Sequence Impedance-Based Stability Analysis of Droop-Controlled AC Microgrids

In this paper, the sequence impedance-based stability analysis is applied to droop-controlled converters with an inner cascaded voltage and current control. In general, the stability analysis of converter-dominated grids by means of Thevenin and Norton equivalent representations offers a method to analyse the behaviour of large converter systems. First, an analytical model of a voltage-controlled converter is implemented. A sequence impedance model is then proposed, which not only predicts the effect of the droop control on the system's stability, but also reveals its frequency coupling effect. Furthermore, a small converter cluster of two droop-controlled entities is analysed with respect to their impedances. These models and the stability of the converter cluster are validated in time-domain simulations. The close correlation between sequence impedance model and time-domain simulation confirms the effectiveness of the derived model.