GE P741

A
pplication Notes P74x/EN AP/N
a7
MiCOM P74
1, P742, P743 (AP) 6-
41
AP
FIGURE 25: ZONE 2 TRIPPED, FAULT STILL PRESENT
Treating this as an open bus coupler circuit breaker as before 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. Remember that in this
example zone 2’s limit extended up to the circuit breaker but this zone has been tripped
already.
The circuit breaker is now open and the fault current would flow to feed the fault. The
differential current in the main zone 2 would equal zero, as the current is flowing into zone 1
whereas the current measured will be equal to the fault current i
fault
.
Zone 2 I
diff
= I
10
+ I
11
= i
diff
Z2 = 0
Zone 1 I
diff
= I
7
+ I
8
=i
diff
Z1 = i
fault
> (I
D
>2 + k2 x I
Bias
)
Check zone I
diff
= I
7
+ I
8
+ I
10
+ I
11
=i
diff
Z1 = i
fault
> (I
D
CZ>2 + kCZ x I
Bias
)
Hence, the system reacts to the continuing presence of the fault and trips the zone 1 as the
check zone I
diff
> (I
D
CZ>2 + kCZ x I
Bias
) and the zone I
diff
> (I
D
>2 + k2 x I
Bias
).
In this example it can be seen that the opposite zone is tripped first but the dynamic topology
reacts to the changed scheme and subsequently trips the adjacent main zone.
Zone
Zone
1 CT Coupler with CB closed - Fault clearance - Stage 2
(zone 2 tripped but fault still present.)
Check zone
I
diff
= Σ i
diff
= i
7
+ i
8
+ i
10
+ i
11
= i
Fault
I
diffZ1
= I
fault
I
diffZ2
= 0
i
8
i
7
i
10
I
11
CT
removed
P0704ENb