
Power Transformer 
The harmonic current can cause extra heating to transformers due to skin effect in coil conductors, extra eddy currents in core laminations, and excessive hysteresis (molecules rubbing against one another, similar to microwave oven operation).
After careful study using computer generated forecasting method, engineers developed a "krated transformer" that is designed to handle and mitigate the possible severity of the harmonics currents and its effect in the electrical system. A harmonic current content of k4 is one that would cause heating equal to that which would have been caused by 1.140 times the load current had it all been fundamental (60Hz) current.
The correct manner of determining the required krating of a transformer can be demonstrated by the following example:
Example: If the measured (or computerforecast) current is as follows, what Krating of transformer would be required to carry this load?
 True rms amperes = 73.3 amperes
 h1 = 52.45 amperes
 h3 = 42.27 amperes
 h5 = 24.97 amperes
 h7 = 9.44 amperes
 h9 = 3.72 amperes
 h11 = 5.51 amperes
 h13 = 4.77 amperes

Formula 
Consider IPU = Ih / IRMS
Using the above formula, we can come up with the following table of results.
From the result, the transformer should have a kfactor of 8.84 or above.
Harmonic Current Flow Thru Transformer
While most harmonic currents travel through transformers from the harmonic current–creating loads to the electrical power supply, some are captured within the transformer. The balanced triplen harmonic currents of the third, ninth, and fifteenth harmonics are captured within the delta winding of a transformer, where they simply circulate and heat the delta winding.
The only triplen harmonics that travel through a deltawye, wyedelta, or wyedeltawye transformer are unbalanced triplen harmonic currents.
Thus, the best way to eliminate the large portion of the harmonic currents is simply to insert a transformer with a delta coil winding into the power system to the load. Also, the alternative way to “cancel” the fifth and seventh harmonic currents from several loads is to connect some of them to deltadelta and some to deltawye transformers, causing a 30° phase shift and a vector addition to almost zero of fifth and seventh harmonic currents.
This is the exact method used when installing a 12pulse variablespeed drive (VSD) or variable frequency drive (VFD) instead of a 6pulse drive, for the 12pulse drive requires another transformer winding that is 30° phaseshifted from the first transformer’s secondary winding.