08-Presentation Distance Protection

Distance Protection

J. Royle

> Distance Protection - January 2004

Distance Protection

X Popular, widely used on Sub-Transmission and Transmission Systems X Virtually independent of Fault Current Level (ZS/ZL ratios) X Fast Discriminative Protection:- Zone 1 or ‘Aided’ Distance Scheme X Time Delayed Remote Back-Up:- Incorporated at little extra cost

> Distance Protection - January 2004

Advantages of Distance Protection

X Measures Z, X or R correctly irrespective of System Conditions

X Compare this with Instantaneous Overcurrent Protection:-

> Distance Protection - January 2004

Advantages of Distance Protection

ZS = 10

ZS = 10

Ω ZL = 4

Ω

Ω

IF1 115kV

50

IF1 = 115kV/√3(5+4) = 7380A ∴ Is > 7380A

> Distance Protection - January 2004

F1

Advantages of Distance Protection X Consider with one source out of service:ZS = 10

Ω F2 IF2

50

IF2 = 115kV/√3 x 10 = 6640A ∴ Is

7380A

> Distance Protection - January 2004

- IMPRACTICAL

Simplified Line Diagram

L

R

L C

R

L C

XL = jWL XC = at FN (50Hz)

L

> Distance Protection - January 2004

R

L C

-j WC XC = large :-

R

R

Basic Principle of Distance Protection

ZS

ZL

Generation

IR Distance Relay

> Distance Protection - January 2004

21

VR

Impedance Seen By Measuring Element

jX ZL

R

> Distance Protection - January 2004

Basic Principle of Distance Protection

ZS

VS

Relay PT.

ZLOAD

VR

Impedance measured

> Distance Protection - January 2004

ZL

IR

ZR =

Normal Load

VR = Z L + Z LOAD ΙR

Basic Principle of Distance Protection ZL ZS

VS

IR

ZF

VR

ZLOAD

Fault

X Impedance Measured ZR = VR/IR = ZF X Relay Operates if ZF < Z

where Z = setting

X Increasing VR has a Restraining Effect ∴VR called Restraining Voltage X Increasing IR has an Operating Effect > Distance Protection - January 2004

Plain Impedance Characteristic

jX

ZL

Impedance Seen At Measuring Location For Line Faults

R TRIP

> Distance Protection - January 2004

STABLE

Impedance Characteristic Generation

IF

jIX

zF

IZ VF

V3

V1

V2

IR Trip

TRIP

STABLE

Spring

Restrain

Ampere Turns :

Operate VF

IZ

Trip Conditions : VF < IFZ

Voltage to Relay = Current to Relay = Replica Impedance =

V I Z

Trip Condition :

S2 < S1

where : S1 = IZ ≈ Z S2 = V ≈ ZF

> Distance Protection - January 2004

Basic Principle of Distance Protection ZP

I1/I2 IR 21

VR

V1 V2

VFP

X Relays are calibrated in secondary ohms :VFP x V2 /V 1 VFP Ι1/ Ι 2 Z R = VR/ ΙR = = x V1/V 2 ΙFP x Ι 2 / Ι1 ΙFP ZR = Z P x > Distance Protection - January 2004

C.T. RATIO V.T. RATIO

Example

ZP = 4Ω; V1/V2 = 115kV/115V; I1/I2 = 600/5A

C.T. RATIO ZR = ZP x V.T. RATIO

ZR(5) = 4 x 600/5 x 115/115x103 = 0.48Ω -5A Relay ZR(1) = 2.4 Ω

> Distance Protection - January 2004

- 1A Relay

Input Quantities for ∅-∅ Faults

FAULT

VRESTRAINT

IOPERATE

A-B

VA - VB

IA - IB

B-C

VB - VC

IB - IC

C-A

VC - VA

IC - IA

X VRESTRAINT & IOPERATE are selected inside the relay X No setting adjustments are required apart from Z1 = Phase Replica Impedance

> Distance Protection - January 2004

Input Quantities for Phase to Earth Faults

FAULT

VRESTRAINT

IOPERATE

A-E

VA ?

