A Systematic Stability Enhancement Method for Microgrids With Unknown-Parameter Inverters

With massive electronic inverters, microgrids are threatened by the instability problems caused by the impedance interactions among inverters and the network. For the microgrids with black-box inverters (whose parameters are unknown due to industry secrets), it is hard to assess, much less enhance, the stability of such systems. This article proposes a systematic impedance-based stability assessment and enhancement method for the microgrids with black-box inverters. First, the return-ratio matrix Gnet of the system with both current-controlled and voltage-controlled inverters is formulated based on the nodal admittance matrix. And then, the sensitivities of the critical eigenvalues of Gnet are calculated with respect to individual admittances/impedances of inverters, which can identify the “trouble maker(s).” Moreover, the low voltage active damper (LVAD) is proposed for the stability enhancement of the system. An eigenvalue perturbation sensitivity analysis method is presented to calculate the sensitivities of the critical eigenvalues with respect to nodal parallel admittances, which identifies the optimal installation position for LVAD, and accordingly provides the guidance for the design of LVAD. The effectiveness of the proposed method is verified using a modified IEEE 6-bus system in PSACD/EMTDC and RT-Lab platforms.