Study on Decomposition Gases by Partial Discharge in Insulation System for Self-contained Fluid Filled Cables

The dissolved gas analysis (DGA) is one of the main diagnostic techniques for oil-filled electrical equipment. Though self-contained fluid filled (SCFF) cables have been maintained by DGA, some faults occurred on SCFF cable joints in Japan. Therefore, it is important to improve DGA for SCFF cables. In this viewpoint, the characteristics of decomposition gases by partial discharges in the insulation system for SCFF cables were experimentally investigated. The insulation layer of test samples was composed of sheets of oil-impregnated insulating paper and an oil gap that simulated a butt gap in SCFF cables. Partial discharges were generated in the oil gap in short or long periods, and the oil gap and light emission due to partial discharges were observed. In addition, after partial discharge generation, the insulating oil in the oil gap was taken and hydrogen and hydrocarbon gases in the insulating oil were detected by DGA. As the result, in the short period case, partial discharges occurred through insulating oil, and main decomposition gases were H
2
and C
2
H
2
. On the other hand, in the long period case, partial discharges occurred in bubbles, and main decomposition gases were H2, CH4 and C2H6. It was estimated from this result that the difference of the main decomposition gases between two cases depended on whether partial discharges mainly occurred through insulating oil or in bubble.