Breakdown mechanism in C3<.sub>F8/CO2 gas mixture under non-uniform field on the basis of partial discharge properties

This paper investigates simultaneous measurements of partial discharge (PD) current and the corresponding light emission phenomena of C₃F₈/CO₂ gas mixture under a non-uniform field with AC high voltage application in order to discuss the PD properties and breakdown mechanism of new gaseous dielectrics as SF₆ substitutes. Perfluoro carbon (PFC) gas mixture is a possible candidate as SF₆ substitute because it almost fulfills the environmental aspects, dielectric properties, boiling point, and other requirements of SF₆ substitute. The authors simultaneously measured PD phenomena such as time-sequential PD current pulses and corresponding light emission images before breakdown during the positive half cycle of the applied AC voltage where most breakdowns occurred. Consequently, it was found that there were three kinds of current pulse patterns, which depended on the gas pressure and the applied voltage amplitude. It was also confirmed that breakdown occurred around the voltage peak, i.e. 90 degrees. of the applied voltage, irrespective of the PD pattern. Furthermore, the simultaneous measurements enabled the authors to classify the discharge area in the N-shaped characteristic of the breakdown voltage based on discrimination of discharge types between streamer and leader discharges. Namely, the location where leader discharge is generated by the transition from streamer discharge in the N-shaped characteristic was indicated. These experimental results and the simultaneous measurement technique will help clarify the breakdown mechanism for new gases or gas mixtures as SF₆ substitutes. The obtained results will also contribute to insulation diagnosis for next-generation gas-insulated power apparatus with the newly developed gaseous dielectrics.