Dielectric Spectroscopy for Diagnostics of Water Tree Deteriorated XLPE Cables

A high voltage dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of non-linear dielectric response in the frequency domain. Today’s commercially available system is capable of resolving low loss and small variation of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated ageing test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated XLPE cables can be recognised and classified into different types of responses related to the ageing status and breakdown strength. The influence of termination and artefacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

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