The application of HVDC transmission is an effective way to send power without affecting power flow, fault current level or voltage stability in the existing ac transmission system. Specifically, the self-commutation dc system is one of the candidates to realize robust power transmission.

A self-commutation dc system with a Gate Turn-Off (GTO) thyristor with a capacity of 37 MW was developed and put into operation in the 500-V Shin-Shinano substation in 1999 for test-use as a 50-Hz and 60-Hz multi-terminal back-to-back interconnection. The dc system has operated without any major failures, and its performance has gone as planned. Incidentally, in the Shin-Shinano substation, 50 Hz of the eastern Japan system and 60 Hz of the western Japan system are originally interconnected through a 600 MW conventional dc back-to-back converter facility with 500-kV ac systems. Figure 4 shows the schematic diagram for the three-terminal dc systems at the Shin-Shinano substation.