Abstract

Parallel droop-controlled inverters with renewable energy sources are widely employed in islanded ac microgrids, where dynamic interactions among them may cause small-signal stability issues. Since the active power-frequency droop scheme is applied, the dynamic interactions among inverters exist not only in bus voltage and transmitted current, but also in variable system fundamental frequency. This paper introduces a novel small-signal terminal characteristic model for droop-controlled inverter. Besides conventional impedance and admittance, a new set of terminal characteristics is proposed to characterize the dynamics of fundamental frequency. Furthermore, the small-signal model of parallel inverters is constructed based on the terminal characteristics of individual inverters. Covering the fundamental frequency interactions, a stability prediction approach based on generalized Nyquist criterion is proposed for parallel droop-controlled inverters. Besides the product of impedance and admittance, an additional term is added in the system return ratio, which consists of the proposed terminal characteristics associated with the fundamental frequency. Finally, experimental results validate the effectiveness of this proposed stability prediction approach.