Research On The Control Strategy Of Grid-Connected And Off-grid Operation Of Inverter Based On VSG In Microgrid

In view of the stability problems caused by the increased penetration of new energy power generation in microgrid,virtual synchronous generator(VSG)control is adopted.In other words,the mathematical model of the synchronous generator is introduced into the inverter control strategy,so that the inverter has the characteristics of the synchronous generator,which can make the energy power generation device connected to the power grid through the power electronic converter play a supporting role in the stable operation of power grid.In this paper,the inverter in microgrid is taken as the research object,and VSG control is analyzed from three aspects:offgrid mode,grid-connected mode and seamlessly switch from off-grid to grid-connected or from grid-connected to off-grid mode.Firstly,in the off-grid mode,the rotor motion equation is introduced into the VSG power loop control,so that the inverter has the performance of the synchronous generator's rotational inertia and damping according to the mathematical model of synchronous generator.In this mode,compare with the droop control of the simulated synchronous generator,the VSG control has better dynamic characteristics.Secondly,in the grid-connected mode,a grid-connected mathematical model based on VSG control is established,and the influences of rotational inertia and damping coefficient on grid-connected active power oscillation are analyzed through the small signal model.Then combining the dynamic and steady-state characteristics of the VSG grid-connected system,the values of J and D parameters are restricted.At the same time,the relationship between J and D parameters and angular frequency is derived according to the rotor motion equation,and an improved parameter self-adjusting control strategy is designed.By which,the J and D parameters can be matched to the corresponding inertia and damping according to the change of angular frequency during the self-adjustment process.The problem of grid-connected active power oscillation based on VSG control is solved,when the inverter is switched from off-grid mode to grid-connected mode.Finally,a simulation model by PSIM software and a three-phase full-bridge inverter grid-connected experimental platform are built.The current parameter self-adjusting control strategies for suppressing active power oscillation are analyzed and compared,and a variety of operating conditions are completed,such as off-grid,grid-connected,parameter self-adjusting control strategy comparison and seamlessly switch from off-grid to grid-connected or from grid-connected to off-grid,the feasibility and effectiveness of the control strategy designed in this paper are verified through simulation and experiment results.