IA ?

B-E

C-E

> Distance Protection - January 2004

Neutral Impedance Replica Vectorial Compensation

Replica impedance circuit :IRA

Z1

∑IZN Z1 Z1

Z1 = Phase replica impedance ZN = Neutral replica impedance IRA passes through Z1 IRN passes through ZN

N

IRN

ZN

> Distance Protection - January 2004

ZT = Z1 + ZN

Neutral Impedance Compensation For a single phase to ground fault the total earth loop impedance is given by :- (Z1 + Z2 + Z0)/3 = ZT ZT = (Z1 + Z2 + Z0)/3 = Z1 + ZN ZN = (Z1 + Z2 + Z0)/3 - Z1 = (2Z1 + Z0)/3 - Z1 = - Z1 + 3 = KN Z1

Z0 3

where KN = (Z0 - Z1) 3Z1 > Distance Protection - January 2004

Neutral Impedance Vectorial Replica Compensation Line CT’s A ZPH

IAZPH

ZPH

IBZPH

ZPH

ICZPH

ZN

INZN

B

C

Set Z PH = Z F1 Set Z N = (Z F0 - Z F1 ) 3 Usually ∠ Z N = ∠ Z PH for OHL’s > Distance Protection - January 2004

Neutral Impedance Replica Compensation

For cables ∠Z0 ≠ ∠Z1 ∴ VECTORIAL COMPENSATION MUST BE USED KN = Z0 - Z1 = ⏐KN⏐ ∠∅N 3Z1

> Distance Protection - January 2004

Characteristics

> Distance Protection - January 2004

Distance Characteristics

jX

jX

jX

jX Zn

Zn

R MHO

Zn

R

Zn R Zn′

Zs

IMPEDANC E

CROSSPOLARISED MHO

R OFFSE T MHO

jX Zn

Zn

Zn R

LENTICULA R

> Distance Protection - January 2004

R

R QUADRILATERAL

POLYGON

Self Polarised Mho Relays

jX

X Very popular characteristic X Simple

RESTRAIN

X Less sensitive to power swings

Z

X Inherently directional X Operates for F1, but not for F2

OPERATE

X Mho = 1/OHM

F1 ϕ

Settings :Z = reach setting ϕ = characteristic angle > Distance Protection - January 2004

F2

R

Neutral Impedance Replica Vectorial Compensation Vectorial compensation allows for ∠ZN ≠ ∠ZPH which is especially important for cable distance protection where ∠ZN < ∠ZPH and ∠ZN is sometimes negative. jX ZPH

ZN ZE

ZE = R

> Distance Protection - January 2004

Earth-loop impedance for ∅ - earth fault on a cable

Offset Mho Characteristic

jX Z

X Normally used as backup protection

R -Z’

> Distance Protection - January 2004

X Operates for zero faults (close up faults) X Generally time delayed (as not discriminative)

Mho Relays

Directional circular characteristic obtained by introducing VPOLARISING X VF → self polarised X VSOUND PHASE → fully cross-polarised X VF + xVS.F. → partially cross-polarised X VPRE-FAULT → ‘memory’ polarised Purpose for this is to ensure operation for close up faults where measured fault voltage collapses

> Distance Protection - January 2004

Quadrilateral Characteristic

jX ZL Z1

Load RF RS

> Distance Protection - January 2004

R

Lenticular Load Avoidance Characteristic

jIX

Lenticular characteristic created from two offset Mho comparators Aspect ratio = a/b b

a

IR

> Distance Protection - January 2004

Lenticular Characteristic

X

Z3 a

Aspect ratios a/b 0.41 0.67 1.00

b Load impedance area R Z3 reverse

> Distance Protection - January 2004

Forward Offset Characteristic Z3

X Rf Z2

Forward blinder Z1 Load area R

X Enhanced resistive coverage for remote faults > Distance Protection - January 2004

