Effect of aging on the low frequency dielectric spectrum of XLPE

Dielectric permittivity measurements of XLPE (cross-linked polyethylene) over the frequency range from 10^-4 Hz to 10 Hz were performed using samples from a set of power cables covering a range of 0 to 14 service years. The measurement system consists of a computer-controlled time-domain spectrometer based on a sensitive electrometer. It basically measures the depolarization current from the device under test after it has been subjected to a high voltage for a sufficiently long time. A Fourier transform of the voltage and current is then applied for interpretation of the results in terms of dielectric permittivity in the frequency domain. By working with the depolarization current, the dielectric spectrum obtained is not influenced by the DC conductivity of the sample; in this way, one can clearly see the small dielectric losses present in the samples in the 10^-3-to 10 ^-2-Hz frequency range. It was found that the frequency of maximum absorption in the dielectric loss spectrum is related to the number of service years