DC current interruption in HVDC SF6 gas MRTB by means of self-excited oscillation superimposition

The Kii-Channel HVDC Link under construction in Japan is equipped with metallic return transfer breakers (MRTBs) in one converter station. A new MRTB for an interrupting current of 3500 A d.c. was developed. To interrupt a d.c. current, a method that produces a current zero point by superimposing a self-excited oscillatory current on a d.c. arc current was employed. For this purpose, a circuit was coupled in parallel to a SF6 gas circuit breaker. Developing a SF6 gas circuit breaker that has a large arc voltage drop gradient against currents permitted a large oscillatory current to be generated. A modified Mayr-type dynamic arc equation was newly presented. The d.c. interruption limits calculated using this equation agreed with the measured values.