Zones of Protection

> Distance Protection - January 2004

Zones of Protection Time

Z3A

T3

Z3C

Z2A

T2

Z2C

Z1A A

Z1B

Z1C B

C

D

T2 Z2B

Z1A = 80% of ZAB Z2A = 120% of ZAB Z3A(FORWARD) = 120% of {ZAB + ZCD} > Distance Protection - January 2004

Zones of Protection jX

Z3A

D

C B

Z2A Z1A

A

> Distance Protection - January 2004

R

Zone 1 X FAST OPERATION Trips circuit breaker without delay as soon as fault within Zone 1 reach is detected. X REACH SETTING Cannot be set to 100% of protected line or may overreach into next section. Overreach caused by possible errors in :CTs VTs ZLINE information Relay Measurement > Distance Protection - January 2004

Zone 1

Possible Overreach

ZONE 1 = ZL ZL F

Possible incorrect tripping for fault at ‘F’ ∴ Zone 1 set to ∼ 0.8ZL ZONE 1 = 0.8ZL ZL

> Distance Protection - January 2004

Zone 1 Settings for Teed Feeders

Z1C = 0.8ZAC

A

C Z1A = 0.8ZAB

Z1B = 0.8ZBA B

Z1C

Z1A Z1B

> Distance Protection - January 2004

Zone 1 Settings for Direct Intertrip Schemes

Z1A A

ZL B

Z1B

Z1A

Send

Receive Trip ‘B’

Receive

> Distance Protection - January 2004

Send

Z1B

Zone 1 Settings for Direct Intertrip Schemes

Effective Zone 1 reaches at A and B must overlap. Otherwise :- No trip for fault at ‘F’

A

Z1A

F

Z1B

∴ Effective Z1A and Z1B must be > 0.5ZL Settings for Zone 1 > 0.8ZL are o.k.

> Distance Protection - January 2004

B

Minimum Zone 1 Reach Setting

Dictated by :Minimum relay voltage for fault at Zone 1 reach point to ensure accurate measurement. Minimum voltage depends on relay design typically 1 → 3 volts.

> Distance Protection - January 2004

System Impedance Ratio :- SIR SIR = ZS/Zn where :-

ZS = Source impedance behind relay Zn = Reach setting

VRPA = Minimum voltage for reach point accuracy Can be expressed in terms of an equivalent value of SIRMAX SIRMAX = ZS MAX Zn MIN ∴ Zn MIN ≡ ZS MAX SIRMAX > Distance Protection - January 2004

Zone 2 X Covers last 20% of line not covered by Zone 1. X Provides back-up protection for remote busbars. Z2G TIME Z1G G

Z1H H F

To allow for errors :Z2G > 1.2 ZGH Zone 2 is time delayed to discriminate with Zone 1 on next section for faults in first 20% of next section. > Distance Protection - January 2004

Zone 2 Zone 2 on adjacent line sections are not normally time graded with each other Z2G

Z2H

Z1G

‘G’

Z1H

‘H’ F

X Overlap only occurs for faults in first 20% of following line. X Faults at ‘F’ should result in operation of Z1H and tripping of circuit breaker ‘H’. If ‘H’ fails to trip possible causes are :Î Z1H operates but trip relays fail. Z2H may operate but will not trip if followed by the same trip relays. Fault must be cleared at ‘G’ by Z2G. Z1H and trip relays operate but circuit breaker fails to trip. > Distance Protection - January 2004

Zone 2

No advantage in time grading Z2G with Z2H Ð Unless Z2H + trip relays energise a 2nd circuit breaker trip coil.

