Moisture in transformers and online dryer performance

Moisture levels in transformers have been long acknowledged as critical in understanding a transformer´s state of reliability. Moisture reduction in a transformer can be done by applying several industry accepted methods. The method this paper will focus on is an online dryer system. Case studies are presented that review the results of online dryers. Benefits include leaving the transformer energized and processing for a longer time period. This method of dehydration gives moisture in the solid insulation time to migrate into the oil and subsequently be removed by the online dryer. The results experienced were positive. After the moisture levels stabilized at new, lower levels, the testing conducted several months, even years later revealed no further increase in moisture levels. This is an indication of successful moisture removal from the solid insulation. In one of the case studies, electrical testing confirmed this improvement.