On the ageing evaluation of multistressed corona suppressing system by partial discharge analysis

The authors describe a procedure for evaluating the performance of a corona suppressing system (CSS) exposed to severe environmental conditions. The test results presented confirm the validity of the diagnostic technique based on partial discharge amplitude distribution (PDAD) analysis for testing insulation ageing conditions. The adopted multistress ageing procedure seems to be quite severe when applied to CSS. The experimental results suggest some improvements of the specimens to use for the test: complete coils are preferred to insulated bars, and stainless steel should be used for the supporting structure. The PDAD diagnostic tool was found to be very suitable and sensitive. At the first step of the ageing, small-magnitude partial discharges inside the groundwall can be detected. Then, it is possible to follow the growth of surface phenomena, which start at the external part of the CSS and, in the final stage of ageing, cause slot discharges