This scheme uses two main buses, and
each circuit includes two bus selector disconnect switches. A bus-tie
circuit connects to the two main buses and, when closed, allows
transfer of a feeder from one bus to the other bus without
deenergizing the feeder circuit by operating the bus selector
disconnect switches.
This arrangement allows the operation
of the circuits from either bus. In this arrangement, a failure on
one bus will not affect the other bus. However, a bus tie breaker
failure will cause the outage of the entire system.
The circuits may all operate from
either the no. 1 or no. 2 main bus, or half the circuits may be
operated off either bus. In the first case, the station will be out
of service for bus or breaker failure. In the second case, half the
circuits will be lost for bus or
breaker failure.
Operating the bus tie breaker in the
normally open position defeats the advantages of the two main buses.
It arranges the system into two single bus systems, which as
described previously, has very low reliability.
Relay protection for this scheme can be
complex, depending on the system requirements, flexibility, and
needs. With two buses and a bus tie available, there is some ease in
doing maintenance, but maintenance on line breakers and switches
would still require outside the substation switching to avoid
outages.
In some cases circuits operate from
both the no. 1 and no. 2 bus, and the bus-tie breaker is normally
operated closed. For this type of operation, a very selective
bus-protective relaying scheme is required to prevent complete loss
of the station for a fault on either bus.
Disconnect-switch operation becomes
quite involved, with the possibility of operator error, injury, and
possible outage. The double-bus, single-breaker scheme is relatively
poor in reliability and is not normally used for important
substations.
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