance the efficiency of energy cascade utilization, but also be used as an effective complementary of power grid and improve the reliability of power supply and power quality. It is one of the latest cutting-edge research topics in the field of electrical engineering at home and abroad. This paper firstly elaborates the background and the basic concept of microgrid, then describes the current domestic and international situation of microgrid research, finally the key problems and development prospects of microgrid technology are discussed. Index Terms - distributed generation, microgrid, reliability, stability I. INTRODUCTION As the energy crisis and environmental issues have become increasingly prominent, it has become the general consensus of human society to develop low carbon economy and construct ecological civilization to achieve sustainable development. The development of clean and renewable energy has become an important strategy of economic and social sustainable development for all the countries in the world. In order to coordinate the contradiction between grid and distributed generation, and maximize the advantages of distributed generation in economy, energy and environment, the scholars put forward the concept of microgrid. Microgrid combines distributed power, load, energy storage devices and control devices, forming a single and controllable power supply system. It can reduce the feeder loss, and increase the reliability of local power supply and improve energy efficiency. The United States, Europe, Japan and many other countries combined with their reality and put forward the concept of microgrid and actively carried out relevant research. So far, microgrid has achieved fruitful results in theory and application, and gradually develops in the intelligent direction. It has become an integral part of smart distribution grid and an important part of smart grid construction. II. THE PROPASAL OF MICROGRID In recent years, environmental pollution, air quality and other issues have become increasingly prominent, which have a great relationship with the extensive use of fossil fuels. Coal, oil are non-renewable resources and will be used up by mankind one day. So, to develop and use renewable new clean 978-1-4799-7098-8/15/$31.00 ©2015 IEEE 76 the two parts are backup of each other. etc. such as sensitive. while the main grid can also provide power for microgrid. Figure 1 shows the photovoltaic generation. and provides flexible. The interface between distributed power and power grid usually uses power electronic devices which can flexibly control active power. it is very urgent and significant to develop microgrid technology rapidly. such as the US. which uses a lot of modern power technology. have carried out research on microgrid. For large power grids. this mproves the reliability of power grid. and improve the reliability of power supply to meet users’ various demand for electricity. microgrid can choose different operation modes.1 Basic structure of microgrid As a useful complement to the single power grid. Microgrid has some characteristics as follows: 1) Uniqueness: Microgrid is a small system consists of microsource and load. etc. And there are many types of loads. 3) Controllability: According to operating conditions. non-sensitive. microturbines and other forms of micro power. THE MEAINING OF MICROGRID According to the current situation of the development of power system and the technical characteristics of the microgrid. wind power. Microgrid and large grid exchange energy by PCC (Point of Common Coupling). 1) The flexibility and dispersion characteristics of the microgrid make it possible to guarantee the power supply of important load in case of large area blackout caused by accident. microgrid has great potential for wide application. At present. 4) Interactivity: As an independent generation equipment microgrid can provide strong support for the main grid when necessary. energy saving and comprehensive energy for end users. 2) Diversity: The composition of microgrid is diverse. microgrid can meet their specific needs. reducing feeder loss and maintaining stability of local voltage. controlled. independent and decentralized system. thus contributing to economic development of these areas. 4) Microgrid can use distributed energy and solve the difficult problem of power supply in remote areas where it is unsuitable to build power grid. some of the major countries of the world. For users. non-controlled type. energy storage devices together. microgrid can be regarded as a controllable power unit. Japan and China. wind. fuel cells. so microgrid is economical. solar power.With the application of new technologies. Distributed energy is set according to need and cooperates with users as much as possible. At the same time. and puts gas turbines. 