The Research Of Control Strategy For Micro-grid Island/grid-connected Operation Mode And Smooth Switching Process

With the rapid development of new and clean distributed generation(DG), its shortages such as uncontrollable and intermittent are increasingly exposed. Micro-grid which is composed of DGs can just solve this problem. Micro-grid could operate in grid-connected mode and islanding mode, so its control strategy is an important researching area. Currently, the researchs on micro-grid control are mostly of grid-connected mode or islanded mode, but the depth researchs of the switching between the two modes are relatively less. The smooth switching control is an important prerequisite to achieve security and stability of micro-grid. This paper focuses on the smooth switching control between grid-connected and islanded operation modes. The main contents are as follows:(1)The research of micro-grid smooth switching control strategyThe reasons for transient oscillations in micro-grid operation modes switching are analyzed. According to the analysis above, a micro-grid smooth switching strategy is presented. Among them, the switching from islanded to grid-connected is based on synchronization of controller output state and the switching from grid-connected to islanded is based on synchronization of d-q rotating coordinate of controller.(2)Improve the pre-synchronous controller and parameters design of dual loop controllerThe traditional pre-synchronous controller directly adjusts the voltage and frequency, which may cause the system to oscillate, so the traditional pre-synchronous controller is improved further. Then analyze the output impedance of the dual loop control system with voltage and current loop and give the method of parameters design.(3)Simulation and modes switching control of micro-grid with wind-PV-ES sourceAnalyze the mathematical model of three typical micro-sources: wind turbine, photovoltaic and energy storage. Meanwhile simplify the models and give the control system of micro-sources. Then achieve the simulation model in the Matlab / Simulink. On this basis, simulink and analyze some typical control strategies of micro-grid operation modes and modes switching. The validity of the proposed control strategy is demonstrated by the simulation results.