10 dp schemes

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Distance Protection Schemes

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Z3

Z3

Z1

Z1

Z2

Z2

Z3

Z2

Z1 1

.T2

T3

Trip

T2

1

. T2

T3

Trip

T2

Basic Distance Scheme

Z1

Z2

Z3

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Basic Distance Scheme - Disadvantages

Zone 1 set to 80% leaves 2 end zones

Faults in end zone results in instantaneoustripping at one end and time delayed at the

other

Time delay may lead to system stabilityproblems

Sequential clearance leaves no dead time for

high speed A/R cycle (transient fault becomes

permanent)

Longer clearance times - More damage

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Zone 1 Extension Scheme

Z3

Z2

Z3

Z1X

Z1X

Z2

Z1

Z1

Z3

Z2

Z1 1

.T2

T3

Trip

&

1

Z1X

A/R

Z2

Z1

Z3

1

. T2

T3

Trip

&

1

Z1X

A/R

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Zone 1 Extension - Advantages

No signalling channel required (may be used

as temporary replacement for carrier aided

scheme when comms. channel out of service)

Provides fast fault clearance at both ends for a

transient fault anywhere along the line length

Allows the use of high speed A/R cycle

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Zone 1 Extension Scheme - Disadvantages

Tripping can occur for external faults (but will be

followed by an autoreclose)

Basic distance scheme logic applies following

reclose (i.e. potential for time delayed clearancefor permanent faults)

Only suitable to systems where autoreclose isused (for example can not be used on cable

circuits)

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Loss of Load Accelerated Trip

Z3

Z3

Z1

Z1

Z2

Z2

T1 = 40ms (allows for slowest pickup of Zone 2)

T2 = 18ms (prevents LOL trip for external fault withCB pole scatter)

Z2

.

.&

.

.

.

&

&

.

1

.

LDA

LDB

LDC

0

T2

T1

0Trip

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Loss of Load Scheme

Fast fault clearance without the need for a

signalling channel

Only applicable where 3 phase tripping is used

Only operates for unbalanced faults

Load current (above the current detector

settings) must exist prior to the fault to arm thescheme

Can be used as back up to signal aided scheme

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Channel Dependant Schemes

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Unit Protection Schemes

Provide high speed clearance for all faults on line

(for example current differential)

Do not provide inherent back-up protection

SOLUTION IS TO PROVIDE DISTANCE PROTECTION FOR

BACK-UP PROTECTION OF ADJACENT LINES WITH AN AIDED

TRIPPING SCHEME FOR HIGH SPEED PROTECTION OF WHOLE

LINE

Requires the use of an ON/OFF signalling

channel between line ends (i.e. HF/VF/Fibre

Optic/Radio)

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Types of Aided Tripping Schemes

Acceleration

Transfer tripping

Direct

Permissive Underreach

Permissive Overreach

Blocking

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Direct Transfer Trip

Z3

Z3

Z1

Z1

Z2

Z2

Z3

Z2

Z1 1

.T2

T3

Trip

Z3

Z2

Z11

. T2

T3

Trip

Tx

Rx

Tx

Rx

Send Logic : Z1

Trip Logic : Rx

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Direct Transfer Trip - Advantages

All faults anywhere along the protected line can

be cleared instantaneously at both line ends

Scheme can be advantageous for protecting 3

terminal lines due to ease of application

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Direct Transfer Trip - Disadvantages

A very secure signalling channel is required :-

incorrect operation leads to false tripping

Circuit breakers at both line ends must be closed

and contribute fault current to obtain high speedfault clearance

If the channel fails only the Basic scheme logicwill be provided

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Acceleration Scheme (for Reach Stepped Relay)

Z3

Z3

Z1

Z1

Z2

Z2

Change

Z1

reach

Z3

Z1/Z2 1

.

T2

Trip

T3

1

. T3

T2

Trip

Tx

Rx

Tx

Rx

1

.

1

.

