Sel SEL-300 Series User guide

Date Code 20200326 SEL Application Guide 2020-04
Use SEL-300 and SEL-400 Series Relays to
Set Up POTT and DCB Schemes
Jared Candelaria, Hardesh Khatri, and Avinash Maddela
INTRODUCTION
The first SEL-300 series relay, the SEL-321-1 Phase and Ground Distance Relay, was introduced
in 1993 and has been used to provide pilot protection on transmission lines ever since. Now, more
than 20 years later, SEL-300 series relays are still widely used. However, as utilities upgrade their
systems, SEL-300 series relays are being replaced by relays with additional functionality, such as
SEL-400 series relays. It is not uncommon for these upgrades to occur on a tie line, where each
end of the line is owned by a different utility. Occasionally, one utility will upgrade its terminal to
match a new standard relaying package, while the other utility will choose to keep their existing
protection. Dissimilar relays on either end of the line do not maintain conventional pilot protec-
tion; however, it is possible to maintain pilot protection. SEL does not recommend connecting dis-
similar relays for pilot protection.
This application guide details how to program SEL-300 and SEL-400 series relays to maintain
pilot protection coordination by using either a permissive overreaching transfer trip (POTT) or a
directional comparison blocking (DCB) scheme. The SEL-400 series relay covered in this applica-
tion guide is the SEL-421-5 Protection, Automation, and Control System. The SEL-300 series
relays covered in this application guide include the SEL-311C-1 Transmission Protection System,
the SEL-311L Line Current Differential Protection and Automation System, and the SEL-321-1
Phase and Ground Distance Relay.
For the examples in this guide, we use the SEL-421-5 to represent all the SEL-400 series relays.
For the SEL-300 series relays, we consider two cases, one with the SEL-311C-1 representing all
variations of the SEL-311 relays and the other with the SEL-321-1. The reason for considering the
SEL-321-1 relay separately than the other SEL-300 series relays is that the internal logic of the
SEL-321-1 is different compared to the other SEL-300 series relays. This guide shows two pilot
protection scheme examples, one between the SEL-421-5 and SEL-311C-1 relays, and the other
between the SEL-421-5 and SEL-321-1 relays.
We assume the reader of this guide has a basic understanding of pilot protection, so this application
guide does not go into detail on the schemes. For an in-depth discussion of POTT and DCB
schemes, refer to [1] and [2].
NOTE: POTT and DCB schemes are not available on the SEL-311A Phase and Ground Distance Relay and the
SEL-311B Distance Relay With Recloser.
SAMPLE SYSTEM
Figure 1 shows a typical two-terminal system. Relay 1 is an SEL-421-5 and Relay 2 can be an
SEL-311C-1 or an SEL-321-1. The two relays communicate through a fiber-optic connection to
implement a POTT or DCB scheme.
Application Guide Volume I AG2020-04

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SEL Application Guide 2020-04 Date Code 20200326
PORT SETUP
To ensure the relays can properly communicate with each other, set up a serial port on each relay
for MIRRORED BITS®communications. For direct fiber communications, use fiber-optic transceiv-
ers (e.g., the SEL-2800 Fiber-Optic Transceiver or SEL-2812 Fiber-Optic Transceiver With
IRIG-B) to convert serial to fiber. The SEL-311C-1 and SEL-421-5 support two separate
MIRRORED BITS communications channels (Channels A and B), while the SEL-321-1 supports one
channel. The SEL-321-1 channel is compatible with either of the SEL-421-5 channels. Table 1,
Table 2, and Table 3 provide example setting values for Port 2 on the SEL-421-5, SEL-311C-1,
and SEL-321-1, respectively. Use Table 1 and Table 2 for pilot protection between the SEL-421-5
and the SEL-311C-1, and use Tabl e 1 and Table 3 for pilot protection between the SEL-421-5 and
the SEL-321-1. The port setup is identical for both POTT and DCB schemes.
There will be a communications delay between the data sent by the local relay and the data
received by the remote relay. This delay depends upon the selection of the baud rate and the data
processing speed of the relay. Choosing a higher baud rate can reduce this delay (for more infor-
mation on the relationship between baud rate and one-way data delay, refer to Table 10 in [3]).
Selecting the correct baud rate depends on the mode of communications between the two relays. If
a direct fiber-optic cable is used, industry practice is to use a higher baud rate. Generally, fiber is
capable of operating at higher baud rates because it has a faster data transfer capability. When you
use other modes of communication, lower baud rates should be considered based on the data trans-
fer capability of the communications medium.
Figure 1 MIRRORED BITS Communications Via a Direct Fiber-Optic Connection
TX
RX
RX
TX
Optical Fiber
Relay 1 Relay 2
Table 1 SEL-421-5 Port 2 Settings (Sheet 1 of 2)
Setting Setting Description Setting
Value Comment
EPORT Enable Port Y The port must be enabled to set up communications.
PROTO Communications Protocol MBAaEnable MIRRORED BITS communications on Channel A.
MBT Using Pulsar 9600 Modem? N We are not using a Pulsar 9600 Modem.
SPEED Communications Baud Rate 38400bThe maximum baud rate for the SEL-421-5 is 38400.
RTSCTS Enable Hardware Handshaking N Not needed for relay-to-relay communications.
TX_ID MIRRORED BITS Transmit Identifier 2 Must match the receive identifier of the remote relay.
RX_ID MIRRORED BITS Receive Identifier 1 Must match the transmit identifier of the remote relay.
RBADPU MIRRORED BITS Receive Bad Pickup
Time
60
(default)
Channel error has to last 60 seconds before RBADxc
asserts.
CBADPU PPM MIRRORED BITS Channel Bad
Pickup
1000
(default)
Ratio of channel downtime to total channel time.
TXMODEdTransmission Mode PeMust be P for connections to devices that are not SEL-400
series relays.

