Furans
Furans are a family of organic compounds which are formed by degradation of paper insulation. Overheating, oxidation, and degradation by high moisture content contribute to the destruction of insulation and form furanic compounds.

Fur Furaldehyde Analysis (FFA)
Direct measurement of these properties is not practical for in-service transformers. However, it has been shown that the amount of 2-furaldehyde in oil (usually the most prominent component of paper decomposition) is directly related to the DP of the paper inside the transformer.

Paper in a transformer does not age uniformly and variations are expected with temperature, moisture distribution, oxygen levels and other operating condition. The levels of 2-furaldehyde in oil related to the average deterioration of the insulating paper. Consequently, the extent of paper deterioration resulting from a “hot spot” will be greater than indicated by levels of 2-furaldehyde in the oil.

For typical power transformer, with an oil to paper ratio of 20:1, the 2-furaldehyde levels have the following significance:

HPLC Test
High Pressure Liquid Chromatography will measure all the furaldehydes.

(2-Furaldehyde, 5-Hydroxy-Methyl Furaldehyde, 2-Furyl Alcohol, 2-Acetyl Furan, 5 Methyl-2-Furaldehyde etc.

De Polymerization of paper in oil
Degrees of Polymerization (DP) is a test done on the paper to reveal its mechanical strength. An inverse relationship exists between the DP (paper test) and the Furals (oil test); the higher the Furals in the oil, the lower the DP of the cellulose paper. This relationship holds, however, only if the paper is degrading evenly. The DP of the paper usually varies within the solid insulation.

Since it is difficult to sample paper from an operating transformer, Fural Analysis in the insulating oil can indirectly reveal the status of the solid insulation.

The mechanical properties of insulating paper can be established by direct measurement of its tensile strength or degree of polymerization (DP). These properties are used to evaluate the end of reliable life of paper insulation. It is generally suggested that DP values of 150-250 represent the lower limits for end-of-life criteria for paper insulation; for values below 150, the paper is without mechanical strength.

Analysis of paper insulation for its DP value requires removal of a few strips of paper from suspect sites. This procedure can conveniently carried out during transformer repairs. The results of these tests will be a deciding factor in rebuilding or scrapping a transformer.