A novel HVDC line protection based on the ratio of harmonic current

Owing to the fact that the present differential protection for HVDC transmission line is of poor reliability and operates with long time delay, a novel HVDC line protection method based on the ratio of harmonic current is proposed in this paper. The ratio of harmonic current measured at DC filter to harmonic current measured at DC line is analyzed under the fault superimposed network. It is found that for internal faults the ratios of harmonic current at rectifier side and inverter side are both close to one, while for external faults the ratio of harmonic current at one side is close to one and the ratio at the other side is relatively large. Thus the ratio of harmonic current can be used to identify the internal faults from external faults. The faulted pole is also identified by harmonic current. A large number of simulation results show that the proposed protection method can identify the fault accurately and is insensitive to fault location and fault resistance.