The effects of Harmonics on transformers

In a typical Delta-Wye transformer, the triplen harmonics

(i.e. odd order harmonic that are multiples of three, such as the 3^rd, 9^th, 15^th etc), will circulate only within the primary delta winding and will not be observed in the power source before the transformer.

Other odd harmonics, such as the 5^th, 7^th etc. do appear in the power source.

Triplen harmonics in the wye secondary are in phase in the neutral so add together.

What is K-Factor ?

An indicator of the transformers ability to handle non-sinusoidal loads

ANSI/IEEE Standard C57.110 Recommended Practice for Establishing Transformer Capability
When Supplying Nonsinusoidal Load Currents, provides a method of quantifying harmonic currents.

Recognized K-Factor values – K-4; K-9; K-13; K-20; K-30; K-50

Underwriters Laboratories selected these values as an industry standard.

An eddy current loss multiplier

The numbers assigned to the K-Factor values represent the number by which the winding eddy current losses of the transformer must be multiplied, to obtain the total value of winding losses when subjected to a non-sinusoidal waveform.