Pre-breakdown phenomena in XLPE under highly divergent field conditions

Prebreakdown phenomena were studied in XLPE (cross-linked polyethylene) of the kind used in the fabrication of extruded power cables. Specimens having a point-plane electrode geometry, obtained by insertion of steel Ogura needles in rectangular blocks of XLPE, have been prepared by means of an ad hoc experimental procedure. A video-recording technique made it possible to observe the treeing growth in real time. Experimental results are presented and discussed. Also discussed is a phenomenological model which considers a further descriptive parameter, E_b, regarded as the electric field at the surface of an ideal equipotential sphere corresponding to the treeing damage structure at the instants immediately proceeding the breakdown, neglecting the contribution of the space charge