Electrical signals from water trees-polarization and depolarization currents

If polymer insulated medium voltage cables are used in wet environments, after some time water trees are generated. As these objects may decrease the breakdown voltage, the interesting questions for utilities are, how many water trees exist in the insulation, what are their lengths and last but not least is there a risk for the safe operation of the cable. One chance to get some answers on these questions without digging out the cables is to apply an AC or a DC voltage to the cable and to measure the loss tangent, the shape of the AC current and/or the polarization and depolarization currents produced by the water trees. Comparing the different methods, the evaluation of the polarization and conduction currents produced by the water trees seems to be more sensitive than the change of the loss tangent, at least in the early stages of the water treeing phenomenon. Furthermore it is known that after termination of the voltage load, water diffuses out of the water trees and hence the electrical signals from water trees are decreased. To get a better understanding of these phenomena the authors aged commercial cable insulations in the laboratory under accelerating conditions (using a temperature gradient in the insulation), applied a DC voltage and analyzed the polarization and depolarization currents