Distribution Transformer Winding Losses Due to Nonsinusoidal Currents

Increased transformer winding losses are an important consideration in determining the overall impact of harmonic currents in a power system. Frequently, losses due to harmonic currents are assumed to vary with the square of frequency. The purpose of this paper is to compare winding loss calculations from a finite element method with measured losses in single-phase distribution transformers and to test the principle of superposition of harmonic losses due to nonsinusoidal currents. The results confirm both the finite element method and the principle of superposition. Furthermore, it is shown that, due primarily to inaccuracy in measuring 60-Hz eddy current losses, application of the commonly accepted frequency squared rule can yield overly pessimistic loss predictions for typical power system harmonic frequencies.