A current-comparator technique for measuring harmonic distortion of the loss current in high voltage cable insulation

Water trees, a type of deterioration that develops in high voltage cable insulation in a humid environment, introduce harmonic distortion in the waveform of the loss current component of the capacitive current of cable insulation. The measurements of the total harmonic distortion could be used as a diagnostic tool to assess the operating conditions of power cables, in particular polymeric-type cables, and their remaining insulation life. A current-comparator technique to accurately measure the total harmonic distortion, including test results obtained on laboratory-aged specimens and short lengths of field-aged cables containing water trees, is presented.