Transformer derating for harmonic currents: A wide-band measurement approach for energized transformers

Power system transformers must often be derated when supplying harmonic cur rents to nonlinear loads, IEEE Recommended Practice C57.110 extrapolates tr ansformer loss for harmonic frequencies based on de winding resistance and rated eddy-current loss, This paper describes an improved derating techniqu e based on direct measurements performed at fundamental and harmonic freque ncies. The measurements can be performed regardless of whether the transfor mer is energized and in service or deenergized and out of service, For ener gized transformers, the measurement can be performed regardless of whether the transformer is fully loaded or unloaded. Measurements of several distri bution transformers show the improvement of this method over C57.110.