VOLTAGE BALANCING OF SYMMETRIC HVDC MONOPOLE TRANSMISSION LINES AFTER EARTH FAULTS

摘要

A voltage balancing unit (330) for a symmetric monopole high voltage direct current (HVDC) transmission line (303) interconnecting two voltage source converters (VSCs) (301, 302), the transmission line comprising first arresters (321–324) having a first switching impulse protective level (SIPL), is provided. The voltage balancing unit comprises a pair of second arresters (331, 332) having a second SIPL which is less than the first SIPL, and switching means (333, 334) being arranged for temporarily connecting, in the event of a voltage unbalance on the transmission line, the first arresters between either pole (304, 305) and ground. The invention makes use of an understanding that a voltage unbalance on the transmission line may be removed by temporarily connecting second arresters, with a significantly lower SIPL than the first arresters, for limiting the pole voltage to a level close to the normal voltage. Further, a method of voltage balancing is provided. Voltage unbalances may arise as a consequence of lightning induced earth faults (311).