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Electric charge accumulation on HVDC insulators in compressed-SF6-insulated transmission lines

This paper describes measurements of the magnitude and distribution of residual fields around a tri-post insulator in compressed SF6, after being subjected to sustained dc voltages of several hundred kV, of negative and positive polarity. The effects of geometry associated with insulator leg fittings, particle-trap slots and residual dust were studied. Comparative measurements were also made on an annular polyethylene insulator in the same coaxial system, and on uniform thickness disks of polyethylene and filled-epoxy resin in both uniform and non-uniform fields in air. The decay of the residual field involved (in some cases) field reversal leading to the conclusion that some part of field measured initially was due to a slow polarization or bulk charging of the dielectric rather than conduction through and/or over the surface of the insulator and drift of ions through the gas. This work was carried out under Department of Energy Contract DE-AC01-79ET-29059 for the design of a ±600kVdc gas insulated transmission line.