Research on Parameters of Step-stress Test to Obtain Life Exponent of XLPE-material used in DC Cable

DC life exponent n of XLPE (Cross-linked Polyethylene) insulation is an important basis of DC cable insulation design and factory test parameters selection. Usually, the statistical time of the insulation failure under different constant electric fields is usually used to adopt the power function, which can obtain the coefficient n. Because this method is time-consuming and inefficient, the step-stress is often used instead of constant-stress to find the equivalent n. However, the step-stress test results are affected significantly by the test parameters (the starting voltage, step-size and step-duration). At present, there is no uniform standard for step-stress test, and no literature analyzes the errors caused by different test parameters. In this paper, the selection scheme of various step-duration is tried, and the influence of step-duration on life exponent is discussed. Two characteristic values -- the minimum standard variance S_min and the negative second-order derivative, are used to evaluate the equivalence of step-stress calculation. The method of cumulative damage area division is proposed, which is used to analyze reasonability of step-stress parameters.