TM-811-1/AFJMAN
32-1080
and to allow for overloading of transformers with-
the oxygen in the room is above 19.5 percent.
Designs will require provision of an SF, gas
out sacrificing transformer life.
(a) Dual thermal rating. Transformers with
sensor, controls, alarm, and calibration of the
sensor system to indicate the unsafe level of SF,
a dual thermal rating of 55/65 degrees C will
gas for personnel.
permit operation of the transformers at 112 per-
cent loading in a maximum daily average ambient
4-7. Substation Equipment.
temperature of 30 degrees C.
a. Power transformers. Power transformers will
(b) Forced-air-cooling. Only single-stage fan
be the outdoor, liquid-filled type. A more detailed
cooling is available for the smaller sizes of power
discussion of transformers is presented in chapter
transformers. Single-stage air-cooling will provide
8.
an additional 15 percent capacity to units base
(1) Quantity of substation transformers. The
rated 750 kVA to 2000 kVA and 25 percent for
quantity of substation transformers to be installed
units base rated 2.5 to 10 MVA. Either single-or
in an existing substation will depend on the
dual-stage forced-air-cooling can be obtained for
present configuration and features of the substa-
units base rated 12 MVA and above, and will
tion, and on any requirements of the utility and
provide a 33.3 percent increase in the transformer
the Using Agency. The number of transformers to
capacity for each of the two stages of cooling.
be installed in new substations should be two of
Single-stage cooling will be specified for all trans-
like design, ratings, and characteristics when the
formers when that option is available, and when
maximum substation capacity is 40 MVA or less.
the selection of that option is more cost effective
A larger quantity may be required for substations
than increasing the self-cooled rating of transform-
with a greater capacity to comply with reliability,
ers to accommodate peak demands of limited dura-
availability, or maintainability criteria of the Us-
tion. Provisions for future second-stage cooling will
ing Agency. The exact number of power transform-
be specified for transformers when the option is
ers may be determined by the utility if transform-
available. Second-stage cooling may be specified to
ers are to be supplied by the utility. Coordination
be provided initially when load demands are ex-
with the utility and the Using Agency will be
pected to increase substantially in early years
required when requirements imposed by the utility
following construction of the station, because of
or the Using Agency dictate the design of new or
planned expansion of facilities at the installation.
(3) Example of determining station capacity.
modified substations. In any instance, a new sub-
station should be constructed with not less than
Figure 4-2 contains an example of determining
two transformers of ample capacity to prevent the
the capacity of a new substation, based on the
outage of one transformer from causing a complete
assumptions given. The example and the preceding
loss of power to an installation. This is not meant
assumes that power transformers will be installed
to always require 100 percent redundant trans-
in a daily average ambient temperature of 30
former capacity. Unless 100 percent normal opera-
degrees C or less. The capacity and features of
tion is required with one unit out of service by the
power transformers will be determined and se-
Using Agency, each transformer should be sized
lected in accordance with industry practices and
standards when transformers are to be installed in
from 50 to 75 percent of load for two transformers,
or from 33 to 67 percent of load for three trans-
a higher ambient temperature region, or when
formers.
other assumptions made do not suit actual site
(2) Capacity of substation transformers. The
conditions or standard transformer designs. Un-
capacity of a new or modified substation trans-
usual service conditions will be determined and
formers will be adequate to supply all installation
compensation will be made in specifying substa-
or project demands determined during design. The
tion equipment.
(4) Load-tap-changing (LTC) transformers.
capacity of substations will be sufficient to accom-
Transformers may be equipped with manual tap
modate expected load growth in later years. Load
growth should be based on increases of one to five
changer mechanisms, operated under de-energized
conditions, or automatic LTC mechanisms to com-
percent of the estimated peak load per year, when
pensate for voltage changes under varying load
more exact load growth information is not avail-
conditions. Automatic LTC transformers provide a
able. The base capacity or rating of new transform-
convenient method of compensating for voltage
ers will be the self-cooled rating for a 55 degrees C
unit. Increased capacity of individual transformers
changes in the primary or secondary voltage sys-
tems. However, failure of such automatic LTC
will be obtained by specifying a dual thermal
provisions may cause the outage of the associated
rating and forced-cooling provisions when avail-
power transformer during the period required to
able and necessary to accommodate load growth,
4-6
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