Imbalance protectiom is the main protection of capacitor C1 in AC filter of high voltage converter station.In this paper

bi-directional counting principle of C1 imbalance protection is introduced and the overvoltage of elements after the fault of capacitor is calculated and analyzed.The operaion setting of the protection section is studied based on the calculation result and tripping statistical analysis of C1 imbalance protection.It is found that the current protection setting is not favaourable for reliable operation of AC filter and would make output of segment II protection out of function.The adjustment suggestion about setting of section II and III is given respectively throughtout overvoltage performance analysis of capacitor elements and with combination of actual operation experience of AC filter.The protection setting after adjustment can reflect very well section action logic and reduce the AC tripping accidents of AC filter.