A Communication-Less Secondary Control Method for Parallel Grid-Forming Inverters in Islanded Microgrids Based on Mode Switching

The grid-forming inverters based on droop control show greatly vitality in microgrid applications owing to their excellent power sharing and frequency supporting ability. However, to obtain good power sharing dynamics required by the grid code, the droop control will introduce inevitable frequency and voltage deviations. Thus, an additional secondary control is indispensable to cancel these deviations so that the power quality requirement can be satisfied. In this article, a mode switching based communication-less secondary control method is proposed for islanded microgrids. In this method, after being triggered by obvious frequency or voltage deviations, the secondary control switches to the first operating mode and uses proportional regulator to maintain power sharing among grid-forming inverters. Subsequently, the secondary control switches to the other two modes in turn and adjusts droop offsets to dissolve the frequency and amplitude deviations of point-of-common-coupling voltage. The mode switching is controlled by preset timer so that no communication is required. Compared to the traditional methods, the proposed method can greatly improve the frequency and voltage regulation accuracy and avoid the accurate power sharing performance of droop control being deteriorated by the accumulated integral error. Meanwhile, the influence of the trigger delay can also be obviated. Hardware experiments are used to verify the effectiveness of the proposed method.