Direct Evaluation of Transformer 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 e carried out during transformer repairs. The results of these tests will be a deciding factor in rebuilding or scrapping a transformer.

Furaldehyde Analysis

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 a typical power transformer, with an oil to paper ratio of 20:1, the 2-furaldehyde levels have the significance shown in the following table.

Furaldehyde Content (PPM)

DP Value



700 - 1,200


0.1 - 1

450 - 700

Moderate Deterioration

1 - 10

250 - 450

Extensive Deterioration



End of Life

Other Diagnostic Compounds

The presence of phenols and cresols in concentrations greater than 1ppm indicate that solid components containing phenolic resin (laminates, spacers, etc.) are involved in overheating.