Selection of transformers for commercial buildings

Standards dealing with the application of transformers subjected to harmonic load currents are clarified, and application guidelines are provided. The impact of harmonic currents in commercial building systems needs to be considered, due to the increased use of electrical equipment that draw nonsinusoidal current from the AC line. A transformer subjected to harmonic load currents should have a K-factor rating higher than that of the load it supplies. It is preferable to use a K-factor-rated transformer, rather than derating a general-purpose transformer. If the transformer is not K-factor rated, its maximum load kVA should be limited in accordance with ANSI/IEEE Standard C57.110, which applies to general-purpose transformers that are subjected to a load current with total harmonic distortion greater than 5%. Most applications for K-factor-rated transformers require either K=4 or K=13. Higher K-factor specifications should be examined for erroneous assumptions regarding load characteristics. The ANSI/IEEE Std. C57.110 derating calculation is very sensitive to errors in the input data; typical values and rules of thumb should be avoided.<>