Previously, we have discussed how furan analysis may be used to determine whether the solid insulation in a transformer has broken down and whether the condition that caused that degradation continues to be active. This week we will talk about what levels of furans we consider to be significant. As you may expect, the significance of the results will depend at least in part on the reason for testing the unit in the first place. Let’s talk first about the most obvious time when testing for furanic compounds may be recommended. As we described in our article a couple weeks ago, that is when you suspect that there may be a problem where furan testing can help you confirm that the solid insulation is breaking down.
In this event, if you have baseline data or if you have other past history, any increase in total furans, and especially the presence of specific furans other than 2-furaldehyde, may be significant. Even a small increase (on the order of 25 to 50 ppb for 2FAL, 20 to 25 ppb for the others) may indicate a significant, ongoing breakdown in the solid insulation. Closer monitoring will be indicated while the underlying cause for the accelerated aging or degradation of the paper is determined.
Where we do not have past history for a furans analysis that is being performed to follow up on other abnormal results or for a baseline determination, we have to make our judgment and recommendations based on incomplete information and some general guidelines. New transformer oil in a newly installed transformer may have some minor background furan content, typically up to approximately 20 ppb. Even transformers that have been in service for decades may continue to maintain these low furan levels if conditions are such that aging of and damage to the solid insulation is kept to a minimum.
For in-service transformers, we consider levels up to 100 ppb to represent normal aging (AC – an ACceptable level). Above 100 ppb, and up to 250 ppb, we consider such levels to indicate probable accelerated aging (QU – a QUestionable level). We consider levels above 250 ppb to represent significantly accelerated aging (UN – an UNacceptable level). In addition to using these general guidelines, we also evaluate whether 5M2F, 2FOL, 2ACF, and 5H2F are present as indicators of an active condition and consider the age and what we may know of the history of a unit before making a recommendation for monitoring or further diagnostic efforts.
For example, 400 ppb of furans, with only 2FAL, in a thirty year old transformer the first time it is tested may not be of significant concern. The proper recommendation may be to use a shorter that normal retest interval of three or six months (depending on the critical nature of the unit) for the furanic compounds analysis and/or dissolved gas analysis just to establish that there is no active overheating condition. On the other hand, a six or seven year old transformer with a little over 100 ppb of furans including some 2FOL (or one of the other three “active” furan derivatives) may be of much greater concern. In that case, particularly if the other results confirm conditions conducive to paper breakdown, further diagnostic efforts with an objective of completely defining and correcting the problem would be more appropriate.
We consider furan levels higher than 1000 ppb to represent substantial and permanent damage to the solid insulation. Increases above these levels may indicate conditions that are worsening to the point where the integrity of the solid insulation becomes suspect. Why this may be the case will be discussed in the next article, where we also will discuss how we can use furan results to calculate an estimated DP of the paper that makes up the solid insulation.