TMS-811-1/AFJMAN
32-1080
ASSUME LOADING
Estimated
peak load . . . . . . . . . . .
.
15 MVA
Estimated
peak load duration. . . . . . .
.
8 hours
Estimated
constant load . . . . . . . . .
.
7.5 MVA
Estimated
constant load duration. . . . .
.
16 hours
Estimated
load growth (not compounded). .
.
3 percent per year
Estimated
life of substation. . . . . . .
.
25 years
ASSUME INITIAL PROVISIONS OF
Two 7.5 MVA transformers (55/65 C temperature rise and 25 year
life), which when provided with forced-air-cooling will raise capacity
to 9.375 MVA at a 55 C temperature rise and to 10.5 WA at a
65 C temperature rise. Also assume space for future installation
of a similar third unit.
AVAILABLE EXTRA CAPACITY AT 30 C AMBIENT
1.
Running transformers at 65 C (8.4 MVA) rather than
55 C temperature rise . . . . . . . . . . . . . . . . . . 12%
2. Using IEEE Std C57.92 peak load factors for normal
life expectancy -
Table 3(a) for self-cooled (OA) operation . . . . . . . 18%
Table 3(e) for forced-air-cooled (FA) operation . . . . 13%
CALCULATE TIME WHEN EACH CAPACITY INCREASE IS REQUIRED
1. Length of time original capacity is acceptable:
Total peak load capacity - 2 x 7.5 MVA x 1.12 x 1.18 - 19.8 MVA
Peak load growth - 15 MVA + (15 MVA x 3% per year x 11 years) = 20.0 MVA
Add forced-air cooling in eleventh year.
2. length of time fan cooling capacity is acceptable:
Total peak load capacity - 2 x 10.5 MVA x 1.13 - 23.7 MVA
Peak load growth - 15 MVA + (15 MVA x 3% per year x 20 years) = 24.0 MVA
Add additional forced-air-cooled unit in twentieth year.
3. Ability of three units to handle capacity for 25-year life:
Total peak load capacity - 3 x 10.5 MVA x 1.13 - 35.6 MVA
Peak load growth - 15 MVA + (15 MVA x 3% per year x 25 years) - 26.3 MVA
TO PROVIDE FOR ASSUMED LOADING
1.
Initial design . . . . . . . . . . . . . . . . Install two 7.5 MVA units
2.
Eleventh year. . . . . . . . . . . . . . . . . Add forced-air-cooling
3.
Twentieth year . . . . . . . . . . . . . . . Add third 7.5 KVA unit
4.
At end of 25-year life . . . . . . . . . . . . Units 74% loaded
US Army Corps of Engineers
Figure 4-2. Example of Sizing Substation Transformer Capacity.
"arrangement one" in figure 4-3. Such an ar-
repair the automatic LTC mechanism. Separate
voltage regulation equipment, therefore, is the
rangement allows for the least expensive method
preferred method of voltage regulation when a
of adding new transformers or switchgear in the
substation is equipped with only two transformers
future. Where the double-ended configuration
or a larger number of transformers that are
shown in figure 4-3 is used, the substation will be
incapable of supplying daily power demands dur-
configured to be served from two different trans-
mission line sources. To increase the operational
ing the outage of one automatic LTC transformer.
availability, c o n s i d e r bringing two different
Specification of automatic LTC transformers, or
manual tap changer for de-energized operation
sources into the substation if the sources are
available from the same commercial utility. Where
(TCDO) features in conjunction with separate
three-phase voltage regulators, should consider the
service is available from a commercial loop or
effects of power factor correction capacitors when
network system, the configuration will include
installed in the substation to improve the power
provisions to serve the substation from either side
of the loop or network source. Additional costs will
factor. Capacitors do not regulate voltage unless
be justified based on the facility mission, availabil-
they are automatically switched. However, they do
increase the voltage level.
ity requirements, and/or an analysis of operation
(5) Transformer arrangement. Transformers
maintenance requirements which demonstrate sig-
nificant increases in the availability factor (outage
will be arranged for connections as shown in
4-7
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