GE P741

P74
x/EN AP/Na7
Application Notes
(AP) 6
-38 MiCOM P74
1, P742, P743
AP
When 2 CTs are used in the coupling and the coupler CB is open and a coupler isolator is
open, the coupler CTs measurements are not taken into account and the zone is extended
from each bar feeder CT to the open CB coupler and to the open coupler isolator.
There is one zone for BB1 to the open breaker and one zone for BB2 to the open isolator.
6.5.4 CTs on one side of bus coupler, CB closes before status acquisition
FIGURE 22: CT’S ON ONE SIDE 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.
Treating this as an open bus coupler circuit breaker the topology algorithm will have
extended Zone 1(with the area located between the CT and the circuit breaker). This then
fully replicates 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 main zone 1 and in main zone 2 will be equal
in magnitude but opposite in sign (
fault
).
When the check zone element is calculated, the differential currents seen in zone 1 and 2,
which result from the discrepancy in the plant status, can be seen to be cancelled out.
Zone 1 I
diff
= I
7
+ I
8
= i
diff
Z1 = -i
fault
> (I
D
>2 + k2 x I
Bias
)
Zone 2 I
diff
= I
10
+ I
11
=i
diff
Z2 = +i
fault
> (I
D
>2 + k2 x I
Bias
)
Check zone I
diff
= I
7
+ I
8
+ I
10
+ I
11
=(-i
fault
) + (+i
fault
) = 
Again the system retains its stability for discrepancies in plant status (even for switch onto
fault).
BB1
BB2
I
ExtFault
through CB
CB CLOSED
but auxiliary
contact OPEN
CT removed
from Zone 1
Zone 1
Zone 2
I
diffZ1
= - i
ExtFault
I
diffZ2
= + i
ExtFault
1 CT coupler and CB Closes on to external fault before Status Acquisition
Check zone
I
diff
= Σ i
diff
feeder = i
7
+ i
8
+ i
10
+ i
11
= [- I
ExtFault
] + [I
ExtFault
] = 0
i
7
i
8
i
11
i
10
P0701ENb