Pilot protection

1. Pilot Protection
of
Transmission Lines
By
Suraj K
Sumanth V
Kiran R

2. Outline
•Transmission line introduction
•Typical protection schemes
•Need for pilot aided schemes
•Various pilot schemes
•Redundancy considerations
•Desirable attributes of pilot relay

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. 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. Typical Protection Schemes
Short Lines
• Current differential
• Phase comparison
• Permissive Overreach Transfer Trip (POTT)
• Directional Comparison Blocking (DCB)

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. 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. 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. B
U
S
B
U
S
Communication Communication
ChannelChannel
Local RelayLocal Relay Remote RelayRemote Relay
Need For Pilot Aided Schemes

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. Distance-based Pilot Protection

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. 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.  
Bus
Line
Bus
Zone 1
Zone 1
DUTT Scheme

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. Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
To protect end of
line
&
Local Trip
Zone 2
Rx PKP
OR
Zone 1
PUTT Scheme

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. Bus
Line
Bus
Zone 1
Zone 2
Trip
Line
Breakers
OR
t
Rx
Tx
AND
(Z1)
(Z1)
o
Zone 1
Zone 2
Zone 2
Zone 1
POTT Scheme

19. POTT Scheme
POTT – Permissive Over-reaching Transfer TripPOTT – Permissive Over-reaching Transfer Trip
B
U
S
B
U
S
End End
ZoneZone
Communication ChannelCommunication Channel

20. Local Local RelayRelay
Remote Remote RelayRelay
Remote Relay Remote Relay
FWD IFWD I
GNDGND
Ground Dir OC FwdGround Dir OC Fwd
OROR
Local Relay – Z2Local Relay – Z2    
ZONE 2 PKPZONE 2 PKP
Local Relay Local Relay
FWD IFWD I
GNDGND
Ground Dir OC FwdGround Dir OC Fwd
OROR
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
POTT TX
ZONE 2 PKPZONE 2 PKP
POTT RX
Communication Communication
ChannelChannel
POTT Scheme

21. POTT TX 4
POTT TX 3
POTT TX 2
POTT TX 1A to GA to G
B to GB to G
C to GC to G
Multi PhaseMulti Phase
Local RelayLocal Relay Remote RelayRemote Relay
POTT RX 4
POTT RX 3
POTT RX 2
POTT RX 1
C
o
m
m
u
n
i
c
a
t
i
o
n
s

C
h
a
n
n
e
l
(
s
)
POTT Scheme

22. Local RelayLocal Relay Remote RelayRemote Relay
POTT TXZONE 2    ORZONE 2    OR
GND DIR OC FWDGND DIR OC FWD
Communication Communication 
ChannelChannel
TRIPTRIP
GND DIR OC REVGND DIR OC REVGND DIR OC REVGND DIR OC REV
POTT RX
  Start Start 
TimerTimer
Timer Timer 
ExpireExpire
GND DIR OC FWDGND DIR OC FWD
POTT Scheme
Current reversal example

23. Local RelayLocal Relay
OpenOpen
Remote RelayRemote Relay
Remote FWD Remote FWD
IIGNDGND
POTT TX
Remote – Z2Remote – Z2
Communication Communication
ChannelChannel
POTT RX
OPENOPEN
POTT TX
Communication Communication
ChannelChannel
POTT RX
TRIPTRIP
POTT Scheme
Echo example

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. Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1 Zone 4
LocalRemote
Weak
system
Hybrid POTT

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. Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
LocalRemote
DCB Scheme

28. B
U
S
B
U
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Blocking (DCB)

29. Directional Comparison Blocking (DCB)
Internal Faults
Local RelayLocal Relay Remote RelayRemote Relay
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
TRIP Timer TRIP Timer
StartStart
FWD IFWD I
GNDGND
GND DIR OC FwdGND DIR OC Fwd
OROR
Dir Block RXNONO
TRIPTRIP
ExpiredExpired

30. Local RelayLocal Relay Remote RelayRemote Relay
Remote Relay – Z4Remote Relay – Z4
Zone 4 PKPZone 4 PKP
REV IREV IGNDGND
GND DIR OC RevGND DIR OC Rev
OROR
DIR BLOCK TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
Dir Block RX
Communication Communication
ChannelChannel
FWD IFWD I
GNDGND
GND DIR OC FwdGND DIR OC Fwd
OROR
TRIP Timer TRIP Timer
StartStart
No TRIPNo TRIP
Directional Comparison Blocking (DCB)
External Faults

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. Bus
Line
Bus
Trip
Line
Breakers
T x1(Un-Block)
Fo rward
Forward
T x2(Block)
Forward
Rx2
Rx1
t
o
AND
t
o
AND
AND
AND
Lo cko ut
(Block)
(Un-Blo ck)
DCUB Scheme

33. B
U
S
B
U
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Unblocking
(DCUB)

34. Directional Comparison Unblocking
(DCUB)
Normal conditions
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication
ChannelChannel
GUARD2 TX GUARD2 RX
NO Loss of GuardNO Loss of Guard
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
NO PermissionNO Permission
NO Loss of GuardNO Loss of Guard
NO PermissionNO Permission
Load CurrentLoad Current

35. Directional Comparison Unblocking
(DCUB)
Normal conditions, channel failure
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication
ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
Loss of GuardLoss of Guard
Block Timer StartedBlock Timer Started
Loss of GuardLoss of Guard
Block Timer StartedBlock Timer Started
Load CurrentLoad Current
NO RX
NO RX
Block DCUB Block DCUB
until Guard OKuntil Guard OK
ExpiredExpired
Block DCUB Block DCUB
until Guard OKuntil Guard OK
ExpiredExpired
Loss of ChannelLoss of Channel

36. Directional Comparison Unblocking
(DCUB)
Internal fault, healthy channel
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication
ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
Loss of GuardLoss of Guard
PermissionPermission
TRIP1 TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
TRIP1 RX
TRIP2 TX
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
ZONE 2 PKPZONE 2 PKP
TRIP Z1TRIP Z1
TRIP2 RX

37. Directional Comparison Unblocking
(DCUB)
Internal fault, channel failure
Local RelayLocal Relay Remote RelayRemote Relay
GUARD1 TXGUARD1 RX
Communication Communication
ChannelChannel
GUARD2 TX GUARD2 RX
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
TRIP1 TX
Local Relay – Z2Local Relay – Z2
Zone 2 PKPZone 2 PKP
NO RX
TRIP2 TX
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
ZONE 2 PKPZONE 2 PKP
TRIP Z1TRIP Z1
NO RX
Loss of GuardLoss of Guard

Loss of ChannelLoss of Channel
Loss of GuardLoss of Guard
Block Timer StartedBlock Timer Started
Duration Timer StartedDuration Timer StartedExpiredExpired

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-of-
3), potentially at expense of dependability.
• Redundancy of instrument transformers, battery systems, trip
coil circuits, etc. also need to be considered.

39. B
U
S
B
U
S
End ZoneEnd Zone
Communication Channel 1Communication Channel 1
Communication Channel 2Communication Channel 2
Loss of Channel 2Loss of Channel 2
AND Channels:AND Channels:
POTT Less ReliablePOTT Less Reliable
DCB Less SecureDCB Less Secure
OR Channels:OR Channels:
POTT More ReliablePOTT More Reliable
DCB More SecureDCB More Secure
More Channel SecurityMore Channel Security More Channel DependabilityMore Channel Dependability
Redundant Communications

40. Redundant Pilot Schemes

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