lineprotection basics june2008

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Electrical faults in the power system

� Transmission lines 85%

� Busbar 12%

� Transformer/ Generator 3%

100%

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Fault types

� Transient faults� are common on transmission lines, approximately 80-85%

� lightning are the most common reason

� can also be caused by birds, falling trees, swinging lines etc.

� will disappear after a short dead interval

� Persistent faults� can be caused by a broken conductor fallen down

� can be a tree falling on a line

� must be located and repaired before normal service

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Fault types on double circuit lines

� Simultaneous and Interline faults� On parallel line applications a problem

can occur with simultaneous faults.

� A full scheme relay is superior when the protection is measuring two different fault types at the same time.

L1

L3

L3

L1

L2

L2

~~ Z<

L2-N

L1-N

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Main requirements on line protection are:

• SPEED• SENSITIVITY• SELECTIVITY• DEPENDABILITY• SECURITY

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Measuring principles

� Over current protection

� Differential protection

� Phase comparison

� Distance protection

� Directional- wave protection

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Over current protection

� Are normally used in radial networks with system voltage below 70 kV where relatively long operating time is acceptable.

� On transmission lines directional or non-directional over current relays are used as back-up protections.

I > I > I >

I >

block

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Dependent Time Overcurrent Relays

CHARACTERISTICS OFDEPENDENT TIME OVERCURRENT RELAYS

0.1

1.0

10.0

100.0

1 10 100

Current (multiple of setting)

Ope

rate

Tim

e [s

] Long T ime Inverse

Extremely Inverse

Normal Inverse

Very Inverse

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Two or Three Phase Over current Relays

� Two phase over current relays and one residual over current relay give complete protection on power lines and cables

� A third phase relay provides back-up protection

� In case of a D/Y-connected transformer, the fault current in one phase may be twice that in the other two phases and it may be necessary to provide three phase over current relays

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Directional Over current Relays

� Relays on radial lines do not need directional element

� Directional elements are useful on parallel lines, on looped lines, and in meshed networks

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Directional Residual Overcurrent Relays

Reverseoperation

Forwardoperation

Upol-3U03I0D

0.6 3I0Dx

3I0 >

φ φ φ φ = the characteristic angle of zerosequence source impedance

φ=65φ=65φ=65φ=65

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Directional Residual Overcurrent Relays

� Residual voltage polarization requires a sensitive directional element

� Third harmonics in voltage must not cause incorrect operation of the directional element

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Pilot wire differential protection� Pilot wires can be in soil or on towers.

� The resistance in the wires will limit the use on longer lines. The use is mostly restricted to distances up to 10 km.

� High sensitivity

� Can be used on short lines

� Very useful on series compensated lines

� Insensitive to power swing

� Weak source no problem

Why differential protection?

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Idiff = Delta Current = 0

Differential protection - operating principle

Idiff = Delta Current > 0

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Digital differential communication

Digital communication with optical fibers or by multiplexed channels

L1L2L3

DL1

DL2

DL3

DL1

DL2

DL3

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Phase comparison

� Phase comparison relays compare the angle difference between the two currents at both ends of the line.

� The measured time for zero crossing is transmitted to the other end.

� Normally a start criteria is added to the phase angle requirement.

I1 I2

e1

e2

e2

e1-

φφφφ>

φφφφ>

I1 I2

load

I2

I2

I1func-tion

αααα

αααα

φφφφφφφφ

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Directional wave protection

� The basic principle of directional wave protection is to observe the polarities of the instantaneous change in voltage and current. Here by one can determine the direction of a fault with respect to the location of the measurement.

� Tripping is achieved when both protections detects a fault in forward direction.

~ ~

A B

F

I U

+ +

- -

- +

+ -

Trip

0

0

1

1

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� Why:

� Local current and voltage: No need for communication

� Fault on protected line: Reach independent of fault current level

� Impedance characteristics can be chosen with different reach for different impedance phase angles.

� Enables remote back-up protection

Application of distance protection

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The principle of distance protection

ZK=Uk/ Ik

Uk=0Uk

IkZ<A B

metallic faultZk

The impedance is proportional to the distance!

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The principle of distance protection• Power lines have impedances of 0,3- 0,4 ohm/ km

and normal angles of 80 - 85 degrees in a 50Hz systems.

