Electrical Treeing and the Principle of Parsimony
A representative of a certain off-line partial discharge testing firm is saying that injection can cause electrical trees and faults. His 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 like a barrier. This is the first I have heard of this, and on the surface the notion seems outlandish. What would the Frog say?
Alarmed in Atlanta
I heard that one too at an ICC meeting several years ago. I’m surprised anyone is still croaking that tune. The principle of parsimony that underpins all of science is that the simplest explanation, requiring the fewest assumptions is most likely to be right. The reason you find the notion outlandish is because of this principle.
Let’s review the facts and two possible explanations. The annotated images nearby (taken from the Spring 2004 ICC meeting minutes, Cable PD Facts – Field Experience, M. S. Mashikian) illustrate a beautiful water tree and two equally as attractive electrical trees astride the water tree, near its base. The author is suggesting with the blue arrows that the electrical tree avoids the water tree in its growth pattern. This suggestion is bizarre for two reasons. First the electrical trees are clearly associated with the water tree. The alternative explanation, that they sprung up there randomly would require the “willing suspension of disbelief” to quote a former New York Senator and now Secretary of State. (It’s true that she was wrong, but she was politically motivated. I, on the other hand, seek only truth and understanding. Amphibians would make poor politicians, because we simply call them as we see them.) Secondly, upon closer inspection both electrical trees have a substantial branch that heads right into the water tree – just the opposite of what is being suggested. There is 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 it is less concentrated. Electrical trees grow where the field is most concentrated.
It is common knowledge that water trees normally precede electrical trees. It is possible to induce electrical tree growth before the presence of water trees, but that takes a great deal of voltage. This, of course, is precisely what the off-line PD guys do – use a great deal of voltage. Except for the folks at this single testing firm, experts agree that water trees are a leading cause of electrical trees. In order to claim that water trees somehow moderate the fields around electrical trees we would have to ignore all of the evidence to contrary.
What does happen when silicone fluids react with and displace the water that is associated with electrical trees? A thorough answer is available from the CIGRÉ Canada, Conference on Power Systems, Vancouver, October 17-19, 2010, paper “Cable Rejuvenation Mechanisms: An Update” at …
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.
Why would anyone suggest such a bizarre explanation? Could it be that the superior economics of rejuvenation compared to diagnostic testing are limiting the growth of the firm that is making the claim? Check out “Diagnostic Testing of Stochastic Circuits” from the March/April Edition of IEEE Electrical Insulation Magazine for a complete analysis. That paper is available at …
So, Alarmed in Atlanta, use the principle of parsimony and the available evidence to determine if electrical trees are moderated by the presence of water trees or if they are caused by water trees and exasperated by the application of voltage two and a half times greater than operating voltage.