Virtual Flux Droop Control with Constant Switching Frequency for Power Sharing between Parallel Inverters in Islanded Microgrids

Droop control method is a popular method for power sharing in islanded microgrids with parallel-connected inverters. Virtual flux droop control is a simple strategy based on the direct flux control (DFC) and hysteresis controllers, in which the active and the reactive powers are proportional to the phase angle and the amplitude of the virtual flux (integral of the output voltage of the inverter), respectively. However, because of the hysteresis controllers, it suffers from a drawback that is the variable switching frequency, which leads to the voltage and current ripples. In this paper, novel phase angle and virtual flux controllers with simple structure have been proposed which lead to a fixed switching frequency, while there is no need for the space vector modulation. Therefore, the ripples of voltage and current have been reduced. In order to evaluate the effectiveness of the proposed method, simulation studies have been carried out in Matlab/Simulink. The results show that the proposed controllers can reduce the voltage and power ripples, while the switching frequency has been fixed.