Circulating current analysis and robust droop multiple loop control method for parallel inverters using resistive output impedance

For parallel multi-inverters in island microgrid, the difference of equivalent output impedance and line impedance affects greatly on power sharing and circulating current restraining. From power transmission characteristics of parallel inverters, the influence of power sharing among resistive inverters was analyzed in this paper, where the conventional droop control was applied. Based on the analysis of circulating current characteristics of resistive inverters, a robust droop multiple loop control method was proposed, which included the outer power loop and the inner voltage and current loop. In the outer power loop, a robust droop controller is adopted to reduce the effects on accurate power sharing due to the impedance difference. Introducing virtual complex impedance including resistive component and inductive component, the equivalent output impedance of inverters is redesigned as pure resistance. Quasi proportional-resonant (QPR) control is applied to realize zero steady-state errors control of the output voltage with wide bandwidth for parallel inverters, which will reduce further the deviation of output voltage and restrain circulating current. The output voltage feedforward control and the proportional control of the capacitor current are adopted to improve the transient response and current disturbance. The effects on the equivalent output impedance in the different control mode and parameters were analyzed comparatively, and the proper parameters were selected. Simulation and experimental results show the correctness and validity of the proposed control method. © 2013 Chinese Society for Electrical Engineering.