Derating of distribution transformers for non-sinusoidal load currents using finite element method

Transformers are normally designed and built for utilizing at rated frequency and perfect sinusoidal load current. Supplying non-linear loads by transformer leads to higher losses, early fatigue of insulation, premature defectiveness and reduction of the useful life of the transformer. To prevent these problems, rated capacity of transformers supplying non-linear loads must be reduced. In this paper, a 50-kVA three-phase distribution transformer is modeled using the finite element (FE) method and its losses are estimated under rated frequency and load and also under nonlinear loads. An equivalent rating of the transformer is estimated based on the harmonic loss factor and it is compared to the recommended standard rating. This comparison shows that the estimation of derating of the transformer supplying non-linear loads using the standard recommendations is acceptable, but it is slightly conservative.