| HVDC transmission will play an important role in transmitting power fromwest to east and interconnecting national network because of its unique advantages.But the relay protection could not clear fault quickly when fault occurred andvarious unpredictable reasons occurred such as element failed or DC power supplydisappeared. So it's necessary to install protection in superior substation whichcould be used as remote back-up protection to clear fault. Negative sequencedistance protection, elliptical distance protection and low current protection wereused as back-up protection to cleat HVDC system fault, DC voltage and currentcombined with differential protection was used as back-up protection to cleat ACsystem fault. And simulation results prove these protections operate reliable andcan be realized easily due to its simple logic.
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