Design, manufacture, test and delivery of a 230 kV extruded irradiated crosslinked polyethylene cable. Final report

A project was initiated to develop a 230 kV solid dielectric cable for use in underground transmission. The dielectric is to be polyethylene, crosslinked by electron bombardment. Compared to the more conventional chemically crosslinked polyethylene, the irradiated cable is expected to contain less sensitive defects and thus be more suitable for a 230 kV rating. A toroidally shaped diode was developed to provide a uniform radiation dose to a thick-walled coaxial cable. The diode is to receive an output wave form obtained by ringing a Marx generator into a peaking capacitor. Initial evaluation of the toroidal diode was performed on thin plaques and tapes of insulating and semi-conducting polyethylene polymers. Additionally, some miscellaneous ethylene plastics were briefly investigated. Using a 4.8 MV Van de Graaff pulse generator in conjunction with several diode configurations, 15 to 35 kV extruded HMW-PE cables were irradiated. Dose rate, temperature, and pressure effects were evaluated. It was found that with limited dose rate it was possible to produce excellent crosslink density and uniformity at room temperature and atmospheric pressure. A subsequent 60 Hz voltage endurance test on an irradiated cable sample indicated it had long term, high stress capability. An engineering study conducted to determine an acceptable irradiator system design is reported. It was estimated that a 7 MV peak voltage at a rate of 2 to 3 pulse/sec can be provided by a Marx generator/peaking capacitor and should be capable of crosslinking a polyethylene wall thickness of approximately 2.5 cm. Based on the accumulated test results and on the performance of the 7 MV irradiator predicted, it appears feasible to continue the work effort into the next scheduled phase