Transient stability emergency control for AC/DC grids considering successive commutation failures

Abstract

The prevalence of Line-Commutated Converter (LCC) HVDC makes the coupling and interaction phenomenon between AC system and DC system of particular complicated. There are significant challenges to determine control strategy using state-of-the-art approaches especially when discrete events (e.g. switching and state resetting) happen frequently due to successive commutation failures (CFs). An enhanced transient stability emergency control (TSEC) for AC/DC grids based on the optimal control is presented to maintain the transient stability and mitigate the successive CFs caused by AC faults. In the proposed TSEC method, an extinction advance angle constraint is added to the TSEC model to mitigate the successive CFs. The emergency power control of LCC-HVDC is employed as an additional control measure to reduce generator-tripping and load-shedding amount. The direct sequential approach is adopted to obtain the optimal solution of TSEC to improve the calculation efficiency. In the direct sequential approach, the modified adjoint sensitivity analysis (MASA) with the consideration of adjoint jump conditions is utilized to increase the gradient calculation accuracy. The effectiveness of the control strategies and the accuracy of the sensitivity calculation are verified on two test systems.