Electrical Tree Growth from Needle-shaped Voids in LDPE

Electrical tree growth is a key degradation process in the failure of high voltage cables, but the subtleties behind established methods of reproducing the processes in the laboratory remain unclear. In this work, electrical treeing tests are performed in needle-plane, low-density polyethylene (LDPE) samples with and without initial conical (i.e. needle-shaped) voids around the needle tip. Different growth characteristics are seen among trees originating from needle tips with voids of various lengths (100-250 μm). Without initial voids, a branch tree develops consisting of stem channels and leaf-type channels, and partial discharge (PD) magnitudes increase almost linearly with tree length. Taking the void-free sample as a benchmark, trees form from voids either grow faster with PD events of higher magnitudes but without leaf-like channels, or develop into a distinct dense branch tree consisting of filamentary channel clusters accompanied by smaller PDs. Phase resolved partial discharge (PRPD) plots reveal that while the accelerated tree growth is dominated by high-magnitude PD clusters, the dense branch structure developed from voids is attributed to low-magnitude PD clusters which are associated with discharges within voids. The influence of voids on electrical treeing highlights degradation threats posed by small voids. The reported PD characteristics may also provide guidance for interpreting measurements of discharges during treeing degradation in insulation materials.