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Date Code 20200326 SEL Application Guide 2020-04
RMByFLfMIRRORED BITS RMB Channel Fail
Status
gUser-defined value transmitted when ROK = 0.
RMByPUfMIRRORED BITS RMB Pickup
Debounce Messages
1 Number of received messages before RMBypicks up.
RMByDOfMIRRORED BITS RMB Dropout
Debounce Messages
1 Number of received messages before RMBydrops out.
aIf PROTO = MBGA or MBGB, the RX_ID and TX_ID settings move from Port settings to Group settings. The MB8 protocol
option is not available in the SEL-421-5 relay, but if the relay is communicating to another relay with MB8 protocol, set
PROTO = MBA or MBB, and set STOPBIT = 2. See [4] for further information.
bWhen you use a fiber-optic cable for communications, consider using the maximum baud rate of 38400.
c
x = A or B.
dThe TXMODE setting provides compatibility with SEL devices that are not SEL-400 series relays. The SEL-421-5 sends
messages faster than SEL-300 series relays and other SEL devices can process the messages. This can lead to loss of data
and a failure to communicate properly. When you set TXMODE to P, the relay sends new MIRRORED BITS messages every 3 ms
even if the selected data speed (SPEED setting) allows more frequent messages.
eP stands for Paced mode. Paced mode provides speed compatibility. This setting must be P for connections to devices that
are not SEL-400 series relays.
f
y = 1 to 8.
gTo maintain security during a loss of communications in a POTT scheme, you can set the default state of the permissive
receive bit (RMB1A) to 0, thus RMB1FL = 0. In a DCB scheme, you can set the default state of the block bit (RMB1A) to 1, thus
RMB1FL = 1.
Table 2 SEL-311C-1 Port 2 Settings (For Communications Between SEL-421-5 and SEL-311C-1 Relays)
(Sheet 1 of 2)
Setting Setting Description Setting Value Comment
EPORT Enable Port Y The port must be enabled to set up communications.
PROTO Communications Protocol MBAaEnable MIRRORED BITS communications on
Channel A.
SPEED Communications Baud Rate 9600bThe SEL-311C-0 is limited to 38400, and the
SEL-311C-1, -2, -3 are limited to 57600.
RTSCTS Enable Hardware Handshaking N Not needed for relay-to-relay communications.
TXID MIRRORED BITS Transmit Identifier 1 Must match the receive identifier of the remote
relay.
RXID MIRRORED BITS Receive Identifier 2 Must match the transmit identifier of the remote
relay.
RBADPU MIRRORED BITS Receive Bad Pickup
Time
60 (default) A channel error must last 60 seconds before
RBADxcasserts.
CBADPU PPM MIRRORED BITS Channel Bad
Pickup
1000 (default) CBAD asserts if the ratio of channel downtime to
total channel time exceeds this value.
RXDFLT MIRRORED BITS Receive Default
Status
XXXXXXXXdThe default state in place of received data in an
error condition.
Table 1 SEL-421-5 Port 2 Settings (Sheet 2 of 2)
Setting Setting Description Setting
Value Comment

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SEL Application Guide 2020-04 Date Code 20200326
When you have set up the ports on both relays and connected the communications cable, verify the
status of the channel. Target the ROKA Relay Word bit in the SEL-421-5 and the SEL-311C-1 by
entering the TAR ROKA command, and target the ROK Relay Word bit in the SEL-321-1 by
RMByPUeMIRRORED BITS RMB Pickup
Debounce Messages
1 Number of received messages before RMBypicks
up.
RMByDOeMIRRORED BITS RMB Dropout
Debounce Messages
1 Number of received messages before RMBydrops
out.
aIf PROTO = MBGA or MBGB, the RXID and TXID settings move from Port settings to Group settings. MBA and MBB use a
seven-data bit format for data encoding. The other options are MB8A, MB8B, MBGA, and MBGB, which use an eight-data bit
format. For direct fiber applications, MBA and MBB work adequately; however, select the MB8A, MB8B, MBGA, or MBGB
protocol if additional communications interface equipment is used on the channel. See [4] for further information.
bThe SEL-311C-1, at a baud rate of 9600, sends MIRRORED BITS four times per power system cycle. Ensure the baud rate of the
SEL-311C-1 matches the baud rate of the SEL-421-5.
c
x = A or B.
dTo maintain security during a loss of communications in a POTT scheme, you can set the default state of the permissive
receive bit (RMB1A) to 0, thus RXDFLT = XXXXXXX0. In a DCB scheme, you can set the default state of the block bit (RMB1A)
to 1, thus RXDFLT = XXXXXXX1.
e
y = 1 to 8.
Table 3 SEL-321-1 Port 2 Settings (For Communications Between SEL-421-5 and SEL-321-1 Relays)
Setting Setting Description Setting Value Comment
PROTOCOL Communications Protocol MBaEnable MIRRORED BITS communications.
SPEED Communications Baud Rate 9600bThe SEL-321-3, -4 and the SEL-321-5 are limited
to 9600.
RTS_CTS Enable Hardware Handshaking N Not needed for relay-to-relay communications.
RBADPU MIRRORED BITS Receive Bad
Pickup
60 (default) A channel error must last 60 seconds before
RBADxcasserts.
CBADPU PPM MIRRORED BITS Channel Bad
Pickup
1000 (default) CBAD asserts if the ratio of channel downtime to
total channel time exceeds this value.
TX_ID MIRRORED BITS Transmit Identifier 1 Must match the receive identifier of the remote
relay.
RX_ID MIRRORED BITS Receive Identifier 2 Must match the transmit identifier of the remote
relay.
RXDFLT MIRRORED BITS Receive Default
Status
XXXXXXXXdThe default state in place of received data in an
error condition.
RMByPUeMIRRORED BITS RMBxPickup
Debounce Messages
1 Number of received messages before RMBypicks
up.
RMByDOeMIRRORED BITS RMBxDropout
Debounce Messages
1 Number of received messages before RMBy drops
out.
aMB uses a seven-data bit format for data encoding. The other option is MB8, which uses an eight-data bit format. For direct
fiber applications, MB works adequately; however, select the MB8 protocol setting if additional communications interface
equipment is used on the channel. See [5] for further information. In the SEL-321-0, -1, if PROTOCOL = MBG, you can set
RMB
x from Group settings instead of Global settings.
bThe SEL-321-1, at a baud rate of 9600, sends MIRRORED BITS two times per power system cycle. Ensure the baud rate of the
SEL-321-1 matches the baud rate of the SEL-421-5.
c
x = A or B.
dTo maintain security during a loss of communications in a POTT scheme, you can set the default state of the permissive
receive bit (RMB1A) to 0, thus RXDFLT = XXXXXXX0. In a DCB scheme, you can set the default state of the block bit (RMB1A)
to 1, thus RXDFLT = XXXXXXX1.
e
y = 1 to 8.
Table 2 SEL-311C-1 Port 2 Settings (For Communications Between SEL-421-5 and SEL-311C-1 Relays)
(Sheet 2 of 2)
Setting Setting Description Setting Value Comment

