However the harmonic loading of the CPC transformers also requiredanalysis. The harmonics listed in the table would add to the heating of the transformer. Derating a transformer is a safe practicefor transformers already installed, however, proper engineering design will overrate a transformerin the original design for the appropriate harmonic loading. K-Factor is discussed in the IEEE Standard C57.110, and is a measure to rate harmonic loadingon a transformer. The primary winding has to have sufficient conductor capability to withstandthe high third harmonic circulating currents, as well as the ninth and fifteenth harmonics.
Thesecondary windings can use multiple, smaller, parallel conductors instead of single larger ones.
This reduces the skin effect andAC resistance at frequencies greater than design. Thetransformer's neutral, to the point of connection of the phase windings, has to be larger to sustainthe third zero sequence currents generated by the non linear devices. A distribution transformer in a switch mode power supply environment has to be designed forharmonic loads and has several challenges. The third harmonic is a circulating, zero sequenceharmonic current. The secondary windingsmust be capable of withstanding the skin effect of higher frequency harmonics.