DC conductivity of voltalit epoxy spacers in SF6

Abstract

In order to determine the stresses in high-voltage DC cable accessories containing a combination of Voltalit (a proprietary epoxy insulation), SF6 and paper/oil insulation, a knowledge of the surface and bulk conductivity of the Voltalit is required. It is also necessary to determine the coefficients governing the dependence of both conductivities with temperature and stress, and the variation of conductivities with time after the application of a step function of direct voltage. Although the bulk conductivity of Voltalit has been determined previously, the surface conductivity in an environment of SF6 is not available. The time dependences of both bulk and surface conductivities after the application of a direct voltage are also unknown. Measurements are made over the range of stress and temperature expected in cable accessories, and equations relating both bulk and surface conductivities with stress and temperature determined. The variation of conductivity with time after the application of a step voltage is also presented.List of symbols E = stress, kVmm ~1 9 = temperature, °C a = bulk conductivity, Sm" 1 o s = surface conductivity, S t = time, min p = resistivity, flm

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