3) Microgrid can provide excellent power quality. thus improving the utilization of energy. Figure 2 shows it mainly consists of loads and small micro sources based on power electronic technology and the capacity of 77 . it first proposed the concept of microgrid in the white paper called microgrid concept written for the DOE and the California Energy Commission [6]. the main difference with large grid is its flexible schedulability. microgrid includes energy storage devices. and is directly connected to user side. reactive power output and voltage output of generation. EU. especially the development of power electronic interface and modern control theory. A. 5) Independence: Microgrid can operate independently under certain conditions and guarantee the local demand for electricity to some extent. perfect control strategy makes the reliability improved and the security guaranteed. This reduces the losses caused by natural disasters and makes up for the weakness of large grids in terms of safety and stability. Microgrid is smaller. Microgrid Research of the US The CERTS of the US is the most famous institutions of microgrid research. there are both traditional power and renewable energy. III. MICROGRID RESEARCH STATUSES Fig. such as increasing local reliability. IV. this is a necessary condition for the stable operation of the system. fuel cells. thus improving reliability of power supply. the concept of microgrid emerged. There is no transmission and distribution losses and corresponding investment of transmission and distribution system caused by long distance transmission of energy. and improve its utilization rate and promote the development of the whole industry. 2) Microgrid can maximize the absorption of scattered micro sources. it can act within a few seconds in order to meet the needs of the external transmission and distribution grid [5]. Some of them are connected to the nearby thermal load and can provide heat for local users. the security of power supply. and introduced the technical implementation of the program in 2006 [7]. including 14 organizations from seven EU member states. Microgrid Research of EU Taking into account the own needs of electricity market.3 The Sendai microgrid 78 . Fig. the Portuguese EDP company and the Spanish LABEIN company have established pilot projects of microgrid in their respective countries and research on microgrid. and it was completed in 2006 [10]. protection and control strategies and economic benefits of microgrid. Currently. ABB. Over the years. EU mainly finances and promotes two microgrid projects: Microgrids and More Microgrids. the future development plans of microgrid are described in detail. and established three microgrid demonstration projects in Aomori. In addition. As the 2020 and subsequent power development goal of Europe. The “Mad River” microgrid undertaken by the northern part of the US power system is America's first microgrid demonstration project. etc. Japan carried out the research of microgrid. Then funded by the EU’s Sixth Framework Programme (2002 . but its development goals mainly located in the diversification of energy supply. accessible. the goal is to achieve technology and business access of a plurality of microgrids in the power market environment. the Italian CESI company. protection. the program points out that the future power grid should have the characteristics of flexible. Europe proposed a "Smart Grid" target in 2005. The NEDO started the distribution grid projects with renewable energy in 2003. the DOE has proposed a phased plan that is to place and use micro distributed generation systems in the form of microgrid in the development strategy of “Grid2030”. Japan has also set up the NEDO to coordinate domestic universities. C. Japan classifies independent power system supplied by traditional power as the category of microgrid research. control. and communication have been initially formed. the object of study developed to parallel operation of a plurality of microgrids.2 Schematic diagram of CERTS microgrid B. This concept has been successfully tested in the laboratory of Wisconsin University. Microgrid Research of Japan Based on the reality background of increasing domestic energy shortage and load. and explored the formulation of regulations and legislation on microgrid and became a successful example of microgrid projects in the United States. the Danish OESTKRAFT company. Germany. forming a large Fig.which is less than 500 kW. The EU’s Fifth Framework Programme (1998-2002) funded the pioneering research of microgrid. aiming to increase the penetration of micro generation devices by expanding and developing the concept of microgrid. reliable and economic. Some basic theories of operation. greatly expanding the definition of microgrid of the US CERTS. and introduces the control method based on power electronic technology. the NEDO also established a demonstration project of reliability services of power quality. Japan has established a number of microgrid projects at home [9]. the utilization of new energy has been the focus of development in Japan. research team which is led by Athens National University of Science and Technology. including Simens. experimental platforms of different scale have been established in Greece. and environmental protection. and other manufacturers and some power enterprises and research teams of EU member states.2006). reducing pollution and meeting users’ individual power needs. It examined the method of modeling and simulation. In Sendai. the research team was further expanded. security. enterprises and the State Key Laboratory to research New energy and its applications. At the same time. Based on this framework. Aichi and Kyoto Prefecture. and Spain [8]. Figure 3 shows the Sendai microgrid. manufacturers and power company from Europe. and did the steady-state and dynamicstate analysis of system. 