Research On The Switching Control Method Of Microgrid Connected And Off-grid Modes

Microgrid is the effective way to solve the contradiction between the DG and the utility grid. The application of microgrid technology has improved the utilization of the renewable energy, and reduced the environmental pollution caused by the traditional energy. Microgrid must be able to smooth switching between the connected grid mode and islanded mode to give full play to the efficiency of distributed generation, and continue to supply power for the local load. Research on the control strategy of microgrid is of great significance to the development of electric power. In this thesis, the low voltage microgrid system is the research object, which mainly focuses on the smooth switching ability of the microgrid.Firstly, selects the type of the inverter for the microsource, builds a mathematical model of the VSI. Then analyzes the PQ control, V/F control and droop control which is the microgrid inverter control strategy commonly used. Focus on the analysis of droop control used conditions, analysis of the stability and dynamic response performance of microgrid which with the different DG droop response time under certain conditions. When the DG with the high drop response time access to micro network, will cause the instability of the other DG in the microgrid. Especially the low voltage microgrid with the resistive, which is more sensitive to droop response time.Secondly, improve the micro source VSI current mode control strategy. Utility the Proportion Resonance regulator as the current controller of the micro source on the basis of micro source VSI current control mode, and design the voltage controller and frequency recovery algorithm, realize the stability of the microgrid system voltage and frequency.Moreover design a power sharing algorithm based on the current amplitude droop with eliminating the need for power measurement and feedback, increases the system response speed. The control strategy can make microgrid implement islanded operation in current control mode, and can unify the control mode of the microgrid in two operations,avoiding the unnecessary operation transition impact due to the switching of the controller mode during the switching of the microgrid operation modes. In order to further improve the smoothness of the microgrid transition, has implemented the the design of the subregional load shedding method and the microgrid synchronization controller.Finally, according to the improved microgrid control strategy establishes the simulation model of the low voltage microgrid system. Then the control strategy is analyzed by the numerical simulation respectively which for single DG, microgrid islanded operation, and the switching of microgrid operation modes. Simulation results show that the control strategy of microgrid islanded operation and dual operating mode switching has a good control effect,and the microgrid has a good dynamic performance in the case of load shedding and the transient process of the operation mode switching.