Influence of Temperature on DC Electrical Tree Initiation in Silicone Rubber

The operating temperature of DC power cable has a critical impact on the electrical performance of silicone rubber (SIR) in the new types of extra-high voltage prefabricated cable accessories. We conducted tests to reveal the relationship between temperature and DC electrical tree initiation characteristics considering different defect types. The DC electrical tree initiation voltage and electrical tree shapes were measured via a digital micro-imaging system under 20, 60, 90 and 120℃, respectively. Results show that only two shapes such as single-branch trees and branch trees can be observed under the DC conditions. In intact needle-tip specimens, it is found that average tree initiation voltage decreases monotonically with temperature increasing. In air gap crack and needle coexisting specimens, tree initiation voltage is obviously lower than that of intact needle-tip specimens. The average tree initiation voltage and proportion of electrical tree shapes both show non-monotonic trend as the temperature rises in SIR specimen with air gap defects. Furthermore, the tree initiation voltage in air gap crack and needle coexisting specimens is consistent with secondary tree initiation voltage. Combined with experiments of differential scanning calorimetry (DSC), it is concluded that partial segment relaxation of SIR due to the rise of temperature leads to the decrease of tree initiation voltage. More attention should be paid to temperature since DC power cable system has to operate for long time at high temperature which has great effects on the DC electrical tree initiation characteristics of SIR. Moreover, air gap crack and needle coexisting specimens can be used to simulate the secondary tree initiation, which provides a reference for cable insulation design. © 2018, High Voltage Engineering Editorial Department of CEPRI. All right reserved.