Fundamental investigations on some transformer liquids under various outdoor conditions

This paper deals with humidity uptake of some transformer liquids and its influence on electrical breakdown. The investigations were performed under some selective outdoor conditions on different insulating liquids, e.g. mineral oil, esters and silicones used in HV transformers. The temperature was set at 23 or 60 °C while the relative air humidity was varied between 20 and 90% to simulate climatic variation. The subject is important in HV transformers with an air breathing system since, even in such transformers where normally the air passes through a dehydrating apparatus with silica gel, a bad run can induce an accident. The scenario considered corresponds to such a defect in the breathing filter system, so that the insulating liquid in the transformer comes into contact with atmospheric humidity. The work presented is in two parts. The first part relates the evolution of the breakdown voltage in terms of the insulating liquid humidity content. The second part describes the evolution of insulating liquid humidity content in terms of surrounding relative air humidity. This evolution is observed during 30 days and the results are discussed regarding the ac electric strength according to International Electrotechnical Commission (IEC) or Verband der Elektrotechnik Elektronik Informationstechnik e.V. (VDE) standards. A correlation between the surrounding relative air humidity, temperature and humidity uptake rate is derived