1 New methodology for remanent life assessment of oil-immersed power transformers W. FLORES, E. MOMBELLO 1 , J. JARDINI 2 , G. RATTA Instituto de Energía Eléctrica, Engineering Faculty, National University of San Juan, ARGENTINA SUMMARY A new methodology for the estimation of the possible remanent life of a power transformer is presented in this paper, considering several types of uncertainties which are involved in the solution of this problem. The premise is to consider the transformer as individual equipment and not as a part of a group of transformers, so as to get its remanent life considering for this purpose the condition of the insulating paper and the location of the substation in which the transformer is installed regarding the power system network. An interval of remanent possible life is estimated, using intervals of possibility, which are obtained from experts' opinion. The proposed evidences (input variables) to be taken into account in order to obtain these intervals are the following: the risk of short circuits external to the transformer regarding the condition of the paper insulation, the diagnosis obtained from the dissolved gas analysis of the transformer tank oil and another variables that influence the condition of the transformer insulation, the risk due to the load and temperature and the loss of life of the transformer. Each one of these parameters is considered as an influence index taken as condition level evidence in obtaining a possible interval of remanent life. The external short circuit risk index considers the stochastic nature which is present in external faults, as well as the degree of polymerization of the insulating paper. The external short circuit risk index is obtained by contrasting the insulation paper condition with the probability that the transformer withstands the short-circuit current flowing along the winding during an external fault. In order to assess the risk, this probability and the value of the degree of polymerization of the insulating paper are regarded as inputs of a type-2 fuzzy logic system (T-2 FLS), which computes the fuzzy risk level. The index related to the diagnosis by means of dissolved gases analysis and another variables, evaluates the condition of the transformer insulation and the result is given using words easy to understand by the maintenance personnel. This tool is a translator that converts the diagnosis from the dissolved gas analysis of the transformer oil in words, obtaining a diagnosis expressed in a simple form which is easy to understand. These two indexes are derived using T-2 FLS which allow to take into account the uncertainty present in the words. In the same way, in the evaluation of the condition of the transformer insulation the recommended procedures given by the present international standards are used, such as IEC 60599 method and Duval's triangle. Furthermore, the risk index due to load and temperature is the one proposed for Weihui Fu et al., which is based on the history of load and temperature of the transformer in order to obtain a risk level for the near future. The methodology is tested shown useful results. KEYWORDS Diagnostic reasoning, Fuzzy diagnostic, Power transformers life, Power transformer insulation, Remaining life assessment. 1 Correspondence to: mombello@iee.unsj.edu.ar 2 University of São Pablo, Brazil 21, rue d’Artois, F-75008 PARIS http : //www.cigre.org A2_203_2010 CIGRE 2010