Evaluation of voltage endurance characteristics for new and aged XLPE cable insulation by electrical treeing test

The voltage endurance coefficient of solid insulating materials can be determined by electrical treeing tests. The measured value is influenced by the test conditions, and its effectiveness remains to be further demonstrated. In this study, samples were made from the XLPE insulation of three 110 kV AC cables, two new and one operated for 16 years. In treeing tests performed with a pin-plane electrode system, the tree initiation times were measured under both progressive and constant voltage, and voltage endurance coefficients were determined by the residual voltage method. The material properties of XLPE were measured using the FTIR, DSC and TSC methods. For the same sample, the voltage endurance coefficient measured with a 10 μm needle electrode was larger than that measured with a 5 μm electrode, indicating the influence of space charges on the electrical field around the needle tip. For the insulation samples of the two new cables, the measured voltage endurance coefficients were different, and the difference became more significant when the rise rate of the progressive voltage decreased. The superior resistance to electrical aging of the insulation with a higher voltage endurance coefficient was confirmed via material property measurements. For the insulation samples of the aged cable, along with a decrease of the progressive voltage rise rate, the tree initiation voltage increased, which was opposite to the test results obtained for the new cable insulation samples. The voltage endurance coefficient could not be measured using the residual voltage method because the trees always initiated at the rising edge of the constant voltage, except for a low value measured at the lowest constant voltage. The abnormal performance of the aged cable insulation might be the result of increased defects and impurities caused by operation aging, which enhanced the space charge effect, especially in the pin-plane electrical field of the electrical treeing test.