A Pilot Line Protection Scheme Based on the Phase of Specific Frequency Current

For the sake of the current differential protection for HVDC transmission line is of poor reliability and operates with long time delay, research on a novel pilot line protection scheme is urgent needed. The phase features of superimposed current at specific frequency is used to identify the line faults. For the internal fault, The phase of the harmonic current we measured at the current protection unit on the DC line positively related with the harmonic current we measured at DC filter on the same side, while for external fault the correlation between the phase of the two harmonic currents on one side is different from another. According to this feature, a pilot line protection scheme based on the phase of specific frequency current is proposed. In this paper, we use S transform to extract the specific harmonic current and the Correlation analysis to determine the correlation between the two currents. Simulation results show that the protection scheme proposed is able to identify the internal fault and external fault correctly and quickly and is of capability to distinguish fault with high transition resistance.