The microgrid is an effective carrier for distributed generation to access large power grids.When there are multiple distributed generation in the microgrid,there will be multiple inverters operated in parallel.When multiple parallel inverters are operated with droop control,there is no communication signal line between each inverter unit.The system construction is convenient and flexible,and it is easy to achieve power sharing among multiple inverters.In this paper,in the grid-connected mode,when the line impedance is not matched and the grid voltage am plitude fluctuates,in order to improve the power sharing effect,the improvement of the droop control is studied.Firstly,the power transfer equation of the inverter when the equivalent line impedance is inductive,resistive and resistive is derived.Bas ed on this,the droop control equation under resistive and inductive line impedance is constructed,and the resistive connection line is There is no corresponding droop control equation for power coupling.For the multi-inverter used droop control system,when the line impedance is not matched,the active power output of inverter with the resistive connection line and the reactive power output of inverter with inductive connection line cannot be effectively equally divided.When there is a deviation from the inverter output voltage,the system will generate a circulation.In order to make the inverter with the connection line of resistance use the droop control equation,improve the power sharing effect and reduce the system circulation,the virtual impedan ce can be introduced in the droop control.Secondly,since the signal of a single inverter in the dq coordinate system is DC,it is convenient to design the control system and establish its small signal model in the dq coordinate system.Under the small-signal model,due to the coupling of the d-axis and q-axis components of the voltage and current,a double-closed-loop decoupling control of voltage and current is proposed.At the same time,the parameters of the voltage and current loop and the LC filter parameters of the system are robustly designed.The correctness of the voltage and current loop is verified in the simulation system.Finally,for the grid-connected mode,the traditional droop control is caused by the line impedance mismatch between the inverters and the amplitude fluctuation of the incorporated grid voltage,which causes the system instability and reactive power can't be effectively equally divided.An improved droop control based on virtual complex impedance and inverter output voltage and bus voltage deviation is proposed as feedback amount.The stability analysis and parameter configuration of the system were carried out by using the established micro-grid small-signal model with two inverters in parallel.Then the microgrid model with two inverters was built on the simulation experiment platform,and the correctness and effectiveness of the improved droop control were verified by simulation experiments. |