The effects of harmonics on
transformers are
• Increased copper losses
• Increased iron losses
• Possibly resonance between
transformers
• windings and line capacitance
• Insulation stress
• Neutral overheating due to triplen
harmonics
The copper losses and iron losses in
the presence of harmonics can be computed. The application
of general equations assumes that the
transformer is a linear device which it is not. However, for normal,
operating conditions and normal levels of harmonics, this is a
reasonable approximation.
However, the increase of hysteresis
losses due to harmonics is only a fraction of the eddy current
losses. Voltage harmonics result in higher transformer voltage,
therefore higher insulation stress. This is not a problem since most
transformers are insulated for much higher voltage levels than the
overvoltages due to usual levels of harmonics.
There is a certain degree of
interaction between voltage and current harmonics for transformers
designed to operate near the saturation point (knee of the saturation
curve). It is possible a small level of voltage harmonic to generate
a high level of current harmonics. This phenomenon depends on
specific harmonic and phase relationship to the fundamental.
To address the overheating of
transformers due to harmonics, the ANSI/IEEE published a standard
C57.110-1998, “Recommended practice for establishing transformer
capability when supplying nonsinusoidal load currents,” which was
reaffirmed in 2004. This standard establishes methods for determining
derating factors for transformer capability to carry nonsinusoidal
load currents.
In 1990, Underwriters Laboratory (UL)
established the method for testing transformers that serve nonlinear
loads. The UL test addresses coil heating due to nonlinear loads and
overheating of the neutral conductor by assigning a “K“ factor to
the transformer. The K-factor is meant to apply to transformers
serving general nonlinear loads. UL has devised the K-factor method
for labeling and rating the ability of dry-type transformers to
withstand the effects of harmonics.
The K-factor rating indicates the
transformer’s ability to tolerate the additional heating caused by
harmonics. The K-factor is based on the methodology similar to that
discussed in the ANSI/IEEE C57.110 standard. The K-factor can be
calculated as the sum of the product of each harmonic current squared
and that harmonic number squared for all harmonics from the
fundamental to the highest harmonic of consequence.
When K-factor is multiplied by the
stray losses of the transformer, the result represents the total
stray losses in the transformer caused by harmonic currents. To
obtain the total load losses, the total stray losses are then added
to the load losses. It should be obvious that the K-factor for linear
loads (absence of harmonics) is 1.
Also, the K-factor does not mean that
the transformer can eliminate harmonics. Harmonics increase heating
losses in all transformers, and some of these losses are deep within
the core and windings and some are closer to the surface. Oil-filled
transformers react differently to the increased heat and are better
able to cool whereas dry-type transformers are more susceptible to
the harmonic current effects and are so labeled. The UL test
addresses coil heating due to nonlinear loads and overheating of the
neutral conductor.