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Date Code 20200326 SEL Application Guide 2020-04
entering the TA R 2 0 command. These bits, when asserted, indicate that the MIRRORED BITS com-
munications channel is operational and ready to transmit and receive data. Figure 2, Figure 3, and
Figure 4 display the expected responses to the commands for each relay.
RELAY ELEMENTS
Pilot schemes use forward-looking overreaching elements and reverse-looking elements. To imple-
ment the pilot scheme, you can use distance elements, directional overcurrent elements, or both.
Table 4 lists the most commonly used relay elements for pilot schemes.
POTT OVERVIEW
A POTT scheme is a communications scheme in which an asserted overreaching element at the
local terminal must receive permission to trip from the remote terminal. When a forward-looking
overreaching zone (Zone 2) asserts, it can only trip the breaker at a high speed if permission has
been received from the remote end. A reverse-looking zone (Zone 3) blocks permission to the
remote terminal.
For example, referencing Figure 5, if a fault occurs at Point F1, the overreaching Zone 2 element at
Relay 1 asserts while the reverse-looking Zone 3 element remains deasserted. This makes the out-
put of AND 1 (shown in Figure 6) assert, sending a permissive trip (PT) signal (key) to Relay 2,
=>>TAR ROKA <Enter>
ROKA RBADA CBADA LBOKA ANOKA DOKA * *
1 0 0 0 0 0 0 0
Figure 2 Status of the ROKA Bit in the SEL-421-5
=>>TAR ROKA <Enter>
LBOKB CBADB RBADB ROKB LBOKA CBADA RBADA ROKA
0 0 0 0 0 0 0 1
Figure 3 Status of the ROKA Bit in the SEL-311C-1
=>>TAR 20 <Enter>
RBAD CBAD LBOK ROK * * * TOP
1 0 0 0 0 0 0 0
Figure 4 Status of the ROK Bit in the SEL-321-1
Table 4 Common Pilot Scheme Relay Elements
Relay Element Protective Zone
M2P Zone 2 mho-phase forward-distance element
Z2G Zone 2 mho ground-distance element
67G2 Level 2 residual-ground forward-overcurrent element
67Q2 Level 2 negative-sequence forward-overcurrent element
M3P Zone 3 mho-phase reverse-distance element
Z3G Zone 3 reverse mho ground-distance element
67G3 Level 3 residual-ground reverse-overcurrent element
67Q3 Level 3 negative-sequence reverse-overcurrent element

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SEL Application Guide 2020-04 Date Code 20200326
which causes the bottom input of AND 3 to be true. At the same time, Zone 2 at Relay 2 also
asserts and the Zone 3 element remains deasserted. This causes the output of AND 4 to assert,
sending a key to Relay 1 and making the bottom input of AND 2 true. Once each relay receives a
key from the remote end, the outputs of AND 2 and AND 3 assert, causing the relays to trip with
no intentional delay.
Now imagine that the fault did not occur at Point F1 but rather at F2. In this scenario, Zone 2 at
Relay 1 asserts, and Zone 3 deasserts. AND 1 asserts, causing a key to be transmitted and the top
input of AND 2 to assert. At this point, Relay 1 is waiting on a key from Relay 2 to trip. At
Relay 2, the Zone 2 element deasserts while the Zone 3 element asserts. This means that AND 4
does not become true. Therefore, a key is not sent to Relay 1, which prevents a high-speed trip. At
Relay 2, even though a key has been received on the bottom input of AND 3, the top input of
AND 3 deasserts, preventing a high-speed trip.
NOTE: This section provides an overview of POTT schemes. For a more in-depth discussion, refer to [1].
POTT SETUP
To setup a POTT scheme in the SEL-421-5, SEL-311L, or SEL-321-1 relays, you must enable the
logic in the relays. The settings in the relays are similar. Table 5, Table 6, and Table 7 provide a
summary of how to set up the POTT logic in the SEL-421-5, SEL-311C-1, and SEL-321-1, respec-
tively.
Figure 5 MIRRORED BITS Communications Via Direct Fiber
Figure 6 Simplified POTT Logic
Optical Fiber
Zone 3
Zone 2 Zone 3
Zone 2
TX
RX
RX
TX
Relay 2Relay 1 F1 F2
12 3 4
Zone 2
Zone 3 AND 1 AND 2 TRIP
Key
Receiver
Zone 2
Zone 3
AND 4
AND 3TRIP
Relay 1 Relay 2
Key
Receiver
Key
Transmitter
Key
Transmitter

