tutorial 6 application of pilot scheme

Application of Pilot Schemes

J. GosaliaDoble Engineering Company

Presentation Topics

2

Pilot Schemes

What is Pilot

protection scheme?

Basic Principle

Types of pilot

schemes

Comparison of different schemes

10th India Doble Power Forum Conference 119

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End Zone Faults

Zone 1 setting at A & B = 80% of the line length Fault at F1: seen in Zone 1 at A & B. Cleared instantaneously Fault at F2:

Cleared inst. at station B but in with Zone 2 time delay at A. Fault remains on the line for Zone 2 time delay

End Zone fault it is not cleared instantaneously

CA BF2

G GF1

End Zone Faults : Pilot Scheme

Fault on the protected line should be cleared 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 pilot scheme Each end communicates with the other end to clear faults

on the protected line instantaneously


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Direct Transfer Trip Scheme : DTT

21

Z1

TripZ2

Z321

Z1

TripZ2

Z3

G G

SignalingChannel

SignalingChannel

Direct Transfer Trip Scheme : Logic

21

Z1

TripZ2

Z3

Signal ReceiveSignal Send

Mal operation of the signaling equipment may result in an unnecessary tripping

To avoid mal operation, signal receive can be supervised by the distance element Z2 before issuing trip command


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Permissive Under Reach TT Scheme

21

Z1

TripZ2

Signal Receive Signal Send

&

Z2t

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

G GZ1

Z2Z1

Permissive Under Reach TT Scheme

21

Z1

TripZ2


&

Z2t

For end Zone fault with the remote breaker open, fault will not be cleared inst.Use 52b contact to send the cont. permissive signal to remote end for open breaker.

G G

52bComm


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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 is fed via line CD and through B. Protection at B now sees the fault in Zone 2 but has no

permissive signal from terminal A to trip Fault is cleared after Zone 2 timer times out.

G

A

C

B

D

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 permissive

signal 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


&

Z2t

52b

td

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

ZR

Zs

Zone 1

Zone 2


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POTT Scheme

21

Z1

Z2

Z3

G G

&Trip 21

Z1

Z2

Z3

&Trip

SignalingChannel

SignalingChannel

POTT : Logic

21

Z1

Z2

Z3

&Trip

Signal Receive

Signal Send


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POTT : Fault Resistance Coverage

Zs

Zone 1

Zone 2

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

&Trip

Signal Send

Signal Receive

tp

Adding a time delay tp in Signal Send will prevent mal operationtp > Fast PU local Z2 time slowest reset time of remote Z2 time

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 the

permissive signal from B If the local breaker is open and receives permissive signal from

remote end, the permissive signal can be Echoed back to remote 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 &Signal Send

Signal Receive

52b

T1 timer avoid sending Echo if breaker is tripped by other protection

T2 avoids continuous Echo

POTT : Logic

Signal Receive

Signal Send

21

Z1

Z2

Z3

& Trip

FSK logicTrip and

Guard Freq. When Permissive signal is

received from Remote End guard frequency is lost and trip freq is received

Permissive Trip is issued by signaling equipment to protection when guard frequency is lost and trip frequency is received

PLCC equipment transmit guard and trip freq. over power line


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POTT : Caution

Line TrapG

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 or attenuated due to fault on the power line

Line Trap

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 trip

signal 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 issued

if 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 to the

fault : Z2 at remote end does not pick up and no permissive signal is sent at local end

Fault on the line is not cleared instantaneously

G

G

Z1

Z2

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 blocking signal to

remote end to block tripping Unlike POTT scheme where guard signal is sent

continuous Z3 is set reverse looking

Z3 Picks up and sends the blocking signal indicating fault is beyond the protected line section

Blocking Scheme

G

G

Z1

Z2

Z3

Fault near the remote terminal is seen as Zone 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 signal No trip at local terminal


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Blocking Scheme : Logic

Z3&

Signal Send

Signal Receive

Z2

&Trip local Br.

tp

G

G

Z1

Z2

Z3

Blocking Vs. Permissive Scheme

Signal Receive: Blocks local inst. Z2 tripping

Signaling channel is normally OFF. only ON when Z3 picks up

When Z2 picks up, it waits for a time delay tp before issuing trip to ensure remote terminal has a time to send block signal

Signal Receive trips Z2 instantaneously

Signaling channel is ON all the time sending Guard frequency and changes to trip freq. if Z2 picks up

No intentional time delay for tripping if permissive signal is received


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Blocking Vs. POTT : Comparison Intentional time delay

added to avoid over tripping

Failure of the communication channel may results in over tripping for an external fault

Blocking scheme is more dependable

Single On/Off signaling channel in use operating at single frequency in both direction

No intentional time delay on trip for a fault on the line

Failure of the communication channel may results in a failure to trip for a fault on the line

POTT scheme is more secure

Separate signaling channel for each direction of signaling

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

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 Auto Reclosing protection 95% of all the faults are transient fault

Phase distance and time over current protection in blocking scheme Phase distance works like any other blocking scheme Ground distance protection has limited fault resistance coverage Little or not fault coverage for short line and/or strong source Time over current protection for ground fault provides higher fault

resistance coverage

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 only

breaker C will trip and breaker A does not trip If the fault is transient, interruption in power transfer is

minimal

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

Zone 1 Ext. Scheme: Pros & Cons

Transient faults on the line are cleared instantaneously without the need for elaborate communication equipment

Permanent end Zone faults are cleared in Zone 2 time

Transient faults in the next line section will create unnecessary tripping and reclosing of the circuit breaker This is the main disadvantage when interconnected

high voltage power system when several feeders can terminate at a single bus


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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. ground

time 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 clear

the fault on the line

G GZ1

21P50/51G

A B C D

21P50/51G

Ground Fault

36

Z1A B

GG

50

67&

tp

50

67&

tp


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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 67

Local End: (Forward fault) Block signal is sent to remote end by non directional ground

time over current element (50 HS) The block signal is cancelled by the ground directional

element sees the forward fault. Remote End: (Forward Fault)

High set is picked up (50 HS) and directional element picked up & block signal is received

Wait for a time delay tp for block signal to be removed from the remote end

Block signal from the local end is cancelled by the directional element of the local end

Once the block signal is cancelled remote end trips.


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tutorial 6 application of pilot scheme

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