The solid insulation in a transformer is made of paper, which can be damaged by moisture or heat or acids. Andy Shkolnik explains what happens to cellulose as it weakens, and how the degree of polymerization test can quantify the damage done.
Transcription:
What is the degree of polymerization test'? That is a test that's done on an actual paper sample from a transformer. Paper insulation is made out of ordinary paper that comes from trees. So therefore it's actually made up out of cellulose. Cellulose is a natural polymer where a polymer of course is a chain-like molecule that has repeating molecules of the same thing again and again and again. So the cellulose that makes up the transformer insulating paper, when it's brand new, it has about a thousand units of this particular repeating glucose monomer and the length - or that thousand elements to the chain - that is the degree of polymerization. The reason why that's important is because the longer it is, the stronger the paper is mechanically. And that's one of the functions of the paper insulation in a transformer is to provide mechanical strength against the forces that occur in a normally operating transformer.
The paper helps hold the transformer together, so it has to have mechanical strength. So it has a lot of mechanical strength when the paper is young because that degree of polymerization, that chain length is a thousand units long approximately. But over the course of the life of the transformer, heat, moisture, oxygen and acids and other decay products all conspire to break down the paper insulation. And you can kind of think of those bad actors as kind of randomly snipping those long cellulose chains. And so as the transformer ages, that chain length or that degree of polymerization gets slowly shorter and shorter. And in fact, IEEE has defined end of useful life as when the degree of polymerization gets to only 200 units long. So that's the reason why some people would want to test paper for a degree of polymerization to see what stage of aging or life that paper is.
However, the test is not actually done very often because if you think about it in a working transformer, you would have to take a sample from the coils and that would damage them a lot. So you'd have to deal with how am I going to repair this? And so most people don't want to go to that trouble. It's a lot of trouble. Some people will test the paper insulation from the leads, since that's easier to get to, and easier to repair. But from actual samples from the windings, we don't see those very often. And when we do see them, it tends to be more of a forensic situation where the transformer has already failed. And someone wants to see, was it due to weak paper. So, they'll look at the failure spot and send in samples. But generally then because of those reasons, we don't see many samples for degree of polymerization. However, it's still an important concept because we have an indirect way of measuring the remaining life in the paper insulation. And that's through the furan tests. So that's a chemical test on the transformer oil or whatever fluid is in the transformer. And what these furans are is they are breakdown products of paper and insulation. So as heat, moisture, oxygen and acids are snipping those cellulose chains. These class of compounds known as furans are produced at the same time. So we can measure them to see the extent of paper degradation.
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