Study On The Control And Stability Of Grid-connected Inverters Based On Droop Control

As a link between distributed generation and distribution network,microgrid can work in grid-connected mode or island mode.New energy is mostly connected to the microgrid through inverters.Inverters with droop control are widely used in the microgrid.In grid-connected mode,the change of grid state will affect the Droop Controlled Grid-connected Inverter(DC-GCI),which leads to system instability.In this paper,DC-GCI with three-loop control structure of "droop-voltage-current loop" is taken as the research object,and droop control and voltage-current double closed-loop control strategies are studied respectively.Firstly,the droop control formula is deduced through detailed theoretical calculation.Then,a dynamic phasor model based on power transmission is established.The influence of droop coefficient and transmission line impedance parameters on system stability is analyzed through root locus diagram,and the droop coefficient is designed.In addition,the small signal space state vector model of grid-connected system is established,the dynamic characteristics of droop control are analyzed,and the stability of the system is verified.Finally,the mathematical model of three-phase voltage source inverter is established,the voltage and current double closed-loop control strategy is studied,and its parameters are designed.In view of the stability problem of DC-GCI caused by grid fluctuation under gridconnected mode,this paper conducts a detailed study.Because of the linear relationship in droop control,the power flow of DC-GCI is very sensitive to the fluctuation of grid frequency and voltage magnitude.The grid fluctuation will cause the output power of DC-GCI to change and make it suffer instantaneous inrush current.Limiting link in PI controller of voltage loop is usually used to limit the output current and maximum output power of DC-GCI.However,because the frequency fluctuation will lead to the saturation of the limiting link,the phase angle of the output of the active droop loop can not be consistent with the phase angle of the grid voltage,and the system can not stable operation.Limiting link can not alleviate the impact of grid frequency fluctuation on DC-GCI.Therefore,it is a challenge to limit the current meanwhile ensuring the stable operation of DC-GCI.To solve the above problems,a modified droop control strategy is proposed and its small signal model is established to analyze its stability.Under the fluctuation of grid frequency and voltage magnitude,the modified droop control and the limiting link of voltage loop PI controller can effectively suppress the inrush current and ensure the stable operation of DC-GCI.Among them,the modified droop control can effectively stabilize the output power of the system and suppress the steady-state output current.The limiting link in the PI controller of voltage loop can effectively restrain the transient current and compensate the slow response speed of the droop loop.The main contributions of this paper are verified by simulation and experiment.