Dielectric spectroscopy-a diagnostic tool applied to water-treed cable

Considerable effort has gone into developing polymer formulations and cable designs to minimise failures through water tree growth. However, diagnostic techniques are still required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 15 kV, XLPE insulated, coaxial cable is used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After a duration of 8 weeks, a high density of trees was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10^-5 to 10⁵ Hz. Insertion loss measurements were also made in the frequency range of 3×10⁵ to 3×10⁹ Hz. From both types of measurement it was possible to distinguish between the cables containing trees and those that were untreed