The selection of the ac supply system
voltage typically begins at the service entrance of the facility. In
most commercial environments in the U. S., the utility supplies
three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V.
In industrial environments, the utility
may supply three-phase power at even higher voltages such as 4160 V,
13 800 V and higher. The magnitude of the voltage will typically
depend on the size of the facility, the load conditions, and the
voltage ratings of the utilization equipment in the facility.
In some cases, the facility owners may
design, install, and maintain their own medium-voltage electrical
distribution system.
Recommended practice is to provide
distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V)
rather than at the actual utilization equipment level of most
electronic load equipment (208 Y/120 V). Electrical distribution
systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following
benefits over 208 Y/120 V systems:
a) The source impedance of 480 Y/277 V
systems are typically less than 208 Y/120 V systems. This
characteristic provides a more stable source with better voltage
regulation, and minimizes voltage distortion due to the nonlinear
load currents.
b) 480 Y/277 V systems are less
susceptible to on-premises generated disturbances. Step-down
transformers (and other power enhancement devices) for 208 Y/120 V
utilization equipment help attenuate disturbances originating on the
480 V system.
c) 480 Y/277 V systems distribute power
at lower currents, which result in lower heat losses in feeders. 480
Y/277 V systems may also decrease material and labor costs associated
with installing long feeder circuits.
Step-down transformers (and other power
enhancement devices) may be located physically close to the
electronic load equipment to minimize the buildup of common-mode
voltage.
Delta-connected transformer primaries
trap balanced triplen harmonic currents generated on the secondary
side by nonlinear electronic load equipment. This action serves to
reduce distortion of the voltage waveform at the 480 Y/277 V level.
It is not recommended practice to
step-up the voltage from the service entrance by means of a locally
installed transformer in order to obtain a higher power system
voltage for the electrical distribution system serving electronic
load equipment. Although this can be done in certain cases, it is
also possible that less satisfactory results can occur than if the
system voltage at the service entrance was used.
Due to the generally lower impedance of
480 Y/277 V distribution systems, higher short-circuit currents may
be available throughout the system. Overcurrent protective devices
with higher interrupting capabilities and equipment with higher
withstand ratings may be required.
In some situations, electrical
distribution at 208 Y/120 V is unavoidable. This may be due to
limitations of the utility or facility to provide higher voltages. As
previously noted, nonlinear electronic load equipment may cause
undesirable voltage distortion that can adversely affect the entire
premises. In these situations, a system analysis may be performed to
determine proper mitigation techniques such as the installation of
isolation transformers, and other power conditioning or filtering
equipment located close to the electronic load equipment.
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