Experimental investigation of treeing growth in polyethylene insulation

Experimental investigation of PE insulation distortion in power electrical fields was carried out by means of optical registration of treeing growth in electrode system "needle-plate". Experiments were performed at following conditions: radius of the needle tip was 2 /spl mu/m; distance between needle and plate -1, 5 mm, applied voltage -6, 7, 8, 9 kV. Carried out experiments have shown some typical features of treeing growth in PE under electrical stress. After initial stage, which is accompanied by increase of fractal dimension, the next stage follows. Stabilization of fractal dimension occurs at this stage. Duration of this stage depends on the level of electrical stress. Period that precedes the breakdown is accompanied by the decrease of fractal dimension of treeing and strong increase of its leading branch length. Another peculiarity of PE treeing growth is increase of its boughs' thickness. If time of the breakdown was less then 20 minutes, branches that are thicker near the needle tip were still thin at their ends. Upon prolong electrical stress treeing boughs have thickened all along. This process may lead to electrical field decrease in the growth area of a treeing and almost complete cessation of treeing penetration inside insulation gap. Treeing with weakly ramified thin boughs are the most dangerous from the point of view of insulation gap breakdown. If treeing has not caused insulator gap breakdown for short time (less then 2-3 minutes), its growth may substantially slow down even upon big stress (U-9 kV). These processes are accompanied by thickening of treeing branches and may be connected with it. Bushing-like ramified treeing with strongly thickening branches are typical for smaller stress (U=6 kV). Such treeing may grow during much longer time without insulation gap breakdown.