Factors affecting partial discharges in SF6 gas impregnated polymer film insulation

The partial discharge characteristics of SF₆ gas-impregnated PET (polyethylene terephthalate) and PP (polypropylene) film insulation were studied. Experimental results indicate that it is essential to distinguish partial discharges at the electrode edge from discharges in the gas-filled mid-dielectric void because they have different characteristics. Apart from the common influencing factors such as relative permittivity and the total thickness of the film insulation, the mid-dielectric discharge also depends on the thickness of the single sheet while the electrode edge discharge is a function of the electrode edge radius. Usually it is the partial discharge at the electrode edge that limits the design stress, but the mid-dielectric discharge can be a limiting factor if the total thickness of the film insulation is below a critical value. It is also confirmed that partial discharges occurring in the gas-filled mid-dielectric void can be calculated with satisfactory accuracy