Effect of DC discharges in mineral oil on degradation characteristics of oil-impregnated pressboard

Oil discharge characteristics have been studied extensively, yet there have been few studies regarding the effect of oil discharges on pressboard. It is not yet known whether a single discharge of large magnitude or the accumulation of many small-magnitude discharges damage oil-impregnated pressboard, nor has any researcher to date identified the precise mechanism of pressboard failure. The present study was conducted to determine the oil discharge magnitude at which damage to pressboard occurs and the mechanism of the subsequent pressboard failure. The harmful oil discharge level was quantified by the comparison of the DC breakdown voltages between new and discharge-treated pressboards. Exposure to a 7.8 nC oil discharge repeated 100 times was found to cause a decrease in the DC breakdown voltage of the pressboard. Oil discharges of 13.1 nC repeated 50 times and of 26.8 nC repeated 10 times also degraded the pressboard. A single discharge of large magnitude will degrade the pressboard at a smaller integrated charge value than will several discharges of small magnitude. A "dual-parameter grid diagram" was proposed to numerically quantify the harmful oil discharge level. Further observation of the discharge-treated pressboard indicated that the oil discharge created a "whitish area" and "wormhole-like" traces on the surface of the pressboard, due to the vaporization/decomposition of the transformer oil and the carbonization of the cellulose, respectively. Subsequent discharges will propagate along the gaseous wormhole into the pressboard, and finally, break down the pressboard in bulk.