C4
Distance Protection Schemes
Network Protection & Automation Guide
Network Protection & Automation Guide
Chapter
C4
Distance Protection Schemes
1.
Introduction
235
2.
Zone 1 extension scheme
236
3.
Transfer trip schemes
237
4.
Blocking over-reaching schemes
240
5.
Directional comparison unblocking scheme
242
6.
Comparison of transfer trip and blocking relaying schemes
242
Distance Protection Schemes
1. Introduction
Unit schemes of protection that compare the conditions at the
two ends of the feeder simultaneously (e.g., line differential
protection) positively identify whether the fault is internal or
external to the protected section and provide high-speed
protection for the whole feeder length. This advantage is
balanced by the fact that the unit scheme does not provide the
back up protection for adjacent feeders given by a distance
scheme.
Conventional time-stepped distance protection is illustrated in
Figure C4.1. One of the main disadvantages of this scheme
is that the instantaneous Zone 1 protection at each end of the
protected line cannot be set to cover the whole of the feeder
length and is usually set to about 80%. This leaves two ‘end
zones’, each being about 20% of the protected feeder length.
Faults in these zones are cleared in Zone 1 time by the protection
at one end of the feeder and in Zone 2 time (typically 0.25 to
0.4 seconds) by the protection at the other end of the feeder.
The most desirable scheme is obviously a combination of the
best features of both arrangements, that is, instantaneous
tripping over the whole feeder length plus back-up protection
to adjacent feeders. This can be achieved by interconnecting
the distance protection relays at each end of the protected
feeder by a communications channel. Communication
techniques are described in detail in Chapter [D2: Signalling
and Intertripping in Protection Schemes].
This situation cannot be tolerated in some applications, for two
main reasons:
a. faults remaining on the feeder for Zone 2 time may cause
the system to become unstable
b. where high-speed auto-reclosing is used, the nonsimultaneous opening of the circuit breakers at both ends
of the faulted section results in no ‘dead time’ during the
auto-reclose cycle for the fault to be extinguished and for
ionised gases to clear. This results in the possibility that a
transient fault will cause permanent lockout of the circuit
breakers at each end of the line section
The purpose of the communications channel is to transmit
information about the system conditions from one end of the
protected line to the other, including requests to initiate or
prevent tripping of the remote circuit breaker. The former
arrangement is generally known as a ‘transfer tripping scheme’
while the latter is generally known as a ‘blocking scheme’.
However, the terminology of the various schemes varies widely,
according to local custom and practice.
Even where instability does not occur, the increased duration
of the disturbance may give rise to power quality problems,
and may result in increased plant damage.
Relay A
end zone
Z3G
Z2A
Z1
Z1A
B
A
Time
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F
0
Z1 B
C
Z2
Z3
Z2 T
Z3 T
0
1
Trip
0
Z2B
Z3B
Relay B
end zone
(b) Trip circuit (solid state logic)
(a) Stepped time/distance characteristics
Figure C4.1:
Conventional distance scheme
235
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Distance Protection Schemes
2. Relay performance
This scheme is intended for use with an auto-reclose facility,
or where no communications channel is available, or the channel
has failed. Thus it may be used on radial distribution feeders,
or on interconnected lines as a fallback when no communications
channel is available, e.g. due to maintenance or temporary
fault. The scheme is shown in Figure C4.2.
The Zone 1 elements of the distance relay have two settings.
One is set to cover 80% of the protected line length as in the
basic distance scheme. The other, known as ‘Extended Zone
1’or ‘Z1X’, is set to overreach the protected line, a setting of
120% of the protected line being common. The Zone 1 reach
is normally controlled by the Z1X setting and is reset to the
basic Zone 1 setting when a command from the auto-reclose
relay is received.
If the fault is transient, the tripped circuit breakers will reclose
successfully, but otherwise further tripping during the reclaim
time is subject to the discrimination obtained with normal Zone
1 and Zone 2 settings.
The disadvantage of the Zone 1 extension scheme is that
external faults within the Z1X reach of the relay result in tripping
of circuit breakers external to the faulted section, increasing
the amount of breaker maintenance needed and needless
transient loss of supply to some consumers. This is illustrated
in Figure C4.3(a) for a single circuit line where three circuit
breakers operate and in Figure C4.3(b) for a double circuit line,
where five circuit breakers operate.
