Decomposition of SF6-free Insulation Gases Due to Partial Discharges

Suitable SF₆-free insulation gases, such as mixtures with C4-fluoronitrile, should not only perform well regarding insulation and arc quenching, but also fulfill other requirements, such as stability against decomposition. One source for decomposing gases comes from partial discharge activity, which may be present in technical applications due to defects, such as free conductive particles. In this paper, an experiment series is presented, which subjects insulation gases to corona discharges. Different gas mixtures at different pressures were exposed to high voltage DC corona discharges for several weeks to determine the charge-based C₄F₇N decomposition by-product formation rates. The experiment is first assessed theoretically regarding expected decomposition rates and ion currents. Then measurement results are presented, covering decomposition and decomposition by-product formation rates. The decomposition rate in (mol/C) can be used to estimate the impact of partial discharge activity on the C₄F₇N concentration for real high voltage equipment contaminated with particles. Even for pessimistic scenarios, the C₄F₇N depletion over a 40-years’ time horizon is marginal and does not affect the performance of the switchgear.