Aging of transformer insulating materials under selective conditions I. Fofana 1,3, * ,y , H. Borsi 2 , E. Gockenbach 2 and M. Farzaneh 3 1 Canada Research Chair, tier 2, on Insulating Liquids and Mixed Dielectrics for Electrotechnology, University of Quebec in Chicoutimi, 555, Boulevard de l’Universite ´, G7H 2B1, Chicoutimi, Quebec, Canada 2 Division of High Voltage Engineering, Institute of Electric Power Systems, Schering Institute, Leibniz Universita ¨t Hannover, Hanover, Germany 3 International Research Centre on Atmospheric Icing and Power Network Engineering (CenGivre), University of Quebec in Chicoutimi, Quebec, Canada SUMMARY In today’s economic climate, it is important to know the condition, by means of suitable diagnostic tests, of the oil impregnated paper usable as primary insulation in equipment such as transformers, switchgear, bushings, cables, and their accessories. The aim of this paper is regarded as a main task to study the parameters that mostly influence the ageing process of oil/paper insulation used in transformers with preset moisture levels. A comparison is made between the performances of cellulose and Aramid papers. It is shown that Aramid paper is much less sensitive to water than cellulose paper. However, the addition of air (oxygen), via acid formation and oxidation in the oil, has a direct influence on the increase of the loss factor for both papers. The catalysts, that represent the metallic components in the transformer, accelerate the ageing process of the cellulose papers, while no influence on the ageing process of Aramid was observed. Oil ageing without a solid partner is insignificantly influenced by water, but accelerated by air-oxygen, via the moisture and acid formation, and oxidation processes. A direct influence on the increase of the loss factor and the decrease of electric strength particularly at low temperatures is to be noted. Copyright # 2006 John Wiley & Sons, Ltd. key words: aging; aging factors; transformers; power transformers; liquid insulation; paper insulation 1. INTRODUCTION The purpose of electrical insulating materials is to insulate components of an electrical apparatus from each other and from the ground, at the same time providing mechanical support for the components. The choice of those materials is a fundamental factor in the efficiency, reliability, and operating life of electrical apparatus, as they are always in contact with conductors. Insulating oils are used in power EUROPEAN TRANSACTIONS ON ELECTRICAL POWER Euro. Trans. Electr. Power 2007; 17:450–470 Published online 8 November 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/etep.134 *Correspondence to: I. Fofana, Canada Research Chair, tier 2, on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), University of Quebec in Chicoutimi, 555, Boulevard de l’Universite ´, G7H 2B1, Chicoutimi, Quebec, Canada. y E-mail: ifofana@uqac.ca Copyright # 2006 John Wiley & Sons, Ltd.