A transformer can run abnormally hot with no apparent overload. The culprit: harmonics generated by modern loads. Here is how to take them into account.
A linear load (a classic motor, a heater) draws a roughly sinusoidal current. A non-linear load draws a distorted one, rich in harmonics, components at multiples of 50 Hz: server switch-mode power supplies, UPS systems, variable-speed drives, LED lighting.
The problem is not the average power but the frequency. Part of a transformer's losses, the eddy-current losses, grows with the square of frequency: a high-order harmonic therefore heats far more than a 50 Hz current of the same amplitude. The result: with plenty of non-linear loads, a "normal" transformer overheats and ages prematurely, even without ever exceeding its nameplate rating.
The K-factor characterises a transformer's ability to carry harmonic loads without overheating. Standard levels such as K-4, K-13 or K-20 are common: the higher the number, the richer the harmonic environment the unit tolerates. The right level follows the load profile: a heavily IT-driven site justifies a high K, a mixed site a moderate one.
The subject is central for data centers and IT in general, where switch-mode supplies are everywhere. It also concerns industry with many drives, and any strongly "electronic" site. Conversely, an essentially resistive load, or large direct-on-line motors, are barely affected.
We build the K-factor into the sizing: depending on your load profile, we steer you towards a suitable K-rated unit or a controlled derating. State the presence of non-linear loads in your request; it is a parameter that changes the outcome. Units are specified to IEC / EN 60076. See also the sizing guide.
General overview; the K level and any derating are determined from an actual analysis of the installation's harmonic profile.
A transformer designed to carry the harmonics of non-linear loads without overheating or derating, thanks to specific design choices (windings, reinforced neutral, loss management). The K-factor (K-4, K-13, K-20 and so on) indicates its level.
Because harmonics sharply increase certain losses (eddy-current losses), which grow with the square of frequency. Heating can therefore exceed the design figure even at a normal average power.
When non-linear loads dominate (data centers, lots of UPS or drives), a K-rated transformer is usually more effective than simply oversizing. For a mixed site, a moderate derating can be enough. The load profile decides.
Ideally from a measurement or an estimate of the installation's harmonic content. Failing that, reason by site type. State the nature of your loads in your request and we will point you to the right level.