Transient stability analysis and enhancement control strategies for interconnected AC systems with VSC-Based generations

This paper analyzes the transient angle stability mechanism of interconnected AC power systems with voltage source converter (VSC) based generations and presents a stability improvement control strategy for the VSCs. Firstly, the mathematical model of the prototype AC system with both synchronous generator (SG) and VSC is developed, considering the dynamics of the pre-fault, fault-on, and post-fault systems. For the heterogenous three-machine system, the analytical method for the critical clearing time (CCT) is proposed for transient stability assessment. Then, the evolution of the transient stability margin of the AC system is studied, considering the impact of the VSCs. Under the conditions that VSCs can keep synchronism with the grid during the fault, reducing the active current of the VSCs benefits the transient angle stability of SGs. Based on the mechanism analysis, an enhancement control strategy is developed. It achieves active and reactive current modulation of the VSC using the frequency measurements of the critical SGs (CSGs) in the AC system. The proposed method devotes to boosting the electromagnetic power of CSGs during the entire transient process. Finally, time-domain simulations in PSCAD/EMTDC demonstrate the theoretical analysis. The effectiveness and robustness of the enhancement controller are further validated by the time-domain simulations in a 2-area interconnected system with 4 SGs and a modified New England 39-bus test system.