What Are Furanic Compounds and How Do They Form in Transformers?

This is Part 1 of a seven-part series on furanic compounds in transformers. Here we will cover how furanic compounds form in transformers as solid insulation breaks down. Part 2 will look at which compounds are tested and what the results tell us.

Why Test the Liquid Instead of the Paper?

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 — without even de-energizing the transformer. The information you can obtain from furan data is extremely important.

How Depolymerization Generates Furans

Furans form when the paper that makes up the solid insulation breaks down, or depolymerizes. During this process, the cellulose molecules break into shorter polymer chains and release a glucose monomer molecule.

DP, Chain Length, and Tensile Strength

As depolymerization progresses, the average polymer chain length in the paper decreases — this can be measured and reported as the DP (degree of polymerization). Shorter polymer chains result in weaker paper, meaning the tensile strength of the paper is also reduced.

How Furans Enter the Insulating Liquid

The glucose monomer released during depolymerization undergoes a chemical reaction and becomes one of a handful of derivatives of 2-furaldehyde. These derivatives are known collectively as “furans” or, more precisely, “furanic compounds.” They form within the paper as it degrades, but are also partially soluble in the insulating liquid. As a result, furans partition between the paper and the insulating liquid — and once in the liquid, they can be detected through standard testing. There is no significant source of furanic compounds in operating transformers other than the breakdown of cellulose.

Other Potential Furan Sources

Some oil refiners use tetrahydrofuran — a related compound — during the transformer oil refining process. However, this has never been found to be a significant source of furanic contamination, with the exception of a small number of cases reported overseas.

Summary:

• Furanic compound analysis allows solid insulation condition to be assessed from a liquid sample — without taking the transformer offline or obtaining a paper sample directly.
• Furans form when cellulose depolymerizes. As the polymer chains in the paper shorten, DP (degree of polymerization) decreases and tensile strength is reduced.
• The glucose monomer released during depolymerization converts into furanic compounds — derivatives of 2-furaldehyde that are partially soluble in the insulating liquid and detectable through standard testing.
• Cellulose breakdown is the only significant source of furanic compounds in operating transformers. Tetrahydrofuran used in oil refining has not been found to contribute meaningfully to furanic contamination under normal circumstances.

In This Series:

• Part 1: What Are Furanic Compounds and How Do They Form in Transformers?
• Part 2: How Do Furanic Compounds Indicate Active Paper Degradation in Transformers?
• Part 3: When Should You Test for Furanic Compounds in Transformer Oil?
• Part 4: When Should Furanic Compound Testing Be Routine?
• Part 5: What Furanic Compound Levels Are Diagnostically Significant?
• Part 6: How Are DP and Insulation Life Estimated from Furans Data?
• Part 7: How Does Oil Processing Affect Furanic Compound Results?