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Pilot Protection
of
Transmission Lines

By
Suraj K
Sumanth V
Kiran R
Outline
•
•
•
•
•
•

Transmission line introduction
Typical protection schemes
Need for pilot aided schemes
Various pilot ...
Transmission Lines
• A Vital Part of the Power System:
• Provide path to transfer power between generation and load
• Oper...
Transmission Lines
• Classification of line length depends on:
 Source-to-line Impedance Ratio (SIR), and
 Nominal volta...
Typical Protection Schemes
Short Lines
•
•
•
•

Current differential
Phase comparison
Permissive Overreach Transfer Trip (...
Typical Protection Schemes
Medium Lines

• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreac...
Typical Protection Schemes
Long Lines

• Phase comparison
• Directional Comparison Blocking (DCB)
• Permissive Underreach ...
Introduction
• Nonpilot protection using overcurrent and distance relays, contain a
fundamental difficulty :
It is not pos...
BUS

BUS

Need For Pilot Aided Schemes

Local Relay

Remote Relay

Communication
Channel
Pilot Communications Channels
• Distance-based pilot schemes traditionally utilize simple
on/off communications between re...
Distance-based Pilot Protection
Pilot-Aided Distance-Based Schemes
 DUTT – Direct Under-reaching Transfer Trip
 PUTT – Permissive Under-reaching Transfe...
Direct Underreaching Transfer Trip
(DUTT)
• Requires only underreaching (RU) functions which
overlap in reach (Zone 1).
•A...
DUTT Scheme
 

Zone 1

Bus

Bus
Line

Zone 1
Permissive Underreaching
Transfer Trip (PUTT)
• Requires both under (RU) and overreaching (RO)
functions
• Identical to DU...
PUTT Scheme
Zone 2

Zone 1

To protect end of
line
Bus

Bus
Line

Zone 1

Zone 2

Rx PKP
Zone 2
Zone 1

&

Local Trip

OR
Permissive Overreaching Transfer
Trip (POTT)
• Requires overreaching (RO) functions (Zone 2).
• Applied with FSK channel:
...
POTT Scheme
Zone 2

Zone 1
Bus

Bus
Line

Zone 1

Zone 2

(Z1)

Tx

Zone 1

(Z1)

OR

Rx
AND

Zone 2

t

o

Trip
Line
Brea...
POTT Scheme
POTT – Permissive Over-reaching Transfer Trip

BUS

BUS

End
Zone

Communication Channel
POTT Scheme Relay
Local

Local Relay
FW IGND
D
Local Relay – Z2  

Remote Relay
FW IGND
D

Remote Relay – Z2

TRIP

Commun...
POTT Scheme

POTT RX 2
POTT RX 3
POTT RX 4

Local Relay

Communications
Channel(s)

POTT RX 1

POTT TX 1 A to G
POTT TX 2 ...
POTT Scheme
Current reversal example
TRIP

Local Relay

GND DIR OC FWD
GND DIR OC REV

Timer 
 Start  Communication 
Expir...
POTT Scheme
Echo example
Remote FWD
IGND

Open

Remote – Z2

OPEN

Communication
Channel

POTT RX

Local Relay

POTT TX

T...
Hybrid POTT
• Intended for three-terminal lines and weak infeed conditions
• Echo feature adds security during weak infeed...
Hybrid POTT
Zone 2

Zone 1
Remote

Local
Weak
system

Bus

Bus
Line

Zone 1

Zone 2

Zone 4
Directional Comparison Blocking
(DCB)
• Requires overreaching (RO) tripping and blocking (B)
functions
• ON/OFF pilot chan...
DCB Scheme
Zone 2

Zone 1
Remote

Local
Bus

Bus
Line

Zone 1

Zone 2
Directional Comparison Blocking (DCB)

BUS

BUS

End Zone

Communication Channel
Directional Comparison Blocking (DCB)
Internal Faults
Local Relay – Z2

FWD IGND

TRIP Timer
Start
Expired

TRIP

Zone 2 P...
Directional Comparison Blocking (DCB)
External Faults
Local Relay – Z2

FWD IGND
TRIP Timer
Start

Remote Relay – Z4
REV I...
Directional Comparison Unblocking
(DCUB)
• Applied to Permissive Overreaching (POR) schemes
to overcome the possibility of...
DCUB Scheme
Forward

