Research On Control Strategy Of Microgrid In-verter Based On Virtual Synchronous Machine

In recent years,with the vigorous development of distributed power generation,a large number of power electronic devices lacking inertia and damping exist in the micro-grid system based on new energy power generation.During grid-connection,the inertia and damping of the power system are reduced,which affects the stability of the power system.As a traditional power generation mode,Synchronous Generator(SG)can improve system stability by providing inertia and damping.Virtual Synchronous Generator(SG),VSG)is based on the synchronous generator with reference to its static characteristics of the power Angle change process,the simulation of SG’s moment of inertia and damping coefficient adjustment process to coordinate the control of the inverter to work,improve the dynamic stability of the power and frequency of the microgrid system.As a bridge between DC distributed power generation system and AC microgrid,inverter control strategy directly affects the dynamic stability of microgrid system.In order to improve the stability of microgrid system,the following researches are carried out on the control strategy of microgrid inverter based on VSG control method:Firstly,the working principle and mathematical model of the virtual synchronous generator are described,and the active power ring and reactive power ring in the VSG overall control strategy are designed.Then on the basis of the stability of the output power,voltage and frequency of the system,aiming at the problem of harmonic current affected by the dead time of the inverter,the VSG current loop controller which adopts the repetitive control and PI control in parallel is designed.After the stability of the system parameters,the harmonic content of the current in the connecting point is further suppressed,and the quality of grid-connected power is guaranteed.Secondly,in order to solve the problem of long transient time and large dynamic adjustment overshoot caused by traditional VSG constant moment of inertia and damping coefficient,an adaptive control strategy based on VSG moment of inertia and damping coefficient parameters is designed,which improves the dynamic adjustment characteristics of VSG output power and frequency compared with the constant parameter control.Based on the closed-loop characteristic equation of the system,the parametric root locus diagram is drawn.The analysis shows that the moment of inertia and the damping coefficient both affect the dynamic adjustment process of the system.In order to verify the control strategy’s frequency regulation ability and current mutation inhibition when the system load is disturbed and ensure the stable operation of the load,this paper simulates the influence of the load on the frequency and current when the load is connected to the system.The simulation results show that the adaptive control strategy can effectively reduce the frequency fluctuation value by 0.03 Hz when the load is connected to the system.The current mutation during load access is effectively inhibited.Thirdly,in view of the current impact in the traditional smooth switching process,the mode switching between parallel and off-grid is studied,and the corresponding VSG pre-synchronization control strategy is adopted to track the phase,amplitude and frequency of the grid voltage without error,so as to make the current smooth transition in the switching process,and to address the security risks brought by the non-planned island.An island detection method is designed to ensure the safe and seamless switching between parallel off-grid modes.Finally,the simulation model of the virtual synchronous generator system is built on the Matlab/Simulink platform,and the current distortion rate of the connecting point is compared when the current loop uses different controllers,and the stability performance of the system is compared respectively under the adaptive control and the traditional control.At the same time,the change waveform of the current,voltage and other system parameters when the off-grid switching is simulated.In summary,the simulation results verify the correctness of the theoretical analysis and the effectiveness of the control strategy.