4.Application of Pilot Schemes

8/6/2019 4.Application of Pilot Schemes

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Knowledge Is PowerSMApparatus Maintenance and Power Management

for Energy Delivery

Application of Pilot Schemes

J. GosaliaDoble Engineering Company


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for Energy Delivery

Pilot Protection Scheme

Pilot

ProtectionSchemes

Types of pilotschemes

Types of pilotschemes

Principle of

operation

Principle ofoperation

Comparison ofdifferent schemes

Comparison ofdifferent schemes

What is Pilot

protectionscheme?

What is Pilot

protectionscheme?


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for Energy Delivery

End Zone Faults

Zone 1 setting at A & B = 80% of the line length

Fault at F1: Fault is seen in Zone 1 at A & B. Cleared instantaneously

Fault at F2:

Cleared instantaneously at station B but cleared with Zone 2

time delay from Station A.

Fault remains on the line for Zone 2 time delay

End Zone fault it is not cleared instantaneously

A CB

F2

G GF1


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for Energy Delivery

End Zone Faults : Pilot Scheme

Fault on the protected line should becleared instantaneously :

Continuity of supply Limit the Damage & shock to electrical system

Stability of the system

Minimize Revenue loss $$$$........Faults on the protected line including end

zone fault can be cleared by employing pilotscheme Each end communicates with the other end to

clear faults on the protected line instantaneously


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for Energy Delivery

Direct Transfer Trip Scheme : DTT

21

Z1

Trip

Z2

Z3 21

Z1

Trip

Z2

Z3

G G

SignalingChannel

SignalingChannel


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for Energy Delivery

Direct Transfer Trip Scheme : Logic

21

Z1

Trip

Z2

Z3

Signal Receive

Signal Send

Mal operation of the signaling equipmentmay result in an unnecessary tripping

Mal operation of the signaling equipmentmay result in an unnecessary tripping

To avoid mal operation, signal receivecan be supervised by the distanceelement Z2 before issuing trip command

To avoid mal operation, signal receivecan be supervised by the distanceelement Z2 before issuing trip command


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for Energy Delivery

Permissive Under Reach TT Scheme

21

Z1

TripZ2

SignalReceive

Signal Send

&

Z2t

Signal is sent to remote terminal if Z1 operates.If signal is received from remote terminal then tripping isissued instantaneously if local Zone 2 see the fault

Signal is sent to remote terminal if Z1 operates.If signal is received from remote terminal then tripping isissued instantaneously if local Zone 2 see the fault

G GZ1

Z2Z1


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for Energy Delivery

Permissive Under Reach TT Scheme

21

Z1

TripZ2

Signal Receive Signal Send

&Z2t

For end Zone fault with the remote breaker open, fault will not becleared instantaneously.Use 52b contact to send the continuous permissive signal toremote end for open breaker.

For end Zone fault with the remote breaker open, fault will not becleared instantaneously.Use 52b contact to send the continuous permissive signal toremote end for open breaker.

G G

Comm52b


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for Energy Delivery

PUTT Scheme : Caution

Fault near terminal A is not seen by Zone 1 at terminal B No current flows through terminal B Fault is seen as Zone 1 Fault at terminal A

Breaker at A trips instantaneously

Now the fault current if fed via line CD and through B.

Protection at B now sees the fault in Zone 2 but has nopermissive signal from terminal A to trip

Fault is cleared after Zone 2 timer times out.

G

A

C

B

D


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Knowledge Is PowerSMApparatus Maintenance and Power Managementfor Energy Delivery

PUTT Scheme : Solution

As breaker A opens the permissive signal to B resets

Now the fault is fed through breakers C,D and B

Zone 2 at B picks up but has no permission from A to trip

Solution : Add a time delay on drop out on a permissivesignal receive

The drop out delay >= Zone 2 pick up time + safety margin

G

C D

A B


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PUTT : Logic

21

Z1

TripZ2

Signal Receive Signal Send

&

Z2t

52b

td


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PUTT : Limitation

For Short line followed by a long line: Fault with the fault resistance may not be seen in Zone 1

PUTT scheme fails to clear the fault on the line

Fault is cleared in Zone 2 time

For Short line followed by a long line: Fault with the fault resistance may not be seen in Zone 1 PUTT scheme fails to clear the fault on the line

Fault is cleared in Zone 2 time

ZR

Zs

Zone 1

Zone 2


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POTT Scheme

21

Z1

Z2

Z3

G G

SignalingChannel

&

Trip 21

Z1

Z2

Z3

&

Trip

Signaling

Channel


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POTT : Logic

21

Z1

Z2

Z3

&

TripTrip

Signal ReceiveSignal Receive

Signal SendSignal Send


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POTT : Fault Resistance Coverage