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Date Code 20200326 SEL Application Guide 2020-04
Table 5 SEL-421-5 POTT Settings
Setting Setting Description Settings Value Description
ECOMM Enable Communication
Scheme
POTT Enables the POTT logic in the SEL-421-5.
Z3RBD Zone 3 Reverse Block Time
Delay
5 (default) Prevents a POTT scheme misoperation during a current
reversal.
EBLKD Echo Block Time Delay 10 (default) Prevents the echoing of a received PT for a settable delay
after the dropout of the local permissive elements.
ETDPU Echo Time Delay Pickup 2 (default) Sets a minimum time requirement for the received PT
before echo begins.
EDURD Echo Duration Time Delay 4 (default) Limits the echo duration to prevent channel lockup.
EWFC Weak-Infeed Enable N (default) Allows echo keying in the event of a weak terminal or an
open breaker.
Table 6 SEL-311C-1 POTT Settings
Setting Setting Description Settings Value Description
ECOMM Enable Communication
Scheme
POTT Enables the POTT logic in the SEL-311C-1.
Z3RBD Zone 3 Reverse Block Time
Delay
5 (default) Prevents a POTT scheme misoperation during a current
reversal.
EBLKD Echo Block Time Delay 10 (default) Prevents the echoing of a received PT for a settable delay
after the dropout of the local permissive elements.
ETDPU Echo Time Delay Pickup 2 (default) Sets a minimum time requirement for the received PT
before echo begins.
EDURD Echo Duration Time Delay 4 (default) Limits the echo duration to prevent channel lockup.
EWFC Weak-Infeed Enable N (default) Allows echo keying in the event of a weak terminal or an
open breaker.
Table 7 SEL-321-1 POTT Settings
Setting Setting Description Settings Value Description
EPOTT Enable Permissive Overreach-
ing Transfer Trip
Y Enables the POTT logic in the SEL-321-1.
Z3RBD Zone 3 Reverse Block Time
Delay
5 (default) Prevents a POTT scheme misoperation during a current
reversal.
EBLKD Echo Block Time Delay 10 (default) Prevents the echoing of a received PT for a settable delay
after the dropout of the local permissive elements.
ETDPU Echo Time Delay Pickup 2 (default) Sets a minimum time requirement for the received PT
before echo begins.
EDURD Echo Duration Time Delay 4 (default) Limits the echo duration to prevent channel lockup.
EWFC Weak-Infeed Enable N (default) Allows echo keying in the event of a weak terminal or an
open breaker (requires EVOLT = Y).

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SEL Application Guide 2020-04 Date Code 20200326
POTT LOGIC
After you have enabled each relay for a POTT scheme, you must program the elements that qualify
a communications-assisted trip. To program the logic, consider the following:
➤In the SEL-421-5 and SEL-311C-1, the setting for the communications-assisted trip
condition is TRCOMM, and in the SEL-321-1, the setting is MTCS. SEL recommends
setting TRCOMM and MTCS by using the same elements so that each terminal
responds similarly to fault conditions.
➤Note the polarization choices for the directional element in the relays. The
SEL-321-0, -1, -2, -3, -4 only have negative-sequence voltage (Q) polarization; the
SEL-321-5 has both negative-sequence voltage (Q) and zero-sequence voltage (V)
polarization. The SEL-421-5, SEL-311L, and SEL-311C-1 allow you to choose a com-
bination of negative-sequence voltage (Q), zero-sequence voltage (V), or zero-sequence
current (I) polarization in any order. To prevent miscoordination, set the SEL-421-5 to
use the same directional elements used by the SEL-300 series relay on the other end of
the line.
➤Each relay must be able to transmit a PT signal. In the SEL-421-5 and SEL-311C-1,
send the permissive signal via TMB1A. In the SEL-321-1, send it via TMB1. Set each
of these elements to KEY.
➤The relays must be programmed to map the key from the remote end into the POTT
logic. Map the PT to the received Mirrored Bit. In the SEL-421-5 and SEL-311C-1,
program PT1 to RMB1A. In the SEL-321-1, program RMB1 to PT.
➤A tripping output must be assigned to trip the respective circuit breaker of each relay. In
the SEL-421-5, set OUT201 to TRIP; in the SEL-311C-1, set OUT101 to TRIP; and in
the SEL-321-1, set OUT1 to 3PT, as shown in Figure 9 and Figure 10.
➤You should wire a breaker contact status to each relay. In the SEL-421-5, wire 52A to
IN201; in the SEL-311C-1, wire 52A to IN101; and in the SEL-321-1, wire 52A to IN1.
It is not required to wire the 52A contact status for DCB and POTT schemes, but it is
beneficial to wire the 52A contact for other elements in the relay.
Table 8 SEL-421-5 POTT Logic
Setting Setting Description Setting Value Comment
TRCOMM Communications-Assisted Trip
Conditions
M2P OR Z2Ga
aNegative-sequence and ground-directional overcurrent elements can be added to the TRCOMM equation, along with the mho
phase- and mho ground-distance elements.
Elements that allow high-speed communications-
assisted tripping.
TMB1A Transmit Channel A Mirrored
Bit 1
KEY Send a PT to the remote end.
PT1 Permissive Trip 1 Equation RMB1A Receive a PT from the remote end.
OUT201 Output Contact 201 Equation TRIP Trip the output to the circuit breaker.
52AA1 Circuit Breaker Status Equation IN201 Assign to the circuit breaker 52a status.
Table 9 SEL-311C-1 POTT Logic (Sheet 1 of 2)
Setting Setting Description Setting Value Comment
TRCOMM Communications-Assisted Trip
Conditions
M2P + Z2Ga
(default)
Elements that allow high-speed communications-
assisted tripping.
TMB1A Transmit Channel A Mirrored
Bit 1
KEY Send a PT to the remote end.

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Date Code 20200326 SEL Application Guide 2020-04
PT1 Permissive Trip 1 Equation RMB1A Receive a PT from the remote end.
OUT101 Output Contact 101 Equation TRIP Trip the output to the circuit breaker.
52A Circuit Breaker Status Equation IN101 Assign to the circuit breaker 52a status.
aNegative-sequence and ground-directional overcurrent elements can be added to the TRCOMM equation, along with the mho
phase- and mho ground-distance elements.
Table 10 SEL-321-1 POTT Logic
Setting Setting Description Setting Value Comment
MTCS Mask for Trip Communications
Scheme Variable
M2P + Z2Ga
(default)
aNegative-sequence and ground-directional overcurrent elements can be added to the TRCOMM equation, along with the mho
phase- and mho ground-distance elements.
Elements that allow high-speed communications-
assisted tripping.
TMB1 Transmit Mirrored Bit 1 KEY Send a PT to the remote end.
RMB1b
bBecause PROTOCOL = MB, set RMB1 in Global settings.
Receive Mirrored Bit 1 PT Receive a PT from the remote end.
OUT1 Output Contact Logic OUT1 3PT Trip the output to the circuit breaker.
IN1 Input Contact 1 Assignment 52A1 Assign to the circuit breaker 52a status.
Table 9 SEL-311C-1 POTT Logic (Sheet 2 of 2)
Setting Setting Description Setting Value Comment
Figure 7 Simplified POTT Logic Between the SEL-421-5 and SEL-311C-1 Relays
TRCOMM
TRIP
PT1
(RMB1A)
Key
(TMB1A)
TRCOM
M
TRIP
PT1
(RMB1A)
Key
(TMB1A)
SEL-421-5 SEL-311C-1
Zone 3
3PO
(Three-Pole Open)
Zone 3
3PO
Figure 8 Simplified POTT Logic Between the SEL-421-5 and SEL-321-1 Relays
TRCOMM
TRIP
PT1
(RMB1A)
Key
(RMB1A)
MTCS
TRIP
PT1
(RMB1)
Key
(TMB1)
SEL-421-5 SEL-321-1
Zone 3
3PO
Zone 3
3PO

