Research On Control Strategy Of Grid-Connected Inverter Based On Virtual Synchronous Generator

With the continuous improvement of grid-connected permeability of new energy power generation and the large number applications of power electronic equipment,the grid presents the characteristics of "low inertia and underdamping".Therefore,grid-connected inverter control becomes the key of "double high" grid.The virtual synchronous generator(VSG)simulates the inertia and damping performances of the synchronous generator(SG),and has naturally friendly grid-connected characteristics,which is widely used in grid-connected inverter control.However,when the traditional VSG is connected to the grid,there is the problem that the steady-state and dynamic characteristics of the active output power cannot be taken into account and the system order will be increased under different disturbances.Based on the VSG equivalent model,this paper establishes the transfer function between VSG different variables,and quantitatively analyzes the relationship between VSG parameters and system performance.And proposes the VSG grid-connected active power control strategy based on dual PD control inertia damping compensation and fractional inertia damping compensation.The thesis main work includes the following four parts.Ⅰ.VSG system modeling and control structure designThe VSG equivalent model is established.The control structure of gridconnected inverter based on VSG is determined.The power loop,voltage and current double closed loop and LCL filter are designed.Ⅱ.Research on the performance comprehensive analysis method for VSG control systemTo further study the relationship between VSG parameters and system performance,the transfer-function with the active power reference value,reactive power reference value and grid frequency as inputs,and output active power and reactive power as outputs is established.The effects of virtual impedance ZV,damping coefficient D,moment of inertia J,active droop coefficient Kf and reactive droop coefficient Kq on system performance are analyzed.The relationship between the steady-state and dynamic performance of the system and the parameters is clarified,and the correctness of the relevant conclusions is verified by simulation.Ⅲ.Research on VSG grid-connected active power control strategy based on dual PD controlAiming at the problems that the contradiction typical VSG parameters J and D values in the steady-state and transient performance adjustment of active power,and active power is prone to oscillation.The VSG grid-connected active power decoupling control strategy based on dual PD control inertia damping is proposed.PD control links are connected in series in the VSG inertia forward channel and damping feedback channel to change the VSG grid-connected active closed-loop structure under the condition of introducing few parameter variables,and realizes the decoupling control of power transient and steady-state characteristics.According to the closed-loop pole-zero distribution diagram,the influences of D and differential coefficient K on the transient characteristics are analyzed,and the constraint conditions of dual PD control parameters are given.The theoretical analysis and simulation results show that the control strategy eliminates the influence of D on VSG steady output,enhances the system dynamic performance,decreases the overshoot and reduces the regulation time.The current THD value of dual PD inertia damping control system is 1.13%,which improves the gridconnected power quality.Ⅳ.Research on VSG grid-connected active power control strategy based on fractional inertia damping compensationAiming at the problems that the dual PD control strategy will increase the system order and the integer-order controller parameters design has small adjustable range.The control strategy of VSG grid-connected active power based on fractional inertia damping compensation is proposed,the fractional virtual inertia is introduced,and fractional PDλ controller is connected in series in the VSG inertia forward channel and damping feedback channel.Through the Bode diagram,the effects of λ and differential coefficient Kd on the transient and steadystate performance of active power are analyzed,the parameters λ and Kd are optimized by frequency domain method,and the parameters values are determined.The effects of different D,λand Kd on the active power output characteristics are compared by simulation.The results show that the control strategy can increases the freedom of power transient characteristic regulation and better suppress power and frequency fluctuations.Its current THD value is 1.09%,effectively optimizes the grid-connected power quality.