Pilot protection

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


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


POTT TX ZONE 2 ORZONE 2 OR

GND DIR OC FWDGND DIR OC FWD


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


POTT RX

OPENOPEN

POTT TX


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



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




REV IREV IGNDGND

GND DIR OC RevGND DIR OC Rev

OROR

DIR BLOCK TX



Dir Block RX


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


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



GUARD2 TX GUARD2 RX


Loss of GuardLoss of Guard

Block Timer StartedBlock Timer Started


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



GUARD2 TX GUARD2 RX



PermissionPermission

TRIP1 TX



TRIP1 RX

TRIP2 TX

TRIPTRIP



TRIP Z1TRIP Z1

TRIP2 RX

Directional Comparison Unblocking (DCUB)Internal fault, channel failure



GUARD2 TX GUARD2 RX


TRIP1 TX



NO RX

TRIP2 TX

TRIPTRIP



TRIP Z1TRIP Z1

NO RX


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

Pilot protection

Technology

Transcript of Pilot protection