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
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.
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.
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