03-Earth FAult and FAult resistance_new

© Siemens AG 2006

Distance Protection: Earth-Faults and Fault Resistance Power Transmission and Distribution

TLQ 2004 Distance Protection – Earth Faults and fault resistance

© Siemens AG 2006Power Transmission and Distribution

Distance protection: Earth fault in system with solid, isolated or compensated system neutral earthing

G

BA C

D

Z1

Z2...

D

ZT

Neutral Earthing :

Peterson Coil or Isolated or Solid

During single phase earth fault:

The short circuit current magnitude depends on the neutral earthing method.



Earth Fault Current - Pick-Up Characteristic

Measuring errors and non-symmetry may not cause incorrect pick-up by earth fault current threshold



Earth Fault Detection Logic

Normal pick-up: 3I0

Heavy load on long line: 3I2

For very small earth current: 3U0 (isolated or compensated system)



Earth fault detection during one pole open condition

During the 1 pole open condition, load current flows in the earth path.

Magnitude comparison of the remaining 2 phases prevents incorrect pick-up



Phase-to-Earth loop:

Phase-to-Phase loop:

Distance measurement Fault loop formulas

2121 LLLLLL IIjXRV

RL + j XLIL1

RE + j XE

VL1 VL2 VL3

IL2

IL3

IE

Relay location

Line and earth impedance are measured

Only the Line impedance is measured

EL

ELLE

L

ELLL

EELLEELLL

EEELLLL

IX

XIjXI

R

RIRV

XIXIjRIRIV

jXRIjXRIV

111

111

11

)()(



Numeric impedance calculation, ph-ph-loop

Infeed

L1

L2

L3

E

Rfwd Xfwd(Lfwd)

Rret Xret(Lret)to remote line end

fwd

ret

Ufwd Uret

relaylocation

faultlocation

U U = X

L3L2

L3L2mL3-L2

-

-

III

L3L2

L3L2L3-L2

-

- e=

II

U UR R

L3L2

L3L2L3-L2

-

II -

U U = Z

With the measurement of phase to phase voltages and currents the fault impedance (impedance to fault location) is correct calculated



EEL

EPh

L II

k

UZ

Uph-E

G

RL XLL1

L

L2

L3

RE XE

E = (L1 + L2 + L3)

L

E

L

L0L

E 3 Z

Z

Z

ZZk

Numeric impedance-calculation, Ph-E-loop (1)

EL

ELLEEL LEPh I

Z

ZIZZIZI U

Calculation of the complex impedance

Residual compensation factor



Uph-E

G

RL XLL1

L

L2

L3

RE XE

E = (L1 + L2 + L3)

Numeric impedance-calculation, Ph-E-loop (2)

Separate calculation of the resistance and reactance

Separate residual compensation factors

EL

ELLE

L

ELLL

EEELLLL

IX

XIjXI

R

RIRV

jXRIjXRIV

111

11

L

E

E, R R

Rk

L

E

E, X X

Xk

This solution is the preferredin Siemens Relays



Distance measurement (Ph-E-loop) influence of fault resistance

UPh-E

XL

L RL

RF

XE

E RE

K

X

R

X

ZL ZPh-E

RF

1+kE

E-L assume LFELLE-Ph IIII R + Z + Z = U

E

F

E

L

E

LEEL

E-PhE-Ph

+ 1 +

+ 1

+ 1 =

k

R

kZZ

Zk

UZ

II

)-Ej(

L

E

FL

L

E

FLE-Ph

L

EE

Le1

R

1

R then , to adapted setting If

ZZ

Z

ZZ

ZZZ

Zk

Also an additional measuring error in the X-direction

This method is not used by SIEMENS



Distance measurement (Ph-E-loop) - influence of fault resistance at separate residual compensation factors

UPh-E

XL

L RL

RF

XE

E RE

K

X

R

ZL

ZPh-E

RF

1+kE,R

LFEEELLLE-Ph III R +X j + R - X j + R = U

with IE = - IL

RE

FL

L

E

L

EPh

Ph-E + k

RR

RR

+

I

U

R,11

Re

L

L

E

L

EPh

Ph-E X

XX

+

IU

X

1

Im

No measuring errorin the X-direction



Short-circuit with fault resistance and infeed from both sides - equivalent circuit

D

ZLB

UA

EA

RF

A

B

EBA

X

ZL

A

B RF

RF

R

FBALA R + + Z = U A III

FBFLAA R + R + Z = U II

FA

BFL

A

AA R + R + Z =

U = Z

I

I

I

The fault resistor RF is seen larger



Fault locating: distance-to-fault measurement with arc compensation

ISCUSCRF

ZL

RF

ZL

ISCR

X

ZSC

(USC)

