Harmonic currents generated by nonlinear loods can cause problems in the power system. Transformer are particularly vulnerable to overheating and premature failure. To protect against transformer overheating caused by harmonic, designers can specify derated equipment, i.e. oversized transformer that will run at a fracton to their rated capacity, or K-Factor transformer specifically designed to accomodate harmonic currents.

K-Factor transformers are preffered because they have additional thermal capacity of known limits, design features that minimize harmonic current losses, and neutral and terminal connections sized at 200% to normal. K Factor transformers allow operation up to nameplate capacity without derating. Another good reason for this choice is cost : a K-Factor transformer is cheaper than an oversized transformer.

K Factor is a weighting of the harmonic load currents according to their effects on transformer heating as derived from ANSI/IEEE C57.110. A K-Factor of 1.0 indicates a linear load (no harmonics. The higher the K-Factor, the greater the harmonic heating effects.

When a non-linear load is supplied from a transformer, it is sometimes necessary to derate the transformer capacity to avoid overheating and subsequent insulation failure.

K Factor transformer are designed to reduce the heating effects of harmonic currents created by loads like those in the table below. The K-Factor ratung is an index of the transformer’s ability to withstand harmonic content while operating within the temperature limits of its insulating system.

To help get around the problem of successfully applying derating factors to conventional transformers, the K-Factor is used by transformer designers to develop transformer made especially for non-linear loads and the extra heating caused by harmonic currents. Transformer come in basic K-Factors such as 4,9,13,20,30,40 and 50.