DC Microgrid StabilityAffected by Aggregated Constant Power Loads Based on Impedance Method

Abstract

Accession of aggregated CPLs such as electric vehicles may lead to oscillation instability of DC microgrid, which challenges the stable operation of DC system. In this paper, the mechanism of instability risk caused by aggregation of CPLs is revealed, and a stability evaluating method based on Nyquist criteria is proposed. First, based on the assumption that all the dynamic models of CPLs are identical, a reduced-order model of high-order DC microgrid consisting of N CPLs is derived. In the reduced order model, single CPL is connected to a constant DC voltage source via the N times of impedance of tie line and output impedance of source subsystem, while the negative impact of N on stability is defined as Impedance Multiplication Effect. Second, without losing generality, situations of N CPLs with different dynamic models are taken into consideration. Then, based on analysis above, a stability estimation method is proposed, which can quantify the instability risk brought out by aggregation of CPLs and estimate the maximum number of CPLs. Finally, study cases are conducted to demonstrate the correctness of proposed method.