Bus

Bus
Line

Forward

(Un-Block)

(Block)

Trip
Line
Breakers

Tx1

Tx2

Forward

(Block)

Rx2

AND...
Directional Comparison Unblocking
(DCUB)

BUS

BUS

End Zone

Communication Channel
Directional Comparison Unblocking
(DCUB)
Normal conditions

Load Current

FSK Carrier
GUARD1 RX

FSK Carrier
GUARD1 TX

Lo...
Directional Comparison Unblocking
(DCUB)
Normal conditions, channel failure

Load Current

Loss of Channel
FSK Carrier
NO ...
Directional Comparison Unblocking
(DCUB)
Internal fault, healthy channel
Local Relay – Z2

Remote Relay – Z2

TRIP

TRIP Z...
Directional Comparison Unblocking
(DCUB)
Internal fault, channel failure
Local Relay – Z2

Remote Relay – Z2
Loss of Chann...
Redundancy Considerations
• Redundant protection systems increase dependability of the

system:
• Multiple sets of protect...
Redundant Communications

BUS

BUS

End Zone

AND Channels:
POTT Less Reliable
DCB Less Secure

OR Channels:
Communication...
Redundant Pilot Schemes
Pilot Relay Desirable Attributes
•Integrated functions:
• weak infeed
• echo
• line pick-up (SOTF)
•Basic protection eleme...
Pilot protection
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Pilot protection

