Chapter 1
Overview of microgrid
Abstract
Overview of the microgrid, describes the history, current status, and trends of microgrids.
Keywords
microgrid
DG
history
current status
prospect
A microgrid is a single, controllable, independent power system comprising distributed generation (DG), load, energy storage (ES), and control devices, in which DG and ES are directly connected to the user side in parallel. For the macrogrid, the microgrid can be deemed as a controlled cell; and for the user side, the microgrid can meet its unique demands, for example, less feeder loss and higher local reliability. Being capable of autonomous control, protection, and management, a microgrid can operate either in parallel with the main grid or in an intentional islanded mode.
A microgrid can be considered as a small electric power system that incorporates generation, transmission, and distribution, and can achieve power balance and optimal energy allocation over a given area, or as a virtual power source or load in the distribution network. Also, it can consist of one or more virtual power plants (VPPs) to meet the demand of a load center, which can be important offices, factories, or remote residences where the traditional way of electricity supply is expensive. Compared with traditional transmission and distribution (T&D) networks, a microgrid has a much more flexible structure.
1.1. History
In 2001, Professor R.H. Lasseter of the University of Wisconsin-Madison proposed the concept of the “microgrid.” Later, the Consortium for Electric Reliability Technology Solutions (CERTS) and the European Commission Project Micro-Grid also gave their definitions of a microgrid.
In 2002, the National Technical University of Athens (NTUA) built a small laboratory microgrid project known as the NTUA Power System Laboratory Facility for tests on the control of distributed resource (DR) and load with multiagent technology.
In 2003, the University of Wisconsin established a small laboratory microgrid (NREL Laboratory Microgrid) with a capacity of 80 kVA, for tests on the control of various types of DRs in different operation modes; another 480 V laboratory microgrid was established in the Walnut test site, Columbus, Ohio, for tests on the dynamic characteristics of various components of a microgrid.
In the same year, multiple demonstration projects were built across the world, including the 7.2 kV microgrid in Mad River Park, Vermont, USA; the 400 V microgrid in Kythnos Islands, Greece; as well as the Aichi, Kyotango, and Hachinohe projects in Japan.
In 2004, the CESI RICERCA test facility was built in Milan, Italy, which can be restructured into different topologies for steady-state and transient-operation tests and power quality analysis.
In 2005, the Imperial College London control and power research center was set up in London, UK, for distribution network prototype tests and load tests.
Over the same period, multiple demonstration projects were successively built all over the world, including Japan’s Sendai system (2004), Shimizu Microgrid (2005), and Tokyo Gas Microgrid (2006); Spain’s Labein Microgrid (2005); USA’s Sandia National Laboratories (2005) and Palmdale’s Clearwell Pumping Station (2006); and Germany’s Manheim Microgrid (2006).
Since 2006, the microgrid has been successively incorporated into China’s 863 Program (State High-Tech Development Plan) and 973 Program (National Basic Research Program). In 2006, Tsinghua University began studies on the microgrid and established a laboratory microgrid encompassing DG, ES, and loads utilizing the facilities in the National Key Laboratory on Power System and Generating Equipment Safety Control and Simulation under the Department of Electrical Engineering.
In 2008, Tianjin University and Hefei University of Technology conducted tests and studies on the microgrid. Tianjin University focused on scientific dispatch of various energy resources in the hope of improving energy efficiency, meeting various demands, and improving reliability, while Hefei University of Technology placed the focus on operation control and energy management.
In 2010, the State Grid Corporation of China (SGCC) built a demonstration project in Zhengzhou for study on operation control of a microgrid combining distributed PV (photovoltaic) generation and energy storage and engineering application and another in Xi’an for study on control technologies for microgrid combining distributed generation/energy storage.
In 2010, the China Southern Power Grid Company built a distributed energy supply – combined cooling and power (CCP) demonstration project in Foshan as a subject under China’s 863 Program.
1.2. Current situation of microgrid outside China and analysis
The world’s power sector has been facing great challenges like increasing loads, environmental issues, low energy efficiency, and users’ higher requirements on power quality. Microgrids can utilize and control DG in an effective, flexible, and smart manner, and hence, can best address these problems. Many countries are now carrying out studies on the microgrid and their own concepts and goals of a microgrid. As a new technology, the microgrid is showing distinct features in different countries.