Send Logic : Z1Trip Logic : Rx + Z2

Z1/Z2

Z3

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Permissive Schemes

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Permissive Underreach Scheme

Z3

Z3

Z1

Z1

Z2

Z2

Tx

Rx

Tx

Rx

Send Logic : Z1

Trip Logic : Rx + Z2

Z3

Z2

Z1 1

.T2

T3

Trip

T2

&

1

. T2

Z3

Z2

Z1

T3

Trip

T2

&

100

0100

0

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Permissive Underreach Scheme

A

21

C

B

D

A

C

B

D

Send

Fault

Fault

Race between relay at D

picking up and signal

send from relay at C

resetting, following

opening of breaker at C

If signal send from Cresets before relay D

operates then aided

tripping will not occur

To prevent this a 100ms

delay on drop off of the

signal send is used in

the scheme logic

21

21 21

Rx + Z2

Rx + Z2

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Permissive Underreach Transfer Trip - Advantages

Only a simplex signalling channel required

Scheme is very secure as signalling channelonly keyed for internal fault (Zone 1 initiation)

Permissive Underreach Transfer Trip -

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Permissive Underreach Transfer Trip -Disadvantages

If one terminal of the line is open then only

Basic scheme logic will apply

If there is a weak infeed at one terminal then

only Basic scheme logic will apply

If signalling channel fails then only Basic

scheme logic will apply

Resistive coverage is governed by Zone 1

setting (may be limited on short lines)

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Permissive Overreach Scheme Internal Fault

Z3

Z3

Z2

Z1 1

.T2

T3

Trip

&

T2

Z3

Z2

Z11

. T2

T3

Trip

&

T2

Rx

Tx

Rx

Tx

Z3

Z1

Z1

Z2

Z2

Send Logic : Z2


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Permissive Overreach Scheme External Fault

Z3

Z3

Z2

Z1 1

.T2

T3

Trip

&

T2

Z3

Z2

Z11

. T2

T3

Trip

&

T2

Rx

Tx

Rx

Tx

Z3

Z1

Z1

Z2

Z2

Send Logic : Z2


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Permissive Overreach Scheme (CB Echo Logic)

Z3

Z3

Z2

Z1

T2

T3

Trip

&

T2

Z3

Z2

Z1

T2

T3

Trip

&

T2

&

1

Rx

Tx

CB open &

1

Rx

Tx

CB open

Z3

Z1

Z1

Z2

Z2

1

.

1

.

Send Logic : Z2

Trip Logic : Rx + Z2Open terminal echo : CB Open + Rx

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Permissive Overreach Scheme (WI Echo Logic)

Z4

Z2

Z1

Z1Z2

Send Logic : Z2

Trip Logic : Rx + Z2Open terminal echo : CB Open + Rx

Weak Infeed echo : Z4 + RxZ4

Z3

Z2

Z1

T2

T3

Trip

T2

Z3

Z2

Z1

T2

T3

Trip

T2

&

Z4

CB open &

Z4

CB open

1

& &

&Rx

Tx &Rx

Tx

1

.

1

.

1

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Permissive Overreach Weak Infeed Trip Scheme

Z4

Z2

Send Logic : Z2


Open terminal echo : CB Open + Rx

Weak Infeed echo : Z4 + Rx

Weak Infeed trip : Z4 + LDOV reset + Rx

Z1

Z1Z2

Z4

Z3

Z2

Z1

T3

Trip

&

T2

&LDOV

Z3

Z2

Z1

T3

Trip

&

T2

&

&Rx

TxZ4

CB open &Rx

Tx

LDOV

Z4

CB open

& &

1

.

1

.

1 1

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Permissive Overreach Transfer Trip - Advantages

Provides better resistive coverage, especially

on short lines, where MHO measuringelements are used

For cases where one line terminal is open,open breaker echo logic can be used

For cases of weak or zero infeed at one lineterminal weak infeed logic can be used

(reverse looking zone required)

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Permissive Overreach Transfer Trip - Disadvantages

Duplex signalling channel required

Scheme is theoretically less secure then PUR

as signalling channel is keyed for external

faults

If signalling channel fails then only Basic

scheme logic will apply

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Blocking Schemes

Bl ki S h I t l F lt

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Blocking Scheme - Internal Fault

Z3

Z2

Z3

Z2

Z1

T3

Trip

&

T2

&Rx

Tx

1

.

