Transformers are normally designed and built for use at rated frequency and prefect sinusoidal load current. Non-linear loads on a transformer lead to higher losses, early fatigue of insulation, premature failure and reduction of the useful life of the transformer. To prevent these problems, the rated capacity of a transformer, which supply 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 conditions, as well as under non-linear loads. An equivalent rating of the transformer is estimated based on the harmonic loss factor and 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 slightly conservative.