1.2.1. USA
The United States is where the concept of “microgrid” originated, and its definition is the most authoritative among all others. The architecture proposed by CERTS consists of power electronic technologies-based micro sources with a capacity of 500 kW or below and loads, and integrates power electronic technologies-based control schemes. Power electronic technologies are indispensable to smart and flexible control and the basis for the “plug and play” and “peer to peer” control and design concepts. CERTS’s preliminary study results have been verified with the laboratory microgrid. The Department of Energy (DOE) took microgrid engineering seriously. In 2003, then US President Bush set the goal of grid modernization, that is, to widely integrate IT technologies and communication technologies into power systems to achieve grid smartness. In the later published “Grid 2030,” the DOE developed power system study and development plans for the coming decades, in which the microgrid is an important part. On the microgrid meeting convened in 2006, the DOE gave detailed accounts of its microgrid development plans. In view of grid modernization, improving reliability for critical loads, meeting various customized quality demands, minimizing the cost, and realizing smartness will be the focus of the United States’ future microgrid.
Figure 1.1 shows the microgrid model proposed by CERTS. This model shows that power electronics interfaces are provided for all micro sources, including PV, wind, small rotary machines, and various types of ESs. The core equipment is a smart static switch that controls the connection to and disconnection from the main grid. For each type of micro source, digital, smart relay protections are used to isolate the protected area from faults, and protection equipment is interconnected via special digital communication links.
Figure 1.1 Microgrid model proposed by CERTS.
1.2.2. Japan
Given the increasing energy shortage and load, Japan studied the microgrid concept with the aim of diversifying energy mix, reducing pollution, and meeting customized demands. In Japan, independent power systems based on traditional sources are also considered as a microgrid, which is a huge extension to the CERTS’s definition. On this basis, Japan has implemented multiple microgrid projects. In addition, Japanese scholars put forward the concept of Flexible Reliability and Intelligent Electrical Energy Delivery System (FRIENDS), that is, to add flexible AC transmission systems (FACTS) to the distribution network to make full use of their advantages in quick and flexible control, optimize the energy mix of the distribution network, and meet varying power quality demands. So far, FRIENDS has become an important form of deployment of microgrids in Japan, and some researchers are considering including the system in combined heat and power systems for better environmental friendliness and higher energy efficiency. Japan has been committed to using new energy for many years. It set up the New Energy & Industrial Technology Development Organization (NEDO) to coordinate studies and use of new energy among universities, companies, and national key laboratories.
1.2.3. European Union
Considering market demands, power supply security, and environmental protection, the European Union (EU) proposed the “Smart Power Networks” program in 2005, and released the strategies in 2006. It called for efficient and close synergy of centralized generation and DG by making full use of distributed energy resource (DER), smart technologies, and advanced power electronic technologies, and called upon all sectors to actively participate in the electricity market and work together to promote the development of grids. Microgrids will be a major part of the European electricity networks thanks to its smartness and diversified energy mix. Currently, theories on operation, control, protection, security, and communications have been established and verified with the laboratory microgrid. The future focus will be more advanced control strategies, standards, and demonstration projects to build the foundation for large-scale integration of DG and transition from the traditional grid to the smart grid. Figure 1.2 shows the microgrid model proposed by the EU with the efforts of ABB, Fraunhofer IWES, and SMA (Germany); ZIV (Spain); The University of Manchester (the UK); EMforce (Holland); and NTUA (Greece).
Figure 1.2 Microgrid model proposed by the EU.
CB, circuit breaker; SWB, switch board; G, micro source; L, load; MV, medium voltage; LV, low voltage.
CB, circuit breaker; SWB, switch board; G, micro source; L, load; MV, medium voltage; LV, low voltage.
The microgrid model shown in Figure 1.2 has a more complete structure, where not all micro sources have power electronics interfaces, all protection equipment is digital and smart, and interequipment communication is via controller area network (CAN). Centralized and decentralized monitoring is configured. In centralized monitoring, the central monitoring unit communicates with various switches, gives orders, and sets the switch action range. The monitoring mode is easy and cheap, but has the disadvantage that operation of all switches relies on the central monitoring unit, the failure of which will cause collapse of the entire protection system. A decentralized monitoring system is composed of multiple central monitoring units fulfilling different functions. When one unit fails, the others will automatically take over, thus avoiding system collapse. This mode offers high reliability but calls for more investment.
