The goals of this presentation are to:
• Describe transmission line communications-assisted tripping schemes.
• Discuss the advantages and disadvantages of these schemes.
• Demonstrate scheme performance using real-world event data.
• Discuss and answer any questions regarding specific scheme settings in SEL
relays.
1
In networked transmission systems, power system faults on a protected line segment
are seen from both ends of the line. Multizone distance protection, where relays on the
protected line are time coordinated with relays on remote lines, is commonly referred
to as step-distance protection. With step-distance protection, line-end faults must be
cleared from the remote terminal with a coordinating time delay, resulting in the
delayed clearing of the fault from that line terminal.
However, various communications-assisted protection schemes have been developed
to provide high-speed tripping from both ends of the line. High-speed clearing of faults
along the entire line segment is desirable for several reasons and may even be required.
When a short circuit exists on a power system, the ability to transfer power across the
power system is reduced. Reducing the time that the short circuit exists on the power
system decreases the likelihood of the power system becoming unstable. High-speed
reclosing is another means of improving power system stability. In order to reclose
quickly, both terminals must clear the fault at high speed.
In step-distance applications where there is a long line adjacent to a short line, it may
not be possible to coordinate the reach of the Zone 2 element of the long line with the
reach of the Zone 1 element of the short line. Therefore, the short line may have to be
cleared at high speed for coordination reasons.
Other advantages of clearing all of the faults quickly include reduced duration of the
voltage sag caused by the short circuit, which minimizes the impact on power quality,
and reduced through-fault duty on power transformers and insulator damage on
account of sustained arcing.
2
Pilot protection schemes require a communications channel between each terminal on
the line to achieve high-speed fault clearing anywhere on the protected line.
Information about the status of each terminal on the line is transmitted to the remote
end(s). As a result, all terminals can make tripping decisions based on more complete
data.
3
Optical fiber is becoming more and more available. A dedicated fiber-optic channel
can consist of a direct point-to-point fiber connection or a multiplexed fiber link.
Synchronous optical network (SONET) can be part of a wide-area communications
network for voice and data traffic that provides a nondedicated communications link.
A concern is how channel delays may change as a network reconfigures for a link
failure. The effects of channel delays on different schemes vary. Also, if optical power
ground wire (OPGW) is used for direct fiber, the possibility of simultaneous faults and
channel failures on scheme performance must be analyzed.
Microwave systems can be either digital or analog. These systems are often part of a
wide-area communications network for voice and data traffic as well. Analog systems
generally use audio tone sets to put the teleprotection information into a voice channel.
Channel delays for audio tone sets on analog microwave can be 8 to 20 milliseconds.
Digital microwave systems can provide channel delays in the 3- to 4-millisecond
range. Both fiber-optic and microwave systems offer large capacity (bandwidth) and
power-line noise immunity. Microwave systems may be subject to signal fade during
inclement weather.
A power-line carrier system provides a reliable point-to-point path for sending
teleprotection information. However, the equipment to couple the signal to the highvoltage power line (line tuner, coupling capacitor, line trap) can be expensive,
frequency availability is increasingly a problem, and the communications link is
subject to power-line noise. Also, the teleprotection scheme used must be designed to
work if the channel is lost during an internal fault that short-circuits the
communications channel. Power-line carrier channel equipment usually comes in two
types: on/off and frequency shift key (FSK). The type used depends upon the needs of
the teleprotection scheme.
4
In modern systems, the relay interface to the communications channel is digital, using
a proprietary protocol. As an example, SEL MIRRORED BITS® communications
technology communicates the status of 8 bits. The advantage of these systems is that
more information can be exchanged, which allows for the inclusion of control to go
along with the protection. These modern systems also simultaneously monitor the
health and availability of the communications channel.
IEC 61850 represents a standardized protocol that can be implemented in a device
from any manufacturer in order to share high-speed data over an Ethernet network.
On/off carrier sets pass 1 bit of information. The units either transmit a signal or they
do not.
FSK carrier sets always transmit something. Under normal conditions, they transmit a
guard tone. When keyed, they shift the transmission frequency to a trip tone. The sets
often include security measures, such as trip-after-guard and guard-before-trip logic, to
ensure the integrity of the communications channel.
