MIL-HDBK-1003/7
arrangement is suitable up to approximately 10,000 kVA. If the bus operates at a
voltage of 13.8 kV, this arrangement is the best for stations up to about 25,000 or
32,000 kVA. For larger stations, the fault duty on the common bus reaches a level that
requires more expensive feeder breakers, and the bus should be split.
b) The bus and switchgear will be in the form of a factory fabricated metal
clad switchgear as shown in Figure 22. For plants with multiple generators and outgoing
circuits, the bus will be split for reliability using a bus tie breaker to permit
separation of approximately one-half of the generators and lines on each side of the
split.
c) A limiting factor of the common type bus system is the interrupting
capacity of the switchgear. The switchgear breakers will be capable of interrupting the
maximum possible fault current that will flow through them to a fault. In the event
that the possible fault current exceeds the interrupting capacity of the available
breakers, a synchronizing bus with current limiting reactors will be required.
Switching arrangement selected will be adequate to handle the maximum calculated short
circuit currents which can be developed under any operating routine that can occur. All
possible sources of fault current; i.e., generators, motors, and outside utility
sources, will be considered when calculating short circuit currents. In order to clear
a fault, all sources will be disconnected. Figure 26 shows, in simplified single line
format, a typical synchronizing bus arrangement. The interrupting capacity of the
breakers in the switchgear for each set of generators is limited to the contribution to
a fault from the generators connected to that bus section plus the contribution from the
synchronizing bus and large (load) motors. Since the contribution from generators
connected to other bus sections must flow through two reactors in series fault current
will be reduced materially.
d) If the plant is 20,000 kVA or larger, and the area covered by the
distribution system requires distribution feeders in excess of 2 miles, it may be
advantageous to connect the generators to a higher voltage bus and feed several
distribution substations from that bus with step-down substation transformers at each
distribution substation as shown in Figure 24.
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