Z1A
Z1ext A
A
Z3A
B
C
Z2A
Z1ext A
Z1A
A
B
Z1ext B
C
Z1extB1
Z1C
Breakers
marked thus
auto-reclose
Z2 B
(a) Distance/time characteristics
(a) Fault within Zone 1 extension reach of distance relays
(single circuit lines)
Auto-reclose
Reset Zone 1ext
Z1extA
Z1A
&
A
B
Z1ext B
Zone 2
Z2 T
0
Zone 3
Z3 T
0
D
C
Z1ext D
1
Zone 1
Z1ext B2
Z1ext C
Z1B
Z3 B
Zone 1ext
Z1B2
Z1B1
Z1D
Z1B
Z1C
1
Z1extC
Trip
Z1P
Z1extP
P
L
(b) Simplified logic
N
Z1extN
Figure C4.2:
Zone 1 extension scheme
M
Z1N
Z1ext M
Z1M
Z1ext L
Z1L
(b) Fault within Zone 1 extension reach of distance relays
(double circuit lines)
On occurrence of a fault at any point within the Z1X reach, the
relay operates in Zone 1 time, trips the circuit breaker and
initiates auto-reclosure. The Zone 1 reach of the distance relay
is also reset to the basic value of 80%, prior to the auto-reclose
closing pulse being applied to the breaker. This should also
occur when the auto-reclose facility is out of service. Reversion
to the Z1X reach setting occurs only at the end of the reclaim
time. For interconnected lines, the Z1X scheme is established
(automatically or manually) upon loss of the communications
channel by selection of the appropriate relay setting (setting
group in a numerical relay).
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Figure C4.3:
Performance of Zone 1 extension scheme in conjunction with
auto-reclose relays
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Distance Protection Schemes
3. Transfer tripping schemes
A number of these schemes are available, as described below.
Selection of an appropriate scheme depends on the
requirements of the system being protected.
3.1 Direct under-reach transfer tripping scheme
The simplest way of reducing the fault clearance time at the
terminal that clears an end zone fault in Zone 2 time is to adopt
a direct transfer trip or intertrip technique, the logic of which
is shown in Figure C4.4.
A variant of this scheme, found on some relays, allows tripping
by Zone 3 element operation as well as Zone 2, provided the
fault is in the forward direction. This is sometimes called the
PUP-Fwd scheme.
Time delayed resetting of the ‘signal received’ element is
required to ensure that the relays at both ends of a single-end
fed faulted line of a parallel feeder circuit have time to trip when
the fault is close to one end. Consider a fault F in a double
circuit line, as shown in Figure C4.6.
A contact operated by the Zone 1 relay element is arranged
to send a signal to the remote relay requesting a trip. The
scheme may be called a ‘direct under-reach transfer tripping
scheme’, ‘transfer trip under-reaching scheme’, or ‘intertripping
under-reach distance protection scheme’, as the Zone 1 relay
elements do not cover the whole of the line.
Signal send
Z1
Signal
send
Z2 T
Z3 T
Z3
Z3 T
0
0
1
Trip
&
T
(a) Signal logic
Trip
0
Signal receive
Figure C4.4:
Logic for direct under-reach transfer tripping scheme
A fault F in the end zone at end B in Figure C4.1(a) results in
operation of the Zone 1 relay and tripping of the circuit breaker
at end B. A request to trip is also sent to the relay at end A.
The receipt of a signal at A initiates tripping immediately
because the receive relay contact is connected directly to the
trip relay. The disadvantage of this scheme is the possibility of
undesired tripping by accidental operation or maloperation of
signalling equipment, or interference on the communications
channel. As a result, it is not commonly used.
3.2 Permissive under-reach transfer tripping
(PUP) scheme
The direct under-reach transfer tripping scheme described
above is made more secure by supervising the received signal
with the operation of the Zone 2 relay element before allowing
an instantaneous trip, as shown in Figure C4.5. The scheme is
then known as a ‘permissive under-reach transfer tripping
scheme’ (sometimes abbreviated as PUP Z2 scheme) or
‘permissive under-reach distance protection’, as both relays
must detect a fault before the remote end relay is permitted to
trip in Zone 1 time.
