for a fast trip, so its dependability is lower than that of a blocking scheme. Blocking and
unblocking schemes are primarily intended for communications with on/off keying and
frequency shift keying power line carrier, respectively, where internal faults can affect
carrier communications. Permissive schemes are applied with other forms of more secure
communications.
12.1.6.1
Blocking scheme
The principle of operation for a blocking scheme is that an overreaching zone is allowed
to trip instantaneously after the settable co-ordination time
tCoord
has elapsed, when no
signal is received from the remote IED.
The carrier send signal in blocking scheme is issued from the reverse directed distance
element. The reverse directed distance element is connected to CS_STOP.
The received signal, which is connected to the CR input, is used to block the release of the
forward looking overreaching zone for external faults and to clear internal faults
instantaneously (after time
tCoord
). The forward overreaching zone to be accelerated is
connected to the input PLTR_CRD, see figure
In case of external faults, the blocking signal (CR) must be received before the settable
timer
tCoord
elapses, to prevent a false trip, see figure
Upon detection of a forward fault the blocking of the carrier send signal is achieved by
activating the input BLKCS. The function can be totally blocked by activating the input
BLOCK, block of trip by activating the input BLKTR.
PLTR-CRD
CR
TRIP
AND
en05000512_ansi.vsd
0
0-tCoord
ANSI05000512 V1 EN
Figure 230:
Basic logic for trip signal in blocking scheme
Channels for communication in each direction must be available.
12.1.6.2
Delta blocking scheme
In order to avoid delays due to carrier coordination times, the initiation of sending of
blocking signal to remote end is done by a fault inception detection element based on delta
quantities of currents and voltages. The delta based fault detection is very fast and if the
channel is fast there is no need for delaying the operation of the remote distance element.
The received blocking signal arrives well before the distance element has picked up. If the
fault is in forward direction the sending is immediately stopped by a forward directed
distance, directional current or directional earth fault element.
Section 12
1MRK 506 335-UUS A
Scheme communication
498
Technical manual
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