Pilot Protection of
Transmission Lines
BySuraj KSumanth VKiran R
Outline
• Transmission line introduction• Typical protection schemes• Need for pilot aided schemes• Various pilot schemes• Redundancy considerations• Desirable attributes of pilot relay
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.
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
Typical Protection SchemesShort Lines
• Current differential• Phase comparison• Permissive Overreach Transfer Trip (POTT)• Directional Comparison Blocking (DCB)
Typical Protection SchemesMedium 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
Typical Protection SchemesLong 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
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’.
BU SBU S
Communication Communication ChannelChannel
Local Local RelayRelay
Remote Remote RelayRelay
Need For Pilot Aided Schemes
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)
Distance-based Pilot Protection
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
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
Bus
Line
Bus
Zone 1
Zone 1
DUTT Scheme
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)
Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
To protect end ofline
& Local Trip Zone 2
Rx PKP
OR Zone 1
PUTT Scheme
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
Bus
Line
Bus
Zone 1
Zone 2
TripLine
BreakersOR
t
Rx
Tx
AND(Z1)
(Z1)
o
Zone 1
Zone 2
Zone 2
Zone 1
POTT Scheme
POTT SchemePOTT – Permissive Over-reaching POTT – Permissive Over-reaching
Transfer TripTransfer TripBU S BU S
End End ZoneZone
Communication Communication ChannelChannel
Local Local RelayRelay
Remote Remote RelayRelay
Remote Remote Relay Relay FWD IFWD IGNDGND
Ground Dir OC FwdGround Dir OC Fwd
OROR
Local Relay – Z2Local Relay – Z2
ZONE 2 PKPZONE 2 PKP
Local Local Relay FWD Relay FWD I IGNDGND
Ground Dir OC Ground Dir OC FwdFwd
OROR
TRIPTRIP
Remote Relay – Z2Remote Relay – Z2
POTT TX
ZONE 2 ZONE 2 PKPPKP
POTT RX
CommunicatCommunication Channelion Channel
POTT Scheme
POTT TX 4
POTT TX 3
POTT TX 2
POTT TX 1 A 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
Com
munications
Channel(s)
POTT Scheme
Local RelayLocal Relay Remote RelayRemote Relay
POTT TX ZONE 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 TimerTimerTimer Timer ExpireExpire
GND DIR OC FWDGND DIR OC FWD
POTT SchemeCurrent reversal example
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 SchemeEcho example
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
Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1 Zone 4
LocalRemoteWeaksystem
Hybrid POTT
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
Bus
Line
Bus
Zone 1
Zone 2
Zone 2
Zone 1
LocalRemote
DCB Scheme
BU
S
BU
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Blocking (DCB)
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 IGNDGND
GND DIR OC FwdGND DIR OC FwdOROR
Dir Block RXNONO
TRIPTRIP
ExpiredExpired
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 IGNDGND
GND DIR OC FwdGND DIR OC Fwd
OROR
TRIP Timer TRIP Timer StartStart No TRIPNo TRIP
Directional Comparison Blocking (DCB)External Faults
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
Bus
Line
Bus
TripLine
BreakersTx1(Un-Block)
Forward
Forward
Tx2(Block)
Forward
Rx2
Rx1
to
AND to
AND
AND
AND
Lockout
(Block)
(Un-Block)
DCUB Scheme
BU
S
BU
S
End ZoneEnd Zone
Communication ChannelCommunication Channel
Directional Comparison Unblocking (DCUB)
Directional Comparison Unblocking (DCUB)Normal conditions
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 TXGUARD1 RX
Communication Communication ChannelChannel
GUARD2 TX GUARD2 RXNO Loss of GuardNO Loss of Guard
FSK CarrierFSK Carrier FSK CarrierFSK Carrier
NO PermissionNO PermissionNO Loss of GuardNO Loss of Guard
NO PermissionNO Permission
Load CurrentLoad Current
Directional Comparison Unblocking (DCUB)Normal conditions, channel failure
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 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
ExpiredExpiredBlock DCUB Block DCUB until Guard OKuntil Guard OK
ExpiredExpired
Loss of ChannelLoss of Channel
Directional Comparison Unblocking (DCUB)Internal fault, healthy channel
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 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
Directional Comparison Unblocking (DCUB)Internal fault, channel failure
Local RelayLocal Relay Remote RelayRemote RelayGUARD1 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 GuardBlock Timer StartedBlock Timer StartedDuration Timer StartedDuration Timer StartedExpiredExpired
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.
BU
S
BU
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
Redundant Pilot Schemes
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