79 . “The research on integration and control technology of power supply system with distributed pluripotent complementary energy microgrid” is researched by HeFei University of Technology and Canada New Brunswick University together. there is a big difference in the response characteristics of microgrid disturbance in various energy storage systems. Figure 4 shows microgrid has many operation modes. and other institutions have begun to research on microgrid. The Interactive Mechanism of Microgrid and Power System with High Permeability.D. Chinese Academy of Sciences Institute of Electrical Engineering. and supply power for important load. and keep the stability of voltage and frequency [13]. the interaction between microgrid and distribution grid system will be very complicated. The Distributed Energy Storage Mechanism for Safe and Stable Operation of The Microgrid.4 Operation mode switch of microgrid A. It built a small power grid by using solar and wind power to generate electricity. It is necessary to fully understand the impact of distributed storage system on the operation characteristics of microgrid. They made a lot of researches and experiments on the control method of distributed generation and seamless switching of microgrid. In microgrid some distributed power. and can keep a building lighting during power outage. there is still a big gap between China and them in the strength of research and the results achieved. Tsinghua University began to explore and research the field of microgrid. Currently. Hefei University of Technology. belongs to intermittent power. Taking into account the diversity of distributed energy storage system. Tsinghua University. China has made some progress in the research on Microgrid. THE KEY TECHNOLOGIES AND PROBLEMS OF MICROGRID Microgrid integrates the input of a variety of energy. the US and Japan. It serves to stabilize the system disturbance and maintain the dynamic balance of generation and load. the power can flow in both directions. Huazhong University of Science and Technology. and is a complex system with mutual coupling of chemistry. The 2007 International Science and Technology Cooperation Program of Anhui province. Tianjin University. The photovoltaic engineering research center of HeFei University of Technology as the Chinese executive unit established an exemplary independent microgrid system on campus. the action mechanism of the security and stability of the microgrid will be very different. distributed energy storage supports independent and stable operation of the microgrid and becomes an important part of it. microgrid can be connected with distribution grid by grid-connected control and switch to grid-connected mode of operation again. After the elimination of grid failure. When the microgrid operates independently. and control and management strategies of autonomous control and operation of microgrid. during the grid-connected mode. there is also a certain randomness in the changes of various types of load in microgrid. thermodynamics. But compared with the large research teams composed of research institutions. At present. Xi'an Jiao Tong University and other units are involved in the research. Tianjin University undertook the National Key Basic Research and Development Program ("973" Program)-basic research related to distributed generation function system. microgrid can be separated from power grid and change to the islanded mode of C. it used the hardware of State Key Lab of Tsinghua University Department of electrical power. energy storage devices and load. The Planning Theory and Methods of Microgrid. Microgrid will have an important impact on the operation characteristics of distribution grid system. The development of microgrid faces challenges in many aspects. electrodynamics and other acts. B. and the analysis of the impact should be based on a new method [12]. When power grid fails. and built a experimental platform of microgrid with renewable energy generation. microgrid technology was included in “863 plan in the field of advanced energy technology thematic topics in 2007” of the Ministry of science and technology [11]. When a large number of distributed energy supply system access to distribution grid system in the form of microgrid. V. Fig. operation by protection action and islanding control. It is necessary to further carry out the key technologies of microgrid and promote the application of microgrid in China. such as photovoltaic cells and wind power. Microgrid Research of China The research on microgrid in China is still in its infancy. The Chinese Academy of Sciences built a 200kVA microgrid test system. the output of a variety of products and a variety of energy conversion units. In 2006. Power energy generated by them has obvious characteristics of randomness and uncertainty. They proposed steady-state and dynamic-state calculation method. Because microgrid can not only operate during gridconnected mode but also during islanded mode. during the grid-connected operation. is vigorously developed by American and European countries with its characteristics of high reliability. etc. In the conditions of the system operating constraint. new protection technology needs to be explored. THE DEVELOPMENT TREND OF MICROGRID F. the models are complex and the transient response time scales of different devices are