Circulating Current Control Strategy Based on Equivalent Feeder for Parallel Inverters in Islanded Microgrid

Among parallel inverters in islanded microgrid, circulating current originated by output impedance mismatches and output voltage differences cannot be neglected. Conventional droop control and fixed virtual impedance control are sensitive when the load changes. In this paper, the concept of equivalent feeder is proposed to cover the mismatch of external feeder impedance. The differences of external inductor, local load, and transmission line among parallel inverters are considered. Based on equivalent feeder impedance calculation, an aggregated control strategy of improved droop control and adaptive virtual impedance control is introduced. To improve power sharing accuracy, the equivalent feeder voltage drop of each inverter is calculated and compensated to the reference voltage of droop control. Conventional virtual impedance control is defective to cope with sudden load perturbations, especially when local loads exist. In this regard, the adaptive virtual complex impedance control can adjust to the equivalent feeder impedance, which provides accurate and quick response to load changes and line impedance mismatches. Simulation results show that the proposed control strategy can reduce circulating current effectively as well as improve power sharing accuracy.