A Study on the AC Breakdown Characteristics of K6 Gas according to Moisture Concentration in a Quasi-uniform Field at Atmospheric Pressure

Regulation (EU) 2024/573 bans fluorinated gases in distribution switchgear rated 24kV and below from 2026. K6, a mixture of 10mol% PMVE (perfluoromethyl vinyl ether) in CO₂, is an early-stage SF6 replacement whose breakdown characteristics under varying field utilization factor and moisture content have not been fully reported. This paper analyses the AC breakdown of K6 at atmospheric pressure in a quasi-uniform field. A 4mm diameter sphere-plane gap of 1, 2 and 5mm, giving field utilization factors of 0.73, 0.57 and 0.33, was tested in dry gas below 10ppmv and moist gas at 200ppmv. Ten measurements per condition were fitted to a two-parameter Weibull distribution, and the scale parameter taken as representative. Breakdown voltage increased with gap distance whereas breakdown strength decreased, so the two were least favourable at opposite gaps. Moisture lowered the breakdown strength in all cases, but the difference was statistically significant only at a utilization factor of 0.73.