The development of Japan’s micro grid is based on the problems of China’s increasing energy shortage and load growth. The Japanese government very much hopes that renewable energy can play an important role in Japan’s energy structure. Therefore, micro grid research and pilot are positioned to solve the diversification of energy supply, reduce pollution and meet the personalized power demand of users.
Japan’s New Energy and Industrial Technology Development Organization (NEDO) has made many achievements in micro grid research. In 2003, the “Re industrial Power Grid with Renewable Energy Resources Project” focused on the construction of three pilot projects of micro grid. The research of these three test platforms focuses on the technology and management of clean energy access to local distribution networks, respectively in Aomori County, Aichi and Kyoto. Clean energy accounts for a large proportion in the micro grid of the three regions. At present, Japan is leading the world in the construction of micro grid pilot projects.
Japan NEDO defines microgrid as: microgrid refers to a small system that uses controllable DR in a certain area to provide power according to user needs.
In addition, under the guidance of NEDO, many commercial companies have joined in the research of distributed generation and micro grid technology. Shimi2U, a well-known commercial construction company, cooperates with the University of Tokyo to research the power grid control system using the test system located in the Tokyo Research Center. The definition of microgrid given by the University of Tokyo is: microgrid is an independent system composed of distributed resources (DR), which is generally connected to large systems through tie lines. Due to the imbalance between power supply and demand, microgrid can choose to operate in the mode of mutual supply or off grid with the main network.
SEKISUI has launched a smart house that completely relies on distributed power supply, and recently announced that it will cooperate with NEC to develop the next generation of smart house. In addition to installing distributed power generation system, it will also be equipped with NEC system that clearly displays power usage information.
Mitsubishi defines microgrid as a small controllable system including power supply, thermal energy equipment and load, which is an integral unit and can be connected to the main network for operation. This concept also brings the independent power system powered by traditional power supply into the research category of microgrid, greatly expanding the definition scope of microgrid by CERTS. Mitsubishi has participated in the construction of several micro grid projects and accumulated rich construction experience.
At present, the main microgrid pilot projects in Japan include:
1) Aichi project. It is equipped with 270kW and 300kW molten carbonate fuel cells, 25kW solid oxide fuel cells, four 200kW phosphoric acid fuel cells, 330kW photovoltaic system, and sodium sulfur battery for energy storage. The access voltage level is 200V, and the goal is to control the imbalance between supply and demand within 3% within 10min. The second independent operation test was conducted in September 2007. It can operate both in grid connection and independently. The funding and operating agency is NEDO.
2) Kyotango project. Equipped with 400kW gas turbine, 250kW molten carbonate fuel cell and 100kW lead-acid battery. Far away areas are equipped with two photovoltaic (PV) systems and 50kW small fans, which are connected to 200V voltage level. Connected to the power grid and controlled by the central control system, the goal is to control the imbalance between supply and demand within 3% within 5 minutes. It can operate both in grid connection and independently. The funding and operating agency is NEDO.
3) Hachinohe project. The sewage treatment plant is equipped with three 170kW gas turbines and 50kW photovoltaic (PV), which are connected to 200V voltage level. The power generated is transmitted to four schools, the water conservancy bureau office building and the municipal office building through 5km private wiring. There are also small fans and photovoltaic (PV) in the school, which can be connected to the grid or operated independently. The control objective is to control the imbalance between supply and demand within 3% within 6 minutes. During the test, the target completion rate is 99.99%. In November 2007, it operated independently for one week, and the funding and operating agency was NEDO.
4) Shimizu microgrid project. Four gas turbines (22kW, 27kW, 90kW and 350kW), photovoltaic (PV) (10kW), lead-acid battery (20kW), NiMH battery (400kW · h) and supercapacitor (100kW) are equipped, which can be connected to the grid or operated independently when connected to the 200V voltage level. Focusing on the research of load tracking technology, Shimizu Corp is the funding and operating agency.
5) The Tokyo Gas microgrid project. A total of 100kW, including gas turbine, CHP, PV, wind power generation and battery energy storage. When connected to 200V voltage level, it can be connected to the grid or operated independently. The funding and operating agency is Tokyo Gas.
From the above analysis on the research and pilot project construction of micro grid in Europe, the United States and Japan, it can be seen that although the definition of micro grid in the world is different, it is also general, and micro grid generally has the following basic characteristics:
1) It is close to the end user and suitable for the user’s voltage level, so the overall scale is small;
2) Based on distributed generation, energy storage, control and protection systems are integrated;
3) It can work in both grid connected and off grid operation modes, can self balance, and has little power exchange with external power grids;
4) Power electronic equipment is used as the main power conversion device.
5) The goal is to efficiently accept distributed renewable energy and improve the efficiency of energy comprehensive utilization.
At present, the micro grid pilot projects in Europe, America, Japan and other countries generally aim to carry out the test and verification of micro grid technology, without considering commercial operation.