TM5-811-1/AFJMAN
32-1080
a. Circuit breakers. Circuit breakers are more
extra height, low-profile structures will be in-
costly than other equipment, used singly or in
stalled.
combination, to accomplish line switching and to
(2) Transformers. Unit substations require
protect station elements. However, circuit breakers
less land space, are less visually objectionable and,
will be used for all switching stations and substa-
because of the integrated transformer and second-
tions, when stations are served by more than one
ary connections, are more reliable than transform-
incoming line or contain transformers rated 10
ers located separate from the associated switch-
MVA or above, when economically justified, when
gear.
required to obtain the required degree of reliabil-
(3) Connection to aerial distribution lines. Un-
ity, or when their use is required for coordinated
derground connections from a new or modified
circuit protection or switching to limit the dura-
station to feeders or incoming lines will be pro-
tion and frequency of outages to the installation.
vided when phase-to-phase voltage is less than 35
Circuit breakers will be of the oil or sulfur
kV. Underground installation of cabling enhances
hexafluoride (SF,) type when the incoming line
the appearance of the station installation.
voltage is greater than 35 kV, nominal. When air
and vacuum circuit breakers have adequate con-
4-6. Incoming line Switching Equipment.
tinuous current and interrupting ratings, those
oilless types will be considered for use as line
Equipment required for the switching of incoming
circuit breakers for lines rated at or below 35 kV.
lines, and for the protection of primary station
SF, breakers may also be used at line voltages
elements when required, may be provided by the
below 35 kV. Standard ratings are listed in IEEE
supplying utility or by the Government to meet
Std C37.04 and IEEE Std C37.06. The design of
any requirements of the utility and the needs of
the station will include provisions to isolate circuit
the using agency. The following applies to the
breakers and to bypass them with power fuse
instances where such equipment is provided by the
disconnecting units when required to ensure con-
Government, with the concurrence of the utility.
tinued protection of station buses and equipment
The exact type, ratings and the consequent cost
when circuit breakers are out-of-service. The by-
will depend on the protective coordination re-
pass feature is not required if other circuit equip-
quired, the voltage rating of the incoming lines or
ment can protect station elements when circuit
feeders, the full-load current and the fault current
breakers are inoperative, or if the utility line
availability at the station. Figure 4-1 includes an
breakers afford the required degree of protection.
example of converting fault MVA to symmetrical
Where only one incoming line serves the entire
fault current. Refer to IEEE Std 242 for calcula-
base or installation, disconnect switches and a
tion and application of asymmetrical fault cur-
fused by-pass switch unit will be specified to allow
rents.
US Army Corps of Engineers
Figure 4-1. Converting Utility Company Short-Circuit MVA to Current.
4-4
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