Assesment of field power transformers insulation using aging markers

Laboratory aging results together with real-transformers testing highlight the relevance of the use of chemical markers for the evaluation of the aging state of the transformers cellulosic insulation. In this study, alcohol chemical markers are used for the diagnostic of a set of identical transformers from the point of view of design and operation conditions. The aim of this study is to obtain information about the applicability of the chemical markers during decision-making concerning the priority of the replacement of operative units in a hydropower plant. A first assessment of the situation was carried out, with a diagnosis based on dissolved gas analysis and then on the use of aging markers. In addition, a post-mortem analysis was performed which allowed to correlate the concentration of chemical markers with the aging of the solid insulation, measured directly by the Degree of Polymerization (DP). The preservation of these chemical markers after oil maintenance was also investigated. It was observed that the fingerprint of the alcohol based markers was quickly reconstituted compared to other conventional markers. Later, a second campaign of measurements allowed a new diagnosis of the equipments. This study shows that the chemical markers are a relevant complementary tool of transformers assessment. The post-mortem analysis is a crucial step on the evaluation of cellulosic insulation aging.