Apart from the United States, Japan, and the European Union, Canada, Australia, and some other countries have also carried out studies on the microgrid. From their grid strategies, studies on, and practices in microgrid technologies, it can be clearly seen that the development of the microgrid does not represent a revolution to traditional centralized, large-scale grids, but an improvement of the power sector’s consciousness of services, energy utilization, and environmental protection. The microgrid is an important means for efficient, environment friendly, and quality power supply by large grids in the future, and hence, a beneficial enhancement to the large grid.
1.3. Analysis of current status in China
China started its studies on the microgrid in 2006, later than other countries. Since this year, the microgrid has been incorporated into the national 863 Program and 973 Program:
863 Program subject in 2006: Distributed power supply system technologies, including technologies and equipment relating to integration, control, and protection of distributed power systems, microgrid technologies.
863 Program subject in 2007: Microgrid technologies, including the following:
1. Structure of microgrids interconnecting multiple energy sources, ESs, and loads and networking technologies;
2. Integration, control, and protection technologies;
3. Control technologies for connection to and disconnection from the grid, and for operation in islanded mode and grid-connected mode;
4. Related power electronics and control technologies;
5. Applicable advanced ESs and control technologies;
6. Internal and external power quality control technologies;
7. Technologies for energy exchange and coordination control between microgrid and macrogrid.
863 Program subject in 2008: distributed energy supply technologies, including the following:
1. Energy matching and control technologies;
2. Microgrid-dependent intermittent power sources and key ES technologies.
973 Program subject in 2009: basic research on distributed power supply system, including the following:
1. Operation characteristics of microgrid and its interaction with macrogrid under high penetration;
2. Theories and methodology on planning distribution systems integrating microgrids;
3. Protection and control for microgrids and distribution systems integrating microgrids;
4. Management methods for comprehensive simulation and energy optimization of distributed power systems.
Currently, many universities, research institutes, and large companies are carrying out studies on the microgrid and have constructed demonstration projects.
In 2006, Tsinghua University began studies on the microgrid and established a laboratory microgrid encompassing DG, ES, and loads utilizing the facilities in the National Key Laboratory on Power System and Generating Equipment Safety Control and Simulation under the Department of Electrical Engineering. Furthermore, Tsinghua, in collaboration with the XJ Group, set up a microgrid simulation platform, established steady-state and dynamic mathematical models of various types of DRs and operation in grid-connected mode, built a simulation environment for microgrids integrating DGs and other power systems and with bidirectional flow, carried out studies on microgrid modeling and operation characteristics analysis, development of simulation platform and operation characteristics analysis, and effects of microgrid on grid load model.
Tianjin University undertook the 973 Program project “Basic research on distributed generation systems,” and Huazhong University of Science and Technology, Xi’an Jiaotong University, and some other organizations participated in the following eight subtopics: (1) interaction with the macrogrid under high penetration; (2) effects of distributed ES on security and stability of the microgrid; (3) optimized planning of distribution systems containing microgrids; (4) theories and technologies of protection for microgrids and distribution systems containing microgrids; (5) microgrid interconnection control and coordinated control of various DRs within the microgrid; (6) power quality analysis and control for microgrids and distribution systems containing microgrids; (7) comprehensive simulation of DG-based microgrids; (8) microgrid economic operation theories and energy optimization management methods.
Hefei University of Technology proposed a new inverter power supply for the microgrid based on the synchronous generator machine-electricity transient model, known as the virtual synchronous generator. It can serve as an uninterrupted power supply for critical loads following a microgrid failure. When the microgrid operates in parallel with the grid, the virtual synchronous generators are power controlled to adjust their output power according to dispatch orders; when the microgrid operates in islanded mode, the inverters are voltage and frequency controlled to provide reference for voltage of the microgrid.