SC

ZSC sin SC

XSC= K sin SC = ZSC ·sin SC

USC

ISC

lF

Measurement of the Reactance gives the bestaccuracy



= I1+I2 - I1

ZL1

XR

UARC/ISC1

1 + k0

for : I1 = IE1 = ISC1 and k0 = =

ZLE

ZL

ZL0 - ZL

3 ZL

ZRel = ZL1 +UARC

(1 + k0) · ISC1

Short-circuit with arc resistance and double-sided in-feed,

influence on distance measurement

U1

IE1

I1ZL1 ZL2

I2

IE2ZE1ZE2

UARC

U1 = I1 · ZL1 + IE1 · ZE1 + UARC

I1 + k0 · IE1

ZRel = = +U1

I1 + k0 · IE1

I1 · ZL1 + IE1 · ZE1

I1 + k0 · IE1

UARC

The phase angle difference between the two sources (load influence) causes an additional error in the measured reactance (see angle )



Influence of load flow on the distance measurement for faults with fault resistance

ZL1 ZL2

U1 U2RF

load

RF = fault resistance

U1U2

1

2

SC1

SC2

L

1

2

X

R

2

1

RF

ZL1

ZSC1

SC1

ZSC1 sinSC1

RF X

R

RF

ZL2

ZSC2

SC2

ZSC2 sin

SC2

1

2

RF

ZK1 = ZL1 + RF + RF

2

1

ZK2 = ZL1 + RF + RF

1

2

An Over-reach(left) or an Under -reach (right) is possible.The grading characteristic mustbe adapted.



Estimation of arc resistance

X Variable R/X-setting

R

Worrington formula:

Ohm ml

AI

28700 R

1,4ARC

Rough estimation: UARC = 2500 V/m

Ohm AI

md V/m 2500 ARCR

F

Phase-to-phase distancesd = 3,5 m (110 kV)d = 7 m (220 kV)d = 11 m (380 kV)

Insulator lengths (long-rod insulator)

l= 1x1,3 = 1,3 m (110 kVl= 2x1,3 = 2,6 m (220 kV)l= 3x1,3 = 3,9 m (380 kV)



Ph-PH-E short-circuit with fault resistances,

Measured loop impedances depending on fault location

RTower

L = 0

100 km12.5 GVA 12.5 GVA

L2-L3-E

4

D1 1

L3-E

L2-L3

L2-E

R

X

50 %

100 %

In the case of multiple earth-faults with fault resistances, complex conditions arise for the distance measurement.

A special logic for loop selection is required (blocking of leading phase (L2-E) to prevent an over-reach)



Loop impedances during Ph-Ph-E short-circuit,

depending on fault resistance to earth and load conditions

RE

RPh = 0

D

Ph-Ph-E

10 GVAloadX0

X1= 1

10 GVAX0

X1

= 1

500 kV; l =310 km

25 10 20

40

2

515

10

20

30

400

100

150

200

50742 MW

-742 MW

RE

RE (Ohm)

50 100

X (Ohm)

X (Ohm)

-742 MW

742 MW

laggingphase

leadingphase

The impedance of the leading phase is seen to be too short.Phase-Phase loop isnearly measured correctly(small X-error)



Effective arc resistance „seen“ by the distance relay with double-sided in-feed (example)

ARCLA A U + Z I = U

A

ARCL

A

AA

II

U + Z =

U = Z

iARC

uARC

D RARC6 m

A = 1 kA B

2 4 6 8 10 BkA

RARC

5

10

15with constantarc voltageUARC = 2500 V/m

with current dependentarc voltage

RLB = Ohm/m28 700

ARC1.4

ARC in A

The rate of arc resistance reducing is greater, if the current increasing is considered



Resultant fault resistance on overhead lines with earth-wire

R R R R R R R R R

earthing resistanceof the station

IPh

IE''

IE'

tower footing resistances

earth-wire(s)

phase-conductors

50 10000

1

2

3

4

5

avarage towerfooting resistance

resultant fault resistance Ph-E

60 mm2 steel wire

2 earth-wires, total 60mm2

tower currents

RLNW RTF RLNW

E

On lines with earth-wires the current flows via several parallel tower footing to earth. The resultant phase-earth resistance which is actually effective, is substantially reduced.



Quadrilateral characteristic with load cut-out for high line loadability

• High reach for remote back-up and adapted arc tolerance (good fault-load discrimination)

• High arc compensation even with short lines

X

R

RF

X- and R-reach separately settable at all zones

2

1

X

R

RFRF

load

D

RF 1 + 2

1

2

ZF = ZL + RF + RF

2

1

Quadrilateral characteristic is a good solutionfor adapting on high fault resistances.It provide a substantially better resistance coverageand arc compensation than circular characteristic.

03-Earth FAult and FAult resistance_new

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