7SG16 Ohmega 402 60 Protection Schemes
©2010 Siemens Protection Devices Limited
Chapter 3 Page 13 of 13
2.4 Blocked Overreach
2.4.1 Scheme Operation
This requires a reverse-looking element to allow the sending of a blocking signal.
The fault must be in the region between the two relays (i.e. on the line section) if;
The overreaching Zone 2 element operates and a reverse looking zone at the remote end has
not
operated.
This can be seen in the diagram below;
S/Stn 1
S/Stn 2
Relay A Zone 2
Relay
B
Relay A Zone 1
Relay A Zone 3
Relay B Zone 2
Relay B Zone 1
Relay
A
Relay A Zone 2
AND
NO Relay B Zone 4
Relay A Carrier Aided Trip
Relay B Zone 3
Relay B Carrier Aided Trip
Relay A Zone 3
AND
NO Relay B Zone 4
Relay A Zone 4
Relay B Zone 4
When the Zone 2 instantaneous element picks up, the relay waits for a “blocking signal” to be received. If no
blocking signal is received during a set time delay (known as the Permissive Trip Time) the relay will carry out a
“Carrier Aided Trip”. If, during this time delay, a blocking signal is received, the Zone 2 time delay will remain in
place, and the relay will carry out a Zone 2 trip after the Zone 2 Time delay.
If the fault is in the last section of the line (i.e. outside the Zone 1 reach) the Zone 2 element will operate, but the
remote end relay Zone 4 element not see the fault. Thus, no blocking signal will be sent, and the relay will carry
out a “Carrier Aided Trip” after the
Permissive Trip Time
.
Obviously when applying this scheme the reverse reach of the Zone 4 element must be further than the
overreach of the remote end Zone 2 element.
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.
The scheme also incorporates an
Unstabilise Relay
status input which can be used for intertripping. Energisation
of this status input will initiate a signal send
2.4.2 Settings
Distance Scheme
BOR
Permissive Trip Time
0..60000
(1ms)
SS Pickup
0..60000
(0ms)
SS Dropoff
0..60000
(1ms)
SR Dropoff
0..60000
(1ms)
Status Inputs:
BLOCK MODE INHIBIT, CARRIER RECV GUARD, UNSTABILISE RELAY
Relay Outputs: N/A
