Diagnostic measurements of high voltage insulation using current comparator technology

The 60 Hz dielectric loss measurements in high voltage cable insulation containing water trees are described. A current-comparator-based high voltage capacitance bridge has been employed to isolate the small loss currents from the large quadrature (capacitive) currents. It is shown that water trees introduce higher harmonics into the loss current waveform and that the total harmonic distortion of the loss current can be correlated with the length of water trees. Insulation free of water trees or insulation in which the water trees were completely dried out did not show any harmonic distortion of the loss current waveform. Insulation with partially dried water trees maintained significant harmonic distortion of the loss current but showed reduced dielectric losses. The diagnostic aspect of the measurements described is discussed