Impedance-Based Stability Analysis of Droop-Controlled Inverters Considering the Inner Control, the Interlock Time, and RHP Poles

This article extends a previously developed impedance model of a grid-forming inverter. The model is described in the stationary αβ reference frame. The frequency coupling caused by the interlock time and the droop control is considered. Depending on the droop parameters and the operating point, the impedance model exhibits a pole in the right half-plane (RHP). A formula to approximate the location of this pole is proposed and it is considered in the impedance-based stability analysis of a microgrid with two grid-forming inverters. Laboratory measurements not only validate the impedance model and the stability analysis, but also reveal remaining limitations. The influence of the inner control on the stability is demonstrated by comparing a control design with a cascaded voltage and current control with an open-loop control approach. Additionally, asymmetric droop parameters and the influence of changes in the operating point are discussed.