Small‐signal modelling and analysis of microgrids with synchronous and virtual synchronous generators

Abstract

Abstract In autonomous alternating current microgrids, the grid‐forming virtual synchronous generators can cooperate with the conventional synchronous generators to improve system inertia and frequency regulation capability. However, undesired active power oscillations between the synchronous generators and grid‐forming virtual synchronous generators may trigger their overcurrent protection and even result in a blackout. To explicitly reveal the oscillatory modes over all frequency bands, a high‐fidelity full‐order state‐space model is first developed. A potentially destabilising sub‐synchronous oscillation mode resulting from the interaction between grid‐forming virtual synchronous generators voltage controller and synchronous generators q‐axis damper winding is identified. Other modes reflecting the low‐frequency oscillation and frequency restoration dynamics are also assessed. Subsequently, to make a reasonable trade‐off between the accuracy and simplicity of system modelling, an enhanced quasi‐stationary model dedicated to low‐frequency oscillation evaluation is simplified from the full‐order type. The enhanced quasi‐stationary model features simplicity and low‐order benefits, which makes it more practical for multi‐generator system analysis. Moreover, by considering the dynamics of synchronous generators field winding and excitation system, the enhanced quasi‐stationary model significantly improves the low‐frequency oscillation characterisation accuracy compared with the existing quasi‐stationary model. The two developed models are comprehensively compared with the existing small‐signal models. Real‐time simulations based on RT‐LAB are conducted to verify the correctness of the theoretical analysis and the accuracy of the proposed small‐signal models.

Keywords

• distributed power generation
• modal analysis
• power convertors
• power system stability
• synchronous generators