UFC 3 -520-01
June 10, 2002
4-4.3
K-Factor Transformers.
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
Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents,
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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