Research On Control And Seamless Switching Strategy Of Microgrid Inverter Based On Virtual Synchronous Generator

The increasingly serious energy crisis and environmental pollution have promoted the development of new energy power generation technologies such as wind power and photovoltaic.The microgrid system based on new energy power generation and inverters has become an important complement to the traditional power system.The application of microgrid improves the power grid's ability to absorb new energy resources,while the negative impact of the lack of inertia and damping on the stability of the system has gradually emerged.How to take the advantage of the operation characteristics of synchronous generators,which play an important role in maintaining the stability of power system,for reference to realize the friendly connection has become one of the valuable research directions in the field of microgrid.This dissertation focuses on the research of virtual synchronous generator(VSG)and its seamless switching control.Firstly,several typical schemes of VSG are analyzed.On this basis,the detailed implementation method,which includes active power-frequency and reactive power-voltage controller,is designed by introducing the second-order model of synchronous generator.Besides,the similarities and differences of VSG,synchronous generator and traditional inverter in the physical process,mathematical model and parameters are also clarified.Secondly,in order to solve the problem of excessive frequency recovery time,a variable-parameter virtual synchronous generator control strategy is proposed.The effects of inertia and damping coefficient on output characteristics are analyzed with the establishment of small signal model of VSG.The characteristics of the underdamped,critical damped and overdamped system are compared.By studying the correlation between inertia,damping coefficient,the frequency variation and deviation,A parameter adaptive control strategy is proposed,in which the inertia and damping coefficient are flexibly and reasonably changed for the improvement of frequency response characteristics according to the frequency state.The numeration method of parameters was given,too.Then,the control strategy,by which the fault current is effectively suppressed by controlling output voltage and active power,for VSG under symmetrical voltage sag situation is proposed.The theoretical and simulation analysis on characteristics of output power and current during voltage sag are carried out.Furthermore,Reactive power compensation control,which is beneficial to the voltage recovery,is designed.In addition,calculation method of key parameters in the strategy is given,and the effectiveness of the proposed control strategy is verified by simulation.Finally,the secondary frequency and voltage regulation methods for VSG are designed,after analyzing the primary frequency and voltage regulation characteristics of under off-grid state,to reduce the voltage and frequency deviation.Moreover,in order to decrease the transient current impact when switching,a pre-synchronization controller based on voltage-frequency,phase and amplitude synchronization is designed to realize smooth seamless switching.By building the simulation model on Simulink,the validity of voltage and frequency secondary regulation strategy and seamless switching control method are all verified.