Introduction
The EOL (End of Life) condition of transformers is a very unclear and controversial issue. The data and numerical values found in the literature differ vastly and make it virtually impossible, at the end of the day, to obtain a clear assessment of the situation and ultimately leave the user alone with his or her problems. A closer look at the matter makes it obvious, however, that there can be no such thing as a universal, generally applicable rule. First of all, it is necessary to define what exactly this problem signifies, and to what extent the problem is of a technical or of an economic nature. One thing is certain: the technical conditions of this condition alone are entirely unclear, and all data found in the literature are as much correct as they are incorrect. That is why we shall attempt, in the following, to establish a guideline, which will allow every user to select the most feasible procedure(s) for his or her particular plant. An entirely novel solution to the problem, to be introduced here, will be the so-called PoAB factor (Probability of Advanced Breakdown).
What does the literature tell us?
A widely used method of assessment, which clings to furan analysis, postulates that furan values above 5 mg/kg will lead to a value of below 200 DP, effectively ruling out safe and reliable operation in the future. Other authors take an even more stringent position, postulating 300 DP minimum. And there are other authors who claim that transformers with a 200 DP can be operated for many more years without any problems. Practical experience has shown, however, that transformers with a 400 DP (based on the furan value) have a high tendency to fail and that a closer look at such transformers (DP profile) will usually also reveal an EOL status of insulation. This obviously goes to show that any assessments based on the furan values are more of a hindrance than a help. Needless to say that this assessment methodology becomes even more questionable in cases where the materials used in the transformer are unknown. I.e. whether or not thermally stabilized paper was used, for instance. Matters become altogether complicated when trying to assess younger transformer types, which frequently contain a lot less cellulose than their older counterparts. Another issue, which will open up an entirely new avenue of discussion, is whether or not transformers should be generally assessed on the basis of their paper’s condition. The troubleshooting procedures required in case of cellulose consumption, i.e. in case of a general deterioration of the DP value, are naturally quite labor and cost-intensive in order to restore correct operating condition through a complete retrofit of the coil windings. In such cases, it is necessary to go beyond the conventional assessments according to which this procedure helps recover approximately 80% of the costs of a newly procured transformer. It is also necessary to take into account a number of additional factors for a technically precise as well as economically correct assessment. These issues will be addressed in detail in the following.
Georg Daemisch
