Pilot Protection ofTransmission Lines

Outline

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Transmission line introduction

Typical protection schemes

Need for pilot aided schemes

Various pilot schemes

Redundancy considerations

Desirable attributes of pilotrelay

Transmission Lines

•A Vital Part of the PowerSystem:

• Provide path totransfer power between generation and load

• Operate at voltage levels from 69kV to765kV

• Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.

Transmission Lines

•Classification of line length dependson: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

• Phasecomparison

• Permissive Overreach Transfer Trip (POTT)

• Directional Comparison Blocking (DCB)

Typical Protection SchemesMedium Lines

• Phasecomparison

• 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

• Phasecomparison

• 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, containa 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 controlcable 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’.

BUS

BUS

Communication Channel

Local Relay Remote Relay

Need ForPilot Aided Schemes

Pilot Communications Channels

• Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can alsoutilize 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 ormultiplexed channels)

Distance-based Pilot Protection

Pilot-Aided Distance-Based Schemes

DUTT –Direct Under-reaching Transfer Trip

PUTT –Permissive Under-reaching TransferTrip

POTT –Permissive Over-reaching TransferTrip

Hybrid POTT –Hybrid Permissive Over-reaching Transfer Trip

DCB –Directional ComparisonBlocking Scheme

DCUB–Directional Comparison Unblocking Scheme

Direct Underreaching Transfer Trip (DUTT)

•Requires only underreaching (RU) functions which overlap in reach (Zone1).

• Applied with FSKchannel

•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, withpilot tripping signal supervised by RO (Zone 2)

Permissive Overreaching Transfer Trip (POTT)

• Requires overreaching (RO) functions (Zone 2).

• Applied with FSKchannel:• GUARD frequency sent instand-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

Trip

L ine

B reakers

O R

t

R x

Tx

A N D

(Z1)

(Z1)

o

Zone 1

Zone 2

Zone 2

Zone 1

POTT Scheme

POTT Scheme

POTT –Permissive Over-reaching TransferTrip

End Zone

BUS

BUS

Communication Channel

Local Relay

ZONE 2 PKP

OR

Ground Dir OCFwd

Remote Relay

Remote Relay FWD IGND

OR

Ground Dir OCFwd

Local Relay –Z2

Local Relay FWD IGND

TRIP

Remote Relay –Z2

ZONE 2 PKP

POTT

Communication Channel

RX POTT TX

POTT Scheme

POTT TX 1

POTT TX 2

POTT TX 3

POTT TX 4 Multi Phase

Remote Relay

A to G

B to G

C to G

Local Relay

POTT RX 1

POTT RX 2

POTT RX 3

POTT RX 4

Com

municatio

ns

Chan

nel(s)

POTT Scheme

Local Relay Remote Relay

ZZONONEE22OROR

GNDGND DIDIRR OOCC

FWFWDD

GND DIR OC REVGNDGND DIDIRR OOCC REVREV

POTT

Communication

Channel

RX POTT TX

TSitmaretr ET

Eiximpierre

GND DIR OC FWD

POTT SchemeCur ent reversal example

TRIP

Local Relay

Open

Remote Relay

Remote FWD

IGND

POTT RX POTT TX

Remote – Z2

Communication

ChannelOPEN

POTT TX

Communication

Channel

POTT RX

TRIP

POTT SchemeEcho example

Hybrid POTT

• Intended for three-terminal lines and weak infeedconditions

• Echo feature adds security during weak infeedconditions

• Reverse-looking distance and oc elements used to identify external faults

Bus

Line

Bus

Zone 1

Zone 2

Zone 2

Zone 1 Zone 4

LocalRemote

W e a k

sys tem

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 whenblocking function(s) operate

•Receipt of signal from remoteend blocks tripping relays

• Tripping function set with Zone 2 reach orgreater

•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 Zone

Communication Channel

Directional Comparison Blocking (DCB)

Directional Comparison Blocking (DCB)Internal Faults

Local Relay GND DIR OC Fwd Remote Relay

Local Relay – Z2

SSttaarrtt

Zone 2 PKP

OR

FWD IGND

Dir Block RXNO

TRIP TimerTRIP

Expired

Local Relay Remote Relay

Remote Relay – Z4

Zone 4 PKP

OR

REV IGND

GND DIR OC Rev

DIR BLOCK TX

Local Relay – Z2

Zone 2 PKP

OR

Dir Block RX

Communication

Channel

FWD IGND

GND DIR OC Fwd

TRIP Timer

Start No 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 thefault

•Unblocking provided in the receiver when signalis lost:

• If signal is lost due to fault, at least one permissive RO functions will be pickedup

•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

Trip

Line

Breakers

(Un-Block) Tx1

Forward

Forward

(Block) Tx2

Forward

t

oAND t

o

AND

AND

AND

Lockout

(Block) Rx2

(Un-Block) Rx1

DCUBScheme

BU

S

BU

S

End Zone

Communication Channel

Directional ComparisonUnblocking (DCUB)

Directional ComparisonUnblocking (DCUB)Normal conditions

Local Relay

NO Loss of Guard

NO Permission

Remote RelayNO Loss of Guard

NO Permission

GUARD1 TXGUARD1 RX

Communication

Channel

GUARD2 TX GUARD2 RX

FSK Carrier FSK Carrier

Load Current

Directional ComparisonUnblocking (DCUB)Normal conditions, channel failure

GUARD1 TX

Communication

Channel

GUARD2 TX

FSK Carrier FSK Carrier

GNUOARRXD2 RX

SSttaarrtedted

SSttaarrtedted

Load Current

Loss of Channel

NGUOARRXD1RX

Block DCUB

until Guard OK

Local Relay

Loss of Guard

Block Timer Expired

Block DCUB

until Guard OK

Remote RelayLoss of Guard

Block TimerExpired

Directional ComparisonUnblocking (DCUB)Internal fault, healLthocyalcRhelaanyn–eZl2

Local Relay

Zone 2 PKP

Remote Relay

ZONE 2 PKP

Communication

Channel

GUTARRIDP11TX

TRIP

FSK Carrier

TRIP Z1FSK Carrier

Loss of Guard

Permission

TGRUIAP1RDR1XRX

GTRUIAP2RDT2XTX

Remote Relay – Z2

GTRUIAP2RDR2XRX

Directional ComparisonUnblocking (DCUB)Internal fault, chanLnoceallfRaelialuyr–eZ2

Local Relay

Zone 2 PKP

Loss of Guard

Block Timer Started

Remote Relay

ZONE 2 PKP

Loss of Guard

Communication

Channel

GNOUARRXD1RX GUTARRIDP11TX

TRIP

FSK Carrier

TRIP Z1FSK Carrier

GTRUIAP2RDT2XTX

Remote Relay – Z2

GNUOARRXD2 RX

Loss of Channel

StartedStarted

Duration Timer Expired

Redundancy Considerations

• Redundant protection systems increase dependability of the system:

• Multiple sets of protection using same protectionprinciple 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 ofdependability.

• Redundancy of instrument transformers, battery systems,trip coil circuits, etc. also need tobe considered.

BU

S

BU

S

End Zone

Communication Channel 1

Communication Channel 2

Loss of Channel 2

AND Channels:

POTT Less Reliable

DCB Less Secure

OR Channels:

POTT More Reliable

DCB More Secure

More Channel Security More 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 keythe communication:• distance elements• fast and sensitive ground (zero and negative sequence) directional IOCs with current, voltage, and/or dual polarization