5
For pilot protection schemes, a balance between speed, security, dependability, and
sensitivity is required. All of these performance factors are interrelated. For example,
an increase in speed may reduce security and enhance dependability. Increasing
security may reduce speed and sensitivity. Choosing one type of scheme (for example,
directional comparison blocking [DCB]) over another (such as permissive
overreaching transfer trip [POTT]) may improve dependability and sensitivity while
decreasing speed and security.
An important aspect to pilot protection schemes is the ability to monitor channel health
and performance. How is this done? Can a failure be detected in real time?
Another important aspect to pilot protection schemes is the effect of channel failure on
scheme performance. For example, in power-line carrier applications, a DCB scheme
would overtrip (lose security) for a channel failure. However, if a DCB scheme is
implemented using MIRRORED BITS communications, the communications link can be
monitored in real time and overtripping can be prevented by disabling the DCB highspeed tripping during channel failures.
6
Directional comparison pilot protection schemes are based on sending 1 bit of data
across the communications channel at a very high speed. In some schemes, this 1 bit
tells the other end that it has permission to trip (permissive). In other schemes, the bit
represents a signal to tell the other end not to trip (block). There are many variations,
but the most prevalent schemes include the following:
• POTT
• Permissive underreaching transfer trip (PUTT)
• DCB
• Directional comparison unblocking (DCUB)
7
For all communications-assisted trip schemes:
• Zones 2 and 3 must be enabled.
• Zone 2 is always set forward by default (hard-coded in the relay).
• Zone 3 must be set reversed using DIR3:=R.
• Zones 1, 4, and 5 are optional.
– At least three zones must be enabled, but Zone 1 can be turned off in the reach
settings.
– The reverse Zone 3 elements at one line terminal must be set to overreach the
forward Zone 2 elements at the other line terminal.
8
Directional elements can also play a key role in communications-assisted trip schemes.
Level 2 elements, such as 67G2 and 67Q2, that are set forward by default can be used
to provide more sensitive fault detection for unbalanced faults and can either
supplement or replace ground distance (zone) elements in communications-assisted
trip schemes.
If Level 2 (forward) elements are used, then the corresponding Level 3 elements, such
as 67G3 and 67Q3, that are set reverse by DIR3:=R must be used at the other line
terminal to assure coordination between forward-reaching elements at one line
terminal and reverse-reaching elements at the other terminal.
The reverse-reaching elements at one line terminal must be set more sensitive than the
forward-reaching elements at the other line terminal.
9
10
At the minimum, a POTT scheme requires an overreaching forward element at each
end of the line. This is typically provided by a Zone 2 element set to reach around
120 percent to 150 percent of the line length. If each relay sees the fault in the forward
direction, then it can be determined that the fault is internal to the protected line.
Relay 3 keys permission if it sees the fault in a forward direction. Then Relay 4 is
allowed to trip if it sees the fault in a forward direction and it receives permission from
Relay 3.
A reverse element is typically required in POTT schemes for reasons that are
described later. This reverse element is typically provided by a Zone 3 element set in
the reverse direction. It is important that the reverse Zone 3 element be set to reach so
that it always picks up for faults that can be seen by the remote Zone 2 overreaching
element.
It is important to note that in all of these schemes, an underreaching Zone 1 element is
typically used that trips for faults, within its zone, independent of the pilot protection
scheme.
11
The basic logic for a POTT scheme is shown on the slide.
A trip requires the overreaching Zone 2 elements to be picked up and permission to be
received from the remote end. The receiver (RCVR) is the output of the
communications channel receiver. RCVR asserts when the permissive signal is
received from the remote terminal.
Operation of the overreaching Zone 2 elements keys the transmission of permission to
trip to the remote end. This is accomplished by keying the transmitter (XMTR), which
is indicated by Key XMTR in the figure.
