Research On Control Strategy Of Microgrid Inverter Based On Virtual Synchronous Generator

With the rapid development of renewable energy power generation represented by wind power and photovoltaic,the permeability of distributed power supply in the grid increases rapidly.The micro-grid with power electronic inverter as interface decreases the stability of the system due to the lack of inertia and damping.By simulating the external characteristics of Synchronous Generator,Virtual Synchronous Generator(VSG)technology enables the microgrid to have inertia and damping,which provides certain support for the stable operation of the system.This paper mainly studies the optimization of virtual synchronous generator control strategy,grid-connected/ grid-off/grid-connected switch.Firstly,the mathematical model of the synchronous generator is established to analyze the mechanical characteristics of the rotor and the electrical characteristics of the stator.The control algorithm of the virtual synchronous generator is designed by simulating the external characteristics of the synchronous generator.The power control ring and output voltage control ring of the virtual synchronous generator are designed by combining the characteristic equation and control principle.Then,the control strategy of virtual synchronous generator is optimized.Aiming at the problem that the constant moment of inertia and damping coefficient are not enough to restrain the fluctuation when the system power instruction changes,an adaptive algorithm of inertia damping is designed to improve the dynamic response of the system output by taking advantage of the flexible regulation of inertia and damping coefficient in the VSG power control loop.In view of the long adjustment time of Proportion Integration Differentiation(PID)controller in VSG voltage-current double closed-loop and the slow rate of steady-state of the system,nonlinear PID is used to replace linear PID in traditional voltage-current control loop.The improvement of voltage and current loop control for microgrid virtual synchronous generator is realized.Finally,in view of the current impact caused by VSG’s direct off-grid/grid-connected mode switching,the mechanism process of VSG’s grid-connected/off-grid/grid-connected mode switching is analyzed,and the mode smooth switching conditions are given.Meanwhile,Synchronous Reference Frame Phase Locked Loop(SRF-PLL)cannot be accurately locked in the face of unbalanced power grids.The Second Order Generalized Integral Phase Locked Loop(SOGI-PLL)is designed to improve the performance of the phase locked loop.According to the characteristics that the improved SOGI-PLL can accurately extract the voltage and phase of the grid,the smooth switching strategy of off-grid/grid-connected is designed,which makes the amplitude,frequency and phase of VSG output voltage consistent with that of the large grid before grid-connected,reduces the grid-connected impulse current,and realizes the smooth switching of the micro-grid from off-grid to grid-connected.