| In the context of high percentage of renewable energy and high percentage of power electronics,many distributed power inverters in microgrids with scattered layout and random access do not have communication conditions,which requires grid-connected operation of microgrids,islanding operation and switching between them to solve the communication dependency problem.Synchronous fixed-frequency microgrid uses satellite timing signals to synchronize the output frequency and phase of each inverter,based on the current-voltage sag relationship,to achieve stable microgrid operation without interconnection communication.The existing research is aimed at islanding operation control,and there is a lack of research on grid-connected/islanding operation conversion control technology.To address the above problems,this paper investigates the synchronous fixed-frequency microgrid grid/islanding smooth switching control technique.The electrical quantity change law and characteristics of the switchover process of gridconnected/islanded operation state of distributed power inverters in microgrid under synchronous fixed-frequency current control are analyzed.The output current phase is determined by the satellite timing signal in the islanding state and by the phase-locked loop in the grid-connected state of each inverter in the synchronous fixed-frequency microgrid,and the grid-connected switch of the large grid and microgrid needs to have the function of simultaneous grid-connected.The switch can be controlled directly according to the demand.For the process of grid connection to islanding,a synchronous detection method of islanding state based on satellite timing signal is proposed to enable each inverter in the microgrid to independently identify the islanding state and switch independently to ensure the consistency of each inverter’s operation mode;an overshoot-free tracking method based on tracking differential is proposed to make the transition process meet the grid frequency demand and improve the rapidity and smoothness of switching from grid connection to islanding.For the process of islanding to grid connection,the grid connection state detection method based on frequency offset is proposed to realize the fast switching of each inverter from synchronous fixed frequency current control to PQ control after the microgrid is reconnected to the grid.Finally,simulations and experiments demonstrate the effectiveness and feasibility of the proposed method.The proposed method effectively solves the safe and flexible interconnection between synchronous fixed-frequency microgrid and distribution network under the condition of no communication,which is important to promote the efficient local consumption of renewable energy. |
