UFC 3 -520-01
June 10, 2002
NONLINEAR LOAD DESIGN CONSIDERATIONS.
Analyze planned electrical loads on new projects to determine whether or not
they are considered potential nonlinear loads with high harmonic content. The following
guidelines are provided if nonlinear loads are a significant portion of the total load.
12-6.1.1 Derate transformer, motor, and generator outputs if necessary to prevent
overheating or burnout. Ensure that design documents and equipment nameplates
reflect the derated capability.
12-6.1.2 If standby generators represent the only power source upon loss of normal
power, the generator design must account for nonlinear loads. Generators are
designed to deliver a pure sinusoidal frequency, usually at a frequency of 60 Hertz in
North America and 5 0 Hertz elsewhere. When harmonic currents are drawn through a
generator, losses increase causing greater heat generation. Voltage distortions can
parallel operation of multiple generators. If the generator cannot be protected from the
effect of harmonic load currents, advise the generator supplier of the nonlinear load
environment to ensure that the generator is designed and sized properly. If a significant
portion of the load is nonlinear, it might be necessary to apply a multiplying factor of 1.3
to 1.5 to the generator size to compensate for the expected heat losses. Also, the
generator manufacturer can design the generator to withstand better a harmonic
environment by adjusting the generator pitch and decreasing the subtransient
reactance. Specify a voltage regulator capable of achieving proper voltage regulation in
high harmonic content and distorted sine wave load conditions. If the generator
manufacturer lowers the generator subtransient reactance, ensure that the facility
design remains acceptable for short circuit conditions.
12-6.1.3 Use a single three-phase transformer with common core, delta connected
primary and wye connected secondary instead of three single-phase transformers
connected for three-phase service. Evaluate the use of a k-factor transformer if a
standard transformer has to be derated by more than 10 percent. Compare the cost of
a k-factor transformer to an equivalent standard transforme r. Even if derating of a
standard transformer is not required, select the k -factor transformer if the cost of the two
types is within 5 percent, provided that the lead time of a k-factor transformer satisfies
facility schedule requirements.
12-6.1.4 If common-mode noise is a concern, specify electrostatically shielded
isolation transformers for critical loads and locate each transformer as near to the
served loads as practical to reduce the load requirement and cost of each transformer.
Bond and ground the shield in accordance with the manufacturer's requirements.
Ground the transformer in accordance with the NEC. Refer to paragraph 4 -4.1 for
12-6.1.5 UPS systems must be capable of performing properly with nonlinear loads.
The UPS should be capable of withstanding high crest factors (the ratio of peak current
to RMS current). The UPS should provide a sine wave output with a total harmonic