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SEL Application Guide 2020-04 Date Code 20200326
DCB OVERVIEW
A DCB scheme is a communications scheme in which an asserted overreaching element at the
local terminal must receive a blocking signal from the remote terminal to block a trip. When a
forward-overreaching zone (Zone 2) at the local terminal asserts, it trips at a high speed unless it
receives a block signal from the remote end. A reverse-looking zone (Zone 3) at the remote
terminal sends the blocking signal.
As Figure 5 shows, if a fault occurs at Point F1, the overreaching Zone 2 element at Relay 1
asserts, and Zone 3 deasserts. Because Zone 3 deasserts, Relay 1 does not send a block signal to
Relay 2. The assertion of Zone 2 starts TIMER 1 (see Figure 11). If Zone 2 asserts for longer than
the programmed timer delay (coordination time delay [CTD]) in TIMER 1, the top input of AND 1
becomes true. If the top input of AND 1 is true and Relay 1 has not received a block from the
remote end, Relay 1 issues a trip. At Relay 2, Zone 2 asserts and Zone 3 remains deasserted.
Figure 9 Simplified Wiring Between the SEL-421-5 and SEL-311C-1 Relays
Figure 10 Simplified Wiring Between the SEL-421-5 and SEL-321-1 Relays
52A
SEL-311C-1
(Partial) A01 A17
OUT101 IN101
A02 A18
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-421-5
(Partial) B01 B33
OUT201 IN201
B02 B34
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-321-1
(Partial) 218 201
OUT1 IN1
217 202
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-421-5
(Partial) B01 B33
OUT201 IN201
B02 B34
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)

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Date Code 20200326 SEL Application Guide 2020-04
Because Zone 3 remains deasserted, Relay 2 does not send a block signal to Relay 1, and Zone 2
starts TIMER 2. If Zone 2 asserts for longer than the programmed timer delay (CTD) in TIMER 2,
the top input of AND 2 becomes true. If the top input of AND 2 is true and Relay 2 has not
received a block from the remote end, Relay 2 issues a trip. In this scenario, both relays identify
the fault in their respective Zone 2 and do not receive a block signal from the remote end, which
allows both relays to trip their respective breakers and clear the line at a high speed.
As another example, consider a fault at Point F2 (see Figure 5). In this scenario, Zone 2 at Relay 1
asserts, and Zone 3 remains deasserted. Again, Relay 1 does not send a block signal to Relay 2,
and Zone 2 starts TIMER 1. At Relay 2, Zone 2 desserts, Zone 3 asserts, and the assertion of
Zone 3 sends a block to Relay 1. This causes AND 1 to prevent a high-speed trip. At Relay 2, even
though Relay 1 does not send a block signal, Zone 2 remains deasserted, which causes AND 2 to
prevent a high-speed trip.
NOTE: This section provides an overview of DCB schemes. For a more in-depth discussion, refer to [2].
DCB SETUP
To setup a DCB scheme in the SEL-421-5, SEL-311L, or SEL-321-1 relays, you must enable the
logic in the relays. The settings in the relays are similar. Table 11, Table 12, and Table 13 provide a
summary of how to set up the DCB logic in the SEL-421-5, SEL-311C-1, and SEL-321-1, respec-
tively. These settings only apply to directional starting. This application guide does not discuss
nondirectional starting.
Figure 11 Simplified DCB Logic
Zone 2 TRIP
Block
Receiver
Block
Transmitter
Zone 2
TRIP
Block
Receiver
Block
Transmitter
Relay 1 Relay 2
CTD
AND 1
Zone 3
AND 2
Zone 3
CTD
TIMER 1 TIMER 2
Table 11 SEL-421-5 DCB Settings
Setting Setting Description Setting Value Description
ECOMM Communication-Assisted Trip
Scheme
DCB Enables the DCB logic in the SEL-421-5.
Z3XPU Zone 3 Reverse Pickup Time
Delay
1 (default) Current reversal guard pickup timer.
Z3XD Zone 3 Pickup Extension Time
Delay
6 (default) Prevents a DCB scheme misoperation during current
reversal.
BTXD Block Trip Receive Extension
Delay
1 Sets the reset time of a block trip received condition
after the reset of a block trip input.
21SD Zone 2 Phase and Ground Coor-
dination Time Delay
See Note 1 Delays the M2P and Z2G element outputs.
67SD 67G and 67Q Coordination
Time Delay
See Note 1 Delays the 67N2 element output.