12
In parallel line applications, faults near one end of a line may result in a sequential trip
operation. This sequential trip happens when the instantaneous relay elements trip the
breaker nearest to the fault location. Recall that Zone 1 elements trip independent of
the communications tripping scheme. The breaker farthest from the fault must wait for
a permissive signal. The major problem with this sequential clearing of fault current is
that it creates a current reversal in the healthy parallel line. If the protection for the
healthy line is not equipped to address this current reversal, one terminal of the healthy
line may trip incorrectly.
The figure on the slide shows the status at the inception of the fault. The relaying at
Breaker 3 detects the fault as being within Zones 1 and 2. The instantaneous Zone 1
element issues a trip signal to the breaker, and the Zone 2 elements issue a permissive
signal to the protection at Breaker 4. The protection at Breaker 4 detects the fault
within Zone 2 but must wait for the permissive signal from Breaker 3 before issuing a
permissive trip output.
The Zone 2 element at Breaker 2 also picks up at fault inception and issues a
permissive signal to the protection scheme at Breaker 1. At this time, the Zone 3
elements at Breaker 1 also pick up and identify the fault as being reverse, or out of
section, to its location.
13
After Breaker 3 opens, the fault currents redistribute. When this redistribution occurs,
the Zone 2 elements at Breaker 2 and the Zone 3 elements at Breaker 1 begin to drop
out. If the Zone 2 elements at Breaker 1 pick up before the received permissive signal
resets, Breaker 1 trips as a result of the current reversal.
This scenario can easily occur when ground directional overcurrent relays are used
because they can often see an end zone fault on an adjacent line. It is less of a factor
when ground distance relays are used.
Another factor that contributes to this scenario is the fact that the closing torque of an
electromechanical element is much higher than the opening torque of a spring,
resulting in a large disparity in pickup versus dropout times. This disparity is also true
with numerical relays, but to a much lesser degree.
To address this, a reverse element (Zone 3) is used to detect when the fault is initially
seen behind the relay. A dropout delay prevents the relay from keying permission upon
a transition from reverse to forward. The delay allows the remote terminal forward
element time to drop out.
14
The AND gate 2 in the diagram on the slide represents the simple POTT logic.
AND gate 1 is the added logic to prevent tripping on current reversals. This logic says
that if the breaker is closed and the fault is in the reverse direction, block POTT
tripping and transmitting permission to the remote end.
Timer T1 maintains this block for a period of time, X, upon the drop out of reverselooking Zone 3 elements.
Factors that influence the block timer setting include the following:
• Remote terminal Zone 2 reset
• Channel reset time
To be conservative, some margin should be added to the summation of the times listed
in the above bullets. A safe margin, and a known quantity, is the maximum expected
operating time of a breaker on the parallel faulted line.
15
POTT schemes require permission from both terminals to achieve accelerated trip
times for internal faults along the entire line. When one line terminal is open, its
protective elements are unable to detect an internal fault and cannot send permission to
the remote terminal. This usually requires the end-of-line faults to be cleared by the
time-delayed Zone 2 elements.
The simplest means of dealing with this is to use a 52b contact of the open breaker to
constantly key permission. However, this solution is undesirable for two reasons:
• It results in constant keying of the permissive signal from both terminals after
they open to clear an internal fault.
• If the communications equipment requires guard-before-trip logic for a specified
amount of time before accepting the permissive trip signal and the
communications signal fades for any reason, the communications-assisted
tripping is defeated for a period of time.
16
Echo logic can be used to key permission when it receives a permissive signal from
the terminal that is feeding the fault. Typically, the following conditions must be met
before a received permissive signal is repeated or echoed to the initiating terminal:
• A reverse fault must not have been detected by the reverse-looking elements.
• The permissive signal must be received for a settable length of time.
The first requirement ensures that the fault is not behind the relay location before
transmitting permission to the remote terminal. This also assumes that the Zone 2
elements at the remote breaker detected a fault and sent a permissive signal. The
second requirement prevents the relay from issuing a permissive signal to the remote
terminal because of communications channel noise. It also allows time for the reverselooking elements to operate. The echo timer determines the permissive trip signal
qualifying time. A typical setting is 2 cycles.