237
0
0
1
Z3
Z2 T
Signal
send
Send
circuit
(f1)
Send
circuit
(f1)
Signal
send
Signal
receive
Receive
circuit
(f1)
Receive
circuit
(f1)
Signal
receive
Signalling equipment
-EndA
Distance relay
Z2
Z2
Signal
receive
Z1
Distance relay
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Signalling equipment
-EndB
(b) Signalling arrangement
Figure C4.5:
Permissive under-reach transfer tripping scheme
The fault is close to end A, so there is negligible infeed from
end B when the fault at F occurs. The protection at B detects
a Zone 2 fault only after the breaker at end A has tripped. It is
possible for the Zone 1 element at B to reset, thus removing
the permissive signal to and causing the ‘signal received’
element at B to reset before the Zone 2 unit at end B operates.
It is therefore necessary to delay the resetting of the ‘signal
received’ element to ensure high speed tripping at end B.
The PUP schemes require only a single communications channel
for two-way signalling between the line ends, as the channel
is keyed by the under-reaching Zone 1 elements.
When the circuit breaker at one end is open, or there is a weak
infeed such that the relevant relay element does not operate,
instantaneous clearance cannot be achieved for end-zone
faults near the ‘breaker open’ terminal unless special features
are included, as detailed in section 3.5.
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Distance Protection Schemes
3. Transfer tripping schemes
Z3A
A
F
B
Z2 A
Z1A
A
B
C
Z1B
Z2 B
Z3B
(a) Fault occurs-bus bar voltage low so
negligible fault current via end B
A
F
(a) Distance/time characteristics
B
Z1 & Z2
Open
Z3
Z3 T
0
Z2 T
0
1
1
(b) End A relay clears fault and current
starts feeding from end B
3.3 Permissive under-reaching acceleration scheme
This scheme is applicable only to zone switched distance relays
that share the same measuring elements for both Zone 1 and
Zone 2. In these relays, the reach of the measuring elements
is extended from Zone 1 to Zone 2 by means of a range change
signal immediately, instead of after Zone 2 time. It is also called
an ‘accelerated underreach distance protection scheme’.
The under-reaching Zone 1 unit is arranged to send a signal
to the remote end of the feeder in addition to tripping the local
circuit breaker. The receive relay contact is arranged to extend
the reach of the measuring element from Zone 1 to Zone 2.
This accelerates the fault clearance at the remote end for faults
that lie in the region between the Zone 1 and Zone 2 reaches.
The scheme is shown in Figure C4.7. Modern distance relays
do not employ switched measuring elements, so the scheme
is likely to fall into disuse.
3.4 Permissive over-reach transfer tripping (POP)
scheme
In this scheme, a distance relay element set to reach beyond
the remote end of the protected line is used to send an
intertripping signal to the remote end. However, it is essential
that the receive relay contact is monitored by a directional relay
contact to ensure that tripping does not take place unless the
fault is within the protected section; see Figure C4.8. The
instantaneous contacts of the Zone 2 unit are arranged to send
the signal, and the received signal, supervised by Zone 2
operation, is used to energise the trip circuit. The scheme is
then known as a ‘permissive over-reach transfer tripping
scheme’ (sometimes abbreviated to ‘POP’), ‘directional
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Range change signal
Signal
receive
&
Figure C4.6:
PUP scheme: Single-end fed close-up fault on double circuit
line
Trip
Signal send
(b) Signal logic
Figure C4.7:
Permissive under-reaching acceleration scheme
comparison scheme’, or ‘permissive overreach distance
protection scheme’.
Since the signalling channel is keyed by over-reaching Zone
2 elements, the scheme requires duplex communication
channels - one frequency for each direction of signalling.
If distance relays with mho characteristics are used, the scheme
may be more advantageous than the permissive under-reaching
scheme for protecting short lines, because the resistive
coverage of the Zone 2 unit may be greater than that of
Zone 1.
To prevent operation under current reversal conditions in a
parallel feeder circuit, it is necessary to use a current reversal
guard timer to inhibit the tripping of the forward Zone 2 elements.
Otherwise maloperation of the scheme may occur under current
reversal conditions, see Chapter [C3: Distance Protection, Section
9] for more details. It is necessary only when the Zone 2 reach
is set greater than 150% of the protected line impedance.
