Study On Step Stress Test Method Of XLPE Insulation Electric Life Model And Influencing Factors Of Cumulative Damage

The inverse power function electrical life model(IPM)of cross-linked polyethylene(XLPE)characterizes long-term pressure resistance of XLPE cable insulation.The IPM model parameters,voltage endurance coefficient n and cumulative damage D are important basis for cable insulation design.The constant stress test is a common method to obtain the insulation IPM parameters.However,the large time-consuming limit the application of constant stress test.In contrast,the step stress test has the advantages of short time-consuming and high efficiency,and it is more suitable for obtaining the electrical life model parameters of the cable sample when the number of samples or the test time are limited.Due to the complicated insulation failure mechanism,the test results of the step stress tests are affected by the test parameters such as the press step and press time,and it is difficult to judge the equivalence with the constant stress method.At present,most researchers select experimental parameters based on experience,and there is no standard for reference.And the IPM parameter values obtained by different research institutions are also not comparable.In this paper,the electrical life model of XLPE insulation under DC voltage is selected as the object,and the equivalent method of the step stress and constant stress test results are studied.Taking the main mechanism of XLPE failure under the two test methods as the equivalent basis,the selection method of step stress parameters and the calculation method of IPM parameters are studied;besides,The influence factors of the cumulative damage D in the IPM model are studied,and the relationship between the D value and electrical strength is analyzed to discuss the physical significance of the parameter.The main conclusions obtained include:(1)A parameter selection method for step stress test is proposed.D-t curve,relationship curve S(n)of damage matrix D,and the second derivative of the S(n)curve d~2S(n)/dn~2 are used as the basis for selecting test parameters.It provides theoretical support for the equivalence evaluation of the step stress and constant stress test results obtained by the XLPE electrical life model.(2)The influencing factors of the cumulative damage D in the IPM electrical life model are studied.The results show that the cumulative damage D value required for XLPE insulation to fail has a certain degree of dispersion,when the test sample and test conditions are constant,the D value changes within a certain range.Changes in the electric field and temperature will cause a significant change in the distribution interval of D;when the main mechanism of insulation failure is electrical breakdown,the cumulative damage required is less than the cumulative damage required for failure under electro-thermal breakdown.And an increase in the experimental temperature will lead to an increase in the cumulative damage D.(3)The physical significance of the cumulative damage D is analyzed,and the relationship between the cumulative damage and the decrease in electrical strength is discussed.It is verified that the cumulative damage in the IPM model is positively related to the magnitude of the decrease in electrical strength caused by electrical damage.The mechanism of cumulative damage change is discussed.It is proposed that the rate of electric strength reaches the decreased electric strength of XLPE under different test electric fields leads to the difference in cumulative damage D_c in different VEC regions.The increase in the electrical intensity at high temperature is the reason for the cumulative damage at the test temperature.