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 "k-rated 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 k-4 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 (60-Hz) current.
The correct manner of determining the required k-rating of a transformer can be demonstrated by the following example:
Example: If the measured (or computer-forecast) current is as follows, what K-rating 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