Distance Protection Schemes

Distance Protection Schemes

GRID Technical Institute

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Basic Distance Scheme

Z3 Z2

Z1

Z1 Z2 Z3

Z1

1

Z2

T2

Z3

T3

.

Trip

Trip

1 .

Z1 T2

Z2

T3

Z3

Basic Distance Scheme - Disadvantages





 



Zone 1 set to 80% leaves 2 end zones Faults in end zone results in instantaneous tripping at one end and time delayed at the other Time delay may lead to system stability problems Sequential clearance leaves no dead time for high speed A/R cycle (transient fault becomes permanent) Longer clearance times - More damage

Zone 1 Extension Scheme Z3 Z2 Z1

Z1X

Z1X Z1 Z2 Z3

A/R Z1X

&

1

1

Z2

T2

.

Z3

T3

Z1

A/R Z1X

&

Trip

Trip

1

1

.

T2

Z2

T3

Z3

Z1

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

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 clearance for permanent faults)



Only suitable to systems where autoreclose is used (for example can not be used on cable circuits)

Loss of Load Accelerated Trip

Z3 Z2 Z1

Z1 Z2 Z3

T1 = 40ms (allows for slowest pickup of Zone 2) T2 = 18ms (prevents LOL trip for external fault with CB pole scatter) LDA LDB LDC

& .

0 T1

1 .

.. & .. .

T2 0

Z2

&

Trip

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’ the scheme  Can be used as back up to signal aided scheme

Channel Dependant Schemes

Unit Protection Schemes 

Provide high speed clearance for all faults on line (for example current differential)



Does not provide inherent back-up protection SOLUTION IS TO PROVIDE DISTANCE PROTECTION FOR BACKUP 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)

Types of Aided Tripping Schemes



Acceleration



Transfer tripping – Direct – Permissive Underreach – Permissive Overreach



Blocking

Direct Transfer Trip

Z3 Z2

Z1

Send Logic : Z1 Trip Logic : Rx

Z1 Z2

Tx Rx

Z3

Z1

1

Z2

T2

Z3

T3

.

Trip

Tx Rx

Trip

1 .

Z1 T2

Z2

T3

Z3

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

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 speed fault clearance



If the channel fails only the Basic scheme logic will be provided

Acceleration Scheme (for Reach Stepped Relay)

Z3 Z2

Z1

Send Logic : Z1 Trip Logic : Rx + Z2

Z1 Z2 Tx Rx

Z3

Z1/Z2 Z3

1 T3

Trip

Trip

.

1 .

. 1

T2

Tx Rx

Change Z1 reach

Z1/Z2 T3

1 .

T2

Z3

Permissive Schemes

Permissive Underreach Scheme

Z3 Z2 Z1

Send Logic : Z1 Trip Logic : Rx + Z2

Z1 Z2 Z3

Tx Rx

0

Tx Rx

0 100

100

& Z1

& 1

Z2

T2

Z3

T3

.

Trip

Trip

Z1

1 .

T2

Z2

T3

Z3

Permissive Underreach Scheme

A

B

C

D

 Race between relay at D picking up and signal send from relay at C resetting, following opening of breaker at C

Fault

21

21

Send

Rx + Z2

A

B

C

D Fault

21

21 Rx + Z2

 If signal send from C resets 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

Permissive Underreach Transfer Trip - Advantages



Only a simplex signalling channel required



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

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 for MHO)

Permissive Overreach Scheme Internal Fault Z3

Z2 Z1

Send Logic : Z2 Trip Logic : Rx + Z2

Z1 Z2 Z3

Rx Tx

Rx Tx

&

& 1

Z1 Z2

T2

Z3

T3

.

Trip

Trip

1

.

Z1 T2

Z2

T3

Z3

Permissive Overreach Scheme External Fault Z3

Z2 Z1

Send Logic : Z2 Trip Logic : Rx + Z2

Z1 Z2 Z3

Rx Tx

Rx Tx

&

& 1

Z1 Z2

T2

Z3

T3

.

Trip

Trip

1

.

