How Are DP and Insulation Life Estimated from Furans Data?

This is Part 6 of a seven-part series on furanic compounds in transformers. Part 5 covered what furanic compound concentrations are considered significant and how to interpret those levels diagnostically. Here we will examine how furanic compound concentrations can be used to estimate the degree of polymerization (DP) of the paper insulation — and what that calculation reveals about remaining insulation life. In Part 7, we will bring together the key points from across the series and discuss how oil processing can change furanic compound results and affect how the degree of polymerization and remaining insulation life is calculated.

From Furans Concentration to DP Estimate

Furanic compound concentrations can be used to estimate the degree of polymerization (DP) of the solid insulation — and from there, to calculate how much insulation life has been consumed. The approach involves two sequential calculations: first, estimating DP from the furans analysis results; second, applying a non-linear equation to convert that DP estimate into a percentage of insulation life used. Insulation life remaining is then reported as 100% minus the calculated percentage consumed, starting from an assumed baseline DP of approximately 800 for new paper in a new transformer.

Selecting the Right DP Equation

DP is not estimated using a single universal equation. The appropriate equation depends on the type of cellulose insulation in the transformer — specifically, whether the paper is standard Kraft or thermally upgraded Kraft.

Standard Kraft Paper

For paper that has not been thermally upgraded, DP is estimated using a corrected version of the Chendong equation — a widely used relationship developed by I. Chendong. The version applied here has been adjusted for results reported in ppb by weight (micrograms of furans per kilogram of insulating liquid), rather than the micrograms-per-liter units used by some laboratories.

Thermally Upgraded Kraft Paper

For paper that has been thermally upgraded — for example, through the addition of dicyandiamide — a different equation is used. This equation was developed in-house using laboratory data and reflects the distinct degradation chemistry of thermally upgraded insulation.

How the Two Equations Diverge at High Furan Levels

When graphed across the full concentration range, the two equations appear broadly similar. The practical difference becomes significant only at higher furan concentrations, where the equations diverge meaningfully — and where the distinction between paper types has the most diagnostic consequence.

The critical DP threshold for evaluating insulation integrity is generally cited in industry guidance as 200. The furan concentration corresponding to a DP of 200 differs substantially depending on paper type:

• Standard Kraft paper: 7,337 ppb
• Thermally upgraded Kraft paper: 2,843 ppb

Misidentifying the paper type at these concentration levels would result in a significantly incorrect assessment of remaining insulation life.

Limitations of the DP Estimate

The DP value calculated from furans analysis represents an average across the total volume of solid insulation in the transformer. It does not capture localized degradation. Conditions such as hot spot overheating can produce areas of severe, concentrated paper breakdown with DP values well below the average — weak points that the furan-derived estimate will not reflect.

A Note on Reported Units

Laboratories do not all use the same units for reporting furanic compound concentrations. Results expressed in micrograms per kilogram (ppb by weight) and micrograms per liter are not directly interchangeable, though both are sometimes referred to as ppb. The equations described here are calibrated for results reported in micrograms per kilogram. Applying them to results reported in micrograms per liter without conversion will produce incorrect DP estimates.

Summary:

• Estimating DP and remaining insulation life from furans data involves two sequential calculations — a DP estimate derived from furanic compound concentrations, followed by a non-linear conversion to percentage of life consumed.
• Applying the calculations correctly requires knowing whether the insulation is standard or thermally upgraded Kraft paper, using the appropriate equation for each, and ensuring results are in the correct units before any calculation is performed.
• The resulting DP figure represents a whole-insulation average; localized degradation from hot spot conditions will not be captured by this estimate alone.

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?