scheider electric MiCOM P746, Technical Manual

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Application Notes P746/EN AP/G31
MiCOM P746 (AP) 6-
33
7.3.5 CTs on both sides of bus coupler, CB closes before status acquisition.
wrong isolator or CB Status position
2 CT coupler and CB Closes on to external fault with
CT CLOSED
but auxiliary
contact OPEN
CT removed
from Zone 1
I
ExtFault
through CB
P0843ENa
FIGURE 18: CT’S ON BOTH SIDES OF BUS COUPLER,
CB CLOSES BEFORE STATUS ACQUISITION
As the CB has closed but the status has not yet been refreshed the topology still believes the
CB to be open.
AP
Treating this as an open bus coupler the topology algorithm will have extended the two
zones with the areas located between the CTs and the circuit breaker. These then fully
replicate the scheme up to the open bus coupler CB on both sides.
If the circuit breaker was open no load current would flow through the circuit breaker. The
differential current in the two main zones would equal zero, as the current flowing into the
zones would still equal the current flowing out.
However, if the circuit breaker is actually closed, the external fault current will flow through
the circuit breaker. The differential current in the two main zones will be equal in magnitude
but opposite in sign. (
i
fault
)
When the check zone element is calculated, the differential currents seen in the two main
zones, which result from the discrepancy in the plant status and which are taken into account
for the check zone calculation, can be seen to be cancelled out.
Zone 1
I
diff
=
I
1
+ I
2
=
i
diff
Z1 = -i
fault
> (I
D
>2 and k2 x
I
Bias
)
Zone 2
I
diff
=
I
5
+ I
6
=
i
diff
Z2 = +i
fault
> (I
D
>2 and k2 x
I
Bias
)
Check zone
I
diff
=
I
1
+ I
2
+ I
5
+ I
6
=
(-i
fault
) + (+i
fault
) = 0
Hence, the system retains its stability even when there are discrepancies in plant status.