Areva MiCOM P441, Technical Manual

Page: 230 / 636

P44x/EN AP/E33 Application Notes
Page 56/220 MiCOM P441/P442 & P444
2.10 Distance schemes 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 the Permissive
Overreach, and Blocking schemes to trip the healthy line. A system configuration that could
result in current reversals is shown in Figure 34. 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.
A
C
B
D
A B
Fault Fault
Strong
source Weak
source
L1
L2
L1
L2 C D
P3067ENa
t2(D)
t2(C)
Note how after circuit breaker B on line L1 opens
the direction of current flow in line L2 is reversed.
FIGURE 34 - CURRENT REVERSAL IN DOUBLE CIRCUIT LINES
(See the zone’ description in section 2.4 – unblock/blocking logical scheme)
2.10.1 Permissive Overreach Schemes Current Reversal Guard
The current reversal guard incorporated in the POP 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 relay’s permissive trip logic and signal send logic are inhibited
at substation D (Figure 34). 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 relay at end C
has reset. Otherwise this would cause the relay at D to over trip. Permissive tripping for the
relays at D and C substations is enabled again, once the faulted line is isolated and the
current reversal guard time has expired. The recommended setting is:
tREVERSAL GUARD = Maximum signalling channel reset time + 35ms.
2.10.2 Blocking Scheme Current Reversal Guard
The current reversal guard incorporated in the BOP scheme logic is initiated when a blocking
signal is received 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. Thus referring to Figure 34, the relays in the healthy line are
prevented from over tripping due to the sequential opening of the circuit breakers in the
faulted line. After the faulty line is isolated, the reverse-looking Zone 4 elements at
substation C and the forward looking elements at substation D will reset. The recommended
setting is:
Where Duplex signalling channels are used:
tREVERSAL GUARD = Maximum signalling channel operating time + 14ms.
Where Simplex signalling channels are used:
tREVERSAL GUARD = Maximum signalling channel operating time -
minimum signalling channel reset time + 14ms.