• The line impedance may have to be converted to secondary values with the formula:

A

Z<

B

Z<

ZL=R+jX

Zsec=VTsec

VTprim CTsecCTprim Zprimx x

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Fault resistance

� multi-phase faults� consists only of arc resistance

� earth faults� consists of arc and tower

footing resistance

L1

L3

L3

L1

L2

L2

Footing resistanceRarc =

28707 x L 1.4I

Warrington´s formula

L= length of arc in meters

I= the actual fault current in A

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A

Z<

B

Z<

C

t1

t2t3

Distance protection has different functional zones with different impedance reaches

With a combination of distance reach setting and functional delay for each zone selectivity is relatively easy to achieve.

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A B C

t1

t2t3

t1

t2t3

Z< Z< Z< Z<


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A B C

f1 f2

t1

t2t3

t1

t2t3

Z< Z<


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A B C

f3

t1

t2t3

t1

t2t3

Z< Z< Z< Z<


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Design of distance protection� Switched scheme

� consists of a start relay which detects the type of fault and select (switch) the measuring loop to the single measuring relay. The relevant loop voltages and currents are switched to the measuring unit.

� Full scheme

� has a measuring element for each measuring loop and for each zone

~~Z<

L2-N

L1-N

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Requirements on Distance relay Zones

� Zone-1 � Must not overreach

� Zone-2 � Must overreach

� Must co-ordinate with next section

� Provides back-up for the next busbar

� Provides back-up for the first part of next line

� Zone-3 � Can provide back-up for next line

� Can provide back-up for next busbar

� In feed of fault current at the remote busbar affects the effective reach of the overreaching zones

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Measuring loop for earth faults

� The distance protection relays are always set based on the phase impedance to the fault

Zs RL XL

RN XN

The measured Impedance is a function of positive and zero sequence impedance

IL1UL1

IN

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Measuring loop for two- phase faults

� The distance protection relays are always set based on the phase impedance to the fault

Zs RL XL

UL1-L2IL1IL2

The measured impedance is equal to the positive sequence impedance up to the fault location

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Measuring loop for three- phase faults

• The distance protection relays are always set based on the phase impedance to the fault

Zs RL XL

UL1IL1IL2

The measured impedance is equal to the positive sequence impedance up to the fault location

IL3UL2

UL3

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The earth fault measurement

U= I1Z1+I0Z0+I2Z2Z1=Z2

U= Z1( I1+I2+I0 ) +I0Z0 -I0Z1 I= I1+I2+I0

U=I Z1+I0 ( Z0 - Z1 ) 3I0=IN

U=IZ1+IN(Z0 - Z1

3 )U=I Z1+IN3

( Z0 - Z1 )

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The earth fault measurement

� The current used is thus the phase current plus the residual current times a factor KN = (Z0-Z1) / 3Z1, the zero sequence compensation factor.

� The factor KN is a transmission line constant and Z0/ Z1 is presumed to be identical throughout the whole line length.

� (1+KN) Z1 gives the total loop impedance for the earth fault loop for single end infeed.

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Measurement Loops

Fault Voltage Current

R-Earth VR IR + Kn ⋅3I0S-Earth VS IS + Kn ⋅3I0T-Earth VT IT + Kn ⋅3I0R - S VR - VS IR - ISS - T VS - VT IS - ITT - R VT - VR IT - IRR - S - T Any phase-earth voltage

any phase-phase voltageCorresponding phase currentCorresponding phase-phase current

R - S - T - Earth Any phase-earth voltageany phase-phase voltage

Corresponding phase currentCorresponding phase-phase current

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Directional measurement

� When a fault occurs close to the relay location the voltage can drop to a value where the directional measurement can not be performed.

� Modern distance protection relays will instead use the healthy voltage e.g. for L1- fault the voltage UL2-L3, shifted 90 degrees compared to UL1. This cross polarisation is used in different proportions between healthy and faulty phases in different products.

� At three- phase fault close to the station all phase voltages are low and cross polarisation is not of any use. Instead a memory voltage is used to secure correct measurement.

