Initiation of electrical degradation in high voltage polymeric cable insulation : electroluminescence detection

Ageing detection and indications of insulation degradation are often achieved when ageing is already significant, for example when partial discharge (PD) activity is detected from a large enough micro-cavity. Little is known about the degradation before PD activity occurs. Nowadays, long-term degradation in discharge free situations is increasingly the subject of attention. This is because the size level of defects in the insulation has been highly reduced thanks to progress in technology. However the stress increase, due to a reduction of the cable insulation thickness for practical and economic reasons, could amplify the effect of phenomena which were not considered as important until recently. Internal defects cannot be completely eliminated from high voltage cable insulation and one of cable manufacturers' quality test has revealed the presence of micro-defects in some cross-linked polyethylene (XLPE) transmission cable main insulation. The aim of this study was to investigate the significance of these micro-defects on the degradation of the insulation under ac stresses.In order to achieve that, a better characterisation and understanding of this phenomenon was primordial. A procedure for the observation of this phenomenon was developed. Since these features were a few microns across, they were likely to induce only a low level of degradation and it was unlikely that they would sustain detectable (electrically) discharges. A method for early detection of electrical ageing before the onset of PD activity was therefore necessary and electroluminescence (EL) was selected. An experimental arrangement was integrally designed to carry out EL measurements on insulating samples in a uniform field configuration at room temperature under ac stress. The experimental system, using a very sensitive cooled CCD detector, made it possible to successfully measure (with short integration time) and monitor the integral electroluminescence in LDPE and XLPE cable insulation samples under relatively low voltages, and to determine the spectral range as well as allowing the imaging of this very low level of light. The difference in temporal behaviour and spectral range of the emission between XLPE cable samples containing micro-defects and samples without micro-defects showed that the degradation of the insulation under ac stresses was accelerated by the presence of the micro-defects.