UFC 3 -520-01
June 10, 2002
distortion of less than 5 percent. If the presence of nonlinear loads causes a leading
power factor, the UPS must still operate properly. Determine the total demand
distortion (TDD) that the UPS can create on its input terminals that will be seen as
harmonic distortion to other electrically connected loads. Evaluate the TDD at UPS full-
load and light -load conditions.
12-6.1.6 Specify harmonic filters as necessary to minimize the localized effects of
harmonics. If separate harmonic filters are installed specifically to protect against
offending loads, locate each filter as close to each load as practical.
12-6.1.7 Specify true RMS sensing meters, relays, and circuit breaker trip elements.
12-6.1.8 The end-use equipment has the most impact on the system power quality and
is the easiest to address on an individual level. Wherever possible, e nsure that reduced
harmonic generation specifications are produced for equipment. For example, a simple
input filter on an ASD can reduce the generated harmonic currents by half. Adjustable
speed drives are an example of a type of equipment that can quickly degrade the power
quality as they are added throughout a facility.
Analysis alone will not always adequately predict power quality problems.
Power quality monitoring after the facility is in full operation is needed because many of
the nonlinear loads might not be running during initial system operation. If standby or
emergency power is provided upon loss of normal power, perform monitoring with the
system powered from these sources also. Correct power quality problems as
necessary based on t he results of the monitoring. Refer to IEEE 1159, IEEE
Recommended Practice for Monitoring Electric Power Quality, for additional information
regarding power quality monitoring.
NEUTRAL CIRCUIT SIZING FOR NONLINEAR LOAD CONDITIONS.
12-7.1.1 Nonlinear electrical loads distort the shape of the electrical system voltage
and current waveform, thereby generating harmonics, eddy currents, increased
motors, and generators can overheat, leading to deteriorating equipment performance.
One common result of nonlinear loads is overloading of the neutral conductor because
of the increased neutral current created by the distorted waveform.
12-7.1.2 Single phase nonlinear loads such as computers and electronics equipment
can have significant triplen frequency harmonic currents (3, 9, 15, and so on). When
these single phase loads are combined in a three phase circuit, these harmonic c urrents
appear as zero sequence components, adding in magnitude in the neutral. For
example, if there are 5 amperes of third harmonic current on each phase of a three
phase circuit, the neutral current will include 15 amperes of third harmonic current.
Because of this effect, neutral currents in low voltage systems can be higher than the
phase currents. Figure 12-2 shows one estimate of the change in neutral current as the