Lightning Impulse and AC Breakdown Characteristics of SF6 and its Alternatives

SF6 alternatives with significantly lower environmental impact have been extensively researched. This paper investigates two electrode configurations and test pressures to mimic the conditions as found in MV/HV equipment to systematically compare the breakdown performance of SF6, C3H2F4 and a mixture of 30% CF3I / 70% CO2 under AC and LI stresses. SF6 possesses comparatively better breakdown performance than the other alternatives investigated in this study. C3H2F4 shows “first breakdown” behavior and only the stable data series is considered for evaluation of 50% breakdown voltage, U50. The lower boiling points of C3H2F4 and CF3I limit its potential application to medium-voltage equipment with a low operating pressure. Due to pyrolysis, there is formation of soot and iodine post-breakdown in C3H2F4 and 30% CF3I / 70% CO2 respectively. This will reduce their dielectric performance and could pose health hazard to maintenance personnel.