The timer is used to block the permissive trip and signal send
circuits as shown in Figure C4.9. The timer is energised if a
signal is received and there is no operation of Zone 2 elements.
An adjustable time delay on pick-up (tp) is usually set to allow
instantaneous tripping to take place for any internal faults, taking
into account a possible slower operation of Zone 2. The timer
will have operated and blocked the ‘permissive trip’ and ‘signal
send’ circuits by the time the current reversal takes place.
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Distance Protection Schemes
3. Transfer tripping schemes
Signal send
Z1
Z2 T
Z2
The above scheme using Zone 2 relay elements is often referred
to as a POP Z2 scheme. An alternative exists that uses Zone 1
elements instead of Zone 2, and this is referred to as the POP
Z1 scheme.
0
3.5 Weak infeed conditions
Z3T
Z3
1
0
In the standard permissive over-reach scheme, as with the
permissive under-reach scheme, instantaneous clearance cannot
be achieved for end-zone faults under weak infeed or breaker
open conditions. To overcome this disadvantage, two possibilities
exist.
Trip
&
Signal receive
Signal
Send
circuit
(f1)
f1
f2
Receive
circuit f 2
(f2)
f1
send
Signal
receive
Signalling equipment
-EndA
Send
circuit
(f2)
Signal
send
Receive
circuit
(f1)
Signal
receive
Distance relay
(a) Signal logic
Distance relay
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Signalling equipment
-EndB
(b) Signalling arrangement
Figure C4.8:
Permissive over-reach transfer tripping scheme
The timer is de-energised if the Zone 2 elements operate or the
‘signal received’ element resets. The reset time delay (td) of the
timer is set to cover any overlap in time caused by Zone 2
elements operating and the signal resetting at the remote end,
when the current in the healthy feeder reverses. Using a timer
in this manner means that no extra time delay is added in the
permissive trip circuit for an internal fault.
The Weak Infeed Echo feature available in some protection relays
allows the remote relay to echo the trip signal back to the sending
relay even if the appropriate remote relay element has not
operated. This caters for conditions of the remote end having
a weak infeed or circuit breaker open condition, so that the
relevant remote relay element does not operate. Fast clearance
for these faults is now obtained at both ends of the line. The
logic is shown in Figure C4.10. A time delay (T1) is required in
the echo circuit to prevent tripping of the remote end breaker
when the local breaker is tripped by the busbar protection or
breaker fail protection associated with other feeders connected
to the busbar. The time delay ensures that the remote end Zone
2 element will reset by the time the echoed signal is received at
that end. Signal transmission can take place even after the remote
end breaker has tripped. This gives rise to the possibility of
continuous signal transmission due to lock-up of both signals.
Timer T1 is used to prevent this. After this time delay, ‘signal
send’ is blocked.
A variation on the Weak Infeed Echo feature is to allow tripping
of the remote relay under the circumstances described above,
providing that an undervoltage condition exists, due to the fault.
This is known as the Weak Infeed Trip feature and ensures that
both ends are tripped if the conditions are satisfied.
Z1
Z2
Z3
Signal
receive
Z2 T
Z3 T
&
0
1
Trip
From 'POP' signal
send logic
(Figure 12.8)
0
tp
To 'POP' trip logic
(Figure 12.8)
td
&
Breaker 'open'
0
&
0
&
1
Signal
send
Signal receive
&
Signal send
Figure C4.9:
Current reversal guard logic – permissive over-reach scheme
239
Figure C4.10:
Weak Infeed Echo logic circuit
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Distance Protection Schemes
4. Blocking over-reaching schemes
The arrangements described so far have used the signalling
channel(s) to transmit a tripping instruction. If the signalling
channel fails or there is no Weak Infeed feature provided,
end-zone faults may take longer to be cleared.
Blocking over-reaching schemes use an over-reaching
distance scheme and inverse logic. Signalling is initiated only
for external faults and signalling transmission takes place
over healthy line sections. Fast fault clearance occurs when
no signal is received and the over-reaching Zone 2 distance
measuring elements looking into the line operate. The signalling
channel is keyed by reverse-looking distance elements (Z3
in the diagram, though which zone is used depends on the
particular relay used). An ideal blocking scheme is shown in
Figure C4.11. The single frequency signalling channel operates
both local and remote receive relays when a block signal is
initiated at any end of the protected section.
relay for Zone 2 faults external to the protected section. To
achieve this, the reverse-looking elements and the signalling
channel must operate faster than the forward-looking elements.
