An improved reactive power control strategy for inverters in microgrids

Due to the different system impedance, the conventional droop control scheme cannot achieve accurate sharing and control of reactive power among voltage-source inverters (VSIs) in microgrid (MG). In this paper, a new droop control strategy is proposed to raise the accuracy of the reactive power control. Firstly, the strategy eliminates the conventional droop for the reference voltage of the VSI under no load, and the reactive power is left to be actually shared among VSIs based on the truth that the inductive system impedance presents a natural droop characteristic as the output of reactive power increases. Secondly, a virtual inductance is introduced to make the whole system impedance predominantly inductive even in a low-voltage network with resistive line impedance, preventing the coupling between the real and reactive power and suppressing the circulating current quickly among paralleled VSIs. Finally, a voltage amplitude droop loop with zero steady-state error control is employed at the point where the VSI system is connected to the MG based on the fact that the whole ac bus has approximately the same voltage amplitude in well-regulated or small-scale MGs. The proposed strategy can effectively improve the reactive power sharing and control accuracy. Simulation and experimental results are given to validate the proposed strategy.