Abstract: Capacitive compensation is widely used in power systems at different voltage levels. Gaoling back to back converter station is a key project of State Grid, which connects the Northeast and North China power grids. Mal-operations of the AC filter differential protection occurred after the station was commissioned in 2008. In order to keep the security of the AC filter and capacitor banks, unfavorable transient problems associated with capacitor switching and potential nearby faults were inspected thoroughly. The configuration of the AC filter in GaoLing station was introduced. Based on a fault record, the causes of one of the mal-operation were analyzed. The optimal filter switching time was studied. Based on the analysis, parameters for the point on wave switching equipment were calculated. Improvements was made in the AC filter differential protection.

Key words: high voltage direct current（HVDC） transmission, AC filter bank, synchronous switching, differential protection