> Distance Protection - January 2004

Zone 2 Î Z1H fails to operate. Ð Results in race between breakers ‘G’ and ‘H’ if Z2H and Z2G have the same time setting. Ð Can only be overcome by time grading Z2G with Z2H. Z2G Z2H Z1G ‘G’

Z1H ‘H’

Problem with this :Zone 2 time delays near source on systems with several line sections will be large. End zone faults on lines nearest the infeed source point will be cleared very slowly. > Distance Protection - January 2004

Maximum Allowable Zone 2 Reach to Allow for Equal Zone 2 Time Settings Z2A (EFF) MAX Z1B (EFF) MIN ZL1

A

B

ZL2

Z2A must not reach beyond Z1B i.e. Z2A(EFF) MAX must not reach further than Z1B(EFF) MIN

∴ ∴

Z1BSETTING = 0.8ZL2 Z1B(EFF) MIN = 0.8 x 0.8ZL2 = 0.64ZL2 Z2A(EFF) MAX < ZL1 + 0.64ZL2 1.2 Z2ASETTING < ZL1 + 0.64ZL2 Z2ASETTING < 0.83ZL1 + 0.53ZL2

> Distance Protection - January 2004

Zone 2 Time Settings on Long Line Followed by Several Short Lines Z2G Z3H Z3J Z2H Z2J Z1G ‘G’

Z1H ‘H’

Z1J ‘J’

F

Z2G reaches into 3rd line section. To limit remote back-up clearance for a fault at ‘F’, the time setting of Z2G must discriminate with Z3H. > Distance Protection - January 2004

Zone 3 X Provides back-up for next adjacent line. X Provides back-up protection for busbars (reverse offset). X Actual Zone 3 settings will be scheme specified, i.e. permissive or blocking schemes. X Many modern relays have more than 3 Zones to allow the use of three forward and an independent reverse zone. Z3G REV

Z3G FWD Z2G

Time

Z1G G

Z1H H

K

Typical settings : Z3FWD > 1.2 x (ZGH + ZHK) Z3REV 0.1 to 0.25 of Z1G > Distance Protection - January 2004

Zone Time Coordination - Ideal Situation Zone 1 :- tZ1 = instantaneous (typically 15 - 35mS) Zone 2 :- tZ2 = tZ1(down) + CB(down) + Z2(reset) + Margin e.g. tZ2 = 35 + 100 + 40 + 100 = 275mS Zone 3 :- tZ3 = tZ2(down) + CB(down) + Z3(reset) + Margin e.g. tZ3 = 275 + 100 + 40 + 100 = 515mS Note: Where upper and lower zones overlap, e.g. Zone 2 up sees beyond Zone 1 down, the upper and lower zone time delays will need to be coordinated, e.g. tZ2(up) to exceed tZ2(down). > Distance Protection - January 2004

Under / Overreach

> Distance Protection - January 2004

Under-Reach

Impedance presented > apparent impedance %age Underreach = ZR - ZF x 100% ZR where ZR = Reach setting ZF = Effective reach

> Distance Protection - January 2004

Underreaching Due to Busbar Infeed between Relay and Fault

IA

ZA

IA+IB

IB

Relay Location

VR = IAZA + (IA + IB) ZB IR = IA ZR = ZA + ZB + IB . ZB IA > Distance Protection - January 2004

ZB

Underreaching Due to Busbar Infeed between Relay and Fault ∴ Relay with setting ZA + ZB will underreach with infeed. Relay with setting ZA + ZB + IB . ZB will measure IA correctly with infeed present but if infeed is removed the relay will overreach. Maximum allowable setting dictated by load impedance

> Distance Protection - January 2004

Under-Reach IP

IG+IP

ZG

ZK

F

IG

RELAY

What relay reach setting is required to ensure fault at F is at boundary of operation ? Impedance seen for fault at F = ZG + IG + IP . ZK IG Limit of operation is when Impedance Seen = Reach Setting ∴ Reach setting required = ZG + IG + IP . ZK IG > Distance Protection - January 2004

Over-Reach

Impedance seen < apparent impedance %age Overreach = ZF - ZR x 100% ZR where ZR = Reach setting ZF = Effective reach

> Distance Protection - January 2004

Mutual Coupling

> Distance Protection - January 2004

Mutual Coupling

X Mutual coupling causes distance relays to either underreach or overreach. X Positive and negative sequence has no impact. X Zero sequence mutual coupling can have a significant influence on the relay. X Only affects ground fault distance.