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  • Pilot schemes speed up the clearing of faults that occur on the transmission line and inside the end zone of the local relay by communicating with the relay at the remote end of the line to determine if the fault is actually on the transmission line.
    Therefore, all pilot aided schemes require a communication channel be provided between the two relays. This communication between the relays can consist of a single channel or multiple channels depending on the particular application.
    Over this communication channel, the two relays share information regarding the fault allowing the clearing of faults on the transmission line to occur as fast as possible.
    Communication channels include power line carriers, Microwave radio channels, SONET channels to list a few. It is desirable to have a very secure and reliable communications channel for this purpose.
  • The most common Pilot aided schemes are :
    The DUTT scheme, which stands for Direct Under-reaching Transfer Trip
    The PUTT scheme which stands for Permissive Under-reaching Transfer Trip
    The POTT scheme, which stands for Permissive Over-reaching Transfer Trip
    The Hybrid POTT scheme which stands for the Hybrid Permissive Over-reaching Transfer Trip
    The Directional Comparison Blocking Scheme
    And the Directional Comparison Unblocking scheme
    We will discuss the POTT, DCB and DCUB schemes in the following sections.
  • Direct Under-reaching Transfer Trip
    Under-reaching units at each end.
    Local Zone 1 PKP causes local trip
    Under-reaching units transmit on operation
    Received signal trips remote breaker
  • Permissive Under-reaching Transfer Trip
    Under-reaching units at each end
    Over-reaching units at each end
    Under-reaching units transmit on operation
    Received signal trips ONLY IF (permissive)
    local over-reach element is operated
    Trip = Local Zone 2 + Remote Zone 1
  • Permissive Over-reaching Transfer Trip
    Designed for two terminal lines
    Over-reaching units at each end
    Over-reaching units transmit on operation
    Received signal trips ONLY IF (permissive)
    local and remote over-reach element have operated
    Strategy:
    Communications channel established between relays sending fault/no fault status to the other relay.
    Both relays set up for zone 2.
    If they both see the fault, the fault is on the line. If only one relay sees the fault, the fault is behind the other relay.
  • The POTT pilot aided scheme stands for the Permissive Over-reaching transfer trip scheme; and like other Pilot aided schemes, is used to speed up the clearing of faults that occur in the end zone of a transmission line.
    As for all pilot aided schemes, a communication channel must be provided between the two relays located at each end of the transmission line for the POTT scheme to operate.
  • In the POTT scheme, the Remote relay speeds up the tripping of an end zone fault by sending a permission to Trip key from the Remote relay to the local relay under 2 circumstances.
    ----The first reason that the remote relay will send a Permissive key is when the it detects a fault occurring within it’s over-reaching zone 2.
    This is where the expression “over-reaching” comes from in the term Permissive Over-reaching Transfer Trip.
    ---The second reason that the remote relay will send a permissive key is when it detects that ground directional overcurrent is flowing in it’s forward direction if this feature is enabled and configured.
    -----Therefore either Negative sequence directional overcurrent Forward element or the Neutral directional overcurrent Forward element, if configured, will send a POTT key to the Local relay, if both are configured,----- as well as when the over-reaching zone 2 pickup flag turns ON
    The Local relay POTT logic will only cause the breaker to trip ---if it gets the POTT key from the remote relay in the form of a receive AND, -----the local relay has detected a fault within it’s zone 2 area of protection
    OR, -----it detects that ground directional current is flowing in it’s Forward direction if this function is configured
    ----Therefore either the local relay’s Forward Negative sequence directional overcurrent element or the Forward Neutral directional overcurrent element as well as the picking up of a Zone 2 fault, will cause the POTT scheme to trip the breaker if it receives a permissive key from the remote relay.
  • In conjunction with the Phase selector feature which determines which phase is faulted, the D60 distance elements can determine which phases of the transmission line are faulted.
    Therefore, the Remote D60 has the ability, -----through the communication key signals, to let the Local D60 know which phases are actually faulted.
    ------If the breakers used on the transmission line have the ability to trip single pole, the D60 can trip only the faulted pole of the breaker based on the received fault type.
    The POTT scheme in the UR D60 handles this signaling of which phase is faulted by having the ability of sending up to 4 different POTT key or transmit signals that are available in the relay.
    Therefore, in order to fully utilize this feature the, scheme would need to have a communication channel between the relays that can share more than one bit of information.
    If the communication channel you are using can only send and receive on piece of information such as a power line carrier can, which phase of the transmission line is faulted can not be sent and single pole tripping is less secure specially on evolving faults.
  • Some additional logic has been added to the POTT scheme to add extra security to transmission lines that are connected parallel to other transmission lines.