Z1

Z1Z2

Send Logic : Z3 + Z2


Z3

Z3

Z2

Z1

T2

T3

Trip

&

T2

&Rx

Tx

1

.

Bl ki S h E t l F lt

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Blocking Scheme - External Fault

Z3

Z2

Z3

Z2

Z1

T3

Trip

&

T2

&Rx

Tx

1

.

Z1

Z1Z2

Send Logic : Z3 + Z2


Z3

Z1

Z2

Z3

T2

T3

Trip

&

T2

&Rx

Tx

1

.

Bl ki S h Ad t

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Blocking Scheme - Advantages

Only simplex signalling channel required

Provides better resistive coverage than PUR onshort lines where MHO elements are used

Fast tripping will still be possible at closed end ofline for all fault positions with remote breaker

open

Fast tripping will still be possible at strong infeed

terminal for all fault positions where remote

terminal has no or weak infeed

Bl ki S h Di d t

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Blocking Scheme - Disadvantages

Only 2 forward zones of protection available(unless relay has >3 Zones)

If signalling channel fails then only Basic schemelogic will apply

Current sensitivity is lower as tripping elements(Z2) are controlled by high set current level

detectors (to ensure blocking elements (Z3/Z4)

are more sensitive than tripping elements)

Permissive Schemes vs Blocking Schemes

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Permissive Schemes vs Blocking Schemes

Permissive less reliable - require a signal from

remote relay plus local operation to trip

Blocking less secure - require a signal from

remote relay to prevent a trip

Permissive schemes are marginally faster and

more sensitive (timer plus high set currentelements on Blocking scheme)

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Teed Feeders

Teed Feeders No Zone 1 Coverage

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Teed Feeders - No Zone 1 Coverage

B

AC'A'B'

B'

A'

C

A-A - Zone 1 reach of relay at A

B-B - Zone 1 reach of relay at B

C-C - Zone 1 reach of relay at C

ABC Zone not covered by Zone 1 from any terminal.

Hence schemes reliant on Zone 1 operation would not

work (Z1 ext, PUR)

Teed Feeders Underreaching

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Teed Feeders - Underreaching

B

A CZc

Zb

Za

Ia Ic

Actual impedance to fault from terminal A = Za + Zb

Measured voltage at terminal A V = Ia.Za + (Ia + Ic).Zb

Measured current at terminal A I = Ia

Impedance measured by relay A V/I = Za + Zb + (Ic/Ia).Zb

Relay therefore measures a greater impedance than the actual impedance

and underreaches. Must allow for this underreach when setting Zone 2

elements to ensure correct scheme operation

Teed Feeders - No/Weak Infeed at One Terminal

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Teed Feeders No/Weak Infeed at One Terminal

B

A C

No infeed from terminal B.

Fault therefore not covered by Zone 1 from any

terminal. Hence schemes reliant on Zone 1

operation would not work (Z1 ext, PUR)

Teed Feeders - Problems (1)

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Teed Feeders - Problems (1)

B

A C

Relay at terminal B sees a reverse fault. Noscheme will operate at this terminal. Fault

clearance will be sequential following opening of

breaker A from relay A Zone 1 element.

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Directional Earth Fault (DEF)

Directional Earth Fault Schemes

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DEF schemes are identical to Distance

schemes

DEF Forward replaces Zone 2, DEF Reverse

replaces Zone 3/4 Reverse No equivalent to Zone 1 as the DEF elements

can not have a defined reach, hence no

schemes using Zone 1 can be replicated (Z1ext, PUR)


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21

Ea E

b

67N

21

RF

Independent

signallingchannel67N

Independent signalling channels allows the use of

different schemes for the distance and DEF

elements, for example PUR distance with POR DEF


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21

EaEb

67N

21

RF

Shared Channel

67N

Shared signalling channels limits the use of

schemes for the distance and DEF elements.

Both use the same scheme logic.

10 dp schemes

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