Research Of DC Micro-grid In Island Operation

With the rapid growth of economy and the steady development of the society, the demand for energy is increasing and the environmental problem is becoming more and more serious. The energy and environmental crisi has become an important problem that every country needs to face in the world, in which non-renewable energy is still the main support of economic development. Against this background, the research of micro-grid has an important significance on improving the energy structure and the utilization efficiency of distributed power systems. The micro-grid mainly exists in the form of AC, but as more and more equipments tend to use DC current and the rapid development of DC distributed generation, DC micro-grid meets the wide space of development. In this paper, DC micro-grid island operation is studied.First of all, this paper builds a DC micro-grid which consists of photovoltaic power unit, wind power unit, storage unit and load unit. Using electric vehicle as a more flexible storage unit is also proposed in this paper to improve the system storage capacity and economic efficiency. In addition, island detection for DC micro-grid is analyzed and detection methods are summarized in this paper, because detecting island mode quickly and effectively is a premise of the research of micro-grid island operation.Secondly, the island operation of the micro-grid is divided into seven different states based on the variation of DC bus voltage for comprehensive researching of the micro-grid island operation. Island operation of each state is analyzed and hierarchical droop control strategy based on DC bus voltage is proposed in this paper. The system control strategy is accomplished by distributed control of each converter, and the objectives are that, MPPT control is adopted by photovoltaic cells and wind turbine to achieve full use of renewable energy, the battery and electric vehicle should be timely charged or discharged in the event of power imbalance, whose charging or discharging current must conform to the system control requirement.In the end, simulation results are carried out by Matlab/simulink to demonstrate the effectiveness of the energy control strategy. Two typical examples that DC micro-grid based on sustainable buildings and DC micro-grid based on DC traction power supply system are proposed, and the two kinds of systems are simulated in this paper. The simulation results demonstrate the validity of the proposed control strategy, and also reflect that the research of DC micro-grid in island operation has a certain scalability and realistic significance.