Transient Stability of Low-Inertia Power Systems with Inverter-Based Generation

This paper studies the transient stability of low-inertia power systems with inverter-based generation (IBG) and proposes a sufficient stability criterion. In low-inertia grids, interactions are induced between electromagnetic dynamics of the IBG and electromechanical dynamics of the synchronous generator (SG) under fault. For this, a hybrid IBG-SG system is established and a delta-power-frequency model is developed. Based on the model, new mechanisms different from conventional power systems are discovered from the energy perspective. Firstly, two types of loss of synchronization (LOS) are identified, depending on the relative power imbalance due to the mismatch between the inertia of IBG and SG under fault. Secondly, the relative angle and frequency will jump at the moment of a fault, thus affecting the energy of the system. Thirdly, the cosine damping coefficient results in a positive energy dissipation, therefore contributing to the stabilization of the system. A unified criterion for identifying both two types of LOS is proposed employing the energy function method. The criterion is proved to be a sufficient stability condition in addressing the effects of the jumps and the cosine damping. Simulation results are provided to verify the new mechanisms and effectiveness of the criterion.