Grid-Connected/Off-Grid Control Strategies For Smooth Switching Of VSG

With the increasing shortage of traditional energy supplies,the microgrid systems integrated with distributed small-capacity distributed generation systems,energy storage systems,as well as load and protection devices have become hot topics at home and abroad.The main operating mode of the microgrid can be divided into grid-connected and off-grid modes.Microgrids and conventional distribution networks usually operate under the grid-connected mode,when a fault occurs in a power system or the power quality cannot meet the load requirements,the microgrid can automatically disconnect from the power grid in a short time and transform to the off-grid operation mode to maintain the continuous electricity supply for important loads.In this paper,the virtual synchronous generator control strategy is used to study the smooth switching between the two operating states of the voltage source inverter.Firstly,based on the MATLAB/Simulink simulation software,the two operation models of the microgrid in the virtual synchronous generator control strategy are modeled.The off-grid and grid-connected state distributed inverters respectively operate in Droop mode and indirect power control mode,and the inverter works in PQ mode.The simulation results show that the off-grid and the grid-connected models built in this paper can reflect the characteristics of the actual grid and lays a foundation for further off-grid smooth switching simulation and experimental verification.Secondly,focusing on the off-grid and grid-connected modes of distributed power supply,this paper analyzes the influences of the three major parameters: rotational inertia,damping coefficient and active frequency regulation factor on the dynamic performance of the system when the loads change or there is step signal in the power output.For the distributed power supply integrated into the strong grid,the motion equation of second-order synchronous generator rotor is used to establish a SMIB and small-signal analytical model to study the influences of the above three parameters selection on the system stability.Thirdly,in order to shorten the time of smooth switching and guarantee the normal operation of it,in this paper,the existing pre-synchronization control method has been improved based on the principle of virtual power synchronization,as well as a grid-connected and off-grid smooth switching model for virtual synchronous generator control strategy is also constructed.The simulation results show that the improved pre-synchronization control strategy can ensure that the disturbing signal in the process of switching can be controlled within a certain range,and maintain stability of the voltage frequency of distributed power supply during the smooth switching.Finally,this paper builds a simulation experiment platform,and applies the improved pre-synchronization control algorithm to the grid-connected and off-grid smooth switching experiment and proves the correctness of the parameter analysis and the improved VSG control strategy.