| Summary: | Load dumping overvoltage has a significant effect on insulation coordination of direct current (DC) transmission system. Based on the PSCAD/EMTDC electromagnetic transient simulation programs, this paper has built the electromagnetic transient calculation model of ultra high-voltage direct current (UHVDC) transmission system. Respectively, we have done comparative research on the overvoltage during load dumping under three rated-load operations, which are the bipolar operation with ground return, unipolar operation with metal return, unipolar operation with ground return, concluding that alternating current (AC) load dumping on the inverter side under these three operations can result in overvoltage no less than 50% on the DC polar line, DC valve top, as well as both ends of the converter transformer valve and the upper six pulses bridge also produce a large overvoltage. Besides, we observed a similar overvoltage level between unipolar ground return operation and unipolar metal return operation. These conclusions can provide suggestions for insulation coordination in various operations.
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