4-4.3

4-4.3.1 Transformers are available for high harmonic-content power distribution

systems without derating, often referred to as *k-factor transformers*, and usually have

the following characteristics:

the core flux density, thereby creating higher core losses, higher magnetizing

currents, higher audible noise, and overheating.

Larger primary winding conductors to compensate for additional heating effects.

Individual insulated secondary conductors to reduce stray losses.

Larger neutral connections to compensate for ha rmonic currents causing larger

neutral currents.

4-4.3.2 Evaluate the effect of nonlinear loads as part of the facility design. In some

cases, nonlinear loads can require transformer derating or, in extreme cases, a

transformer designed specifically for nonlinear loads might be required. Also, the

transformer neutral conductors might require sizing for up to 200 percent of rated

current. Excessive harmonic distortion causes higher eddy current losses inside a

transformer, resulting in overheating. IEEE C57.110, *IEEE Recommended Practice for*

states that a transformer should be capable of carrying its rated current provided that

the total harmonic distortion is less than 5 percent. Beyond this amount, derating of the

transformer might be necessary. Newer transformers are often, but not always, already

designed for some level of a higher harmonic distortion environment. Older

transformers likely were not designed for harmonic distortion. Refer to paragraph 12-4

to determine if a transformer requires derating.

4-4.3.3 The k-factor relates transformer capability to serve varying degrees of

nonlinear load without exceeding the rated temperature rise limits. The most common

k-factor ratings are k-4 and k-13. Manufacturers recommend k-4 transformers if the

connected load is 50 percent nonlinear electronic loads and k -13 transformers are

recommended for 100 percent nonlinear electronic loads. This simplified approach

allows the user to avoid calculating actual k-factor values for the facility. Transformer

k-factor ratings greater than k-13 should never be necessary, and the use of such

transformers actually can contribute to harmonic distortion problems because of their

low impedance.

4-4.3.4 In practice, the system k-factor tends to decrease as the overall load

increases. Thus, k-factor measurements taken in lightly loaded conditions can be quite

high, but can be significantly lower on a fully loaded system. Transformer coil losses

decrease with the square of the load and this reduction far exceeds the increased

harmonic distortion variation, the maximum loss point in transformer coils is always at

4-9

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