Zs

Zone 1

Zone 2


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Permissive Overreach TT Scheme

G

A

C

B

D

G

C sees Zone 2 fault, B sees zone 1 fault

Fault current flows C-D-B

D receives permission to trip from C but D sees reverse fault

Breaker B opens as it sees the fault in Zone 1

Now the fault current reverses and flows through D-C-A

D sees Zone 2 fault, C does not see fault and zone 2 resets and

stop sending permissive signal to D If D Zone 2 picks up before Zone 2 at C resets, D can trip

If C Zone 2 is slow to reset it can trip for a fault on parallel line


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POTT : Solution

21

Z1

Z2

Z3

&

TripTrip



tp

Adding a time delay tp in Signal Send will prevent mal operation

tp > Fast PU local Z2 time slowest reset time of remote Z2 time

Adding a time delay tp in Signal Send will prevent mal operation

tp > Fast PU local Z2 time slowest reset time of remote Z2 time


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POTT Scheme :Remote Breaker Open

G G

A sees Zone 2 fault, B does not see fault as it is open

A can not trip instantaneously as it does not receive thepermissive signal from B

If the local breaker is open and receives permissive signal fromremote end, the permissive signal can be Echoed back toremote breaker

Now the breaker A can trip to clear the fault on the line

A B


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POTT : Signal Echo Logic

T1 & T2 &



52b52b

T1 timer avoid sending Echo if breaker is tripped by otherprotection

T2 avoids continuous Echo


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POTT : Logic



21

Z1

Z2

Z3

& TripTrip

FSK logicTrip and

Guard Freq.

When Permissivesignal is receivedfrom Remote Endguard frequency is

lost and trip freq isreceived

Permissive Trip isissued by signalingequipment to

protection whenguard frequency islost and tripfrequency is received

PLCC equipmenttransmit guard andtrip freq. over powerline

When Permissivesignal is receivedfrom Remote Endguard frequency is

lost and trip freq isreceived

Permissive Trip isissued by signalingequipment to

protection whenguard frequency islost and tripfrequency is received

PLCC equipment

transmit guard andtrip freq. over powerline


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POTT : Caution

Line Trap

G

FSK logicTrip and

Guard Freq.

Coupling cap

G

FSK logicTrip and

Guard Freq.

Coupling cap

Guard as well as trip frequency can not be transmitted orattenuated due to fault on the power line

Line Trap


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Un Blocking Scheme

POTT scheme

Trip frequency must be received & guard

frequency must be lost to issue permissive signal

Un Blocking Scheme

Enhanced POTT scheme

If guard frequency is lost then permissive tripsignal is issued only for 100-150 mSecs.

If Z2 is picked up protection trip during this time

After this time permissive trip signal is only issuedif guard frequency is lost & trip frequency received

Very secure and reliable scheme


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POTT Scheme : Weak In feed

Fault near the remote terminal is seen as

Zone 2 at local terminal Needs permissive trip signal from remote end to

clear the fault

Remote end has a week source contributing tothe fault : Z2 at remote end does not pick upand no permissive signal is sent at local end

Fault on the line is not cleared instantaneously

G

G

Z1

Z2


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POTT : Weak In Feed

G

G

Z1

Z2

Z3

Signal Receive

Z3 &

Echo Signal Send

Trip local Br.


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Blocking Scheme

Simple scheme

Very popular in N. America

Comm. channel only sends the blockingsignal to remote end to block tripping

Unlike POTT scheme where guard signal is sentcontinuous

Z3 is set reverse looking

Z3 Picks up and sends the blocking signal

indicating fault is beyond the protected linesection


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Blocking Scheme

G

G

Z1

Z2

Z3

Fault near the remote terminal is seen asZone 2 at local terminal Remote Z3 does not see the fault

No blocking signal is sent

Trip is issued after a time delay tp to clear fault

For fault beyond remote terminal Z3 reverse looking picks up & send block singal

No trip at local terminal


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Blocking Scheme : Logic

Z3

&Signal Send

G

G

Z1

Z2

Z3

Signal Receive

Z2

&Trip local Br.

tp


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Blocking Vs. Permissive Scheme

Signal Receive: Blockslocal inst. Z2 tripping

Signaling channel isnormally OFF. onlyON when Z3 picks up

When Z2 picks up, it

waits for a time delaytp before issuing trip toensure remoteterminal has a time tosend block signal