    ---For example if a fault had occurred on the paralleled transmission line as shown here, the local and remote relays will operate in the following way.
    --The remote D60 will detect that ground current is flowing in it’s transmission line in the Forward direction, and send a permissive key to the Local relay.
    ---The local D60 will detect that ground current is flowing in its reverse direction. This reverse ground over-current detection does not meet the criteria defined by the POTT scheme and thus the Local relay will not Trip.
    After a certain time period, the breaker on the parallel line correctly tripped to attempt to clear the fault. The ground current would now begin to flow through our transmission line in the opposite direction to feed that fault if the remote breaker on the opposite end if it did not open yet.
    The local and the remote D60’s would now operate in the following way.
    ----First the Local D60 would have its Forward Negative Sequence Directional overcurrent identify current the current is flowing in the Forward direction.
    ----At the same time the the local D60 identified the change in direction of ground current, the remote D60 will identify this change as well.
    However, due to the delay in the communications channel, the POTT permissive key will not immediately be removed.
    Since the Negative sequence directional element of the local D60 indicates that the fault is in the forward direction and the local relay is still receiving the permission key, the POTT scheme will cause the breaker to trip,---- shutting down the transmission line when it did not need to be.
  • The POTT scheme has added one more feature to help to speed up the tripping of a faulted transmission line, if one of the ends is open.
    If the breaker on the local end of the transmission line is open for any reason, the local D60 will not detect any current flow into the transmission line and therefore will not detect any faults within it’s zones of protection. If a fault did occur on the line, no key will be sent to the remote D60.
    ----If a fault occurred in the remote D60’s end zone as seen here,--- the remote D60 will detect the fault in it’s zone 2 and send the POTT key to speed up tripping of the local D60.
    Since the local D60 will not send a key because it does not see the fault, the tripping of the remote breaker will not occur until zone 2 of that relay has timed out.
    The additional logic in the POTT scheme which is called the Echo function works in the following way. The Local D60 must first detects that it’s own breaker is open. If it’s local breaker is open, and it receives a POTT key from the remote D60, ----it will send the POTT key sequence it received directly back to the remote relay. The local D60 is telling the remote D60 that it is ok to trip because the local end of the line is already open and clearing of the fault will be much faster, ensuring the system will not become unstable.
    ----The remote relay will then take this echoed POTT key and trip it’s breaker.
    If the Echo function is going to be used, the Line Pickup protection element must first be configured. The description of the Line pickup function will be covered later in the course.
  • Typically applied on power line carrier. The line is sending the signal and is faulted therefor communications isn’t very reliable… we use this scheme even though it is slower then POTT because it will eventually trip line where as POTT requires a reliable comms. link.
    Operation:
    If local zone 2 operates and have not received a block from remote relay local relay will trip.
  • The Directional Comparison Blocking scheme that is available in the D60 is one of the most popular types of tele-protection schemes used in distance applications today.
    The purpose of the scheme is to still speed up the tripping of faults that occur in the end zone of a transmission line, just like the POTT scheme.
    As for all pilot aided schemes, a communication channel must be provided between the two relays located at each end of the transmission line for the Directional Blocking scheme to operate.
  • In the Directional Blocking scheme, the local D60 has an additional delay timer that is started by the detecting of a fault inside it’s zone 2 area of protection, or, ------the detection of ground current flowing in the forward direction. This timer is set considerably shorter than the normal zone 2 delay.
    When this additional timer expires the Local D60 will trip the local breaker unless the------Local D60 receives a block message or key from the remote D60.
  • The remote D60 will only send this blocking key if it detects that the fault is located in it’s zone 4 area of protection, or, it detects that ground current is flowing in the reverse direction. Both of which, would indicate an external fault.
    In the event that the communications channel failed, the local relay will misoperate. This is one of the disadvantages of the DCB scheme.
  • The Directional Comparison Unblocking scheme that is currently not available in the D60, but will be implemented in the near future.
    This is a scheme that was developed to operate ONLY with FSK (Frequency Shift Keying) channels, like Power Line Carriers
    The purpose of the scheme is to still speed up the tripping of faults that occur in the end zone of a transmission line, just like the POTT and DCB schemes.
    As for all pilot aided schemes, a communication channel must be provided between the two relays located at each end of the transmission line for the Directional Comparison Unblocking scheme to operate. This channel is normally a single FSK power line carrier.
    This scheme utilizes principles from the POTT and DCB schemes, making it the most reliable scheme when the communications channel is a power line carrier.
  • Transcript of "Pilot protection"