In practice, this is seldom the case and to ensure discrimination,
a short time delay is generally introduced into the blocking
mode trip circuit. Either the Zone 2 or Zone 1 element can be
used as the forward-looking element, giving rise to two variants
of the scheme.
4.1.1 Blocking over-reaching protection scheme using
zone 2 element
This scheme (sometimes abbreviated to BOP Z2) is based
on the ideal blocking scheme of Figure C4.11, but has the
signal logic illustrated in Figure C4.12. It is also known as a
‘directional comparison blocking scheme’ or a ‘blocking overreach distance protection scheme’.
Z1
Z3 A
Z2 A
Z1 A
Z2
Z2 T
0
1
Trip
C
Z3
Z3 T
0
Z1 B
Z2 B
Signal
receive
Z3 B
&
tp
td
&
(a) Distance/time characteristics
Signal send
Z1
&
Z2
Z2 T
0
Z3
Z3T
0
1
Figure C4.12:
Signal logic for BOP Z2 scheme
Trip
&
Signal receive
Operation of the scheme can be understood by considering
the faults shown at F1, F2 and F3 in Figure C4.11 along with
the signal logic of Figure C4.12.
Signal
send
Send
circuit
Send
circuit
Signal
send
Signal
receive
Receive
circuit
Receive
circuit
Signal
receive
Signalling equipment
-End
Distance relay
(b) Simplified logic
Distance relay
Signal send
Signalling equipment
-End
A fault at F1 is seen by the Zone 1 relay elements at both
ends A and B; as a result, the fault is cleared instantaneously
at both ends of the protected line. Signalling is controlled by
the Z3 elements looking away from the protected section, so
no transmission takes place, thus giving fast tripping via the
forward-looking Zone 1 elements.
(c) Signalling arrangement
Figure C4.11:
Ideal distance protection blocking scheme
4.1 Practical blocking schemes
A blocking instruction has to be sent by the reverse-looking
relay elements to prevent instantaneous tripping of the remote
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A fault at F2 is seen by the forward-looking Zone 2 elements
at ends A and B and by the Zone 1 elements at end B. No
signal transmission takes place, since the fault is internal and
the fault is cleared in Zone 1 time at end B and after the short
time lag (STL) at end A.
A fault at F3 is seen by the reverse-looking Z3 elements at
end B and the forward looking Zone 2 elements at end A.
The Zone 1 relay elements at end B associated with line
section B-C would normally clear the fault at F3. To prevent
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Distance Protection Schemes
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4. Blocking over-reaching schemes
the Z2 elements at end A from tripping, the reverse-looking
Zone 3 elements at end B send a blocking signal to end A.
If the fault is not cleared instantaneously by the protection on
line section B-C, the trip signal will be given at end B for
section A-B after the Z3 time delay.
The setting of the reverse-looking Zone 3 elements must be
greater than that of the Zone 2 elements at the remote end
of the feeder, otherwise there is the possibility of Zone 2
elements initiating tripping and the reverse looking Zone 3
elements failing to see an external fault. This would result in
instantaneous tripping for an external fault. When the signalling
channel is used for a stabilising signal, as in the above case,
transmission takes place over a healthy line section if power
line carrier is used. The signalling channel should then be
more reliable when used in the blocking mode than in tripping
mode.
It is essential that the operating times of the various relays
be skilfully co-ordinated for all system conditions, so that
sufficient time is always allowed for the receipt of a blocking
signal from the remote end of the feeder. If this is not done
accurately, the scheme may trip for an external fault or
alternatively, the end zone tripping times may be delayed
longer than is necessary.
If the signalling channel fails, the scheme must be arranged
to revert to conventional basic distance protection. Normally,
the blocking mode trip circuit is supervised by a ‘channel-inservice’ contact so that the blocking mode trip circuit is isolated
when the channel is out of service, as shown in Figure C4.12.
In a practical application, the reverse-looking relay elements
may be set with a forward offset characteristic to provide
back-up protection for busbar faults after the zone time delay.
It is then necessary to stop the blocking signal being sent for
internal faults. This is achieved by making the ‘signal send’
circuit conditional upon non-operation of the forward-looking
Zone 2 elements, as shown in Figure C4.13.