Signal Receive: Blockslocal inst. Z2 tripping

Signaling channel isnormally OFF. onlyON when Z3 picks up

When Z2 picks up, itwaits for a time delaytp before issuing trip toensure remote

terminal has a time tosend block signal

Signal Receive tripsZ2 instantaneously

Signaling channel isON all the timesending Guardfrequency and

changes to trip freq. ifZ2 picks up

No intentional timedelay for tripping ifpermissive signal isreceived

Signal Receive tripsZ2 instantaneously

Signaling channel isON all the timesending Guardfrequency and

changes to trip freq. ifZ2 picks up

No intentional time

delay for tripping ifpermissive signal isreceived


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Blocking Vs. POTT : Comparison

Intentional time delayadded to avoid overtripping

Failure of thecommunication channelmay results in over trippingfor an external fault

Blocking scheme is more

dependable Single On/Off signaling

channel in use operating atsingle frequency in bothdirection

Intentional time delayadded to avoid overtripping

Failure of thecommunication channelmay results in over trippingfor an external fault

Blocking scheme is more

dependable Single On/Off signaling

channel in use operating atsingle frequency in both

direction

No intentional time delayon trip for a fault on the line

Failure of the

communication channelmay results in a failure totrip for a fault on the line

POTT scheme is moresecure

Separate signaling channelfor each direction ofsignaling

No intentional time delayon trip for a fault on the line

Failure of the

communication channelmay results in a failure totrip for a fault on the line

POTT scheme is moresecure

Separate signaling channelfor each direction ofsignaling

Un Blocking scheme combines security of POTT scheme &

Dependability of Blocking schemeUn Blocking scheme combines security of POTT scheme &

Dependability of Blocking scheme


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Selection of Pilot Scheme?

Deciding factors

Reliability of signaling channels

N. America for 115/230 kV lines,blocking scheme is used frequently

Reliability is quite good

Operating speed Fault clearing time requirement

System operating practice


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Special Schemes

Zone 1 extension scheme

No communication channel is used

Economical, Simple Used along with AutoReclosing protection

95% of all the faults are transient fault

Phase distance and time over current protection in

blocking scheme Ground distance protection has limited fault resistance

coverage

Little or not fault coverage for short line and/or strong

source

Use of time over current for ground fault provides higherfault resistance coverage


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Zone 1 Ext. Scheme

Zone 1 is set to over reach by setting Z1x

Z1x reach of AB is set to 20% in to the line section CD If the fault occurs in line section CD, breaker A and C trips High speed Auto Reclose protection closes the breakers

A & C During the same time, the Z1x reach is pulled back to Z1

If the fault is permanent on the next line section onlybreaker C will trip and breaker A does not trip

If the fault is transient, interruption in power transfer isminimal

G GZ1

Z1x

21/79

A B C D


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Zone 1 Ext. Scheme : Logic

Auto RecloseIn progress

Z1

& Z1 trip.Z1x

&

79


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Zone 1 Ext. Scheme: Pros and Cons

Transient faults on the line are clearedinstantaneously without the need for

elaborate communication equipmentPermanent end Zone faults are cleared in

Zone 2 time

Transient faults in the next line section willcreate unnecessary tripping and reclosingof the circuit breaker This is the main disadvantage when

interconnected high voltage power system whenseveral feeders can terminate at a single bus

Transient faults on the line are clearedinstantaneously without the need for

elaborate communication equipmentPermanent end Zone faults are cleared in

Zone 2 time

Transient faults in the next line section willcreate unnecessary tripping and reclosingof the circuit breaker This is the main disadvantage when

interconnected high voltage power system whenseveral feeders can terminate at a single bus


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Pilot Scheme: 21P and 50/51G

Blocking scheme : 21P as discussed earlier

Ground over current uses 50/51 and directional protection Local End:

Block signal is sent to remote end by non directional groundtime over current element

The block signal is cancelled if the ground directional elementpicks up for a fault on a line (Forward fault)

Remote End:

If high set is picked up (50 HS) and directional element pickedup & block signal is received : Fault is forward

Wait for a time delay tp for block signal to be removed fromthe remote end for a forward fault

If the block signal from the local end is cancelled then theprotection trips

If block signal is not cancelled then fault is on the next linesection and it may trip after a time delay 51G


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Pilot Scheme: 21P and 50/51G

Protection A and B: Block signal is sent to remote end by high speed non dir. groundtime over current element

The block signal is cancelled by the ground directional element

Block signal is received, 50 HS with directional element picked up.

Wait for a time delay tp to see if block signal is cancelled

Once the block signal is cancelled protection A and B trips to clearthe fault on the line

G GZ1

21P50/51G

A B C D

21P50/51G


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4.Application of Pilot Schemes

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