The Institute of Electrical Engineering, Chinese Academy of Sciences, established a 200 kVA laboratory microgrid system, conducted steady-state and dynamic analysis, proposed steady-state and dynamic calculation methods and control and management strategies for microgrid islanded operation, and carried out a lot of studies and tests on control methods of DG and the smooth mode transfer of the microgrid.
Zhejiang University analyzed the active and reactive power circulating current model of parallel-connected inverters in a typical microgrid, and proposed an improved self-regulating droop factor control method considering the problem of instability of output power amplitude and frequency of such inverters adopting traditional droop control strategy.
Sichuan University proposed a multiagent-based uninterrupted substation power coordination system, in which various DRs are capable of automatic compensation for load fluctuation by adopting multiagent coordination strategy, thereby improving load tracing capability and reliability of substations.
In 2009, Zhejiang Electric Power Company developed a laboratory microgrid comprising multiple types of DGs and ESs, which can be flexibly restructured, and therefore can be used to simulate various failures and realize smooth transfer between grid-connected mode and islanded mode. It also carried out tests on various coordination control and protection technologies, quality control, and other advanced applications.
In 2010, Henan Electric Power Company and XJ Group jointly completed SGCC’s demonstration project, “Comprehensive study on operation control of microgrid containing PV and ES and engineering application” (as part of the Golden Sun Demonstration Project launched by the Ministry of Finance of the People’s Republic of China). This project, located in Henan College of Finance & Taxation, comprises a PV system of 380 kW and an ES system of 2 × 100 kW/100 kWh, and covers the dormitories and canteens in the No. 4 distribution area of the college, including three PV circuits, two ES circuits, and 32 low-voltage (LV) distribution circuits. It communicates with the dispatch center of Zhongmu Electric Power Company.
In the same year, Shaanxi Electric Power Company and the XJ Group jointly completed another demonstration project, “Study on control technologies for microgrid containing distributed generation/energy storage.” This project, located at the entrance to Xi’an China International Horticultural Exposition Park, consists of a series of trial projects including smart distribution network, DG, and microgrid, electric vehicle charging station, and experience of intelligent use of electricity, and is intended to exhibit SGCC’s new technologies and findings on smart grid to the public. In this project, a 50 kW PV power system is arranged on the shed of the electric vehicle charging station, and six wind turbine generators with an aggregate capacity of 12 kW and a 30 kW/60 kWh energy storage system are arranged around the charging station.
These two demonstration projects prove that the microgrid system can achieve optimal operation in grid-connected mode and stable operation in islanded mode, automatic transfer between the two modes, and control of power exchange with the macrogrid, all being unique to the microgrid. The two demonstration projects have been in commercial operation.
In 2010, CSG’s “Distributed energy supply – combined cooling and power demonstration project,” a subject under the 863 Program, began commercial operation. This project, located in the Power Supply Bureau of Chancheng District, Foshan, consists of three 200 kW microturbines and one lithium bromide water absorption refrigerator. The system can meet the cooling and power loads of the three buildings in the Bureau, and according to the design objective, the efficiency of primary energy will exceed 75%.
1.4. Prospects
With the advanced IT and communication technologies, electric power systems will develop toward more flexible, cleaner, safer, and more economic smart grids. The smart grid is intended for power systems encompassing generation, transmission, distribution, and consumption, and allows for smart interaction between all links by developing and introducing advanced control technologies, thereby systematically optimizing electricity production, transmission, and consumption. To suit the development of the smart grid, the distribution network has to shift from passive to active, which is favorable to DG and allows for real-time participation of the generation side and user side in optimizing the power system operation. The microgrid is an effective means for an active distribution network, which will help large-scale integration of DG and transition from the traditional grid to smart grid.
The use of various types of DGs and ESs in the microgrid is not only conducive to energy saving and emission reduction, but also significantly motivates the sustainability strategy in China. Compared with traditional centralized power systems, new energy-based DG can largely reduce feeder losses and save investment on T&D networks. It allows for mutual support with the macrogrid, full use of available resources and equipment, and reliable and quality supply, thereby increasing energy efficiency and grid security. In spite of a short history in China, the microgrid technology suits China’s needs to develop renewable energy and seek sustainability, and hence, in-depth studies on the microgrid are of great significance.
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