dispersed. These changes brought by microgrid access to distribution system make the work principle of protection and action logic complex. “973” and other national key research development plans also started to set up projects in order to encourage and support the research on microgrid technology in various universities and research institutes. and is simultaneously competitive in economy. the harmonic level of distribution system will also rise. The Economic Operation and Energy Optimal Management of Microgrid. etc. and flexibility. the theoried and related technologies of microgrid will be verified through 80 . This is the key to the sustainable development of microgrid. In recent years. Although there are some simulation methods can be used for reference. because the physical facilities of microgrid are diverse. The access of microgrid with multiple distributed power and energy storage devices greatly changes the fault characteristics of the distribution system. microgrid technology. microgrid can achieve optimal operation and reasonable distribution of energy by controlling and regulating these variables. With respect to the connected distribution system. The Comprehensive Simulation and Analysis of Microgrid. there are electromagnetic transient process that changes fast in microseconds. the operating mode often needs to be switched. In short. At present. as a more cuttingedge research field. related devices may have different levels of harmonics according to different power electronics technologies adopted. and supply power to distribution system during the grid-connected mode. and makes changes of electrical variables more complex after fault. H. especially in the case of high permeability. Considering the photovoltaic and other renewable energy generation is more economic than conventional power.The purpose of construction microgrid is to achieve the effective management of distributed power. the gridconnected operation of microgrid needs to meet certain conditions of voltage and frequency. these factors should also be considered scientifically in the formulation process of energy optimization strategy. But different from conventional generators. The Protection of Microgrid and Distribution System with Microgrid. there may be some intermittent power supplies. electromechanical transient process that changes in milliseconds and slow dynamic process that changes in seconds [17]. the microgrid can be seen as a virtual generator with unique operating characteristics. VI. are likely to bring problems of power quality to the users access to the system [16]. microgrid needs some special control modes to make it possible to meet the conditions of grid-connected operation [15]. Similar to the gridconnected operation of conventional generator. In addition. environmental protection. the manipulated variables of microgrid are more abundant. E. the economy of the microgrid is particularly important for users. the digital and analog hybrid simulation technology is a hot field of conventional power system research. due to many types of power supplies in the microgrid need to output power with the help of power electronic devices to meet the requirements of frequency and voltage of load. if you want to ensure that the microgrid gives full play to its advantages in energy supply. Meanwhile. and ensure the economic operation of the entire microgrid. there are both synchronous generators and other rotating equipments with a large time constant and fast response power electronic devices. Compared with the conventional way of power supply. G. such as the active power output of distributed power. microgrid must be scientifically planned to create conditions for the construction of microgrid and the economy of operation in terms of the power structure. the interface bus voltage of voltage source inverters and the interface current of current source inventers. D. and maximize the use of renewable energy. thereby affecting the economy of the whole life cycle of microgrid. Compared with conventional power system. Because the operating strategy of energy storage system directly affects its service life. as far as possible to use clean and renewable energy to meet the users’ demand for energy. The Analysis and Control of Power Quality of Microgrid and Distribution System with Microgrid. related theories and methods of hybrid simulation must be targeted to be developed. The Grid-Connected Control and Multi Distributed Power Coordinated Control of Microgrid. It will be very difficult to consider the interaction between them and realize the digital simulation of the dynamic process. this requires to make the necessary adjustments to the traditional method of protection and control in order to meet the requirements of system. due to the types and characteristics of distributed power in microgrid is different. In the next few years. and make it impossible to fail to accurately determine the location of the fault [14]. The traditional fault detection method will be severely affected. the frequent changes of start and stop operation and power output. In microgrid. network structure. the losses of distribution transformer and feeder in distribution system can be reduced by effectively controlling the output of microgrid [18]. In the event of system disturbances. 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