Research On The Control Strategy Of Virtual Synchronous Generator For Micro-grid Inverter

The increasing scarcity of traditional energy sources and serious environmental pollution has brought about the widespread concern of renewable energy power generation technology.The proportion of distributed power sources in the power system is also increasing,and the microgrid has become the hot research areas of power system.However,the microgrid inverter composed of power electronic devices lacks inertia and damping characteristics,which brings great challenges to the stable operation of the power grid.Virtual Synchronous Generator(VSG)technology makes the inverter have the inertia and damping characteristics of the synchronous generator through the control method,and improves the stability of the system.Therefore,this article studies the VSG control strategy of the microgrid inverter.According to the mechanical and electrical equations of the synchronous generator,the second-order mathematical model of the VSG is established.The design of the VSG power frequency controller and the excitation controller is completed by simulating the operating characteristics of the synchronous generator.The connection and difference between VSG and traditional synchronous generator are compared and analyzed.The power frequency characteristics of VSG are analyzed,the effects of rotational inertia and damping coefficient on the dynamic process of VSG output are studied,and the VSG small signal model is established.On this basis,the VSG control parameters and LC filter parameters are designed,and The similarities and differences between VSG control and droop control are compared and analyzed.In view of the shortcomings of the traditional seamless switching control algorithm and the difficulty of obtaining frequency information when the grid voltage fluctuates quickly and accurately with a phase-locked loop,an improved seamless switching control method that introduces virtual resistance is proposed.This method controls the active power of the virtual resistance and voltage deviation to be equal to zero to achieve seamless switching,and the effectiveness of the improved method is verified by simulation.Aiming at the problem of grid current distortion caused by grid voltage distortion,the current harmonic suppression method based on VSG was studied,the cause of grid voltage distortion on grid current was analyzed,and a grid voltage feedforward control method was adopted.Reshaping the grid admittance at harmonic frequencies achieves the purpose of suppressing grid-connected current harmonics,and the effectiveness of this method is verified by simulation.Aiming at the problem that the VSG output is unstable and the power fluctuation at the moment of fault clearing seriously under the severe voltage sag of the symmetric power grid,the low voltage ride-through control method of VSG is studied.The power command adjustment method is used to stabilize the output of the VSG during the failure.The combined method of voltage amplitude control limits the overcurrent generated at the moment of VSG fault,and the effectiveness of the control strategy is verified by simulation.Finally,the VSG power frequency characteristic experiment and the VSG current harmonic suppression experiment were completed on the RT-LAB experimental platform.The experimental results verified the correctness of the theoretical analysis of the VSG power frequency characteristic theory and verified the effectiveness of grid voltage feedforward control strategy for grid-connected current harmonic suppression.