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Section IV. OPERATING UNDER USUAL CONDITIONS
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TM-5-6120-250-12 Substation Trailer Mounted 500 KVA AC 4160-416Y/240V 208Y/120V 3 Phase 50/60 Hertz Manual
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Table 2-2 Maximum Allowable Average Temperature of Cooling Air for Carrying Rated K VA
(2) The load should not exceed 560 amps per
(2) The winding  temperature  with  65
C
feeder or a combined total of 1600 amps for the three
average rise in an average ambient temperature of
feeders breakers in a 208 / 120 volt operational
30  (the permissible top oil temperature f normal life
C
mode.
expectancy) would be 90 C. Assuming the hottest spot
rise did not exceed 200C. NEMA Standard TR98-1964
(3) The load should not exceed 280 amps per
provides a guide for loading
feeder or a combined total of 800 amps for the three
(3) The pressure-vacuum gage is calibrated
feeder breakers in a 416/ 240 volt operational mode.
P'SIG from-10 to + 10 pounds per square in( Gage
readings vary with transformer temperature changes
(4) The circuit breakers should be operated
and would normally indicate a posit pressure. The gage
periodically. See paragraph 3-15 a
may indicate a vacuum with the tank when the
transformer is de-energized or operating under light or
b.  Transformer.  Periodic readings of the meter
no load conditions in a I ambient temperature.  Any
change in temperate without a change in reading is an
and gages on the transformer must be made.  Record
indication leak.
and note any changes from the last and previous
readings. Note the highest reading since the last one on
2-13. De-energizing Distribution and Main Lines
the liquid temperature and winding temperature meters.
a. Feeder Lines (fig. 2-15).
Reset highest reading pointer. If a trend of deviations or
(1) Trip feeder breakers No.
1, 2, and
a sharp change is noticed, notify organizational
mechanically by pushing trip button located to left of
maintenance for corrective action.
breaker charging handle. To de-energize c one feeder
line, the breaker of that feeder line tripped. When the
(1) The liquid level gage has a center
trip button has been pushed, indicator should read open
marking indicating the level at 25 . Dial markings are
C
and the charging hat charge-indicator should read zero.
minimum-25 C
maximum.
On
a
deenergized
transformer the liquid level should indicate near enter at
(2) A check to make sure a line is dead may
normal ambient temperature.  It should indicate near
performed by using the ammeter readings of feeder line.
maximum when fully loaded at operating temperature.
When checking the level at temperatures other than
(3) De-energize all feeder lines when shut
25 C, it should be noted the maximum end of the scale
complete substation down.
represents  the  level  at  approximately  90
C  oil
temperature. The gage indicator travel between 25 C
b. Main Line. Trip main breaker the same I feeder
and 90 C may considered to be proportional to the
breaker in a. above.
temperature.  For example, for an oil temperature of
55-25 =
55 the level would be
C
30 slightly
c. Open Load Bread Switch (fig. 2-1 and .
90-25
65
The opening operation is the same as the clothing (para.
less than halfway between 25 C and maximum.
2-1 11 c).
Section V. OPERATION UNDER UNUSUAL CONDITIONS
operated at nameplate rating and standard ambient
2-14. Operation in Extreme Cold
temperatures over the same period.
The mobile substation is designed to operate at
temperatures down to -300 F. The cabinet interior must
Table 2-1. Loading on Basis of Ambient Temperature.
be continually checked to insure heaters are working.
PERCENT OF RATED KVA
Decrease
Decrease
2-15. Operation in Extreme Heat
Decrease Load For
Load For
TYPE OF COOLING
Each Deg. C
Each Deg C
a. General The
ambient temperature
is an
Higher
Lower
important factor in determining the load capacity of the
Temperature
Temperature
transformer. The temperature rise for any load must be
Self-Cooled OA
1.5
1.0
added  to  the  ambient  to  determine  operating
temperature. Ambient temperature should be used in
b. Cleaning.  Make sure the ventilation screens,
the determination of winding hottest-spot temperature
grills and filters are clean and free from air intake
and the load capacity of the transformer. Use table 2-1
obstruction.
to determine load capacity. This will give approximately
the same life expectancy as if the transformer were
2-30

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