K Factor Rated Transformers include several major design improvements that address the problems caused by non-linear loads and harmonics. They are designed to withstand the effects of harmonic currents without exceeding the temperature rating of the insulation system. The K Factor design compensates for the stresses on a transformer's winding insulation which prevents insulation breakdown and premature failure. The net result is longer transformer life.

Core
A high grade, nonaging, grain-oriented silicon steel with high magnetic permeability provides reduced coreinduction levels, preventing saturation as a result of the higher frequency harmonics and resultant peak voltages. In a core approaching saturation, the current in the coil will increase as voltage drops because the core cannot absorb the additional magnetic flux. This core also provides reduced eddy currents or induced currents in the steel caused by the high ratios of peak-to-RMS currents and voltages found in harmonic loads.

Neutral Bus
The neutral bus is sized and configured to accommodate at least 200% of the rated current. This compensates for the increased neutral currents found in non-linear loads thus reducing heat.

The K Factor
A common industry term for the amount of harmonics produced by a given load is the K Factor. The larger the K Factor, the more harmonics are present. Linear loads, for example have a K Factor of 1. Transformers may carry a K Factor rating to define the transformer's ability to withstand the additional heating generated by harmonic currents.

Calculating the K Factor
All nonlinear waveforms can be broken down mathematically into a fundamental frequency and its harmonics IEEE C57.110 establishes a direct relationship between these harmonics and transformer heating. Underwriters Laboratories has established a similar relationship, the K Factor, which is derived by summing the square of the percentage current at a given harmonic level multiplied by the square of the harmonic order

For example: a load that is 90% of the fundamental, 30% of the third harmonic, and 20% of the fifth harmonic would yield (.9)2 (1)2 + (.3)2 (3)2 + (.2)2 (5)2 or a K Factor of 2.62. This load would require a Transformer with a K Factor rating of 4. Transformers that carry a K Factor rating define the transformer's ability to withstand a given harmonic load while operating within the transformer's insulation class. An analysis of harmonic loads and a calculation of the K Factor must be made to properly apply transformers in any building or facility. Note that the calculated K Factor is not constant since non-linear loads change throughout the day as equipment and lighting is turned off and on. These harmonic loads also change over the life of the building or facility as equipment is added or removed.

Harmonic Currents
Harmonic currents are found in nonlinear loads. These currents are generated by various types of equipment including switching mode power supplies that abruptly switch current on and off during each line cycle. Switching mode power supplies or diodecapacitor power supplies convert AC line volt-age to low voltage DC. This process accomplished by charging capacitors during each line cycle with narrow pulses of current that are time-coincident with line voltage peaks. Examples of this equipment include electronic ballasts for fluorescent lighting, personal computers, printers, fax machines, electronic and medical test equipment, uninterruptible power supplies, and solid-state motor drives.