Current Reversal Guard Logic; Figure 73: Example Of Fault Current Reversal Of Direction - GE MiCOM P40 Technical Manual

Chapter 8 - Carrier Aided Schemes
System condition
Unblock
Signalling Anomaly
The window of time during which the unblocking logic is enabled starts 10ms after the guard signal is lost, and
continues for 150ms. The 10ms delay gives time for the signalling equipment to change frequency, as in normal
operation. For the duration of any alarm condition, Zone 1 extension logic is invoked if the Z1 Ext Scheme setting is
set to operate for channel failure.
4.4

CURRENT REVERSAL GUARD LOGIC

For double circuit lines, the fault current direction can change in one circuit when circuit breakers open
sequentially to clear the fault on the parallel circuit. The change in current direction causes the overreaching
distance elements to see the fault in the opposite direction to the direction in which the fault was initially detected
(settings of these elements exceed 150% of the line impedance at each terminal). The race between operation and
resetting of the overreaching distance elements at each line terminal can cause permissive overreach, and
blocking schemes to trip the healthy line. A system configuration that could result in current reversals is shown in
the figure below. For a fault on line L1 close to circuit breaker B, as circuit breaker B trips it causes the direction of
current flow in line L2 to reverse. Current reversal guard logic is incorporated in this product to prevent POR and
Blocking schemes from tripping incorrectly during current reversal conditions.
Strong
source
E03503

Figure 73: Example of fault current reversal of direction

The current reversal guard incorporated in the permissive overreach scheme logic is initiated when the reverse
looking Zone 4 elements operate on a healthy line.
Once the reverse looking Zone 4 elements have operated, the permissive trip logic and signal send logic are
inhibited at substation D. The reset of the current reversal guard timer is initiated when the reverse looking Zone 4
resets. A time delay tReversal Guard is required in case the overreaching trip element at end D operates before
the signal send from the protection at end C has reset. Otherwise this would cause the product at D to overreach.
Permissive tripping for the products at D and C substations is enabled again once the faulted line is isolated and
the current reversal guard time has expired.
The current reversal guard incorporated in the blocking scheme logic is initiated when a blocking element picks-up
to inhibit the channel-aided trip. When the current reverses and the reverse looking Zone 4 elements reset, the
blocking signal is maintained by the timer tReversal Guard. Therefore, the protections in the healthy line are
prevented from overreaching due to the sequential opening of the circuit breakers in the faulted line. After the
faulted line is isolated, the reverse-looking Zone 4 elements at substation C and the forward looking elements at
substation D reset.
176
Permissive channel
received
No Yes
Yes No
t2(C)
Fault
A
L1 B
C D
L2
Note how after circuit breaker B on line L1 opens
the direction of current flow in line L2 is reversed.
Permissive
Loss of guard
trip allowed
Yes, during a 150 ms
window
No
A
Weak
source C
Alarm
generated
Yes, delayed on pickup by
150 ms
Yes, delayed on pickup by
150 ms
t2(D)
Fault
B
L1
L2
D
P446SV-TM-EN-1
P446SV