Testing Furanic Compounds in Insulating Liquids - Part 1Technical Briefs
How can you evaluate whether the solid insulation in your transformer is breaking down? You could take the unit offline and go through the long, involved process of obtaining a sample of the paper for testing. On the other hand, you can analyze an insulating liquid sample for furanic compounds. You can do that without even de-energizing the transformer, and the information you can obtain from furan data is extremely important.
Furans form when the paper that makes up the solid insulation breaks down or depolymerizes. When this happens, the cellulose molecules break into shorter polymer chains and kick out a glucose monomer molecule. As this happens, the average polymer chain length in the paper (which can be measured and reported as the DP or degree of polymerization) decreases. Shorter polymer chains result in weaker paper, so that the tensile strength of the paper is also reduced.
What happens to the glucose monomer molecule that gets kicked out? It goes through a chemical reaction and becomes one of a handful of derivatives of a chemical compound called 2-furaldehyde. These derivatives are known collectively as “furans” or, more properly, “furanic compounds”. They form in the paper as it breaks down, but they are also partially soluble in the insulating liquid. So, some of the furans will partition between the paper and the insulating liquid. When that happens, we can test for them in the insulating liquid. There is no significant source for furanic compounds in operating transformers other than the breakdown of the cellulose.
NOTE: There are some oil refiners who use tetrahydrofuran, a related compound, during the transformer oil refining process. This has never been found to be a significant source of furans except for a few cases overseas.