Improvement of Dissolved Gas Analysis Technique for Oil-Filled Cable Facilities and Practical Application of Gas Analysis Technique to XLPE Cable Facilities

摘要

Previous Dissolved Gas Analysis (DGA) diagnostic method has been used as a tool for preventive maintenance to inspect abnormality of self-contained oil-filled (SCOF) cable facilities. However, the number of breakdown accidents in SCOF cable facilities especially joint boxes is increasing due to cable core movement by thermal expansion and water infiltration through the insulation flange made of phenol resin in Japan. DGA data for SCOF single-core cable joints and data from dismantling and examining them have been collected even after the criterion was suggested in 1999, and several cases were found in which the internal states of SCOF single-core cable joints as estimated by the criterion did not match the actual internal states. Therefore, we revised the previous DGA criterion with a total of 313 data, including previous data. The new DGA criterion obtained as a result can reveal the internal state more precisely, and became the new standard criterion in 2014. In addition, a DGA criterion for SCOF cable terminal joint boxes and another new DGA criterion focusing on various kinds of dissolved gas amounts and their rates was also considered. Cross-linked polyethylene (XLPE) cables and accessories, which have been widely used since the 1970s in Japan, are continuing to age. Among them, some pre-fabricated terminations have caused dielectric breakdown accidents because of PD deterioration which is derived from the solidification of silicone oil at the interface between the cable and the pre-molded insulation. As a result of dismantling broken-down terminations, it was found that deterioration progressed in many terminations because of PDs at the interfaces between the cables and the pre-molded insulations. When the silicone oil applied at the interface is under a high electric field, the silicone oil solidifies due to occurrence of PDs. The PDs generate gases such as acetylene and ethylene. It is possible to diagnose the degree of deterioration of pre-fabricated terminations by measuring the amounts of acetylene gas detected in the air within their metal casings at the sites. Some utilities are adopting this gas analysis (GA) technique as their inspection method according to the new inspection period and criterion, and detecting the abnormality, which leads to more appropriate preventive maintenance for XLPE cable facilities.