    1. 1. Pilot Protection of Transmission Lines By Suraj K Sumanth V Kiran R
    2. 2. Outline • • • • • • Transmission line introduction Typical protection schemes Need for pilot aided schemes Various pilot schemes Redundancy considerations Desirable attributes of pilot relay
    3. 3. Transmission Lines • A Vital Part of the Power System: • Provide path to transfer power between generation and load • Operate at voltage levels from 69kV to 765kV • Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.
    4. 4. Transmission Lines • Classification of line length depends on:  Source-to-line Impedance Ratio (SIR), and  Nominal voltage • Length considerations:  Short Lines: SIR > 4  Medium Lines: 0.5 < SIR < 4  Long Lines: SIR < 0.5
    5. 5. Typical Protection Schemes Short Lines • • • • Current differential Phase comparison Permissive Overreach Transfer Trip (POTT) Directional Comparison Blocking (DCB)
    6. 6. Typical Protection Schemes Medium Lines • Phase comparison • Directional Comparison Blocking (DCB) • Permissive Underreach Transfer Trip (PUTT) • Permissive Overreach Transfer Trip (POTT) • Unblocking • Step Distance • Step or coordinated overcurrent • Inverse time overcurrent • Current Differential
    7. 7. Typical Protection Schemes Long Lines • Phase comparison • Directional Comparison Blocking (DCB) • Permissive Underreach Transfer Trip (PUTT) • Permissive Overreach Transfer Trip (POTT) • Unblocking • Step Distance • Step or coordinated overcurrent • Current Differential
    8. 8. Introduction • Nonpilot protection using overcurrent and distance relays, contain a fundamental difficulty : It is not possible to instantaneously clear a fault from both ends of a transmission line if the fault is near one end of the line. • Pilot protection is an adaptation of the principles of differential relaying that avoids the use of control cable between terminals. • The term ‘pilot’ refers to a communication channel between two or more ends of a transmission line to provide instantaneous clearing over 100% of the line. • This form of protection is also known as ‘teleprotection’.
    9. 9. BUS BUS Need For Pilot Aided Schemes Local Relay Remote Relay Communication Channel
    10. 10. Pilot Communications Channels • Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can also utilize peer-to-peer communications and GOOSE messaging over digital channels • Typical communications media include: • Pilot-wire (50Hz, 60Hz, AT) • Power line carrier • Microwave • Radio • Optic fiber (directly connected or multiplexed channels)
    11. 11. Distance-based Pilot Protection
    12. 12. Pilot-Aided Distance-Based Schemes  DUTT – Direct Under-reaching Transfer Trip  PUTT – Permissive Under-reaching Transfer Trip  POTT – Permissive Over-reaching Transfer Trip  Hybrid POTT – Hybrid Permissive Over-reaching Transfer Trip  DCB – Directional Comparison Blocking Scheme  DCUB – Directional Comparison Unblocking Scheme
    13. 13. Direct Underreaching Transfer Trip (DUTT) • Requires only underreaching (RU) functions which overlap in reach (Zone 1). •Applied with FSK channel • GUARD frequency transmitted during normal conditions • TRIP frequency when one RU function operates • Scheme does not provide tripping for faults beyond RU reach if remote breaker is open or channel is inoperative. • Dual pilot channels improve security
    14. 14. DUTT Scheme   Zone 1 Bus Bus Line Zone 1
    15. 15. Permissive Underreaching Transfer Trip (PUTT) • Requires both under (RU) and overreaching (RO) functions • Identical to DUTT, with pilot tripping signal supervised by RO (Zone 2)
    16. 16. PUTT Scheme Zone 2 Zone 1 To protect end of line Bus Bus Line Zone 1 Zone 2 Rx PKP Zone 2 Zone 1 & Local Trip OR
    17. 17. Permissive Overreaching Transfer Trip (POTT) • Requires overreaching (RO) functions (Zone 2). • Applied with FSK channel: • GUARD frequency sent in stand-by • TRIP frequency when one RO function operates • No trip for external faults if pilot channel is inoperative • Time-delayed tripping can be provided
    18. 18. POTT Scheme Zone 2 Zone 1 Bus Bus Line Zone 1 Zone 2 (Z1) Tx Zone 1 (Z1) OR Rx AND Zone 2 t o Trip Line Breakers
    19. 19. POTT Scheme POTT – Permissive Over-reaching Transfer Trip BUS BUS End Zone Communication Channel
    20. 20. POTT Scheme Relay Local Local Relay FW IGND D Local Relay – Z2   Remote Relay FW IGND D Remote Relay – Z2 TRIP Communication Channel POTT RX Local Relay ZONE 2 PKP OR Ground Dir OC Fwd POTT TX ZONE 2 PKP Remote Relay OR Ground Dir OC Fwd
    21. 21. POTT Scheme POTT RX 2 POTT RX 3 POTT RX 4 Local Relay Communications Channel(s) POTT RX 1 POTT TX 1 A to G POTT TX 2 B to G POTT TX 3 C to G POTT TX 4 Multi Phase Remote Relay
    22. 22. POTT Scheme Current reversal example TRIP Local Relay GND DIR OC FWD GND DIR OC REV Timer   Start  Communication  Expire Timer Channel POTT RX POTT TX Remote Relay ZONE 2    OR GND DIR OC REV GND DIR OC FWD
