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

Power system transformers must often be derated when supplying harmonic currents to nonlinear loads. IEEE Recommended Practice C57.110 extrapolates transformer loss for harmonic frequencies based on DC winding resistance and rated eddy-current loss. This paper describes an improved derating technique based on direct measurements performed at fundamental and harmonic frequencies. The measurements can be performed regardless of whether the transformer is energized and in service or deenergized and out of service. For energized transformers, the measurement can be performed regardless of whether the transformer is fully loaded or unloaded. Measurements of several distribution transformers show the improvement of this method over C57.110.