Measurement of the harmonic distortion of the insulation loss current as a diagnostic tool for high voltage cable insulation

The 60 Hz insulation loss current measurements in high voltage cable insulation containing water trees are described. To isolate the small insulation loss current from the significantly larger, quadrature (capacitive) current, a current-comparator-based high voltage capacitance bridge was used. It is shown that water trees introduce higher harmonics into the insulation loss current waveform and that the harmonic distortion of the loss current can be correlated with the length of water trees. Current waveforms for insulation free of water trees or insulation in which the water trees were completely dried out showed negligible harmonic distortion. Insulation with partially dried water trees maintained significant harmonic distortion of the loss current even though the loss tangent was very much reduced. The diagnostic aspect of the described measurements is discussed