Evaluation of Moisture Ageing Status of Cross-Linked Polyethylene Insulation in Power Cables at Different Temperatures

Cross-linked polyethylene (XLPE) cables play an important role in power systems to transport electrical energy, and moisture is one of the important predisposing factors for insulation failure in medium and high-voltage cables. In this paper, a temperature-moisture accelerated ageing experiment is designed, and a method for assessing the moisture status of cable insulation is proposed. This is achieved by analysing the frequency-domain dielectric-spectral curves of samples exposed to different levels of moisture and calculating the integral value of the tangent angle of the low-frequency (0.1–1 Hz) dielectric loss as the characteristic parameter. Firstly, according to the principle of time–temperature superposition, the reduced relationship model between ageing time and ageing temperature is established. Secondly, the experimental data are analysed by nonlinear regression method, and the intrinsic relationship between ageing time, temperature, moisture content and ageing characteristic quantity is established, respectively. Based on the above results, the temperature–moisture coupling coefficient is further revised, and the equation of the ageing status of XLPE insulated cables under the two-factor coupling of temperature-dampness is established. Lastly, the accuracy of the model is verified by using laboratory ageing samples and returned cables. The results indicate that the model achieves an accuracy of 95.49% under controlled laboratory conditions. Then the service life of the returned cable is quantitatively analysed, and the calculated results are within the range of the model. This work has important guiding significance for the assessment of ageing status of power cables.