Micom P721, Technical Manual

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 5/52
2. BUSBAR CONFIGURATIONS
There are several busbar arrangements depending on the economics and flexibility of
system operation. In addition to plain single and double busbar schemes, the following are
some other popular arrangements. The following figures show the zone of protection of
several busbar arrangements.
2.1 Single busbar
The single bus, single breaker bus arrangement is the most simple, economical and basic. It
is widely used in distribution and low transmission voltages. As shown in FIGURE 1, this
arrangement provides no operating flexibility. All the breakers must trip for any busbar fault.
FIGURE 1: SINGLE BUSBAR ARRANGEMENT
2.2 Double busbar with transfer
With this double busbar variation, each feeder has isolators to enable switching to main or
reverse/transfer bars, and an additional isolator to enable the feeder breaker to be bypassed.
The reverse bar may then function also as a transfer bar and the bus coupler breaker takes
over the function of the feeder breaker to free it for maintenance.
To apply discriminative busbar protection, suitable auxiliary switches are required on each
isolator to select the CTs for the correct zone, and the trip circuits to the appropriate relays.