Transformer K Factor Rating: Mitigating Transformer Heat Survival

K-factor transformers are used to supply power to loads that generate harmonics. Non-linear loads such as VSDS and inverters used in converting DC power to AC are increasingly common due to their ability to improve energy efficiency. However, the harmonics they produce create challenging operating conditions, such as heat generation, that standard transformers are not designed to handle. As a result, a harmonic study should be conducted to determine the K-factor transformer specification required to handle the demanding environment.

Non-linear loads introduce undesirable effects into electrical systems, primarily in the form of harmonic currents. These currents affect system wiring and are especially problematic for the transformers supplying power to such loads. Harmonic currents generate additional heat and mechanical vibrations within the transformer, potentially reducing its efficiency and lifespan.

Triplen harmonics are the most troublesome harmonics. These are produced by single-phase, non-linear loads such as fluorescent lighting. Triplen harmonics are unique in that they are additive on the neutral conductors, connectors, and bus bars of a three-phase system. For instance, if a third-order harmonic (which is a triplen harmonic) produces 1 amp of current on each phase, the result is 3 amps of 150 Hz current flowing through the transformer’s neutral wire and bus. This phenomenon is referred to as neutral current. In contrast, linear loads, those operating purely at the fundamental 50 Hz frequency without generating harmonics, typically produce little to no neutral current, meaning there is minimal heat buildup in the neutral conductors and transformer components. To mitigate the effects of these harmonics, transformers may incorporate electrostatically shielded windings, which help reduce the transfer of high-frequency noise and harmonic distortion between windings.

Notches are short bursts of current drawn from the power system, which is what allows energy-efficient operation of motors using drives. These notches provide the only power required to keep the motor running within system requirements after startup, and the motors run cooler, using less electrical energy to do the same work. However, these same power-draw notches generate harmonic activity. A 6-pulse variable speed drive will have harmonic activity primarily at the 5th and 7th harmonic (one above and one below the pulse count, so 250 and 420 Hz, respectively). The 6-pulse drive non-linear load generates harmonic currents, which in turn generate heat in the distribution equipment, neutral conductors, and distribution transformers. This is why K-factor transformers are specifically designed, tested, and manufactured to operate with these harmonic currents without exceeding their temperature rise rating.

K Factor Ratings

The K-factor is an index that indicates a transformer's capability to handle harmonic currents without exceeding its rated maximum temperature rise. It reflects the transformer's suitability for powering loads that produce harmonic content. K-factor values range from 1 to 50. A K-factor of 1 is typically used with linear loads. On the other end, a K-factor of 50 is designed for extremely harsh harmonic environments and is usually chosen based on detailed knowledge of the load’s harmonic profile. Commonly used K-factors include 4 and 13, which are frequently specified and installed. Transformers rated with a K-factor are referred to as K-rated, and this rating is listed on the transformer’s nameplate.

K factor rating guideline:

K Factor	Applications	Harmonic Activity
K1	Standard transformer, motors without drives, standard lighting	Little to no harmonic loads (typically less than 15%).
K4	Induction heating, SCR drives, AC drives.	Up to 50% of the load generates harmonics.
K13	Institutional electronically controlled lighting, schools, and hospitals.	50% to 100% of loads generate harmonics.
K20	Data processing equipment, computer server loads, critical care facilities, and hospital operating rooms.	75 to 100% of loads generate harmonics while the transformer is more heavily loaded.
K30-50	Known loads that are consistently generating harmonics.	100% of loads generate harmonics (harmonic signature).

While the term K rated transformer is widely recognised and used, particularly in North America, primarily through the UL (Underwriters Laboratories) 1561 standard and ANSI/IEEE C57.110, the International Electrotechnical Commission (IEC) does not have a direct, equivalent “K-factor” rating system for transformers in the same way. The IEC’s approach, particularly in IEC 60076 series (Power Transformers), address the effects of harmonics in transformers through methodologies and design considerations.

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