On-site diagnostic technique for smart maintenance of power cables

Polymeric insulation of power cables has excellent short-term dielectric properties but is subjected to serious degradation problems due to defects, water, ionic contamination, partial discharges, electrical treeing, etc. Since more than 20% of installed underground power cables are older than their design life-time, utilities are faced with important maintenance related questions whether to repair, refurbish or completely replace old cables. In the era of Smart Grid, accurate assessment of the operating state of cable insulation through reliable diagnostic measurements is crucial to the so called “Smart Maintenance”, i.e. the replacement of only those cables that may adversely affect the reliability of the network in the near future. At the National Research Council of Canada, an on-site testing technique for estimating the degree of degradation of power cables based on the polarization/depolarization current measurements has been developed and tested for two years on underground power network of a local utility. Using a noiseless, high voltage solid state switch, both the high frequency (HF) and the low frequency components of the depolarization current (I
_Dep
) were measured and analyzed. It was observed that the area under the HF component of the I
_Dep
curve, Q
_Dep
, could be linked to the dielectric loss factor, and the intensity of water treeing in cable insulation. Furthermore, it was observed that the I
_Dep
waveforms of new and aged cables could be fitted to an equation, which contains a parameter that correlates with the degree of insulation degradation. This new insulation degradation indicator and the Q
_Dep
characteristics could be used for monitoring the operating condition of cables in service and would help utilities to develop cable replacement programs based on the actual state of the insulation operating conditions. Long term aging (~3.5 yrs) of XLPE miniature cables in NaCl solution has shown a large increase of the area under the polarization current curve, Q
_P
with aging time. Comparing the Q
_P
characteristics of miniature cables with those of cables in the field could also help to assess the degree of insulation aging.