Over the past few weeks, we have discussed testing for furanic compounds. Key points we covered were why furan analysis gives us information on the condition of the solid insulation, when we recommend running the test, the significant values, and how we use the results to estimate the condition of the insulation. Along the way, we received an interesting question that we would like to answer. We have heard variations of this question a number of times at our seminars and in phone conversations, and we even have answered it on-line previously.
To paraphrase from a couple of sources: Do the furan results change significantly after a project to process the oil in a transformer? Does this also affect how the estimation of degree of polymerization is done and how remaining insulation life is calculated?
Furans are polar compounds that are partially soluble in oil. They form primarily in the solid insulation and then some of them migrate out into the oil. The different furanic compounds migrate or partition into the oil at different rates, but all will stay at around 90% or more in the solid insulation, 10% or less in the oil.
To answer the question about processing the oil you must first consider what type of processing is being done. If vacuum processing only is done (for example to dry out or degas the oil), reduction in furans content during the project is relatively small. Vacuum processing will remove some of the furans from the oil, but will not remove any significant amount from the solid insulation. Most of the furans started out in the solid insulation before the project, so they will continue to still be in the solid insulation after the project. As a result, the estimates for degree of polymerization and remaining life calculation are not affected to any great degree.
There are oil processing and transformer maintenance procedures that will significantly change the content of furans. If the unit is subjected to reclamation/regeneration using an adsorbent system such as fullers earth, alumina, or one of the other reclaiming adsorbents that can be reactivated, the degree to which furans will be removed will depend on how well the reclaiming job was performed. Doing the correct number of passes, sufficient to clean up the solid insulation and remove the byproducts from the oxidation of the oil, will remove virtually all of the furans from the oil, as well as 50% to 70% of the furans from the solid insulation.
You can still estimate DP and use the calculations for remaining life, but you need to have or assume data regarding how the furan content was reduced by the processing.
Immediately after the procedure, furans content will be zero or very low. Over the course of the next few months, it will gradually increase to an equilibrium value of 30 to 50% what it had been before the procedure. We recommend that you know the furan content before the job is pursued, and then test it again six months after the project is completed.
Example: Furans before a well-done reclaiming job are 800 ppb. Six months after the job (when the system has returned to equilibrium with regard to furans partitioning into the oil) , the furan content is 320 ppb. That is 40% of the original value, indicating that 60% of the total furans were removed. That is about what would be expected for a good job of reclaiming. After two more years of operations, we can use as an example that the unit has continued to age. After those two years of operation, the furans content has increased to 400 ppb. You would calculate the DP and life remaining based on 800 ppb plus the increase after the project (400 minus 320 = 80). So the calculations would use a furans content of 880 ppb. If this is a unit with thermally upgraded paper, the calculated DP before the project based on 800 ppb would be 389 (48% life remaining). After the two additional years of operation, based on 880 ppb, the calculated DP is 375 (45% life remaining).
If you do not have complete information before and after such a project, estimation may still be possible, but it is considerably more difficult and less certain.
Next week, we are going to start a new topic. See you then!
