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 °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 (Fa«) 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), Vati is 55.7% of that in N samples at 20 °C. In AN samples, Vati and tree shape do not change monotonically and Vati varies in a small range as temperature rises. Vati 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 Fröhlich'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 Vati decreasing. Electrical trees previously generated inside the cable or an air gap created due to mechanical damage cause Vati 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.