> Distance Protection - January 2004

Mutual Coupling Example Under Reach

Z2 ‘Boost’ G/F Z2 PH

Zmo

> Distance Protection - January 2004

Mutual Coupling Example Over Reach

Z2 ‘reduced’ G/F Z2 PH

> Distance Protection - January 2004

Mutual Coupling Example Over Reach

Z1 G/F (optional) Z1 G/F (normal)

Zmo

> Distance Protection - January 2004

Ancilliary Functions

> Distance Protection - January 2004

Switch on to Fault (SOTF)

X X X

X Fast tripping for faults on line energisation, even where line VTs provide no prefault voltage memory

> Distance Protection - January 2004

Voltage Transformer Supervision

X A VT fault and subsequent operation of VT fuses or MCB’s results in misrepresentation of primary voltages X Relay will remain stable as the current phase selector will not pick up X Subsequent system fault may cause unwanted / incorrect tripping X VTS operating from presence of V0 with no I0 or V2 with no I2 is used to block relay if required

> Distance Protection - January 2004

VT Supervision X Under load conditions Š Loss of 1 or 2 phase voltages Š Loss of all 3 phase voltages X Upon line energisation Š Loss of 1 or 2 phase voltages Š Loss of all 3 phase voltages X Digital input to monitor MCB X Set to block voltage dependent functions

> Distance Protection - January 2004

Zone 1 Mho Relay K ZS HH Z1

Ø3

J Ø 2

G

Ø1

ZS G

L

Power Swing Locus

> Distance Protection - January 2004

L O A D

X Will not operate for load or stable power swing X Ø1, Ø2, Ø3, = Angles between system voltages at ‘K’ and ‘L’ Ø increases as power swing approaches relay at G X ‘J’ is point where power swing enters relay characteristic X At ‘J’ the angle between voltages at ‘G’ & ‘H’ is 90° X Normal limit of angle between voltages at ‘G’ & ‘H’ for load is of the order of 30°

Comparison between Stability of Mho and Quadrilateral Impedance Elements during a Power Swing jX Power Swing Locus

θ R

> Distance Protection - January 2004

Illustration of Basic Power Swing Blocking System jX Power Swing Locus ZP Z3

R

> Distance Protection - January 2004

Power Swing Blocking

X A power swing will result in continuous change of current X Continuous output from the relay superimposed current element can be used to block for a power swing X Using this method the relay is able to operate for faults occurring during a power swing

> Distance Protection - January 2004

Directional Earth Fault Protection (DEF) O

High resistance ground faults

O

Instantaneous or time delayed

O

IEC and IEEE curves

O

Single or shared signalling channel

> Distance Protection - January 2004

Transformer Feeders

> Distance Protection - January 2004

Transformer Feeders

ZT ZL

21

Zone 1 = ZL + 0.5ZT T1 = Instantaneous Zone 2 = 1.2 (ZL +ZT) T2 = Co-ordinate with downstream protection Zone 3 T3

> Distance Protection - January 2004

- Back-up use as appropriate

Low Voltage VT, High Voltage CT ZT

ZL

21

* 1 VT may be required to account for phase shift. Example 1 ZT = 10Ω , ZL = 1Ω Set relay Z1 = 0.8 x (ZT + ZL) = 8.8Ω ∴ Z1 does not reach through transformer. Example 2 ZT = 10Ω , ZL = 1Ω Z1 = ZT + 0.8ZL = 10.8Ω with 20% error = 12.96Ω - overreach problem

> Distance Protection - January 2004