K-factor transformers are installed to supply electricity to harmonic generating loads. Non-linear loads continue to proliferate in the market due to their desired benefit of reducing energy consumption.
Harmonic-generating loads present a difficult electrical environment for general purpose transformers to supply electricity; therefore, K-factor transformers are installed.
A K-factor rated transformer has special insulation and winding conductors to withstand harmonics. Manufacturers build these units to endure the extra heat that harmonics cause. K-rated transformers have four unique components that make them extra durable.
Four primary design features distinguish K-factor rated transformers from standard units:
K-factor transformers are not designed to eliminate harmonics. Instead, they are engineered to survive the heat generated by harmonics under continuous operation.
Harmonic currents increase losses in several ways:
Eddy current losses rise with the square of harmonic order
Skin and proximity effects increase conductor resistance
Triplen harmonics overload neutral conductors
Insulation systems experience accelerated thermal aging
To address these issues, K-factor transformers incorporate four key design features:
Harmonics increase the frequency at which the magnetic field reverses polarity. For example, a 5th harmonic causes magnetic reversals at 300 Hz instead of 60 Hz. Standard cores overheat under these conditions.
K-factor transformers use low-loss core materials and optimized flux density designs to minimize excessive heating caused by high-frequency harmonics.
Harmonic currents push electrons toward the surface of conductors, a phenomenon known as the skin effect. This increases resistance and localized heating.
K-factor rated windings use specially shaped or multiple parallel conductors that distribute current more evenly, reducing hot spots and improving thermal performance.
In wye-connected systems, triplen harmonics (3rd, 9th, 15th, etc.) accumulate in the neutral conductor rather than canceling out.
K-factor transformers are equipped with a neutral conductor rated at 200% of phase current, preventing overheating and insulation failure in harmonic-rich systems.
Most K-factor transformers include an electrostatic shield between primary and secondary windings. While it does not mitigate harmonics, it protects against voltage transients, common-mode noise, and electrical disturbances often associated with non-linear loads.
5. Cooling and Thermal Management
K-factor values define the severity of harmonic loading a transformer can safely handle.
Table 1: General K-factor rating guidelines
| K-Factor Rating | Typical Applications | Approximate Non-Linear Load Level |
|---|---|---|
| K-1 |
Standard transformer, general purpose, standard lighting, motors without drives |
Little to no harmonic generating loads, typically <15% |
| K-4 |
induction heating, SCR drives, AC drives |
Up to 50% of loads generate harmonics |
| K-9 | Office electronics, PLCs | Up to 50% |
| K-13 |
Institutional electronically controlled lighting, schools, hospitals, etc |
50-100% of loads generate harmonics |
| K-20 |
Data processing equipment, computer server loads, critical care facilities and hospital operating rooms |
75–100% of loads generate harmonics while the transformer is more heavily loaded |
| K-30–50 |
Known loads that are consistently generating harmonics, extra K-factor strength |
100% of loads generate harmonics, known harmonic signature |
Among these, K-13 is the most commonly specified rating for industrial and commercial facilities.
How do you know if you project need one?
Doing a power quality study will answer this for you. The study will let you know the total harmonic distortion in your system. It should also give you a breakdown of each harmonic frequency present. You could also examine the individual units in your system, and then estimate the total harmonic distortion.
Here’s a common example of where you might need a K-rated unit.
Let’s say you have an older hospital building. In recent years, the hospital has added machines with non-linear loads including cat-scan and MRI machines. Adding too many of these devices could change the total harmonic distortion of the hospital’s power system. Conducting a power quality analysis will tell you if you now need a K-rated transformer.
Here are a few other common applications:
It is important to distinguish between withstanding harmonics and reducing harmonics.
K-factor transformers tolerate harmonic heating but do not reduce harmonic currents
Harmonic mitigating transformers (HMTs) use phase shifting and winding configurations to cancel specific harmonics
K-factor transformers are often preferred when:
Harmonic analysis data is limited
Loads are mixed or constantly changing
System simplicity and reliability are priorities
HMTs are more complex and require detailed system modeling, whereas K-factor transformers offer a straightforward and robust solution.
In some cases, users oversize a standard transformer to reduce operating temperature under harmonic loading. This practice, known as derating, can provide short-term relief.
However:
Oversizing does not address skin effect or neutral overheating
Insulation systems are still not designed for harmonic stress
Long-term reliability remains uncertain
For systems with moderate to high harmonic distortion, a true K-factor rated transformer is the safer and more durable solution, particularly in mining and heavy industrial environments.
Harmonics are a common power system problem, but there are ways of handling them. If any of the above scenarios sound familiar, it’s very possible that a K-rated transformer could help. You'll want to start with a power quality study to know what your system can handle, and what level of harmonics you have.
Even though harmonics are common, they are not something to be ignored. Neglecting them could mean continued overheating and power loss to your transformers. It could also mean expensive repairs, and very expensive replacements.
Using a K-rated unit will strengthen your system against the heat of harmonics.
