Selection and Rating of Transformers

This Application Note discusses the effects of harmonic currents on power transformers. The main consequence is an increase in losses, mainly in the windings. Higher losses mean that more heat is generated in the transformer so that the operating temperature increases, leading to deterioration of the insulation and a potential reduction in lifetime. As a result, it is necessary to reduce the maximum power load on the transformer, a practice referred to as de-rating, or to take extra care in the design of the transformer to reduce these losses.

To estimate the de-rating of the transformer, the load’s K-factor, calculated according to the harmonic spectrum of the load current, may be used. This factor is an indication of the additional eddy current load losses and reflects the excess losses experienced in a traditional wire wound transformer.

Modern transformers may use alternative winding designs such as foil windings or mixed wire/foil windings and for these transformers, the actual increase in losses proves to be very dependent on the construction method.

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