Study of the decomposition of c-C4F8 and its mixture with N2 under 50Hz ac corona discharge

Because of its high dielectric strength, good partial discharge capability, low global warming potential (GWP), none ozone depletion potential (ODP) and no toxicity, fluorocarbon gases(c-C4F8 and its mixture with N2) are possible substitute for SF6, and can be used in electrical equipment. Gas insulated switchgears and transformers using fluorocarbon gas mixtures as dielectric and coolant are new promising power transmission and distribution equipment. To establish the foundation for their safe operation and easily maintenance, more studies are required. The decomposition of fluorocarbon gases subjected to 50 Hz ac corona discharges are presented in this paper. The experiments are conducted on the test model which was designed on our own. The gaseous decomposition products formed during the tests are analyzed and assayed by means of gas chromatography/mass spectrometry (GC/MS). The main identified gaseous by-products are: CF₄, C₂F₆, C₂F₄, C₃F₈ and C₃F₆ for pure c-C₄F₈ gas and CF₄, C₂F₆, C₂F₄, C₃F₈, C₃F₆ and C₂F₃N for mixture gas. In particular the influences of current amplitude, discharge capacity on the formation of main by-products are studied. Meanwhile, the relationship of relative content of decomposition gas with corona discharge amplitude and capacity are obtained. Based on these results and the literature conclusion, criterions for detecting the corona discharge in fluorocarbon insulated equipment is suggested, and it could be helpful to develop a corona discharge diagnostic method for the new type electrical equipment.