Two variants of Blocking scheme are available:
● Blocking 1 (Reversal Guard applied to the Signal Send)
● Blocking 2 (Reversal Guard applied to the Signal Receive)
4.5 AIDED DISTANCE BLOCKING SCHEMES
Two default Blocking schemes are provided:
● Blocking 1
● Blocking 2
The two schemes are similar. Both schemes feature current reversal guard signals used in conjunction with reverse
looking Zone 4 elements. In the Blocking 1 scheme, the current reversal guard signal applies to the send signal,
whereas in the Blocking 2 scheme, the current reversal guard signal applies to the receive signal.
The signalling channel is keyed from operation of the reverse-looking Zone 4 elements. If the remote zone 2
element picks up, it operates after the trip delay if no block is received. Listed below are some of the main features
and requirements for a Blocking scheme:
● Blocking schemes require only a simplex communication channel.
● Reverse-looking Zone 4 is used to send a blocking signal to the remote end to prevent unwanted tripping.
● When a simplex channel is used, a blocking scheme can easily be applied to a multi-terminal line provided
that outfeed does not occur for any internal faults.
● The blocking signal is transmitted over a healthy line, and so problems associated with power line carrier
signals failing are avoided.
● Blocking schemes provide similar resistive coverage to the permissive overreach schemes.
● Fast tripping occurs at a strong source line end, for faults along the protected line section, even if there is
weak- or zero- infeed at the other end of the protected line.
● If a line terminal is open, fast tripping still occurs for faults along the whole of the protected line length.
● If the signalling channel fails to send a blocking signal during a fault, fast tripping occurs for faults along the
whole of the protected line, but also for some faults in the next line section.
● If the signalling channel is taken out of service, the protection operates in the conventional basic mode.
● A current reversal guard timer is included in the logic to prevent unwanted tripping on healthy circuits
during current reversal situations on a parallel circuits.
The Blocking scheme logic is:
● Send logic: Assert carrier if Reverse Zone 4 element picks up
● Trip logic: Trip if the Zone 2 element picks up if no carrier is received, but only after the Aided Signal Delay
time out
The figure below shows the simplified scheme logic.
P543i/P545i Chapter 8 - Carrier Aided Schemes
P54x1i-TM-EN-1 219
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