UFC 3 -520-01
June 10, 2002
4-1.4
Impedance.
4-1.4.1 The transformer impedance is an important design characteristic; the
impedance determines how the transformer will regulate voltage with variation in load.
Additionally, the impedance limits the maximum fault current that can be supplied
through the transformer. Transformer impedances commonly vary between 3 percent
and 6 percent. A high impedance might limit short circuit current at the expe nse of
regulation and a low impedance might provide acceptable regulation at the expense of
higher short circuit currents.
4-1.4.3 Evaluate the selected transformer's impedance rating to ensure that it will not
allow a greater short circuit current in i ts secondary than the downstream protective
devices are capable of interrupting.
4-1.4.3 Impedance affects transformer regulation. As the impedance increases, the
voltage regulation tends to increase. Unless the system requires a tighter tolerance,
design for a voltage regulation range of 2 percent to 5 percent. For sensitive
equipment, tighter regulation requirements might apply; review downstream equipment
voltage requirements to verify that the regulation will be acceptable.
4-1.4.4 Transformers are readily available with an Energy Star rating, which are
intended to reduce energy losses by a more efficient design. Wherever energy-efficient
transformers are used, verify that the available short circuit current does not exceed the
interrupting rating of downstream protective devices.
4-1.5
Number of Phases.
4-1.5.1 Use single-phase transformers on single -phase systems and on single -phase
circuits derived from three-phase systems.
4-1.5.2 Use either three single -phase transformers or one three-phase transformer on
three-phase circuits. A three-phase transformer weighs less; requires less space than
three single-phase transformers of the same type, construction, and total kVA capacity;
and is easier to install. The use of three single -phase transformers has the advantage
that failure of one transformer requires only that the failed transformer be replaced and,
if necessary, the remaining two transformers can still be connected to deliver about 57
percent of the nameplate rating. Failure of a three-p hase transformer requires complete
replacement.
4-1.6
Transformer Taps.
4-1.6.1 Depending on the system conditions, the nominal secondary voltage might
not satisfy the voltage requirements of the loads. General purpose transformers should
be provided with several taps on the primary to vary the secondary voltage. Taps are
connection points along the transformer coil that effectively change the secondary
voltage by changing the transformer turns ratio.
4-4
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