What is Transformer K-Factor?

K-Factor Transformers Deal with Harmonic Loads

Industrial applications today use modern electric systems to meet their high-efficiency requirements. As most electric systems and their loads are non-linear in nature, this tends to increase the level of harmonics in the power supply. Harmonics cause a wide range of problems ranging from principal effects such as overheating to secondary effects like false operation of relays, premature blowing of fuses and circuit breakers, and neutrals of transformers and panel boards running hotter than normal. The higher the frequency of harmonics present in the system, greater is the heat losses in generators, conducting cables, transformers, and motors.

Because of overheating from harmonics, power sources such as transformers cannot operate up to their rated capacity. For instance, a 100 kVA transformer can only operate at 60 kVA if it has to remain within its operating temperature specifications.

K Rating

ANSI/IEEE and UL standards define K rating as a factor to indicate the level of harmonics that the load can generate. The K rating is immensely useful when sizing components and designing electric systems suitable for operating with high levels of harmonics. Typical load K ratings are K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50, with the larger numbers signifying satisfactory operation in the presence of higher harmonics.

For instance, loads such as resistance heaters, transformers, and motors operating at low levels of harmonics have a rating of K-1, while induction heaters and welders generating a substantial level of harmonics have a rating of K-4. To allow power sources to operate satisfactorily, loads with higher K rating need connection to transformers with a corresponding K factor.

Transformer Specs

Sample Transformer Specifications with Listed K-Factor

K Factor Transformers

Manufacturers design the K factor transformer to power harmonic generating loads. Transformers with higher K factor are made of thicker conductors, transposed suitably to withstand higher eddy currents caused by high frequencies. The neutral conductor in these transformers is much thicker than normal to protect against triplen harmonics. To reduce the eddy current losses and heating, designers place electrostatic shielding between the primary and secondary windings. To cancel the skin effect from high-frequency currents, the secondary winding may consist of multiple windings in parallel.

A transformer will deliver its rated capacity as long as the K rating of the load matches the K factor of the transformer. For instance, a transformer rated at 100 kVA and a K factor of 4 will deliver 100 kVA for a load that has a K rating up to 4. At higher K ratings, it is essential to derate the transformer.

Compared to transformers with a K factor of 1, those with higher K factors are expensive and occupy more space because of their larger size.

Importance of K Factor

The K factor designates the transformer’s ability to carry harmonic currents while operating within its specified temperature limits. In a typical installation, the rationale is to compute the K rating of the load, and consequently specify a transformer with a K factor that is equal to or higher than the calculated value.

The above results in the use of a transformer sized to the load without derating. It allows the transformer handle the non-linear load specified, in addition to 100% of the fundamental 50/60 Hz load.

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