As paper degrades, a number of specific furanic compounds are produced and dissolve in the oil. Of these, 2-furaldehyde is prominent, and its concentration has been related to the strength of paper as measured by its degree of polymerization (DP). Measurement of 2-furaldehyde in oil provides a convenient, non-invasive method to assess the condition of the paper insulation.

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. There are two ways to do Fural Analysis:

  • Screening Test (low cost, colour cost)
  • HPLC Test (complete analysis of furaldehydes and phenols)

Screening Test

Powertech has developed a low cost screen test procedure to determine the concentration of 2-furaldehyde in oil from in-service transformers. The screen test allows utilities to develop a large database for very low cost. Only equipment samples that exceed acceptable limits would require HPLC testing.

The test will measure only 2-furaldehyde, which is also the most prominent of the furaldehydes.

High Pressure Liquid Chromatography

This test will measure all the furaldehydes (2-Furaldehyde, 5-Hydroxy-Methyl Furaldehyde, 2-Furyl Alcohol, 2-Acetyl Furan, 5 Methyl-2-Furaldehyde and Phenols, m-Cresol, O-Cresol and Dimethyl Phenols). When to SampleWe recommend that screen testing for paper degradation be performed whenever gas-in-oil sampling is done. In this way, no additional sampling is required, thus minimizing costs. The screen test compliments gas-in-oil testing as a diagnostic technique to identify incipient faults.Transformer oil samples should be analyzed for solid insulation decomposition products when one of these conditions is indicated.

  • Suspected overheating
  • High levels of CO or CO2 gases have been detected
  • Confirmation of paper degradation is required
  • Sudden decrease of IFT without corresponding increase in acid number
  • Sudden increase of moisture in oil or sudden darkening of the oil
  • The transformer is near the end of its expected life
  • Deliberate overloading


We require 20 mL of insulating oil, which should be transported in a small, dark bottle, as Furals are light sensitive. Always sample oil before reclaiming it, since Furals will be absorbed.