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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. A new algorithm based on mathematical morphology is presented for fast discrimination between internal fault and external fault. The method is based on the principle that the waveform caused by internal fault contains much wave peak and wave trough and the waveform caused by external fault changes slightly.