Chemical aspects of the ageing of solid insulating materials submitted to partial discharges

The authors attempt to correlate the characteristics of the electrical signals due to partial discharges (PD) with the chemical aspects of the ageing of the insulating material subjected to these discharges. The samples are 19-μm-thick foils of polyethylene terephthalate, low-density polyethylene, and polypropylene. The PD development both in time and in space is related to the electrical pulse shape and to the chemical composition of the surface subjected to PD. The pulse shape can be related to the ageing of the insulating material. Several ageing steps appear. The surface wettability increases at the very beginning of exposure to PD. Changes of the surface morphology are studied using electron scanning microscopy. The by-products of chemical reactions due to PD have been analyzed by infrared microspectroscopy. The production of carboxylic aid by PD and the effect of this product and water on the ageing processes are also discussed