Study On Negative Impact Of Phase-locked Loop On Grid-Connected Inverters And Corresponding Suppression Strategies Under Weak Grid Conditions

With the rapid development of new energy power generation systems,three-phase grid-connected inverters have been widely used in power systems.However,existing research shows that,as an important part of grid-tied inverters,phase-locked loop(PLL)has negative impact on the analysis and the operation of grid-connected inverters under weak grid conditions.Therefore,it is urgent to analyze the impact mechanism of PLL and propose corresponding suppression strategies.First,an output impedance model of grid-connected inverters needs to be established.In this paper,the small-signal model of PLL is established based on complex transfer functions,and the small-signal model of the inverter main circuit and the inverter control circuit in different dq domain are combined to derive the output impedance model,then,the correctness of the inverter output impedance model is verified through simulations.Second,the negative impact of PLL on grid-connected inverters under weak grid conditions is analyzed.Based on the output impedance model of grid-connected inverters,this paper analyzes the frequency coupling phenomenon caused by PLL and the influence of PLL on inverter stability.The analysis results show that PLL introduces voltage disturbances at point of common coupling(PCC)into grid-connected inverter output currents,which results in the frequency coupling phenomenon and affects the stability of grid-connected inverters under weak grid conditions.In addition,with the increasing of PLL bandwidthes,the frequency coupling phenomenon of grid-connected inverters will gradually increase,and the stability of grid-connected inverters under weak grid conditions will correspondingly decrease.Finally,a grid-connected inverter control method that suppresses the negative impact of PLL is proposed.From the point of view that PLL introduces voltage disturbances at PCC into the grid-connected inverter output currents,this paper proposes to suppress the negative impact of PLL by injecting the voltage disturbances with the same amplitude and opposite phase angle.The analysis and experimental results show that the proposed control method can reduce the frequency coupling caused by PLL and improve the stability of grid-connected inverters under weak grid conditions without affecting the dynamic response of PLL.