TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
Insulation classifications are affected by the insula-
a minimum ambient temperature for dry-type units
tion's temperature rating and by the method of cool-
of not less than minus 30 degree C. These restric-
tions apply if the transformer is to provide its nor-
ing needed to remove the heat from the transformer.
( 1) Liquid-immersed transformers. The core
mal life expectancy.
(2) Insulation temperature ratings. Liquid-
and the coils are immersed in an insulating liquid. A
flammable mineral oii insulation is the most fre-
immersed transformers are rated 65 degree C rise
quently used liquid. Various less flammable liquids
or 55/65 degree C rise. Dry-type transformers are
rated 150 degree C rise, 115/150 degree C rise, or
are used to meet NEC code requirements. Only
80/115 degree C rise. The lower the temperature
flammable and less flammable liquids are accept-
able on military installations. Nonflammable-
rise the lower the rated full-load capacity.
(3) Altitude. The dielectric strength of trans-
insulated liquids, though available, are not consid-
formers, which depend in whole or in part upon air
ered environmentally acceptable. Polychlorinated
for insulation, decreases as the altitude increases
biphenyl (PCB) insulated transformers should have
due to the effect of decreased air density. This ap-
been removed to meet OSHA requirements. Re-
plies to liquid-immersed transformers as well as
placement of liquid-filled transformers in or near
buildings must take into account the latest appli-
e. Location. Transformers can be classified by
cable NEC code restrictions, which might require an
their location, but only those which are most often
existing installation to be modified or a different
installed on military facilities are covered below.
type of insulation to be provided.
(1) Outdoor transformer. This is a transformer
(2) Dry-type transformers. The core and coil are
of weather-resistant construction, suitable for ser-
in a gaseous or dry-compound insulating material.
c. Cooling classes. Distribution and small power
vice without additional protection from the weather.
Industry standards also classify transformers as in-
transformers are generally self-cooled. Other meth-
ods of cooling may be added to provide a greater
door units, which must be protected from the
load capacity than would be available with a self-
(2) Pole-type transformer. An outdoor trans-
cooled unit. Cooling methods include forced air-
former which is suitable for mounting on a pole or a
cooling (fans) for liquid-immersed and dry-type
units; and forced air-cooling and/or forced liquid-
(3) Pad-mounted transformer. This is a unit-
cooling for liquid-immersed transformers. Dry-type
ized or compartmental-type transformer, with en-
units can be ventilated, nonventilated, or sealed.
closed compartments for medium-voltage and low-
Transformers may be provided with the cooling
equipment, may have provisions for adding cooling
voltage cables entering from below, and is mounted
on a pad. The terminology is confusing, and it is
equipment, or may be without future capability for
adding cooling equipment.
recommended that this type of unit be called a pad-
d. Insulation temperature ranges. Transformers
mounted compartmental-type transformer.
(4) Station-type transformer. A unit designed
are designed to carry their normal rated load in
specific ambient temperatures with a maximum
for mounting on a pad and installed in a substation,
stated temperature rise for normal life. If ambients
more often referred to as a substation type.
(5) Unit substat ion transformer. A unit which is
or temperature rises are exceeded under operating
conditions, the transformer life may be decreased. If
mechanically and electrically connected to, and co-
lower temperatures occur, the transformer life may
ordinated in design with, one or more primary or
actually be increased. Overload capabilities of
secondary switchgear lineups or a motor-control
transformers are indicated in ANSI/IEEE C57.91,
center. A primary unit substation has a medium-
voltage secondary. A secondary unit substation has
ANSI/IEEE C57.92, and ANSI/IEEE C57.96.
(1) Ambient temperatures. The ambient tem-
a low-voltage secondary.
(6) Other types. These include submersible,
perature for an air-cooled unit should not exceed 40
degrees C; and the average temperature for any
subway, vault-type, network, sub-surface, and
24-hour period should not exceed 30 degrees C; with
7-4. Transformer inspection and maintenance
larly and permanent records kept of all observa-
tions and tests for both scheduled and unscheduled
inspections. The frequency of inspection should be
Transformers are simple rugged devices which will
based on the importance of the transformer, the
give many years of trouble-free operation if pro-
operating environment, and the severity of the load-
vided with periodic inspections and maintenance.
ing conditions. In addition to the inspection recom-
Inspections of transformers should be made regu-