Anomalies in interpretation of transformer oil tests for thermally upgraded paper — A case history

Continuity of electrical power supply to continuous processes is vital to overall success in an industrial setting. It is therefore critically important to evaluate accurately the degree to which transformer insulation has been exposed to high temperatures and the degree of service aging which has occurred, even at normal temperatures. Concentration of carbon oxides dissolved in the oil is one marker commonly used, and concentration of furans dissolved in the oil is another. Sometimes an apparent anomaly may develop in which the concentration of carbon oxides indicates substantial long term overheating, while at the same time the concentration of furans indicates that there has been little aging of the solid insulation. Actual instances are analyzed and a logical explanation is developed. Guidelines are presented which address the development of this confusing situation and allow for logical conclusions as to the actual condition of the insulation system.