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SEL Application Guide 2020-04 Date Code 20200326
Note 1. When determining the CTD, you must evaluate the operating time of the tripping ele-
ments. You must consider both the source-to-impedance ratio (SIR) and the distance from
the relay. For the SEL-421-5, SEL-311C-1, and SEL-321-1, you can find this information in
their respective instruction manuals. Delay the CTD at the faster relay to coordinate with
the blocking elements of the slower relay. Add all of this to the communications channel
delay, plus additional margin. The CTD must be greater than the communications channel
delay.
In the SEL-421-5 and SEL-311C-1 relays, Z3XPU determines how long Zone 3 elements must
assert before the block is sealed in to provide security for current reversals. The Z3XPU timer is
hard coded in the SEL-321-1 at 2 cycles, which holds in the START bit. The default setting for
Z3XPU in the SEL-311L is 2 cycles, while in the SEL-421-5, the default is 1 cycle. As long as the
relay trip time, along with breaker open time, exceeds 2 cycles, there is no difference between the
two times regarding current reversal security for an external fault. It is not required to set Z3XPU
the same in the local and remote relays; they do not require coordination with each other.
Table 12 SEL-311C-1 DCB Settings
Setting Setting Description Setting Value Description
ECOMM Communication-Assisted Trip
Scheme
DCB Enables the DCB logic in the SEL-311C-1.
Z3XPU Zone 3 Reverse Pickup Time
Delay
1 (default) Current reversal guard pickup timer.
Z3XD Zone 3 Pickup Extension Time
Delay
6 Prevents a DCB scheme misoperation during current
reversal; this also matches the SEL-421-5 for better
coordination.
BTXD Block Trip Receive Extension
Delay
1 Sets the reset time of a block trip received condition
after the reset of a block trip input.
21SD Zone 2 Phase and Ground Coor-
dination Time Delay
See Note 1 Delays the M2P element output.
67SD 67G and 67Q Coordination
Time Delay
See Note 1 Delays the 67N2 element output.
Table 13 SEL-321-1 DCB Settings
Setting Setting Description Setting Value Description
EDCB Enable Directional Comparison
Blocking
Y Enables the DCB logic in the SEL-321-1.
Z3XD Zone 3 Pickup Extension Time
Delay
6 Prevents a DCB scheme misoperation during current
reversal; this also matches the SEL-421-5 for better
coordination.
BTXD Block Trip Receive Extension
Delay
1 (default) Sets the reset time of a block trip received condition
after the reset of a block trip input.
Z2PSD Zone 2 Phase Coordination
Time Delay
See Note 1 Delays the M2P element output.
Z2GSD Zone 2 Ground Coordination
Time Delay
See Note 1 Delays the Z2G element output.
67N2SD 67N Coordination Time Delay See Note 1 Delays the 67N2 element output.
67Q2SD 67Q Coordination Time Delay See Note 1 Delays the 67Q2 element output.

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Date Code 20200326 SEL Application Guide 2020-04
DCB LOGIC
When you have enabled each relay for a DCB scheme, program the elements that qualify a
communications-assisted trip. To program the logic, consider the following:
➤In the SEL-421-5 and SEL-311C-1, the setting for the communications-assisted trip is
TRCOMM, and in the SEL-321-1, the setting is MTCS. SEL recommends setting
TRCOMM and MTCS by using the same elements so that each terminal responds simi-
larly to fault conditions.
➤Note the polarization choices for the directional element in the relays. The
SEL-321-0, -1, -2, -3, -4 only have negative-sequence voltage (Q) polarization; the
SEL-321-5 has both negative-sequence voltage (Q) and zero-sequence voltage (V)
polarization; and the SEL-421-5, SEL-311L, and SEL-311C-1 allow you to choose a
combination of negative-sequence voltage (Q), zero-sequence voltage (V), or zero-
sequence current (I) polarization in any order. To prevent miscoordination, set the
SEL-421-5 to use the same directional elements used by the SEL-300 series relay on
the other end of the line.
➤The calculation of the negative-sequence directional element thresholds (Z2F and Z2R)
is different in the SEL-300 series and SEL-400 series relay instruction manuals. When
coordinating the SEL-300 series relays with the SEL-400 series relays, coordinate the
Z2F and Z2R settings on both line terminals. Calculate (manually or by using the
AUTO or AUTO2 settings) these thresholds for the SEL-400 series relays, and replicate
the same thresholds for the SEL-300 series relays.
➤Each relay must be able to transmit a block signal. In the SEL-421-5 and SEL-311C-1,
send the block via TMB1A, and in the SEL-321-1, send it via TMB1.
➤The relays must be programmed to map the BLOCK element from the remote end into
the DCB logic. Map the BLOCK element to the received Mirrored Bit. In the
SEL-421-5 and SEL-311C-1, program BT to RMB1A, and in the SEL-321-1, program
RMB1 to BT.
➤The SEL-421-4, -5 supports quadrilateral phase elements. If these elements are enabled
on one end, the distance elements in these relays may have different sensitivities than
the SEL-311C-1 and SEL-321-1. Ensure the distance elements coordinate on both line
terminals.
➤A tripping output must be assigned to trip the respective circuit breaker of each relay. In
the SEL-421-5, set OUT201 to TRIP; in the SEL-311C-1, set OUT101 to TRIP; and in
the SEL-321-1, set OUT1 to 3PT, as shown in Figure 14 and Figure 15.
➤Wire a breaker contact status to each relay. In the SEL-421-5, wire 52A to IN201; in the
SEL-311C-1, wire 52A to IN101; and in the SEL-321-1, wire 52A to IN1. It is not
required to wire the 52A contact status for DCB and POTT schemes, but it is beneficial
to wire the 52A contact for other elements in the relay.
Table 14 SEL-421-5 DCB Logic Settings (Sheet 1 of 2)
Setting Setting Description Setting Value Comment
TRCOMM Communications-Assisted Trip Conditions Z2PGS OR
67QG2SaElements that allow high-speed communications-assisted tripping.
TMB1A Transmit Channel A Mirrored Bit 1 DSTRT Send a block signal to the remote end.
BT Block Trip Equation RMB1A Receive a block signal from the remote
end.
OUT201 Output Contact 101 Equation TRIP Trip the output to the circuit breaker.

