Alstom MiCOM P545 - 3.3.2 Permissive overreach schemes; 3.3.3 Permissive underreach schemes

P54x/EN AP/La4 Application Notes
(AP) 6-60 MiCOM P543, P544, P545 & P546
AP
I
a
I
b
I
c
AB
C
Z
at
Z
bt
Z
ct
V
a
=
I
a
Z
at
+
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b
Z
bt
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b
=
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a
+
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c
V
a
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a
P1018ENb
Impedance seen by relay A =
P1018ENa
Figure 19 Teed feeder application - apparent impedances seen by RELAY
3.3.2 Permissive overreach schemes
To ensure op
eration for internal faults in a POR scheme, the relays at the three terminals
should be able to see a fault at any point within the protected feeder. This may demand very
large zone 2 reach settings to deal with the apparent impedances seen by the relays.
A POR scheme requires the use of two signaling channels. A permissive trip can only be
issued upon operation of zone 2 and receipt of a signal from both remote line ends. The
requirement for an 'AND' function of received signals must be realized through use of contact
logic external to the relay, or the internal Programmable Scheme Logic. Although a POR
scheme can be applied to a three terminal line, the signaling requirements make its use
unattractive.
3.3.3 Permissive underreach schemes
For a PUR scheme, the
signaling channel is only keyed for internal faults. Permissive
tripping is allowed for operation of zone 2 plus receipt of a signal from either remote line end.
This makes the signaling channel requirements for a PUR scheme less demanding than for
a POR scheme. A common power line carrier (PLC) signaling channel or a triangulated
signaling arrangement can be used. This makes the use of a PUR scheme for a teed feeder
a more attractive alternative than use of a POR scheme.
The channel is keyed from operation of zone 1 tripping elements. Provided at least one
zone 1 element can see an internal fault then aided tripping will occur at the other terminals if
the overreaching zone 2 setting requirement has been met. There are however two cases
where this is not possible:
 Figure 20 (i) shows the case where a short tee is connected close to another terminal.
In this case, zone 1 elements set to 80% of the shortest relative feeder length do not
overlap. This leaves a section not covered by any zone 1 element. Any fault in this
section would result in zone 2 time delayed tripping.
 Figure 20 (ii) shows an example where terminal 'C' has no infeed. Faults close to this
terminal will not operate the relay at 'C' and hence the fault will be cleared by the zone 2
time-delayed elements of the relays at 'A' and 'B'.