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Distance protection on short lines

� Distance protection with mho characteristic can not see an average fault resistance

RF

XF

jX

R

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Distance protection on short lines

� Quadrilateral characteristic improves sensitivity for higher RF/XFratio

� It still has some limitations:� the value of set RF/XF ratio is

limited to 5

jX

RXF

RF

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Distance protection on short lines� Overreaching permissive

schemes increase the sensitivity

� Weak infeed logic for very high fault resistance

� Independent underreachingzone 1 gives additional advantage

jX

R

RF

XF

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Distance protection on long lines� Load impedance limits the reach

in resistive direction

� High value of RF/XF ratio is generally not necessary

� Circular (mho) characteristic � Has no strictly defined reach

in resistive direction � Needs limitations in resistive

direction (blinder)

R

jX

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Double end infeed

I1 I2

UF RF

UF = RF ( I1 + I2 )

RF ( I1 + I2 )RF1=

I1

U1 U2

I Load

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Resistive fault, double end fed

ZSCA ZSCBk ZL (1-k) ZL

Rf

+EA-

+EB-

IA IB

VA

fA

BAL

A

AA R

III

ZkIV

Z ⋅++⋅==( ) fBAALA RIIIZkV ⋅++⋅⋅=

The fault has more or less fault resistance.If the fault is an arcing fault the fault resistance is normally very small.

The influence of the fault resistance depends on the fault current infeed from the remote line end.

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fA

BALA R

III

ZkZ ⋅++⋅=

The fault resistance seen by the distance protection can be increased compared to its real value.

fR fA

BA RI

II ⋅+

LZk ⋅

UNDERREACH!

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fA

BALA R

III

ZkZ ⋅++⋅=

There is a risk that zone 1 will trip for faults outside its border.

fR

fA

BA RI

II ⋅+

LZk ⋅

OVERREACH!

The apparent fault resistance can also get a phase shift, depending on the load conditions before the load.

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Compensation of overreach in Zone1 due to load

ph - E

R

XFault resistance reach influence

Zone 1 of the REL 5XX/REL 670 terminal has a compensation of the characteristic due to the overreach caused by the load current.In case of active power out from the station the characteristic isautomatically tilted. This is valid only for Ph-E loops.

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Remote faults

� Due to current contribution If2 and If3 in substation B, the distance protection in station A will measure a higher impedance than the "true" impedance to the fault.

� The relay will thus underreach and this means in practice it can be diffcult to get a remote back-up.

Z<

If1

If2

If3

If=If1+If2+If3

ZL

ZF

A B

Um

Um= If1 x ZL+ (If1+If2+If3) x ZF

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Zero- sequence mutual coupling on parallel lines

ZA< overreaching

ZB< underreaching

~ ZOMZL

ZL

~

ZA< ZB<

~

~

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Parallel line out of service and earthed at both ends

∆∆∼ ∼

∆Z = - ZLKOM • ZOM / ZOL

1 + KO•

= - 0.23 ZL

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Parallel line in Service

∆Z =

∼ ∆

DKOM

1 + KO• ZL

= 0.38 ZL

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Distance relay settings for parallel lines

� The influence of zero sequence coupling can be compensated in two different ways

� Different K factor for different Zones within same group setting parameters

� Different groups of setting parameters for different operating conditions

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Communication equipment� Power line carrier (PLC) equipment is based on a

capacitive connection of signals with frequency in range 50- 500 kHz on the power line.

� Radio link is a good and reliable communication equiment, but is rarely used due to the high cost.

� Optical fibres have the advantage to be insensitive to noise and can transmit a huge amount of information.

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Permissive schemes.

PermissivePermissive

UnderreachUnderreach

OverreachOverreach

permission to trip instantaneouslyto an overreaching zone.

The permission is sent byan Under reaching zone

The permission is sent byan Overreaching zone

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Permissive underreaching scheme

CS = ZM1

Trip = ZM1 + ZM2 *(T2 + CR) +ZM3 * T3

ZM2, T2

ZM1, T1

A B

ZM2, T2

ZM1, T1

Permission is sent byan Underreaching zone (ZM1)

Permission to trip instantaneouslyto an overreaching zone (ZM2).

If B has a weak source, it could not see the fault and fail to send the carrier to A.

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Permissive communication schemes

� Communication signal carrier send (CS) is sent to remote end when the fault is detected in forward direction. Tripping is achieved when the commmunication signal carrier receive (CR) is received and the local relay has detected a forward fault.

� In a permissive underreaching scheme the communication signal is sent from a zone that underreaches the remote end.

� In a permissive overreaching scheme the communication signal is sent from a zone that overreaches the remote end.

A

Z< Z<

B

Carrier send CS = Z< forward, under or overreach

Trip = ZM1 + ZM2 (t2 + CR) + ZM3 x t3

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Permissive Underreach Distance Protection

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Permissive overreaching scheme

CS = ZM2

Trip = ZM1 + ZM2 *(T2 + CR) +ZM3 * T3

A B

Permission is sent byan Overreaching zone (ZM 2)

Permission to trip instantaneouslyto an overreaching zone (ZM2).

