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Electrical Treeing and the Effect of Rejuvenation

December 12, 2010

Electrical Treeing and the Effect of Rejuvenation

Q:

According to a representative of an offline partial-discharge testing firm, injection can cause electrical trees and faults. The premise is that the water tree is superimposed over the electrical tree and thus grades the electrical stress around the electrical tree. By curing the water tree, the electrical tree is unleashed to grow to a fault. Basically, the water tree acts as a barrier. Is there merit to this claim?

A:

That theory was proposed at an ICC meeting several years ago. To determine the validity of the theory, one must first consider whether electrical trees are moderated by the presence of water trees or whether they’re caused by wateWater Tree Growth Pathr trees.

These images appeared in the Spring 2004 ICC meeting minutes (in an article titled “Cable PD Facts—Field Experience,” by M. S. Mashikian). They illustrate a beautiful water tree and two equally attrWater Tree 2active electrical trees astride the water tree, near its base. The blue arrows suggest that the electrical tree avoids the water tree in its growth pattern. That suggestion is problematic for two reasons. First, the electrical trees are clearly associated with the water tree; it’s unlikely that they sprung up there randomly.

Second, both electrical trees have a substantial branch that heads right into the water tree — just the opposite of what is being suggested. There’s no doubt that the water tree alters the electrical field in its neighborhood. For every location where the field is concentrated, there has to be a neighboring location where the field is less concentrated. Electrical trees grow where the field is most concentrated.

It’s common knowledge that water trees normally precede electrical trees, and most experts agree that water trees are a leading cause of electrical trees. (It’s possible to induce electrical-tree growth before the presence of water trees, but that takes a great deal of voltage. Of course, that’s what offline PD testing firms do — use a great deal of voltage.) To accept that water trees somehow moderate the fields around electrical trees, we would have to ignore all the evidence to the contrary.

What does happen when silicone fluids react with and displace the water that’s associated with electrical trees? In short, the poor dielectric properties of the water in the water tree are replaced with the superb dielectric properties of silicones and tree-retardant organics. A thorough answer is available in Cable Rejuvenation Mechanisms: An Update by D. Busby and G. Bertini (IEEE Electrical Insulation Magazine, March/April 2009).

For a complete analysis of this topic, check out Diagnostic Testing of Stochastic Cables by G. J. Bertini (IEEE Electrical Insulation Magazine, March/April 2009).