The Calculation of HV Capacitor Unbalance Protection Settings for AC Filters in GeZhouBa Converter Station

AC filters are important harmonics filtering equipment in HVDC transmission. The high voltage capacitor failure in AC filters is incidental. Some elements failure cannot be detected by the traditional protection due to the small fault current. The capacitor unbalance protection is designed for this situation to protect the high voltage capacitor units in the H-bridge against avalanche effects and avoid AC filter life reduction. In this paper, the principle of the capacitor unbalance protection and the unbalance current calculation are introduced in detailed. Both tool and model used for unbalance current calculation are described. The unbalance current in different cases is calculated for AC filters in GeZhouBa converter station. Three levels of capacitor element failure detection are provided: one for alarm, one for delayed trip and one for immediate trip. Some conclusions are made for the protection configuration and principles applied in the later project.