The carrier is sent by both relays for faults on the whole line.

ZM2, T2

ZM1, T3

ZM3, T3

ZM2, T2

ZM1, T1

ZM3, T3

Good for weak-end infeed.Echo carrier signal is sent back from B if a carrier has been received but no fault detected in ZM1, ZM2 and ZM3.

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Permissive Overreach Distance Protection

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Permissive overreaching schemes are adopted for short lines( Also called directional comparison schemes)

Advantages are• Better performance for high resistance faults.• Superior to pilot wire as digital decisions areexchanged and not analogue• Superior to phase comparison which requiresfaithful transmission of phase information.

Permissive Overreach Distance Protection

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Blocking communication schemes

� Communication signal (CS) is sent to remote end when the fault is detected in the reverse direction. Tripping is achieved when this blocking signal is not received within a time T0 (20-40 ms) and the local relay has detected a fault in the forward direction.

A

Z< Z<

B

Carrier send CS = Z< reverse zone

Trip = ZM1 + ZM2 (t2 + CR x T0) + ZM3 x t3

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Blocking overreaching scheme

ZM2, T2

ZM1, T1

A B

Block signal is sent bythe reverse zone (Zone 3)

Overreaching inst. zone to beBlocked by a block signal).

• Carrier is sent when the line is healthy

• Good for short lines, where it is impossible to set 80-90% of the line length.

• Series compensated lines

ZM3, T3

CS = ZM3

Trip = ZM1 + ZM2 * TCR* CR+ (ZM3 * T3 + ZM2 * T2)

ZM2, T2

ZM1, T1

ZM3, T3

Waiting time for the block signal (tCoord)

Block signal.

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Blocking Overreach Distance Protection

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This function is based on condition

3Uo > 20 % of Un / √√√√ 3 and 3Io < 20 % of In

It can be selected to block protection and give alarmor just to give alarm.

Fuse fail supervision is blocked for 200ms following Line energisation in order not to operate for unequalpole closing and also during auto-reclosing.

MCB can also be used.

FUSE FAIL SUPERVISION

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Switch On To Fault (SOTF)� When energizing a power line onto a forgotten earthing, no

measuring voltage will be available and the directional measuring can thus not operate correctly.

� A special SOTF function is thus provided. Different principles can be used, from one phase current to non-directional impedance measuring.

Z<

U=0 V

SOTF condition can either be taken from the manual closing signal activating the (BC) input or it can be detected internally by a logic.

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� A power swing can start by sudden load change or due to a fault somewhere in the network.

� Close to the centre of the power swing, low voltage and thus low impedance will occur.

� A distance protection relay must then be blocked during the power swing.

� This can be done by measuring the transit time of the impedance locus passing two dedicated impedance zones.

� Normally the time used is 35-40 ms.

Power Swing Blocking function

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∆∆∆∆t

∆∆∆∆t = 40 ms

X

R

Power swing locus

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• When power swing detection unit operates any impedencezone can be selected to be blocked or not as required.

• Operation of power swing detection unit is inhibited when zero sequence current is detected. This feature is included to ensuretripping of high resistance earth faults where fault resistance can decrease slowly.

• The residual current inhibit condition ensure PSD will not block due to unbalanced load or residual current experienced with un-transposed transmission lines.


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Stub protection functionIt is not possible for the distance protection relay to measure impedance when the line disconnector is open. Not to risk incorrect operation the distance protection must be blocked and a Stub protection is released.

The Stub protection is a simple current relay.

line discopen

I STUB >

& trip25ms

Bus A

Bus B

> Z+

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Current reversal logic

~~A:1

B:1

A:2

B:2

~~ A:1

B:1

A:2

B:2

Permissive overreaching schemes can trip healthy line without C.R.L1 Fault occurs on line 1

Fault detection by protection A:1 B:1 and A:2

2 Relay B:1 trips CB and sends carrier to A:1

Relay A:2 sees fault in forward direction and sends carrier to B:2

3 Fault cleared at B:1, current direction changed on line 2

4 Carrier from A:2 and forward looking measuring element in relay A:2 does not reset before relay B:2 detects the fault in forward direction and trips, also relay A:1 will trip when receiving carrier from B:1

C.R.L allows slowly resetting communication equipment without risking to tripping the healthy line.

