always required by NEMA or ANSI standards for
vides enhanced protection against low magnitude
smaller distribution transformers, the specifica-
ground fault currents. Therefore, connections will
tions will require taps on transformers where they
be delta-wye, except where other connections are
technically advantageous. When wye-wye connec-
tions cannot be avoided, a three-winding trans-
h. Factory tests.
former will be specified. The tertiary winding will
(1) Standard manufacturer's tests. "Routine"
be specified to be delta connected to provide a path
f a c t o r y tests, in accordance with IEEE Std
C57.12.00, will be specified for all transformers.
ferroresonance is not an acceptable reason for
Optional "design" and "other" tests, in accordance
providing a wye primary connection.
with IEEE Std C57.12.00, will be specified for
f. Impedance. Standard impedances will be used
t r a n s f o r m e r s supplying critical facilities.
to the greatest extent possible for reasons of
"Routine" tests are tests made for quality control
economy. However, industry standards do not spec-
by the manufacturer on every transformer, or
ify the percent impedance for overhead transform-
representative samples, to verify during produc-
ers or for pad-mounted compartmental distribution
tion that the transformer meets the design specifi-
cations. "Design" tests are those tests made to
supply these units with such low impedances that
determine the adequacy of the design of a particu-
normal interrupting duties for protective devices
lar type, style, or model of transformer to meet its
may be inadequate. For this reason, it may be
assigned ratings and to operate satisfactorily un-
necessary to specify minimum impedances for
der normal service conditions or under special
three-phase pad-mounted compartmental and over-
conditions if specified; and to demonstrate compli-
head type transformers. These impedances will be
ance with appropriate standards of the industry.
no more than are required by industry standards
Design tests are made only on representative
for secondary unit substations of the same kVA
samples. "Other" tests are tests so identified in
rating as shown on table 8-5. Where the designer
individual product standards which may be speci-
feels that circumstances warrant specifying a min-
fied by the purchaser in addition to routine tests.
imum impedance for overhead and single-phase
(2) ANSI impulse tests. A reduced wave,
pad-mounted compartmental units, the values of
chopped wave, and full wave impulse test, in
table 8-5 should be used. Values greater than 1.5
accordance with IEEE Std C57.98 will be required
percent for transformers 75 kVA and smaller are
on all transformers rated 200 kVA and above.
Since such a test is mainly for quality control,
tests must be run for each transformer. Prototype
g. Voltage taps. Power transformers for use in
tests do not indicate quality control deficiencies.
transmission to distribution substation applica-
(3) Optional NEMA impulse tests. These tests
tions are normally provided with manual voltage
are expensive and will not be provided for distribu-
taps at no additional cost. Since taps are not
Maximum three-phase short circuit
less than 15 kVa
These impedance values are reproduced by permission from NEMA Standard Publication No. 210-1970(R 1976)
entitled "Secondary Unit Substations."
Transformer short circuit current based on unlimited utility company contribution.
Table 8-5. Standard Secondary Unit Substation Percent Impedances.