Recommendations to improve the efficiency of drying processes in power transformer

Water is a byproduct of ageing reactions in cellulosic insulation. During the life of a transformer water content in paper increases from levels below 0.5% in weight after manufacturing, to levels of 6% in weight at the end of transformer life. High humidity levels may also be found in not so old transformers, for example in those that have been submitted to repairs in field. As it is well known, water is damaging for power transformers. Water accelerates ageing processes, reduces dielectric margin, decreases partial discharge inception voltage and, therefore, increases the probability of unexpected failures. Taking into account that power transformers are very expensive equipment and that the electrical supply guarantee is closely related to their reliability, it seems clear that transformer water content is a variable that must be kept under control. In order to reduce an excessive water content in a transformer, it is possible to submit it to a drying process. Every time is more habitual to process the transformers in the field, in order to reduce the outage period and to reduce the cost of the process. In this paper an study is presented that analyzes different field drying methods that are currently widely used. The efficiency of these methods is studied and compared, and recommendations about drying parameters are given.