Blocking schemes, like the permissive over-reach scheme,
are also affected by the current reversal in the healthy feeder
due to a fault in a double circuit line. If current reversal
conditions occur, as described in section 9.9, it may be
possible for the maloperation of a breaker on the healthy line
to occur. To avoid this, the resetting of the ‘signal received’
element provided in the blocking scheme is time delayed.
The timer with delayed resetting (td) is set to cover the time
difference between the maximum resetting time of reverselooking Zone 3 elements and the signalling channel. So, if
there is a momentary loss of the blocking signal during the
current reversal, the timer does not have time to reset in the
blocking mode trip circuit and no false tripping takes place.
4.1.2 Blocking over-reaching protection scheme using
zone 1 element
This is similar to the BOP Z2 scheme described above, except
that an over-reaching Zone 1 element is used in the logic,
instead of the Zone 2 element. It may also be known as the
BOP Z1 scheme.
4.2 Weak infeed conditions
The protection at the strong infeed terminal will operate for
all internal faults, since a blocking signal is not received from
the weak infeed terminal end. In the case of external faults
behind the weak infeed terminal, the reverse-looking elements
at that end will see the fault current fed from the strong infeed
terminal and operate, initiating a block signal to the remote
end. The relay at the strong infeed end operates correctly
without the need for any additional circuits. The relay at the
weak infeed end cannot operate for internal faults, and so
tripping of that breaker is possible only by means of direct
intertripping from the strong source end.
Z3 G
Z2G
Z1G
Z1H
Z2 H
Z3 H
(a) Distance/time characteristics
Z3
Z2
&
Signal send
(b) Solid state logic of send circuit
Figure C4.13:
Blocking scheme using reverselooking relays with offset
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Distance Protection Schemes
5. Directional comparison unblocking scheme
The permissive over-reach scheme described in Section 3.4
can be arranged to operate on a directional comparison
unblocking principle by providing additional circuitry in the
signalling equipment. In this scheme (also called a ’deblocking
over-reach distance protection scheme’), a continuous block
(or guard) signal is transmitted. When the over-reaching
distance elements operate, the frequency of the signal
transmitted is shifted to an ‘unblock’ (trip) frequency. The
receipt of the unblock frequency signal and the operation of
over-reaching distance elements allow fast tripping to occur
for faults within the protected zone. In principle, the scheme
is similar to the permissive over-reach scheme.
The scheme is made more dependable than the standard
permissive over-reach scheme by providing additional circuits
in the receiver equipment. These allow tripping to take place
for internal faults even if the transmitted unblock signal is
short-circuited by the fault. This is achieved by allowing aided
tripping for a short time interval, typically 100 to 150
milliseconds, after the loss of both the block and the unblock
frequency signals. After this time interval, aided tripping is
permitted only if the unblock frequency signal is received.
This arrangement gives the scheme improved security over
a blocking scheme, since tripping for external faults is possible
only if the fault occurs within the above time interval of channel
failure. Weak Infeed terminal conditions can be catered for
by the techniques detailed in Section 3.5.
In this way, the scheme has the dependability of a blocking
scheme and the security of a permissive over-reach scheme.
This scheme is generally preferred when power line carrier
is used, except when continuous transmission of signal is not
acceptable.
6. Comparison of transfer trip and blocking relaying schemes
On normal two-terminal lines the main deciding factors in
the choice of the type of scheme, apart from the reliability of
the signalling channel previously discussed, are operating
speed and the method of operation of the system. Table C4.1
compares the important characteristics of the various types
of scheme.
Modern digital or numerical distance relays are provided with
a choice of several schemes in the same relay. Thus, scheme
selection is now largely independent of relay selection, and
the user is assured that a relay is available with all the required
features to cope with changing system conditions.
Criterion
Transfer
tripping
scheme
Blocking
scheme
Speed of operation
Fast
Not as fast
Speed with
in-service testing
Slower
As fast
Suitable for autoreclose
Yes
Yes
Security against
maloperation due to:
Current reversal
Special features Special features
required
required
Loss of
communications
Poor
Weak Infeed/Open CB
Special features Special features
required
required
Good
Table C4.1:
Comparison of different distance protection schemes
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