WATER TREEING IN POWER CABLE PHYSICAL BASIS BASIC AND TUTORIALS

 Water treeing can range from predominantly electromechanical in nature to essentially electrochemical. Agreat deal of the early laboratory work was carried out with “water needle” configurations, which produce extremely high electric fields at the tip of a needle-shaped, water-filled cavity.

The electric field at the tip was usually high enough to produce an electrical tree if the cavity were not filled with water, and the water tree grows in hours to days, rather than months to years as for a water tree grown under utility operating conditions.

Dorris, et al. Investigated electrical signals generated by the growth of such water trees. An analysis of their data suggests that the measured electrical signals could be produced by a sudden 0.01 to 0.1 μm extension of the water tree channel.

This work provides clear evidence for the growth of essentially electromechanical trees at very high fields. Such trees probably grow through (i) electrochemical damage in the tree tip region, which weakens the polymer to the point that (ii) electromechanical forces cause a sudden yielding of the polymer and extension of the tree in the range of 0.01 to 0.1 μm.

Because the electric field and resulting electromechanical forces are relatively large [9], relatively little damage to the polymer in the tree tip region is required to reduce the yield stress of the polymer sufficiently that the electromechanical forces cause yielding and extension of the tree channel.

For high-field, water needle-induced water trees, micro- infrared spectra of the resulting water tree indicate relatively little electro-oxidation, which progresses slowly relative to the time frame (days) in which the tree growth takes place under these high field conditions.

Under long-term utility service conditions, the electric field is quite low, typically 1-3 kV/mm, as are the resulting electromechanical forces.

The polymer must undergo substantial electrochemical degradation to reduce the yield stress to the point that the water tree can extend, and micro-infrared spectra of service-induced water trees show evidence of appreciable electro-oxidation in the tree region.

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