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Simultaneous faults

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� On parallel line applications a problem can occur with simultaneous faults.

� A full scheme relay is superior when the protection is measuring two different fault types at the same time.

Simultaneous faults

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Weak end infeedWeak end infeed is a condition which can occur on a transmission line, either when the circuit breaker is open, so there is no current infeedfrom that line end, or when the current infeed is low due to weak generation behind the protection.

lt1

t2t3

CS = ZM2

TRIP = ZM1 + ZM2(CR + t2)

CS (echo)=CR x low voltage x no start forward or reverse

Z< Z<CS

CS (echo)

CR

CR

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∼∼∼∼∼∼∼∼

∼∼∼∼-+

L

FA B

IA IF

IB

ZA ZB

RF

pZL ( I - p )ZL

pZL ( 1- p ) ZL

ZA ZB

Fault Locator Measuring Principle

UA=IA X P ZL + IFA X RF

DA

DA = (I-P) ZL +ZB

ZA+ZL +ZB

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Series compensated system

• Correct direction discrimination at voltage reversal (negative fault reactance)

• variation in resulted line impedance

Consideration for line distance protections

BA

F1

X =70%C X =100%l

R

jX

AB

B´

70%

100%

gap not flashed

gap flashed

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(i) Zone-I: to be set to cover 80-85% of protected line length

(ii) Zone II: to be set to cover minimum 120% of length of principle line section. However, in case of D/C lines 150% coverage must be provided to take care of, under reaching due to mutual coupling effect but, care is to be taken that it does not reach into next lower voltage level.

3.0 SETTING CRITERIA3.0 SETTING CRITERIA

3.1 Reach settings of distance protection3.1 Reach settings of distance protection

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(iii) Zone-III:

For 400kV lines Zone-III to be set to cover 120% of principle sectionplus adjacent longest section subject to a reach restriction so that it does not reach into next lower voltage level.

For 220 kV lines, Zone-III reach may be provided to cover adjacentlongest section if there is no provision of LBB or all protection are connected to single DC source at remote end substation.

(iv) Resistive reach should be set to give maximum coverage subject to check of possibility against load point encroachment considering minimum expected voltage and maximum load. Attention has to begiven to any limitations indicated by manufacturer in respect of resistive setting vis-a-vis reactance setting.

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• A Zone-II timing of 0.3 second is recommended. If a long line is follow-ed by a short line, then a higher setting may be adopted on long line to avoid indiscriminate tripping through Zone-II operation on both lines.

• Zone-III timer should be set so as to provide discrimination with theoperating time of relays provided in subsequent sections with which Zone-III reach of relay being set overlaps.

3.2 Time setting of distance protection3.2 Time setting of distance protection

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3.3.1 • Low set voltage may be set at 110% with a typical time delay of 5

seconds.

• A time grading of 1 second may be provided between relays of different lines at a station.

• Longest time delay should be checked with expected operating timeof overfluxing relay of the transformer to ensure disconnection of line before tripping of transformer.

3.3.2

• High set stage may be set at 150% with a time delay of 100 m second.

3.3 O / V Protection3.3 O / V Protection

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• Decisions pertaining to allowing which Zone to trip and which to blockshould be taken based on system studies on case to case basis.

3.4 Power Swing Blocking Function Associated with Distance Relays3.4 Power Swing Blocking Function Associated with Distance Relays

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Zone-1

� Offers instantaneous circuit-local back-up protection for nearby faults, but not for the entire transmission circuit from both terminals.

� Set to under reach protected circuit to ensure external security

� In case of parallel circuit may be necessary to increase degree of under reach

� For multi circuit lines reach reduces further due to in feeds

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� Zone-2 Reach Setting Criteria

� Should overreach all terminals of the protected circuit by an acceptable margin (typically 20% of highest impedance seen) for all fault conditions and for all intended modes of system operation.

� As far as possible, should be less than Zone-1 coverage of all adjacent lines, to minimize the required Zone-2 time delay setting.

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Zone-2

� Zone-2 time Setting Criteria� Must be set to coordinate with clearance of adjacent circuit faults,

within reach, by the intended main protection or by breaker failprotection

tZ2 = Required Zone-2 time delay

tMA = Operating time of slowest adjacent circuit main protection or Circuit Local back-up for faults within Zone-2 reach

tCB = Associated adjacent circuit breaker clearance time

tZ2reset= Resetting time of Zone-2 impedance element with load current Present

tS = Safety margin for tolerance (e.g. 100ms)

sresetzCBMAz ttttt +++> 22

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Zone-2

� Effect of parallel lines� Where common impedance settings exist for

phase and ground fault impedance elements, or where independent residual compensation settings are not available for each zone of protection the phase fault Zone-2 reach will unavoidably be extended in order to satisfy ground fault reach requirements.

