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Unformatted text preview: dule, which
contains the common alarm circuits and the bias resistors.
The power supplies are also fed in through this module.
All zones of measurement are biased by the total current
flowing to or from the busbar system via the feeders.
This ensures that all zones of measurement will have
similar fault sensitivity under all load conditions. The
bias is derived from the check zone and fixed at 20%
with a characteristic generally as shown in Figure
15.15(b). Thus some ratio mismatch is tolerable. Network Protection & Automation Guide 22.214.171.124 Stability with saturated current transformers
The traditional method for stabilising a differential relay
is to add a resistor to the differential path. Whilst this
improves stability it increases the burden on the current
transformer for internal faults. The technique used in
the MBCZ scheme overcomes this problem. Busbar P rotection Figure 15.17 shows the correlation between the circuit
breakers and the protection modules for a typical double
busbar installation. In practice the modules are mounted
in a multi-tier rack or cubicle. The MBCZ design detects when a CT is saturated and
short-circuits the differential path for the portion of the
cycle for which saturation occurs. The resultant spill
current does not then flow through the measuring circuit
and stability is assured.
This principle allows a very low impedance differential
circuit to be developed that will operate successfully
with relatively small CT's.
126.96.36.199 Operation for internal faults
If the CT's carrying fault current are not saturated there
will be ample current in the differential circuit to operate
the differential relay quickly for fault currents exceeding
the minimum operating level, which is adjustable
between 20%-200% rated current.
When the only CT(s) carrying internal fault current
become saturated, it might be supposed that the CT
saturation detectors may completely inhibit operation by
short-circuiting the differential circuit. However, the
resulting inhibit pulses remove only an insignificant
portion of the differential current, so operation of the
relay is therefore virtually unaffected. • 249 • • 15 • Supervision Differential Current
c =1 c = Check
m = Main
r = Reserve
t = Transfer 1
CT Fault m Enable 1
t Alarm OR Supervision Biased
Differential Enable Trip m =1 Bias Trip
Differential Trip t Out of service
Figure 15.18: Block diagram of measuring unit Busbar P rotection 188.8.131.52 Discrepancy alarm feature • 15 • As shown in Figure 15.18, each measuring module
contains duplicated biased differential elements and also
a pair of supervision elements, which are a part of a
comprehensive supervision facility.
This arrangement provides supervision of CT secondary
circuits for both open circuit conditions and any
impairment of the element to operate for an internal
fault, without waiting for an actual system fault
condition to show this up. For a zone to operate it is
necessary for both the differential supervision element
and the biased differential element to operate. For a
circuit breaker to be tripped it requires the associated
main zone to be operated and also the overall check
zone, as shown in Figure 15.19. to operate the two busbar sections as a single bar. The
fault current will then divide between the two measuring
elements in the ratio of their impedances. If both of the
two measuring elements are of low and equal impedance
the effective minimum operating current of the scheme
will be doubled.
This is avoided by using a 'master/follower' arrangement.
By making the impedance of one of the measuring
elements very much higher than the other it is possible to
ensure that one of the relays retains its original minimum
operation current. Then to ensure that both the parallelconnected zones are tripped the trip circuits of the two
zones are connected in parallel. Any measuring unit can
have the role of 'master' or 'follower' as it is selectable by
means of a switch on the front of the module.
184.108.40.206 Transfer tripping for breaker failure Main zone Check zone S1
+ ve D1 S1 S2 D2 S2 D2 Serious damage may result, and even danger to life, if a
circuit breaker fails to open when called upon to do so.
To reduce this risk breaker fail protection schemes were
developed some years ago. D1
Trip Figure 15.19: Busbar protection trip logic 220.127.116.11 Master/follower measuring units
When two sections of a busbar are connected together
by isolators it will result in two measuring elements
being connected in parallel when the isolators are closed These schemes are generally based on the assumption
that if current is still flowing through the circuit breaker
a set time after the trip command has been issued, then
it has failed to function. The circuit breakers in the next
stage back in the system are then automatically tripped.
For a bus coupler or section breaker this would involve
tripping all the infeeds to the adjacent zone, a facility
that is included in the busbar protection schem...
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