Research On Off-grid Operation Control Of Inverter

As the link between distributed generation and power system,microgrid can work in both grid-connected mode and off-grid mode.As an important interface between the microgrid and power system or load,the control performance of inverter has an important impact on power quality of the system and the stability of the system when microgrid works in the off-grid mode.Therefore,this paper takes LC-type three-phase inverter as the research object,and studies its off-grid operation control.The mathematical model is established for LC-type three-phase off-grid voltage source inverter in three-phase stationary coordinate system,two-phase stationary coordinate system and two-phase rotating coordinate system.The single voltage loop,active damping double-closed loop and traditional double-closed loop control block diagram are established in two-phase stationary coordinate system.And the difference and relationship between the traditional double-closed loop and active damping double-closed loop are analyzed.For the LC filter resonance problem,considering the influence of control delay,the stability of the undamped system and traditional inductance-current-feedback active damping system are analyzed in the discrete domain.Aiming at the problem that the effective damping region of the traditional inductance-current-feedback active damping method is narrow,an improved inductance-current-feedback active damping method is studied.This method extends the effective damping region from(0,f_s/6)to(0,f_s/4).According to the Nyquist stability criterion,the stability of the system is analyzed,and the optimal damping coefficient is designed to improve the robustness of system to the variation of filter parameters.Simulation and experiment verify the validity of this method.The output voltage distortion caused by nonlinear load is analyzed from the point of view of output impedance.A harmonic suppression method of off-grid system based on repetitive control is studied.Since the harmonics caused by nonlinear load are mainly6k±1 times,the odd-order harmonic repetitive control is used,the control parameters are designed,the dynamic performance of the repetitive control is improved,and the influence of the phase lead unit on the stability of the system is analyzed geometrically.Simulation and experiment verify the validity of this strategy.