Classification in iso-attention classes of hv transformer fleets

The largest electrical operators may have in service some hundreds of HV power transformers. It means that failures are to be expected along their operational times with possible heavy consequences in case of fire or explosion or other negative events. The traditional approach for preventing failures and their consequences is normally based on periodical evaluations of some physical, chemical and electrical parameters of the insulating oil and/or of the equipment but these data do not give any information on the overall "state of health" of the units when the observed properties are below the values assumed by the different existing Standards as acceptable limits. Also these data are not related to failure consequence. With this aim, a more complete risk evaluation may be an innovative powerful tool for preventing negative situation due to transformer failures and for classifying these electrical units in iso-attention classes. This risk is formed by three partial components, which are: 1) causes of the failure, 2) probability that a failure may provoke a damage and 3) type and entity of damages (magnitude). This approach may be a useful tool for supporting decisions and addressing periodical investments of electrical operators and utilities having in service transformer fleets. With this aim, approximately 700 different HV transformers of one of the biggest electrical operator have been examined, evaluating the single components forming their risks. This analysis has permitted to establish which transformers may be considered more critical with possibility to better address the successive maintenance strategies.