Once the echoed permissive signal is issued to the remote terminal, its duration must
be limited to prevent a situation where both terminals maintain the permissive signal
channel in a continuous “trip keyed” or constantly “on” state. The echo duration timer
limits the echoed permissive trip signal to a settable duration. The timer should be set
greater than the communications channel operation time plus the remote breaker trip
time.
17
In some applications, with all sources in, one terminal may not contribute enough fault
current to operate the protective elements. If the fault lies within the Zone 1 reach of
the strong terminal, the fault currents may redistribute after the strong terminal line
breaker opens to permit sequential tripping of the weak terminal breaker.
If the currents do not redistribute sufficiently to operate the protective elements at the
weak terminal, it is still necessary to open the local breaker. This prevents the lowlevel currents from maintaining the fault arc and allows autoreclosing at the strong
terminal. When the fault location is near the weak terminal, the Zone 1 elements of the
strong terminal do not pick up and the fault is not cleared at high speed because the
weak terminal protective elements do not operate.
18
Weak infeed logic is required to permit high-speed tripping of both line terminals for
internal faults near the weak terminal. The strong terminal is permitted to trip via the
permissive signal echoed back from the weak terminal. The weak infeed logic
generates a trip at the weak terminal if all of the following are true:
• A permissive trip signal is received for a given time.
• A phase undervoltage or residual overvoltage element is picked up.
• No reverse-looking elements are picked up.
• All breaker poles are closed.
If there is a permissive signal to echo transmit, the breaker is closed, and low voltage
is detected at the weak terminal, the relay trips the local breaker.
The typical phase undervoltage setting is 70 percent to 80 percent of the lowest
expected system operating voltage. The residual overvoltage setting should be set to
approximately twice the expected standing 3V0 voltage. With the 59N element set at
twice the nominal standing 3V0 voltage, the instrument only operates for fault-induced
zero-sequence voltage.
19
Figure 1.93 POTT Logic Diagram in the SEL-421-4, -5 Reference Manual.
20
PUTT logic offers the same basic logic as POTT, but it can be even more secure.
Underreaching elements are used to key permissive trips to the remote terminal.
Because the keying elements are set with a shorter reach, there is slower operation and
less fault resistance coverage.
Because the permissive keying elements can only see faults within the protected line,
there is no danger of misoperation on current reversal situations. The remote terminal
is allowed to trip if it sees the fault as forward with its overreaching element and the
remote end sees it with its underreaching element.
The PUTT scheme should not be used in applications where there is potential for weak
feed conditions on one of the terminals.
21
22
23
In a DCB scheme, each line terminal has reverse-looking elements (typically Zone 3)
and forward-overreaching elements (typically Zone 2). As in the other schemes,
underreaching Zone 1 elements are applied for independent, high-speed clearing of
faults within its zone.
In a DCB scheme, the relay sends a blocking signal to the remote terminal if it detects
a fault in the reverse direction, indicating that the fault is outside of the protected zone.
The logic is configured so the relay trips if it sees the fault in the forward direction and
does not receive a blocking signal from the remote terminal.
24
The slide shows the fundamental logic involved in a DCB scheme. Pilot tripping
occurs for an internal fault if the local Zone 2 forward-overreaching element operates
and the remote Zone 3 reverse-reaching element does not send a block signal within a
settable time.
The channel coordination delay must allow time for the block signal to be received
before the tripping element can operate. If the block does not arrive, or is late, a DCB
scheme may overtrip. This scheme is often used with a power-line carrier and an
on/off transmitter because the only time the signal must get through is when the fault is
not on the protected line.
One way to speed up the issuance of the blocking signal is to use a nondirectional
carrier start. In this case, a high-speed overcurrent element detects the fault and keys
the transmitter. Then the slower directional element stops the signal if the fault is
forward. If the directional element detects that the fault is reverse (out of zone), the
blocking signal has already been sent. This can reduce the required carrier
coordination time delay, resulting in increased security.
25
Loss of a channel is a particular issue with DCB schemes. Because a terminal trips if
the block signal is not received, these schemes can overtrip if the channel fails. This is
complicated when an on/off type carrier set is used to obtain the highest possible
channel speed. An on/off carrier set is off in the normal state. It is turned on to block
the remote end. For this reason, it is usually desirable to use an automatic carrier
check-back system with on/off carrier sets.