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SEL Application Guide 2020-04 Date Code 20200326
52AA1 Circuit Breaker Status Equation IN201 Assign to the circuit breaker 52a status.
ORDERbGround Directional Priority bSet to match the remote relay.
aIn the DCB logic, use Z2PGS instead of M2P OR Z2G to provide individual carrier coordination delay timers for the Level 2
directional elements, and use 67QG2S instead of 67Q2 OR 67G2 to provide carrier coordination delay timers to the Level 2
directional elements.
bMatch the ORDER setting value in the SEL-421-5 to the ORDER setting value in the SEL-311C-1.
Table 15 SEL-311C-1 DCB Logic Settings
Setting Setting Description Setting Value Comment
TRCOMM Communications-Assisted Trip Conditions Z2PGS +
67QG2Sa
aIn the DCB logic, use Z2PGS instead of M2P + Z2G to provide individual carrier coordination delay timers for the Level 2
directional elements, and use 67QG2S instead of 67Q2 + 67G2 to provide carrier coordination delay timers to the Level 2
directional elements.
Elements that allow high-speed
communications-assisted tripping.
TMB1A Transmit Channel A Mirrored Bit 1 DSTRT Send a block signal to the remote end.
BT Block Trip Equation RMB1A Receive a block signal from the remote
end.
OUT101 Output Contact 101 Equation TRIP Trip the output to the circuit breaker.
52A Circuit Breaker Status Equation IN101 Assign to the circuit breaker 52a status.
ORDERb
bMatch the ORDER setting value in the SEL-421-5 to the ORDER setting value in the SEL-311C-1.
Ground Directional Priority bSet to match the remote relay.
Table 16 SEL-321-1 DCB Logic Settings
Setting Setting Description Setting Value Comment
MTCS Mask for Communications-Assisted Tripping M2P + Z2G +
67N2 + 67Q2
(default)
Elements that allow high-speed
communications-assisted tripping.
TMB1 Transmit Mirrored Bit 1 START + M3P
+ Z3G + 67N3
+ 67Q3
Send a block signal to the remote end.
RMB1a
aWhen PROTOCOL = MB, set RMB1 in Global settings.
Receive Mirrored Bit 1 BT Receive a block signal from the remote
end.
OUT1 Output Contact Logic OUT1 3PT Trip the output to the circuit breaker.
IN1 Input Contact 1 Assignment 52A1 Assign to the circuit breaker 52a status.
Table 14 SEL-421-5 DCB Logic Settings (Sheet 2 of 2)
Setting Setting Description Setting Value Comment
Figure 12 Simplified DCB Logic Between the SEL-421-5 and SEL-311C-1 Relays
Zone 3
TRIP
SEL-421-5
TRCOMM
RMB1A (BT)
DSTRT (TMB1A)
Zone 3
TRIP
SEL-311C-1
TRCOMM
BT (RMB1A)
DSTART (TMB1A)

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Date Code 20200326 SEL Application Guide 2020-04
Figure 13 Simplified DCB Logic Between the SEL-421-5 and SEL-321-1 Relays
Figure 14 Simplified Wiring Between the SEL-421-5 and SEL-311C-1 Relays
Figure 15 Simplified Wiring Between the SEL-421-5 and SEL-321-1 Relays
Zone 3
TRIP
SEL-421-5
TRCOMM
BT (RMB1A)
DSTRT (TMB1A)
Zone 3
3PT
SEL-321-1
MTCS
BT (RMB1)
START (TMB1)
52A
SEL-311C-1
(Partial) A01 A17
OUT101 IN101
A02 A18
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-421-5
(Partial) B01 B33
OUT201 IN201
B02 B34
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-321-1
(Partial) 218 201
OUT1 IN1
217 202
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)
52A
SEL-421-5
(Partial) B01 B33
OUT201 IN201
B02 B34
52A
52TC
Circuit
Breaker
(Partial)
(+) (+)
(–)

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SEL Application Guide 2020-04 Date Code 20200326
CONSIDERATIONS FOR PILOT PROTECTION BETWEEN DISSIMILAR RELAYS
Different relays with different sensitivities used in pilot schemes can cause issues with coordina-
tion. See Figure 16 for an example system. The relay installed on Breaker 3 is an SEL-400 series
relay and the relay installed on Breaker 4 is an SEL-300 series relay. When you are using two dis-
similar relays, there may be coordination problems because the relays may have different process-
ing speeds. Also, the SEL-421-3, SEL-421-5, and SEL-421-7 have high-speed distance elements
to reduce operating time. These features make SEL-400 series relays faster than SEL-300 series
relays, and this must be accounted for in the coordination time delays.
If a fault occurs on the line between Breakers 5 and 6 in a POTT scheme, Breaker 3 sends a PT sig-
nal to the relay at Breaker 4 (if the relay at Breaker 3 is set sensitively enough to detect the fault).
If the protection at Breaker 4 is not sensitive enough to detect this reverse fault and Breaker 4 uses
echo logic, as explained in [1], Breaker 4 echoes the received PT signal. The result is that
Breaker 3 trips for an out-of-section fault. This misoperation can be avoided with more sensitive
reverse-reaching protection elements at Breaker 4.
If the blocking relay is slow compared to the tripping relay, miscoordination can occur. To solve
this problem, make sure Zone 3 always overreaches the remote Zone 2 by using Equation 1 to set
Zone 3 of the local relay. Only apply this equation for two-terminal lines.
Equation 1
Now Zone 3 is set so that it overreaches Zone 2 of the remote relay with a margin of 25 percent.
Equation 1 also compensates for any differences in the CT and PT ratios at the local and remote
terminals. Perform this exercise at least twice at each end of the line to make sure that Zone 3 is
defined as explained previously. Doing this prevents tripping for an out-of-section fault.
For a reliable POTT scheme, the relays at each line terminal should have similar sensitivities. If the
sensitivities are not matched, a misoperation or failure to operate could occur. When coordinating
SEL-400 series relays with SEL-300 series relays, consult your fault study to ensure that each
relay has adequate operating quantities for all internal and external faults.
For a more detailed discussion of ground-relaying sensitivities, refer to [6] [7]. These papers iden-
tify relay sensitivity limits and fault resistance coverage for various fixed and settable directional
element limits. They also provide some useful tools for verifying relay settings in practice.
Figure 16 Example System Single-Line Diagram Showing Sensitivity Miscoordination
Zone 2 Zone 3
Zone 3 Zone 2
1 2 4 3654
Bus A Bus B
Z3Local Z2Remote
PTR
CTR
------------


