Sequence Impedance-Based Stability Analysis of AC Microgrids Controlled by Virtual Synchronous Generator Control Methods

Abstract

In this paper, sequence impedance-based modelling is applied to two different grid-forming converters which are based on virtual synchronous generator (VSG) concepts including a dual loop voltage control. The considered controls only differ in the feedback design (PLL-driven or not) of the power-related control loop. In general, impedance modelling is a suitable method to analyse stability issues related to converter controls for use in larger power networks. In this work, the analytical model of a voltage-controlled converter is illustrated first. Sequence impedance models are then proposed, which do not only predict the effect of two different VSG controls on the systems stability, but also reveal its frequency coupling effect and analogy to the classical droop control. In addition, a small power system consisting of VSG-controlled converters is analysed by their equivalent output impedances. These models and the stability of the converter cluster are validated by time-domain simulations and laboratory experiments. The close correlation between sequence impedance model, time-domain simulation and experimental results confirms the effectiveness of the derived models.