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2.3 Permissive Overreach Zone 2 – POR2.
2.3.1 Scheme Operation
Typically (as for the time stepped and PUR schemes) the Zone 1 is set to 80% of the line length, Zone 2 to 120%
of the line length and Zone 3 as delayed back up protection to cover at least the longest adjacent line. Zone 1
has no time delay, Zone 2 has a time delay, and the Zone 3 has a larger time delay.
The fault must be in the region between the two relays (i.e. on the line section) if;
Zone 1 element operates, or
Remote end Zone 2 operates AND local Zone 2 element operates.
This is shown in the diagram below:
The relay is arranged to send a signal when its Zone 2 picks up.
S/Stn 1
Relay A
Relay A Zone
2
AND
The relay will trip instantaneously for a Zone 1 fault. If a signal is received from the remote end, the time delay
will be removed from the Zone 2 element, allowing it to trip instantaneously. The name of the scheme comes
from the fact that a Permissive signal is being sent by the Overreaching Zone 2
This scheme may be used if the Zone 1 reach does not give sufficient resistive coverage, and may be useful on
short lines. Note that when using POR 2, two signalling channels must be available (one in each direction) since
the Zone 2 elements which initiate the signal send will both operate for a fault on the line section.
Where the signalling equipment has an output which indicates that the signalling channel is out of service, this
can be connected to a Status Input called Carrier Recv Guard. On energisation of this status input the relay will
revert to a time stepped distance scheme.
2.3.2 Circuit Breaker Echo.
With the circuit breaker at one end of the line open, there can be no permissive signal from the remote end relay.
If a fault occurs near the open circuit breaker (i.e. outside the zone 1 of the remote end relay) this would normally
be cleared after the Zone 2 time delay. However, where an overreaching zone is used to provide the permissive
signal, the CB Echo feature can accelerate the tripping.
If a permissive signal is received from the remote end AND the local Circuit breaker is open, the relay will send
(or "echo") a signal back to the remote end.
Thus operation of one of the relay Zone 2 elements will initiate a signal send of duration set as POR CB Echo
Pulse Width. On receipt of a signal from the remote end, if the local circuit breaker is open, the relay will echo the
signal back to the remote end relay. This will remove the time delay from the Zone 2 element, allowing tripping
after a short time delay.
©2010 Siemens Protection Devices Limited
Relay A Zone 1
Relay A Carrier Aided Trip
Relay A Zone 2
Relay A Zone 3
Relay B Zone 1
Relay B Carrier Aided Trip
Relay B Zone 2
Relay B Zone 3
7SG16 Ohmega 402 60 Protection Schemes
S/Stn 2
Relay B
Relay A Zone 2
Relay B Zone 2
Chapter 3 Page 10 of 13
AND
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