Rehabilitation of a gassy low pressure liquid filled 115 kV cable

The attempt to degassify a self contained liquid filled (SCLF) cable in Ontario Hydro is described. In September 1989, there was a high pressure alarm on one phase of a SCLF 115 kV cable circuit. The direct buried cable circuit is about 1890 m long with two joints and was installed in 1968. Oil samples from the cable had 10 to 12% gas content with high levels of hydrogen, methane, and ethane, and very low levels of carbon monoxide, carbon dioxide, and acetylene. From these results it was concluded that the gas was generated by low energy electrical discharges and the paper insulation was not degraded. Various utilities and cable manufacturers were contacted to find if they had experienced similar problems. The cable duct oil was flushed and degassified and the successive oil samples were analyzed in order to locate the gassing site. The joints were examined and oil samples were taken through holes drilled in the cable sheath. From the test results the problem site was located to within 300 m. Because of operational demand on the circuit, the cable was put back in service in May 1990 and has been monitored since then. The problems encountered in the investigation are described and the areas for further research are outlined in the paper