Temperature Dependence of DC Electrical Tree Initiation in Silicone Rubber Considering Defect Type and Polarity

In this article, the electrical tree initiation behavior in silicone rubber (SIR) under DC voltage was studied. Tests were conducted to reveal the effects of temperature (20-120 degrees C), defect type and voltage polarity on electrical tree initiation. In samples with an undamaged needle-tip (N samples), we find that the average tree initiation voltage (V-ati) decreases exponentially with increasing temperature by 42%. Single-branch trees and multiple-branch trees are observed. The probability of generating multiple-branch trees decreases with increasing temperature. In samples where an air gap or crack is in the needle (AN samples), V-ati is 55.7% of that in N samples at 20 degrees C. In AN samples, V-ati and tree shape do not change monotonically and V-ati varies in a small range as temperature rises. V-ati for N samples under negative voltage is much larger than that under positive voltage. This polarity effect is reversed in AN samples. Gas discharge theory and Frohlich's theory of dielectric breakdown explain electrical tree initiation behavior at different temperatures. Based on differential scanning calorimetry (DSC) results, we believe that partial segment relaxation of SIR at high temperature leads to V-ati decreasing. Electrical trees previously generated inside the cable or an air gap created due to mechanical damage cause V-ati to of a subsequently generated tree to drop to nearly half of its typical level. This information is useful for the design of HVDC cable insulation.