An automatic carrier check-back system can be programmed to operate several times a
day. There is usually a master check-back unit that keys the local transmitter with a
series of carrier pulses. The slave check-back units monitor their local receiver and
recognize this code as a check-back transmission instead of a fault transmission. They
then respond by keying their local transmitter with an answer code.
If the master hears the answer on its local receiver, it knows that the channel is viable.
If it does not, the master typically alarms supervisory control and data acquisition
(SCADA) that the channel has failed. If an internal fault occurs during a check-back
transmission, the relay asserts its “carrier stop” output, if so equipped. Carrier sets that
have a “carrier stop” over “carrier start” priority turn off the transmitter if the fault is
internal.
26
The channel delay coordination timer blocks the trip signal, so it is desirable to make
this delay as short as possible while maintaining security. Local Zone 2 elements must
be delayed to coordinate with the block trip signal.
Channel coordination is improved by the fact that, for an external fault, the local
reverse Zone 3 element is closer to the fault and the fault is at a lower percentage of
the element reach than the remote forward-reaching Zone 2 element. Thus, the Zone 3
element that is keying the block signal naturally operates faster than the remote
tripping element. This allows the coordination delay to be set roughly to the channel
operate time with the difference in element operate time making up the security
margin.
27
A typical DCB scheme with directional transmitter start logic is shown on the slide. In
this case, the block signal is sent only when the relay sees the fault in the reverse
direction. A stop transmitter output is not required because the stop transmitter is
asserted only when the reverse element is not picked up. There is a Zone 3 extension
delay timer that is part of the current reversal logic, which is discussed later.
Nondirectional carrier start is a modification of the scheme used to reduce channel
delay coordination time for the tripping output.
28
Internal end-zone faults are cleared after the coordinating interval expires.
Speed can be improved by using nondirectional transmitter start. With nondirectional
transmitter start, the relay turns on the transmitter as soon as a fault is detected. This
sends the block signal as soon as possible. Then, if the slower, forward-overreaching
elements pick up, indicating that the fault appears to be in the protected zone, the
transmitter stop asserts and turns off the carrier that is allowing the trip.
29
Stop preference over start is a feature of most on/off teleprotection channel equipment.
This feature is required for nondirectional transmitter start because the transmitter is
keyed for every fault to reduce the channel coordination delay time. However, for an
internal fault, the transmitter start is asserted by the nondirectional element and the
transmitter stop is asserted by the forward-overreaching Zone 2 elements.
30
The graph on the slide shows that numerical and electromechanical nondirectional
elements are very close in operating speed.
31
Here is where the speed improvement comes with nondirectional transmitter start. The
nondirectional element can be almost a half cycle faster. For an internal fault, an
approximately 7-millisecond burst of carrier occurs and then shuts off, allowing the
remote end to trip. However, the remote channel delay coordination timer could be set
to 7 milliseconds less and achieve the same security as the directional start logic.
32
A DCB scheme may also lack security for current reversal.
33
The current reversal situation for a DCB scheme is similar to the one previously
described. The figure on the slide shows the status at the inception of the fault.
The relaying at Breaker 3 detects the fault as being within Zone 1 and issues a trip
signal to the breaker. This trip condition also stops transmission of the block signal.
The relay at Breaker 4 detects the fault within Zone 2 but must wait for its channel
coordination timer to expire before tripping.
The reverse-reaching Zone 3 element at Breaker 1 is picked up, indicating the fault is
initially reversed to its location. The assertion of the Zone 3 element starts
transmission of the block signal to Breaker 2. Simultaneously, the Zone 2 element at
Breaker 2 is picked up. Because the typical carrier coordination timer setting is less
than the breaker operation time, the Zone 2 element is ready to trip but is blocked by
the signal from Breaker 1.
34
After Breaker 3 opens, the fault currents redistribute and the forward-overreaching
Zone 2 elements at Breaker 2 and the Reverse Zone 3 elements at Breaker 1 begin to
drop out.
