General model for the prediction of the characteristics of aged transformer oil

The effect of aging on transformer oil´s physical, chemical and electrical properties was studied using the international testing methods for the evaluation of transformer oil quality. The study was carried out on twelve transformers in the field and for monitoring periods up to eight years. The properties which are strongly time dependent were specified and those which have a great impact on the transformer oil breakdown voltage were defined. A multiple linear regression model for the breakdown voltage as function of the transformer oil properties was developed by combining the results of a group of transformers into that of a single equivalent transformer. This model was validated by predicting the properties of some other transformers and comparing them with the measured values. The comparison showed a good agreement for the results of transformers which have either been used or not in the derivation of the model