Application, design and rating of transformers containing harmonic currents

Semiconductor power converters used as power supplies for both AC and DC motors produce harmonic currents which affect other types of electrical equipment. The effect of these currents on transformers used in such systems is examined here. The authors define the problem, provide guidelines for the design and rating of new transformers, and explain how to derate existing transformers that, as a result of retrofit, are now subject to harmonic currents. It is noted that transformers subject to harmonic currents from SCR circuits have higher losses due to the increased eddy current and stray losses resulting from the high-frequency components of the current. With the aid of IEEE Standard C57.110 losses can be estimated if the eddy current and stray loss components of the load losses are known under sinusoidal conditions and the harmonic spectrum of the load currents. With this information, standard kilovoltampere transformers subject to harmonic currents using hottest-spot loading as a criteria can be derated. Since this increases the losses is considered by a factor of 4, the transformer is essentially derated by a factor twice as high as would be used if average loss density were used as a criterion. It is pointed out that using transformers designed with low eddy current losses and/or circuits designed to decrease the harmonic content of the load currents will greatly minimize the effects of harmonic current on transformers