EVIDENCE OF STRONG CORRELATION BETWEEN SPACE-CHARGE BUILDUP AND BREAKDOWN IN CABLE INSULATION

Many processes have been considered over the years to explain the orig in of breakdown in cable insulation. Such effects as space charge buil d-up, tree growth, charge injection, etc. have all been discussed. Var ious techniques are now available to measure, in a nondestructive way, space charge distributions in insulators, These techniques, for insta nce the pressure wave propagation (PWP) method, can be used under appl ied electric stress and thus make it possible to follow the developmen t of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under de stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8x the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electri cal trees, the breakdown channel being centered on one of them. The st udy of space charge evolution in practical insulations permits an unde rstanding of the role of space charge in de breakdowns. This understan ding enables the development of technologies to suppress this effect a nd hence realize practical de XLPE transmission cables.