On-site diagnostics of medium-voltage underground cross-linked polyethylene cables

Abstract

The polarization-depolarization current measurements were performed on miniature cables (aged in the laboratory), field-aged cables (removed from service and measured in the laboratory), and cables still installed in the local utility power distribution network. The measurements performed on miniature cables revealed significant changes in the area Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> under the HF components of the polarization current with aging time. The percentage increase of Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> could be related to the degree of insulation degradation due to water treeing. The low-frequency component of the depolarization current for field-aged cables was sensitive to polarization time but only when the measurements were performed shortly after switching off of the ac voltage. Virgin and relatively new cables did not show this effect. Cables of similar age and construction but installed in different environments (e.g., in conduits vs. bare ground) gave distinctly different depolarization current waveforms, which indicates that the technique can distinguish the more deteriorated from the less deteriorated cables. An extra year of service caused noticeable changes in the insulation depolarization current characteristics for some cables and almost no change in others. A large change in depolarization current could indicate advancing deterioration of the insulation and increasing probability of failure.