to take into account any nameplate derating for
microsecond or a 1.5 x 40 microsecond full im-
units located in high ambient temperature areas.
pulse voltage wave, dependent upon the industry
Low ambient temperature should not be a deciding
specification. The crest value of the voltage wave
factor in sizing of transformers, except where the
is called the basic impulse insulation level (BILL) of
installation has an average daily ambient of 20
the equipment involved. Standard basic impulse
degrees C or less.
insulation levels have been established for each
(c) Use of ANSI loading factors. The influ-
voltage reference class; however, equipment rated
ence of operating temperatures and load durations
15 kV and below is often built to a so-called
on transformer life may have to be taken into
"distribution-class" BIL rather than to the stan-
account in sizing transformers.
dard or "power-class" BIL. The distribution class
c. Cooling provisions. The rated capacities of
BIL requirement is even lower for ventilated
750 kVA and larger transformers can be increased
dry-type transformers than for liquid-filled trans-
by the addition of cooling fans, with the exception
formers as shown in table 8-4, although BIL levels
o f pad-mounted compartmental units. ANSI
equivalent to those for distribution-class liquid-
C57.12.10 covers both the initial installation of
filled units can be obtained for some voltages at a
cooling fans and the provisions which permit
relatively small cost increase. Ventilated dry-type
addition of future forced-air-cooling. Fan cooling
transformers will always be provided with a BIL
can be controlled by either top-liquid temperature
rating equivalent to liquid-filled units of the same
or winding temperature. The type of control will
rating. Use of external devices such as surge
be left to the option of the manufacturer.
arresters to provide an adequate BIL protective
(1) Main electric supply stations. Forced-air-
level is not acceptable.
cooling will be provided in accordance with the
criteria of chapter 4.
e. Transformer connections. The wye-wye con-
(2) Secondary unit substation transformers.
nection of two-winding transformers requires that
Since secondary unit substation transformers sup-
a fourth wire (neutral or ground) be installed
ply electrical energy for direct utilization by mo-
throughout the length of distribution lines, and
tors, lights, and other devices, initial forced-air-
the solid grounding of the primary and secondary
cooling is not necessary is most cases. Provisions
windings of transformers. Loss of either the pri-
for addition of future forced-air-cooling will be
mary or secondary ground and unbalanced loading
provided only when such cooling equipment is a
can cause interference on communication circuits
cost effective way to satisfy future load increases.
and result in excessive heating of the tanks of
d. Basic impulse levels. Insulation characteris-
three-phase transformers. For those reasons, the
wye-wye connection of two-winding transformers
tics for voltage surges of high magnitude but short
will be avoided whenever possible. A delta pri-
duration, such as lightning or switching surges,
mary connection eliminates objectionable odd har-
are determined by impulse tests. The most com-
monic paths and a wye-secondary connection pro-
mon test is the application of either a 1.2 x 50
Table 8-4. Basic Impulse Insulation Levels
Basic impulse level (BIL)
75 . . . . . . . . . . . . . . . .
15 . . . . . . . . . . . . . . . . .
110 . . . . . . . .
60 . . . . . . . . . . . . . . .
25 . . . . . . . . . . . . . . .
150 . . . . . . . . . . . . . . . .
Reprinted with permission from IEEE Std C57.12.00-1987 "Standard General Requirements for Liquid-Immersed Distribution,
Power, and Regulating Transformers.
Reprinted with permission from IEEE Std C57.12.01-1969 "Standard General Requirements for Dry-Type Distribution, Power,
and Regulating Transformers.
`Rating also applicable to primary and secondary units substations.