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Power system transformers must often be derated where supplying harmonic currents to nonlinear loads. An existing technique uses measurements performed at DC and the fundamental frequency to extrapolate transformer loss for harmonic frequencies. This paper describes an improved measurement technique which allows direct calculation of transformer loss at harmonic frequencies. The measurement is performed on both energized and deenergized transformers in the loaded and unloaded conditions and is supported by theoretical and numerical analysis. Measurements of several distribution transformers show the limitations of the existing derating technique when compared to the method described in this paper.
Kelley et al. (Tue,) studied this question.