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Standard estimation of top-oil temperature uses a thermal model related to load changes and variation of ambient temperature. Attempts have been done to improve the accuracy of top-oil temperature calculations by introducing internal properties into the model i.e. oil viscosity and winding resistance. The interest of this paper is to investigate the effect of external factors on top-oil temperature by looking into the weather, i.e. wind velocity. The results are compared with measurements on a 63MVA-ONAF 55/140 kV transformer unit, which is operated in ONAN cooling mode. The unit is located in subarctic climate, and it is equipped with a monitoring system and a weather station.

Transformer condition should be maintained because of its importance to electricity network. Each utility, in this case PLN P3B Jawa -Bali, has to assure their reliability in order to maintain electrical power system stability by assessing transformer conditions. Once transformer condition attained, appropriate maintenance and operation strategy can be defined. The most important things influence transformer condition is its insulation. Since transformer insulation is also influenced by thermal stress, another stresses also influence insulation condition i.e. chemical, electrical and mechanical stress. We calculate FAA (Factor Accelerated Aging) of the transformer by estimating Hot Spot from Top Oil temperature. Calculated FAA is correlated to Dissolved Gasses Analysis (DGA) and Oil Quality Analysis (OQA). Data used were taken from 150/20 kV 60 MVA power transformer from the year of 2007 until 2008.