The identification of internal and external faults for ±800kV UHVDC transmission line based on S-transform of the polarity wave

There is a smoothing reactor and DC filter between the inverter and the direct current line to form a boundary in the HVDC transmission system. Since this boundary presents the stop-band characteristic to the high frequency transient voltage signals, the high-frequency transient voltage signal caused by external faults through boundary will be attenuated and the signals caused by internal faults will be unchanged. In this paper, a new technique in which the S-transform is used to analyze the transient signals is proposed for distinguishing internal faults from the external faults based on the significantly difference of high-frequency transient voltage on the point of protection.