Circulating-current analysis and power sharing control of parallel inverters with different capacities

This paper proposes control methods to achieve accurate active and reactive power sharing of parallel multi-inverters in proportion to their capacities. Firstly, according to physical properties of parallel inverters´ output power, the interaction mechanism between circulating-current and load-sharing is elaborated. Conditions for a control strategy to result in specified power sharing and minimum circulating-current are discussed based on the assumption that transmission lines are inductive. Limited effectiveness of traditional droop control is deeply analysed. An improved droop control strategy is proposed to achieve accurate proportional power sharing results unaffected by the impedance of lines. Moreover, since lines in microgrid are not inductive in practice, coupling of active and reactive power is observed on lines. Based on improved droop control, a new control strategy with added virtual reactance is furtherly proposed to enable decoupling control of active and reactive power sharing. The effectiveness of theoretical analyses and proposed control strategies in this paper is verified by PSCAD/EMTDC simulation results.