12-6

12-6.1

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

additional information.

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

12-8

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