UFC 3 -520-01
June 10, 2002
12-3.2
IEEE 519, IEEE Recommended Practices and Requirements for Harmonic
Control in Electrical Power Systems, provides the industry-accepted method of
evaluating harmonic voltages and cur rents. IEEE 519 provides system level guidance,
not equipment specific guidance. What this means is that harmonic distortion limits are
established for the facility and the installation of any equipment should not degrade the
system to beyond acceptable levels. Meeting IEEE 519 limits can be easy for a facility
with primarily linear loads and it can be more difficult for a facility with predominantly
nonlinear loads. As an example, some facilities have installed many ASDs for HVAC
system motors in an attempt to reduce energy usage. Unfortunately, each ASD can
generate a large current harmonic distortion and nuisance tripping of other equipment
has occurred at some facilities as a consequence.
12-3.3
IEEE 519 provides the distortion limits for a low voltage system; Table 12-1
summarizes the various limits. Treat IEEE 519 distortion limits as design limits for
interior applications. Hospitals and airports are classified as special applications and
have the most stringent criteria. The criterion of most interest in this table is the voltage
total harmonic distortion (THD) limit of 3 percent for special applications or 5 percent for
general applications. Treat all critical systems as special applications.
Table 12-1. Low Voltage System Classification and Distortion Limits
Special
General
Dedicated
Applications
System
System
Notch Depth
10%
20%
50%
THD (Voltage)
3%
5%
10%
Notch Area (A N
16,400
22,800
36,500
volt microseconds)
12-4
HARMONIC CURRENT EFFECTS ON TRANSFORMERS .
12-4.1
Excessive harmonic distortion causes higher eddy current losses inside a
transformer, resulting in overheating. ANSI 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 a higher harmonic distortion environment. Older transformers likely were
not designed for harmonic distortion. Evaluate the effects of harmonic currents on
transformers in accordance with the following subsections.
12-4.2
Whenever significant nonlinear loads are expected in a facility, evaluate the
system to determine if transformer derating will be required. Transformer derating is not
the same as calculating the transformer k-factor. The k -factor is used to match a
harmonic current condition with a k -factor ra ting. For transformers without a k-factor
rating, derating must be used to determine the maximum fundamental load current that
the transformer can maintain with the additional harmonic currents
12-3
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