CR
CS_STOP
BLOCK
PLTR_CRD
CS
TRIP
CRL
ANSI05000448-1-en.vsd
AND
AND
AND
0
25ms
0-tCoord
0
0
50ms
ANSI05000448 V1 EN
Figure 246:
Simplified logic diagram for blocking scheme
12.5.6.2
Permissive under/overreaching scheme
In the permissive scheme the forward directed ground-fault measuring element sends a
permissive signal to the other end, if a ground fault is detected in the forward direction.
The directional element at the other line end must wait for a permissive signal before
activating a trip signal. Independent channels must be available for the communication in
each direction.
An impedance measuring IED, which works in the same type of permissive mode, with
one channel in each direction, can share the channels with the communication scheme for
residual overcurrent protection. If the impedance measuring IED works in the permissive
overreaching mode, common channels can be used in single line applications. In case of
double lines connected to a common bus at both ends, use common channels only if the
ratio Z
1S
/Z
0S
(positive through zero-sequence source impedance) is about equal at both
ends. If the ratio is different, the impedance measuring and the directional ground-fault
current system of the healthy line may detect a fault in different directions, which could
result in unwanted tripping.
Common channels cannot be used when the weak-end infeed function is used in the
distance or ground-fault protection.
In case of an internal ground-fault, the forward directed measuring element operates and
sends a permissive signal to the remote end via the CS output (sent signal). Local tripping
is permitted when the forward direction measuring element operates and a permissive
signal is received via the CR binary input (received signal).
The permissive scheme can be of either underreaching or overreaching type. In the
underreaching alternative, an underreaching directional residual overcurrent
Section 12
1MRK 506 335-UUS A
Scheme communication
520
Technical manual
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