� This can create Zone-2 back-up co-ordination difficulties, particularly where adjacent sections or transmission circuit are of unequal length

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Zone-2

� Multi terminal circuits� The primary Zone-2 setting criterion must be met with

allowance for the highest apparent impedance seen for a fault at any remote circuit terminal.

� The Zone-2 reach setting may reach very high percentage of the circuit impedance between the closest terminals. The reach may need to be further enhanced to address under reach for ground faults when protecting parallel multi-terminal circuits.

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Zone-2

� Load encroachment� One problem with mho impedance elements is that the

fault resistance coverage varies with the forward reach setting. When applying Zone-1 elements to short lines, fault resistance coverage may be insufficient. It can also be disadvantage for Zone–2 elements to be set with unusually high forward reach setting in relation to the minimum load impedance. It may be possible for the minimum load impedance to encroach upon the Zone-2 operating region.

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Zone-3 Remote back-up

� General� Usually set to provide remote back-up protection for

adjacent sections of a transmission circuit.

� May have independently adjustable forward and reverse reach setting

� Usually forward reach provides remote back-up protection.

� With duplicate main protection, there may be a case for not applying Zone-3 remote back-up protection at all.

� In case of long 400kV lines it may be desirable either to reduce the reach or to block 3rd zone of distance relay for reasons of security.

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Zone-3 remote back-up

� Reach setting� Zone –3 should overreach the remote terminal of the

longest adjacent line by an acceptable margin (typically 20% of highest impedance seen) for all fault conditions and in feed conditions associated with all intended modes of system operation

� Zone-3 reach should be less than the Zone-2 protection coverage of the shortest adjacent transmission circuit and it should not see through power transformers into distribution systems, in order to minimize the required zone-3 time delay setting.

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Zone-3 remote back-up� Time setting

� Must be set to co-ordinate with clearance of faults by adjacent circuit-local back-up protection. Zone-2 distance protection or time delayed over current protection

� The following formula would be the basis for determining the minimum acceptable Zone-3 time setting:

Where:

tZ3 = Required Zone-3 time delay

tMA = Operating time of slowest adjacent circuit local back-up protection

tCB = Associated adjacent circuit breaker clearance time

tZ3reset = Resetting time of Zone-3 impedance element with load current present

tS = Safety margin for tolerance (e.g. 100ms)

sresetzCBMAz ttttt +++> 33

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� Consideration of mutual coupling� As for Zone-2 protection the under reaching effect

of zero sequence mutual coupling for remote ground faults must also be considered.

� Such consideration is quite complex, since there may well be differences in the levels of mutual coupling for the protected circuit and any number of adjacent circuits.

� In addition, some circuit sections may be multi-circuit while other sections may not be.

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� Considerations for intervening fault currents� The under reaching effects are encountered in relation to

adjacent circuit impedance, when applying Zone-3 remote back-up protection.

� These under-reaching effects are particularly difficult to address, since they are variable according to the type of fault.

� Ground faults can invoke additional zero sequence current in feed from transformers with grounded star-connected primary windings and other delta-connected windings.

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Zone-3 remote back-up � Zone-3 load impedance encroachment

� Encroachment due to the minimum load impedance under expected modes of system operation and the minimum impedance that might be sustained for seconds or minutes during abnormal or emergencysystem situations.

� Use of blinders in case of Mho type of elements or by use of polygon type impedance elements.

� In case of long 400kV transmission lines it may be desirable to limit the reach of Zone-3 for reasons of security. In such cases, if the adjacent station has bus bar protection and breaker failure protection, Zone-3 can be dispensed with.

� ESKOM (South Africa) have a practice of not using third zone impedance protection in long line applications . They use enhanced local breaker back-up .

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Zone- 4 substation local back-up

� An additional zone of reverse-looking protection (e.g. Zone-4) to offer substation-local back-up protection.

� The Zone-4 reverse reach must adequately cover expected levels of apparent bus bar fault resistance, when allowing for multiple in feeds from other circuits.

� Sometimes when separate reverse looking element is not available the above is achieved by offset reach of Zone-3 of distance relay.

lineprotection basics june2008

Engineering