    23. 23. POTT Scheme Echo example Remote FWD IGND Open Remote – Z2 OPEN Communication Channel POTT RX Local Relay POTT TX TRIP POTT TX POTT RX Communication Channel Remote Relay
    24. 24. Hybrid POTT • Intended for three-terminal lines and weak infeed conditions • Echo feature adds security during weak infeed conditions • Reverse-looking distance and oc elements used to identify external faults
    25. 25. Hybrid POTT Zone 2 Zone 1 Remote Local Weak system Bus Bus Line Zone 1 Zone 2 Zone 4
    26. 26. Directional Comparison Blocking (DCB) • Requires overreaching (RO) tripping and blocking (B) functions • ON/OFF pilot channel typically used (i.e., PLC) • Transmitter is keyed to ON state when blocking function(s) operate • Receipt of signal from remote end blocks tripping relays • Tripping function set with Zone 2 reach or greater • Blocking functions include Zone 3 reverse and low-set ground overcurrent elements
    27. 27. DCB Scheme Zone 2 Zone 1 Remote Local Bus Bus Line Zone 1 Zone 2
    28. 28. Directional Comparison Blocking (DCB) BUS BUS End Zone Communication Channel
    29. 29. Directional Comparison Blocking (DCB) Internal Faults Local Relay – Z2 FWD IGND TRIP Timer Start Expired TRIP Zone 2 PKP OR NO Local Relay GND DIR OC Fwd Dir Block RX Remote Relay
    30. 30. Directional Comparison Blocking (DCB) External Faults Local Relay – Z2 FWD IGND TRIP Timer Start Remote Relay – Z4 REV IGND No TRIP Dir Block RX Local Relay Zone 2 PKP OR DIR BLOCK TX Communication Channel GND DIR OC Fwd Zone 4 PKP OR GND DIR OC Rev Remote Relay
    31. 31. Directional Comparison Unblocking (DCUB) • Applied to Permissive Overreaching (POR) schemes to overcome the possibility of carrier signal attenuation or loss as a result of the fault • Unblocking provided in the receiver when signal is lost: • If signal is lost due to fault, at least one permissive RO functions will be picked up • Unblocking logic produces short-duration TRIP signal (150-300 ms). If RO function not picked up, channel lockout occurs until GUARD signal returns
    32. 32. DCUB Scheme Forward Bus Bus Line Forward (Un-Block) (Block) Trip Line Breakers Tx1 Tx2 Forward (Block) Rx2 AND AND (Un-Block) t AND o AND Rx1 Lockout
    33. 33. Directional Comparison Unblocking (DCUB) BUS BUS End Zone Communication Channel
    34. 34. Directional Comparison Unblocking (DCUB) Normal conditions Load Current FSK Carrier GUARD1 RX FSK Carrier GUARD1 TX Local Relay NO Loss of Guard NO Permission GUARD2 TX Communication Channel GUARD2 RX Remote Relay NO Loss of Guard NO Permission
    35. 35. Directional Comparison Unblocking (DCUB) Normal conditions, channel failure Load Current Loss of Channel FSK Carrier NO RX GUARD1 RX FSK Carrier GUARD1 TX Local Relay Loss of Guard Block Timer Expired Started Block DCUB until Guard OK GUARD2 TX Communication Channel GUARD2 RX NO RX Remote Relay Loss of Guard Block Timer Expired Started Block DCUB until Guard OK
    36. 36. Directional Comparison Unblocking (DCUB) Internal fault, healthy channel Local Relay – Z2 Remote Relay – Z2 TRIP TRIP Z1 FSK Carrier Local Relay Zone 2 PKP Loss of Guard Permission FSK Carrier TRIP1 RX GUARD1 RX GUARD1 TX TRIP1 TX GUARD2 TX TRIP2 TX GUARD2 RX TRIP2 RX Communication Channel Remote Relay ZONE 2 PKP
    37. 37. Directional Comparison Unblocking (DCUB) Internal fault, channel failure Local Relay – Z2 Remote Relay – Z2 Loss of Channel TRIP FSK Carrier Local Relay Zone 2 PKP TRIP Z1 FSK Carrier NO RX GUARD1 RX GUARD1 TX TRIP1 TX GUARD2 TX TRIP2 TX GUARD2 RX NO RX Loss of Guard Block Timer Started Duration Timer Expired Started Remote Relay ZONE 2 PKP Loss of Guard Communication Channel
    38. 38. Redundancy Considerations • Redundant protection systems increase dependability of the system: • Multiple sets of protection using same protection principle and multiple pilot channels overcome individual element failure, or • Multiple sets of protection using different protection principles and multiple channels protects against failure of one of the protection methods. • Security can be improved using “voting” schemes (i.e., 2-out-of3), potentially at expense of dependability. • Redundancy of instrument transformers, battery systems, trip coil circuits, etc. also need to be considered.
    39. 39. Redundant Communications BUS BUS End Zone AND Channels: POTT Less Reliable DCB Less Secure OR Channels: Communication Channel 1 Communication Channel 2 More Channel Security POTT More Reliable DCB More Secure More Channel Dependability Loss of Channel 2
    40. 40. Redundant Pilot Schemes
    41. 41. Pilot Relay Desirable Attributes •Integrated functions: • weak infeed • echo • line pick-up (SOTF) •Basic protection elements used to key the communication: • distance elements • fast and sensitive ground (zero and negative sequence) directional IOCs with current, voltage, and/or dual polarization
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