Cooperative voltage balancing in islanded microgrid with single-phase loads

Due to the presence of single-phase loads, low voltage microgrids have substantial likelihood of becoming unbalanced. In an unbalanced microgrid fed by a single voltage source converter (VSC), the VSC has to inject a negative current for balancing the load voltage. In a microgrid fed by multiple VSCs, all the VSCs can cooperate in the voltage balancing task. This paper proposes a droop control method for parallel VSCs operating in an unbalanced islanded microgrid. Each VSC has its own controller that regulates the negative current references to compensate for voltage imbalances within the islanded network. By drooping the negative voltage references against negative output currents, parallel converters compensate for voltage imbalance and jointly share the negative currents in proportion to a predetermined ratio regardless of the plant parameters. The proposed droop method is integrated with the voltage-power droop/frequency-reactive power boost (VPD/FQB) control scheme in order to control and share both the positive and negative currents. Simulation results are also provided to validate the proposed method.