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Bus Pr
Bus Protection
CT Trouble
Any differential protection scheme depends on the correctness
of the CT secondary circuits connected to the relay. In addition to
CT saturation and linear CT measurement error, broken or shorted
CT connections cause the relay to see a false differential current.
During normal operations, the false differential current due to an
broken or shorted CT secondary circuit is typically too small to
cause a relay operation. However, during external faults or high load
periods, this false differential current can result in an incorrect relay
operation. The CT Trouble function in the B30 and B90 relays detects
this condition by using a low-set differential element, typically set
around 10% of the least heavily loaded circuit connected to the bus,
that asserts after a settable time delay. The CT Trouble alarm can be
sent via SCADA to operating personnel, or it may be used to block
the differential element.
Multiple segment busbar protection
Multiple segment busbars, such as double busbar and triple
busbar arrangements, are used to balance loads between various
transmission circuits, minimize the physical space required for
a substation, and provide simpler operating procedures when
performing breaker maintenance. The protection challenge for
these busbar configurations actually lies in operations. The busbar
protection must recognize which segment is faulted, and clear only
that segment. Additionally, the busbar protection must not operate
when breakers are transferred between busbar segments.
The figure shows a typical double busbar configuration. For an
internal fault, the busbar protection must identify the faulted bus
segment, and trip the circuit breakers attached to that bus segment.
This requires the busbar protection to use a dynamic bus replica to
track which circuit breakers are connected to the bus segment.
The B90 Bus Differential Relay provides protection of multiple
segment busbars, using a phase-segregated, centralized protection
scheme. The B90 is phase-segregated to simplify the design of the
system. One B90 is used for each phase, and processes only the
AC signals for that phase, eliminating the need for data transfer
and synchronized sampling between the devices. The B90 uses a
centralized protection scheme to also simplify the design of the
system. The relays are in the substation control house, and all
current circuits and control wiring are brought to the control house
as per typical relay installations. No special equipment needs to be
mounted in the substation yard or circuit breaker control cabinets.
Dynamic bus replica
The objective for protecting complex busbar arrangements is
to provide for optimum protection by avoiding blind spots or
unnecessary bus outages. As a rule, this task calls for dynamic
adjustments of boundaries of differential zones of protection, and
can be safely accomplished when using numerical relays. Dynamic
bus replica and CT switching were initially done outside the relay and
as such complicated the bus protection schemes. The B90 and B30
relays provide user programmable logic to create a dynamic bus
replica inside the system, catering for dynamic switching schemes
necessary to make the bus protection scheme secure and reliable.
End-zone fault protection
End fault protection is one use of a dynamic bus replica. When
using line-side CTs, the circuit breaker is included in the bus zone
of protection. When the relay determines the circuit breaker is open
through the use of breaker status contacts, the circuit breaker is
removed from the bus zone. In this case, a fault between the CT
location and the circuit breaker is in a protection blind spot. Since
Dynamic bus
replica knows
breaker status,
isolator status
Bus side CT:
bus protection
must detect this
fault when CB is
End-zone fault:
bus protection
must detect this
fault when CB is
Zone 2 trips CB-3,
CB-4, and T for
this fault, based
on dynamic bus
Re-configurable busbar with zone boundaries