Z1 T2

Z2

T3

Z3

Permissive Overreach Scheme (CB Echo Logic) Z3

Z2 Z1

Send Logic : Z2 Trip Logic : Rx + Z2 Open terminal echo : CB Open + Rx

Z1 Z2 Z3

CB open

&

& Rx Tx

1

Rx Tx

1

&

& 1

Z1 Z2

T2

Z3

T3

CB open

.

Trip

Trip

1

.

Z1 T2

Z2

T3

Z3

Permissive Overreach Scheme (WI Echo Logic) Z4 Z2 Z1

Send Logic : Z2 Trip Logic : Rx + Z2 Open terminal echo : CB Open + Rx Weak Infeed echo : Z4 + Rx

Z1

Z2 Z4

CB open

&

Z4

&

Rx Tx

1

Rx Tx

1

&

Z2

T2

Z3

T3

CB open

&

Z4

& 1

Z1

&

.

Trip

Trip

1

.

Z1 T2

Z2

T3

Z3

Permissive Overreach Weak Infeed Trip Scheme Z4 Z2 Z1

Send Logic : Z2 Trip Logic : Rx + Z2 Open terminal echo : CB Open + Rx Weak Infeed echo : Z4 + Rx Weak Infeed trip : Z4 + LDOV reset + Rx

Z1

Z2 Z4

CB open

&

&

CB open

Z4

&

&

Z4

LDOV

&

&

LDOV

&

&

Rx Tx

1

1

Z1 Z2

T2

Z3

T3

.

Trip

Rx Tx

Trip

1

1

.

Z1 T2

Z2

T3

Z3

Permissive Overreach Transfer Trip - Advantages 

Provides better resistive coverage, especially on short lines, where MHO measuring elements 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 line terminal weak infeed logic can be used (reverse looking zone required)

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

Blocking Schemes

Blocking Scheme - Internal Fault Z3 Z2 Z1

Send Logic : Z3 + Z2 Trip Logic : Rx + Z2

Z1

Z2 Z3

&

& Rx Tx

Rx Tx

&

& 1

Z1 Z2

T2

Z3

T3

.

Trip

Trip

1 .

Z1 T2

Z2

T3

Z3

Blocking Scheme - External Fault Z3 Z2 Z1

Send Logic : Z3 + Z2 Trip Logic : Rx + Z2

Z1

Z2 Z3

&

& Rx Tx

Rx Tx

&

& 1

Z1 Z2

T2

Z3

T3

.

Trip

Trip

1 .

Z1 T2

Z2

T3

Z3

Blocking Scheme - Advantages  Only simplex signalling channel required

 Provides better resistive coverage than PUR on short lines where MHO elements are used  Fast tripping will still be possible at closed end of line 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

Blocking Scheme - Disadvantages

 Historically only 2 forward zones of protection available (unless relay has >3 Zones)  If signalling channel fails supervision must revert the relay back to Basic scheme logic

 Historically 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  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 an accelerated trip  Permissive schemes are marginally faster and more sensitive (timer plus high set current elements on Blocking scheme)

Teed Feeders

Teed Feeders - No Zone 1 Coverage A

A'

B'

C'

C

B' A'

B

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 A’B’C’ 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 A

C Za

Ia

Zc

Zb

Ic

B

   

Actual impedance to fault from terminal A Measured voltage at terminal A Measured current at terminal A Impedance measured by relay A

V I V/I

= Za + Zb = Ia.Za + (Ia + Ic).Zb = Ia = 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 A

C

B

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) A

C

B

Relay at terminal B sees a reverse fault. No scheme will operate at this terminal. Fault clearance will be sequential following opening of breaker A from relay A Zone 1 element.

Directional Earth Fault (DEF)

Directional Earth Fault Schemes

 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 (Z1 ext, PUR)

Directional Earth Fault Schemes

Ea

Eb

RF 21 67N

Independent signalling channel

21 67N

Independent signalling channels allows the use of different schemes for the distance and DEF elements, for example PUR distance with POR DEF

Directional Earth Fault Schemes

Ea

Eb

RF 21 67N

Shared Channel

21 67N

Shared signalling channels limits the use of schemes for the distance and DEF elements. Both use the same scheme logic.