Electrical tree initiation in XLPE cable insulation by application of DC and impulse voltage

Electrical tree initiation behavior of XLPE cable insulation was investigated by application of DC voltage, impulse voltage, repetitive DC voltage, and impulse voltage after DC prestressing. Treeing test samples are made of XLPE specimens with a pin-plane electrode system. The needle inserted has a tip radius of 5 μm and a pinpoint angle of 30 °, and the pin-plane distance is 2 mm. No trees appeared when DC voltage up to 70 kV or -60 kV was applied on the samples continuously or repetitively. Impulse voltage with or without DC prestressing could initiate an electrical tree. The tree initiation ratio increased due to higher impulse voltage, increased DC prestress level and shorter rest time between DC prestress and impulse voltage. The measured initial tree length showed a proportional relation with the impulse magnitude, whereas it was almost independent of DC prestress level and the rest time. The observations are explained by the injection, accumulation, dissipation, trapping and detrapping of space charges in the region around the needle tip.