K - RATED TRANSFORMERS BASIC INFORMATION AND TUTORIALS

What are K-Rated Transformers?

UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.

The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:

a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.

b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.

c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.

For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to

As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.

Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.