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 dc was developed. To interrupt a dc current, a method that produces a current zero point by superimposing a self-excited oscillatory current on a dc arc current was employed. For this purpose, an LC circuit was coupled in parallel to an SF6 gas circuit breaker. Developing an 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 dc interruption limits calculated using this equation agreed with the measured values.