1040 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No. 6, Dec(�mber 2001 Fundamental Investigations on Some Transformer Liquids under Various Outdoor Conditions I. Fofana Research Group on Atmospheric Icing Universite du Quebec a Chicoutimi Quebec, Canada H. Borsi and E. Gockenbach stitute of Electric Power Systems Division of  Engineering Schering·Institute University of Hanover, Hanover, Germany ABSTRACT This paper deals with the humidity uptake of some ansformer liquids and its influence on the electrical breakdown. The investigations were performed under some selective outdoor condions on different insulating liquids, e.g. mineral oil, ester and silicone used in HV trans- formers. The temperature was at 23 or 60 while the relative air humidity was varied between 20 and 90% to simulate climatic variation. The subject is important in a kind of HV transformer wi  air breathing system and, even if in such transformer, where normally the air passes through a dehydrating apparatus with silica gel, a bad run of this apparatus can induce an accident. The scenario considered corresponds to such a defect in the breathing filter system, and thus when the insulating liquid in the transformer becomes in contact with humidity of atmospheric air. 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 one describes the evolution of insulating liquid humidity content in terms of surrounding relave air humidity. This evolution is observed during 30 days and the results are discussed regard- ing the ac electric strength according to Inteational Electrotechnical Commission (lEe) or Verband der Elektrotechnik Elektronik lnfoationstechnik e. V (VDE) standards. A correla- tion between the surrounding relative air humidity, temperature and humidity uptake rate is derived. 1 INTRODUCTION T RANSFORMER life/ aging is related mainly to the degradation of the insulation, caused dominantly by thermal degrading of the insu- lating paper, together with the decomposition of the paper. The by- pducts are water and other substances of partly polarizable and ion- izable character. For example, the life of insulating cellulose paper at 11 0 is calculated as ten years [1]. Aging depends not just on loading, but also is influenced significantly by the type of paper, pulp compo- sition, humidity and oxygen content as well as the acidity level within the insulating liquid [2]. down. Water content increases electric conductivi di:lsipation factor and worsens the elecic strength. A mass transfer process of water results from the equil ibri imbalance, in which at higher tempera- tures moisture leaves the paper to enter the liquid. Water in the liquid originates from air moisture in the case of 'open-breather' or 'liquid- conservator' types, and the thermal decomposition of cellulose-based solid insulati ng materials. It has a strong uence on the life ex- pectancy and load capacity of a transformer. The insulation liquid in the transformer changes volumetrically ac- cording to the variation in its temperature arising from changes in at- mospheric temperature and/ or transformer load. From the expansion coefficient of the liquid and the range of temperature changes (- 20 to 90), its volumetric change is � 9% maximum. Therefore, par- ticular attention has to be given to prevent the introduction of mois- Water is par ticularly detrimental to dielectric properties of both liq- uid and paper insulation systems and thus to their resistance to ag- ing. Moisture in insulating liquid may under fast decreasing temper- ature transients, result in free water that can lead to elecical break-. 1070-9878/ $3.00 © 2001 IEEE