Just as current harmonics can cause additional heating and losses in the magnetic core of a motor, they can produce much the same effect in the iron core of a transformer. However, greater losses from harmonics can occur in the windings of the transformer.

David Chesley, PE, LEED AP, RCDD, Interface Engineering, Portland, Ore.

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Eddy currents are circulating current in the conductors induced by the sweeping action of leakage magnetic field on the conductors. Eddy current concentrations are higher at the ends of transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities. Eddy current losses increase with the square of the frequency of the harmonic content of the current. Transformers that supply power to nonlinear loads generate more internal heat than if the same load was strictly linear in type, meaning that the transformers aren’t capable of meeting their nameplate capacities without running at damaging high temperatures. 

By the early 1980s, commercial buildings had a significantly high percentage of nonlinear loads in proportion to total load; as a result, overheating was regularly seen on heavily loaded transformers. Electrical manufacturers responded with transformers specially designed to handle harmonic-rich loads, called k-rated transformers. 

For this reason, where transformers will be serving nonlinear loads comprising more than 35% to 50% of their nameplate rating, k-rated transformers ensure the transformer does not overheat and possibly fail. 

The k-rating of a transformer is defined as highest k-factor load that the transformer can serve at its nameplate rating. In effect, the higher the k-factor, the more resulting heat the transformer is able to handle without exceeding its load carrying rating. In practical terms, the most common k-rated transformers typically have a k rating of 4, 7, 13, or 20, which corresponds to harmonic producing equipment totaling up to 35%, 50%, 75%, and 100% of downstream load, respectively. 

A key weakness of k-rated transformers is that while they reduce losses within the transformer itself, they do not actually reduce harmonics within the power system. Thus all loads connected to the secondary side of the transformer see the same voltage harmonics, and the upstream building electrical system is exposed to the current harmonics drawn by the transformer. 

Harmonic mitigating transformers (HMTs) are designed to greatly reduce certain harmonics based on their design, and thus reduce exposure to the rest of the electrical system from current harmonics drawn by downstream loads such as VFDs.