Furan Testing - Part 6Technical Briefs
Testing for Furanic Compounds in Insulating Liquids Part 6 – Estimating Degree of Polymerization
In Part 5, we talked about the significant values for furan results. In addition to defining AC, QU, and UN levels, we talked about very high levels that may indicate substantial damage to the insulating system. Here, we are going to talk about calculations that we perform to estimate the condition of the solid insulation by using the furans analysis results. We use a two-step approach to estimate a DP and calculate insulation life remaining.
First, we use the furan analysis values to calculate an estimated degree of polymerization for the cellulose. Then, we calculate the percentage of insulation life used (from an assumed starting point of approximately 800 DP for new paper in a new transformer) using a non-linear equation. Insulating life remaining is reported, considering that this value is 100%, less the calculated percentage of life used.
Degree of polymerization is estimated using one of two equations. Determination of which equation to use depends on whether the paper used as insulation is normal Kraft paper or whether it has been thermally upgraded. DP of Kraft paper that has not been upgraded is estimated using a version of a widely used equation developed by I. Chendong that we have corrected for our reported units of measurement. DP of Kraft paper that has been thermally upgraded uses a different equation than what was developed in our laboratory using our database.
If you simply graph these two equations, there does not appear to be a whole lot of difference between them. The red line graphs the equation for paper that has not been thermally upgraded, while the blue line graphs the equation for paper that has been thermally upgraded by the addition of dicyandiamide:
guidance as 200 degrees of polymerization. The furan level that corresponds to this DP for paper that has not been thermally upgraded is 7337 ppb. The furan level that corresponds to this DP for paper that has been thermally upgraded is considerably lower – 2843 ppb.
Keep in mind that the calculation of DP from furans is really an estimate of the average throughout the total volume of the solid insulation. A number of conditions, especially hot spot overheating, may caused localized degradation and weak spots that have much lower DP than the average would indicate.
A final consideration, and the reason that we modify the Chendong equation before we use it, is that we report results in ppb by weight – micrograms of furans per kilogram of insulating liquid. A number of laboratories use micrograms of furans per liter of insulating liquid, but may still refer to that as ppb.