Thermal ageing of cross-linked polyethylene cables: mechanism and influence on the time-varying property of frequency domain dielectric spectroscopy

In this work, samples of cross-linked polyethylene (XLPE) cables are subjected to accelerated thermal ageing experiments, and the frequency domain dielectric spectroscopy (FDS) test is performed to evaluate the ageing status of XLPE cables accurately on the basis of dielectric characteristics. The FDS results of samples subjected to accelerated thermal ageing experiments at three different temperatures illustrate that dielectric loss does not monotonously increase with the passage of thermal ageing. Instead, dielectric loss first increases, then decreases and finally increases. The results of samples from cables operated for years are in good agreement with those of the former samples. Further study shows that the change in dielectric loss can be mainly attributed to the partial melting of the imperfect crystals in XLPE, the subsequent process of recrystallisation and the process of molecular chain breakage. On the basis of the above results, the rise in the dielectric loss spectrum does not necessarily indicate that XLPE has entered the later stage of ageing. The historical data of FDS tests are necessary to obtain an objective judgement of the ageing state of XLPE cables.