A new method of estimating the inverse power law ageing parameter of XLPE based on step-stress tests

In recent years, HVDC cables have been greatly improved, 500kV XLPE DC cable has been developed, and many XLPE DC cables of lower applied voltage have been used in HVDC systems. However, there still is not a unified standard for testing high voltage DC extruded cable systems, only a recommendation. Furthermore the recommendation only gives a lower limit of ageing parameter (n=10 in the inverse power law tL.V-n=constant), which directly determines the voltage used in the test. A more precise value of ageing parameter is needed in order to improve the testing method. The most common method of estimating the ageing parameter uses constant-stress tests at different voltages (fields) and is based on the inverse power model, but this method takes a lot of time and cannot give usable data when the applied voltage is low. An alternative method is to use step-stress tests based on the cumulative life model. This approach can give usable data quickly, but parameter evaluation is much harder than for the constant-stress tests and the approximations necessary for the data solution unavoidably brings errors. In this paper, a new method of estimating the ageing parameter of XLPE is proposed. Step-stress tests are also used but a new parameter evaluation method is presented, which avoids complicated approximations in the data solution. This new method can greatly reduce the errors and easily give the ageing parameter. A series of step-stress tests on 0.3 mm thick XLPE samples have been carried out to illustrate the method. The values of the ageing parameter, n, obtained from different methods are compared, and it is found that the new method gives a greater consistency between different test protocols, indicating a more accurate estimation of its value. However, a comparison between the new method and the constant stress accelerated life tests is still needed to confirm the accuracy of the new method.