Remote





CTR
PTR
------------


Local


Z1MAGLocal
–•


1.25•=

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Date Code 20200326 SEL Application Guide 2020-04
CONCLUSION
This application guide discusses the settings and configuration requirements to connect two dis-
similar relays for POTT and DCB schemes. SEL does not recommend connecting dissimilar relays
for pilot protection, but if it is necessary, be aware of the capabilities of both relays. Consider the
following important factors:
➤That the directional and protective element sensitivities at both line terminals match.
Use the same protective elements at each line terminal whenever possible, and set the
reverse-reaching elements at a greater sensitivity than the remote overreaching
elements.
➤That the directional elements and pickup settings at each line terminal have sufficient
operating quantities to detect both internal and external faults.
➤If you cannot follow the previous considerations, consider disabling the echo keying
logic at the least-sensitive terminal. Doing this avoids a misoperation when the least-
sensitive terminal does not pick up a reverse-looking element for an out-of-section fault
while the remote relay does.

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SEL Application Guide 2020-04 Date Code 20200326
APPENDIX A: DETERMINING COORDINATION TIME DELAY
This appendix shows example settings for the 21SD timer (Zone 2 Phase and Ground Coordination
Time Delay) and the 67SD timer (67G and 67Q Coordination Time Delay) when the SEL-421-5
and the SEL-311C-0 are used to set up a DCB scheme.
For this example, refer to Figure 5 and assume that Relay 1 is an SEL-421-5 and that Relay 2 is an
SEL-311C-0, and that the relays are connected with a single-mode optical fiber for communica-
tions.
Recommendations for the 21SD Timer
The recommended setting for the 21SD timer is the sum of the following three times:
➤Control input recognition time (including debounce timer)
➤Remote Zone 3 distance protection maximum operating time
➤Maximum communications channel time
21SD Calculation for the SEL-421-5
Control Input Recognition Time
Table 17 shows the MIRRORED BITS data delay times for different baud rates [4].
A data delay is the time between the assertion of the transmit bit in one device and the bit being
received and processed in the other device when the devices are connected back to back. The
SEL-400 series relays have a 1/8-cycle processing interval and most other SEL relays have 1/4-
cycle processing interval.The SEL-421-5 relay with a baud rate of 38400 has a typical data delay
of 8.3 ms if it is communicating with the SEL-311C-0, which has a processing interval of 1/4-
cycles. With this information, the data delay in cycles can be determined as shown:
where 60 is the conversion from seconds to cycles.
Maximum Communications Channel Time
The maximum communications channel time depends on the distance between the two relays that
are communicating with each other and the mode of communication (e.g., single mode fiber-optic,
multimode fiber-optic, serial cable, etc.).
For example, assume that the relays are 32.2 km apart and connected by an SEL-C809 9 µm
Single-Mode Fiber Cable. The typical communications delay for a single-mode fiber-optic cable is
5 µs per 1 km.
Tab le 17 MIRRORED BITS Data Delay Times
Baud Rate Typical Data Delay of 1/8-Cycle Processing
Interval Devices (Maximum)
Typical Data Delay of 1/4-Cycle Processing
Interval Devices (Maximum)
38400 4.2 ms (4.2 ms) 8.3 ms (8.3 ms)
19200 6.3 ms (6.3 ms) 10.5 ms (12.5 ms)
9600 8.3 ms (10.4 ms) 12.5 ms (12.5 ms)
4800 12.5 ms (18.7 ms) 16.7 ms (20.8 ms)
Data Delay 0.0083 s=
Data Delay 60.0000 0.0083 0.5000 cycles=•=

19
Date Code 20200326 SEL Application Guide 2020-04
Use Equation 2 to determine the total communication delay for 32.20000 km.
Equation 2
Remote Zone 3 Distance Protection Maximum Operating Time
This example uses the SEL-311C-0 as the remote relay. Refer to the SEL-311C-0 instruction man-
ual and [7] for more information about the Zone 3 distance protection maximum operating times.
Figure 17 and Figure 18 show the operating curves for the SEL-311C-0. If you use any relay other
than the SEL-311C-0, refer to the respective instruction manual for the operating curves.
Distance 32.20000 km=
Delay Communications 32.20000 km 5.00000 s/km 0.00016 s=•=
Delay 60.00000 0.00016 s 0.01000 cycles=•=
Figure 17 Operating Time Curve of the SEL-311C-0 for a Phase-to-Phase Fault
SIR = 0.1
SIR = 1.0
SIR = 10.0
SIR = 30.0
01020 9030 40 50 60 70 80
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
Fault Location in Percent of Set Reach
Trip Time in Cycles

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SEL Application Guide 2020-04 Date Code 20200326
Consider the worst-case operation time and assume SIR = 1. The maximum operating time of a
distance element is about 1.90 cycles.
Use Equation 3 to determine how long it takes for the remote relay distance element to pick up and
transmit data to the local relay via a MIRRORED BITS channel.
Equation 3
The final calculated value of the coordination time delay timer (21SD) is 2.40 cycles.
21SD Calculation for the SEL-311C-0
Control Input Recognition Time
Table 17 shows the MIRRORED BITS data delay times for different baud rates [4].
The SEL-311C-0 relay with a baud rate of 38400 has a typical data delay of 8.3 ms because its pro-
cessing interval is 1/4-cycles. With this information, use Equation 4 to calculate the data delay in
cycles.
Equation 4
Figure 18 Operating Time Curve of the SEL-311C-0 for a Phase-to-Ground Fault
SIR = 0.1
SIR = 1.0
SIR = 10.0
SIR = 30.0
01020 9030 40 50 60 70 80
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2.25
Fault Location in Percent of Set Reach
Operating Time in Cycles
2
where:
1.90 is the distance element operating time
0.50 is the control input recognition time
0.01 is the data communications delay
Total delay 1.90 0.50 0.01++ 2.40 cycles==
Data delay 0.0083 s=
Data delay 60.0000 0.0083 0.5000 cycles=•=