Research On The Control Technology Of Micro-grids

The traditional centralized bulk transmission network that is of high voltage and long distance has the potential problems of power quality, safety, reliability, economical efficiency and flexibility, which promotes the development of renewable energy technology. As the traditional energy resources such as coal, oil and gas becoming less and less, the research and development of renewable energy, like solar, wind, tide, geothermal, etc. which is clean, and free from contamination, becoming more and more important. The key problem in using of renewable energy is how to convert it to electrical energy and connect to the utility. Micro-grid is a small-sized grid that integrates a number of micro-sources and loads, which is controllable, and can either be connected to distribution grid as a special load, or disconnected from the utility to operate in island model. As a good supplementation to the traditional power supply model, micro-grid represents the new development direction of power system.In order to improve the power reliability of crucial loads, micro-gird is required to operate in island mode. In addition, different energy sources have different characteristic, which need different control methods. When connected to the utility, the micro-sources need only to supply right amount of power to keep power balance, while in island mode, there must be at lest one micro-source in micro-grid to output rating voltage and frequency besides keeping power balance. Several conventional control methods of micro-source are analyzed in this paper. Combined with the characteristics of different micro-sources, two typical control strategies are researched and simulated, considering the necessity of two operation modes of micro-grid. Micro-sources are often connected to network through inverter, which has highly different external characteristics compared with synchronous generator. Researching the technology that how to incorporate inverter-based sources to the utility grid, therefore is of much importance in the development of renewable energy resources, which is the basis of the control of micro-grid. The main aspects of the technology are studied, and the low frequency link of synchronous inverter technology is analyzed in detail through an example of solar generation system.Several inverter-based sources are often operated in parallel to enlarge the capacity of single micro-source. When in parallel connection, the output of the voltage amplitude, frequency and initial phase of every inverter should be equal. If not, it needs to deal with the problem of circulating current between micro-sources. The reason why the circulating current problem arises is analyzed and several ways of synchronous are represented. And the method that is accord with the control strategy of micro-grid is researched.A rubout and reliable communication system is needed in many aspects, such as the switch between grid connected mode and island mode, the cooperation of different micro-sources to keep power balance, etc. Multi-agent system (MAS) is introduced in summary, and the superiority and feasibility of researching and developing control system base on MAS is analyzed.