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Laboratory scale investigations were conducted to assess the feasibility of improving the dielectric characteristics of crosslinked polyethylene (XLPE) insulated cables rated 138 kV and above. Impregnation of slabs cut from 138 kV cables and of model cables with liquids increased the ac breakdown strength, presumably by filling the microvoids (and possibly in some cases acting as a voltage stabilizer) and preventing premature failure due to electrical discharge. The degree of improvement depends on the chemical nature of the impregnant. Vinyl toluene, styrene, styrene oxide and vinyl pyrrolidone were found to increase the dielectric strength substantially and in some cases improve the VT curve. With vinyl toluene, the improved strength is approximately 3750 V/mil compared to 2300 V/mil for the control. In situ polymerization with lauryl methacrylate and vinyl toluene monomers and impregnation with polyethylene wax have proven that the solid polymer is as effective as the liquid monomer. Optical and scanning electron microscopic examination supports the concept of void filling. The volume resistivity of semiconducting shields is not materially affected by the impregnation and polymerization, probably because the carbon black inhibits the polymerization. An economic analysis including raw materials' costs and manufacturing costs suggests that the process of impregnation andmore » polymerization costs somewhat less than lead sheathed, dry cured power cables.« less |
