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The Kii channel HVDC transmission system (1400 MW in the first stage) was commissioned in the year 2000 and transmits power generated in Shikoku Island to the AC 500 kV system on Honshu Island. The transmission line of approximately 100 km between the converter stations consists of two sections-undersea cable and overhead line sections. The transmission lines consist of 2 main lines and 2 return lines. The fault protection function was verified by making artificial grounding faults both for the main lines and return lines during a series of commissioning tests. The characteristics of the protection system of the return circuit including the metallic return transfer breakers (MRTBs) were evaluated from the tests. At some of the points along the line, the grounding arc extinguishes itself before closing MRTB and at other points; the, grounding arc is cleared by closing MRTB. It was confirmed that the clearing time of the grounding arc was consistent with the system design parameters.