If the Zone 3 elements at Breaker 1 drop out and the channel resets before the Zone 2
elements at Breaker 2 drop out, Breaker 2 issues an unwanted trip.
35
The Zone 3 extension timer holds the block signal up for a period of time to allow the
remote Zone 2 elements to drop out before turning off the block signal.
36
37
Figure 1.91 DCB Logic Diagram in the SEL-421-4, -5 Reference Manual.
38
39
The basic POTT scheme requires that relays at both ends of the line see the fault and
transmit a trip signal to each other via a communications channel. Overreaching
Zone 2 elements are then allowed to trip with receipt of the permissive signal. Faults
within Zone 1 reach are cleared by instantaneous elements without regard for the
receipt of the trip signal from the other end. Faults outside Zone 1, but within Zone 2,
must receive a permissive signal from the remote relay to trip at high speed or must
wait for the Zone 2 timer to time out.
The communications medium for transmitting the trip signal to the other end may be
one of many. When using power-line carrier in a permissive scheme, getting the trip
signal through to the remote end can be difficult. In many instances, the signal is
transmitted on the same line that has the fault. This may reduce the signal to the point
of not having it received by the remote end. In these cases, a DCUB scheme can
provide high-speed fault clearing.
FSK channel equipment is required for a DCUB scheme. FSK equipment continuously
sends a guard tone. The loss of the guard tone without the receipt of the trip tone is
recognized as the loss of a channel. Upon detection of a loss of channel, a short
window of opportunity is opened to allow unsupervised tripping. Thus, if the loss of
channel is caused by the fault, the relay is allowed to trip.
40
In a DCUB scheme, frequency shift carrier equipment is used to provide
communication between the two terminals of the line. This equipment continuously
transmits a guard signal on one frequency.
When a fault occurs, the protective relay signals the carrier equipment to shift from the
guard frequency to the trip frequency. The receiver at the other end of the line
monitors these signals. There is a short transition period in which there is no guard
signal and no permissive trip signal being received.
If the permissive trip signal arrives momentarily, then a normal trip occurs. If the
permissive trip does not arrive, then a short period of time is provided that allows a trip
without receipt of the trip signal if the relay sees the fault.
41
Figure 1.97 DCUB Logic Diagram in the SEL-421-4, -5 Reference Manual.
42
For comparison purposes, pilot protection systems can be divided into two groups:
directional comparison systems and current-only systems. The latter group includes
phase comparison and current differential pilot systems. Directional comparison
systems use current and voltage information and exchange logic information through a
channel. Current-only systems do not require voltage information and exchange analog
or digital information about the currents at the line terminals. These basic differences
between the two general types of pilot systems make them complementary—the
advantages of one type are drawbacks of the other type and vice versa.
The advantages of directional comparison systems are summarized on the slide. The
low channel requirements explain why more than 80 percent of the transmission lines
in the United States have directional comparison protection systems.
43
POTT schemes are more secure and less dependable because they fail to trip at a high
speed for a channel failure.
DCB schemes are more dependable because they operate if the block signal is not
received. They are less secure because they overtrip if the channel fails or if the
channel delay increases.
DCUB schemes combine the security of POTT schemes, but they allow tripping for a
window of opportunity to accommodate channel failure during a fault. FSK channel
equipment provides constant monitoring of the channel by continuously transmitting a
guard tone.
44
DCB schemes should not be used with networked communications channels, such as
SONET, where the channel delay can change. For such cases, a high-speed channel,
such as a power-line carrier on/off channel, is needed.
POTT and DCUB schemes do not trip until the permission (or unblock) signal arrives,
so there are no concerns about channel delay for security. Channel delay does affect
ultimate tripping time. Given modern high-bandwidth digital channels, POTT and
DCUB schemes can be as fast, or faster, than DCB schemes, without the security
concerns.
If a fault on the power line can affect the teleprotection channel, a DCB or DCUB
scheme should be used. Examples where a power-line fault could affect the
teleprotection channel include power-line carrier or communications lines sharing right
of way with the protected power line.
45
46