Robust droop multiple loop control method for resistance output parallel inverters

For parallel inverters in island microgrid, the equivalent output impedance and the line impedance affects greatly on power sharing and circulating current among inverters. Based on analysis of power transmission and circulating current of the resistive output inverters, a robust droop multiple loop control method is proposed, which included the outer power loop and the inner voltage and current loop. In the outer power loop, a robust power droop controller is adopted to improve accurate power sharing due to the impedance difference. The virtual impedance including resistive component and inductive component is introduced to the feedback control of inverter output current to make the equivalent output impedance of inverters into pure resistance. Quasi-proportional resonant (QPR) controller is applied to track the output voltage with zero steady-state errors, which reduces the deviation of output voltage and restrains the circulating current. By introducing the output voltage feedforward, the filter capacitor current control is adopted to improve the transient response and current disturbance. Simulation and experimental results verify the validity of the proposed method.