Abstract: This paper investigates a fault case in the UHV AC transmission line series compensation protection system. Through comprehensive analysis of the entire fault occurrence process, the root cause of the high-value unbalance protection operation in capacitors is identified. Based on the fault mechanism, optimization suggestions for the logic modification of the series compensation protection system are proposed. These improvements aim to enhance the robustness of the protection logic and mitigate the impact of similar faults on transmission line reliability.