Evaluation of Transformer Loading Above Nameplate Rating

Abstract

Loading capabilities of power transformers in emergency and planned overload conaiticns are reviewed in the cotext of present day practice and knowledge of the subject. The limitations and defects in the present day practice for deternining the transformer overload limnits are discussed. These defects include the unrealistic determination of hottest spot temperature in the transformer windings, ambiguity regarding the forced oil cooled ratings of the transformers, minimum transformer life at 100% loading per the ANSI guide, and little scope for improving the cooling design based on the practical hottest spot temperature rating for the transformers. An economic program and evaluation of transformer overloading is developed. The transformer overloading and subsequent loss of transformer life can be evaluated on a cost basis including losses. An easily approachable criterion and method is shown to determine the limiting overload on power transformers in the face of growing concern for increasing energy cost. Transformer addition or changeout at a substation can be evaluated with the econcinic model taking into account the cost of loss of transformer life and losses viz a viz load growth projections. A total evaluation of transformer loading capabilities is discussed including carbined effect of thermal aging of insulation, short circuit stresses, and transient overvoltages as well as limitations on the transformer components other than windings.