Appl 15 Distance Prot at Parallel Lines En

8/12/2019 Appl 15 Distance Prot at Parallel Lines En

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Siemens PTD EA Applications for SIPROTEC Protection Relays 2005 1

Line Protection in Transmission Systems

Environmental and cost consciousness are forcingutilities to install more and more parallel lines.The close arrangement of the transmission linesleads to a higher fault rate and to influencing ofthe measuring results. The considerable influenceexerted by parallel lines on the measuring results(of up to 30 % in distance protection) and the re-

medial actions are considered in this applicationexample.

1. Explanation of the term parallel line

1.1 Parallel lines with common positive and

negative-sequence systems

The two parallel lines have the same infeed at bothline ends.

In this arrangement where both systems are con-nected with the same infeed, it is possible (for dis-tance protection) to compensate the influence ofthe parallel line.

In this arrangement of parallel lines the effect canonly be compensated on one side.

Distance Protection with

Parallel Compensation

1.2 Parallel lines with common positive-sequence

and independent zero-sequence system

This arrangement of parallel lines does not influ-ence the distance measurement.

Fig. 3 Parallel lines over the Bosporus

Fig. 1 Parallel lines with same infeed at both line ends

Fig. 2 Parallel line with only one common infeedFig. 4 Parallel line with common infeed at a common tower

LSP2567.t

if


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Siemens PTD EA Applications for SIPROTEC Protection Relays 20052

1.3 Parallel lines with isolated positive and zero-

sequence systems

This is the most unfavorable arrangement for dis-tance protection. Compensation of the inductive

coupling of the circuits is not possible.This arrangement causes a complicated fault volt-age and current distribution due to the inductivecoupling.

2. General

When overhead lines follow parallel paths, a mu-tual, inductive coupling of the current paths ex-ists. In the case of transposed lines, this effect inthe positive and negative sequence system may beneglected for all practical purposes (mutual reac-tance less than 5 % of the self-impedance). Thisimplies that during load conditions, and for allshort-circuits without earth, the lines may be con-sidered as independent.

During earth-faults, the phase currents do not add

up to zero, but rather a summation current corre-sponding to the earth-current results. For thissummated current, a fictitious summation con-ductor placed at the geometrical centre of the

phase-conductors models the three-phase system.Two lines in parallel are modelled by two parallelsingle conductors with an earth return path, forwhich the mutual reactance must be calculated. Inthe case of lines with earth-wires, an additionalcoupling results, which must be considered in thecalculations. The coupling impedance can be cal-culated as follows:

Z f j f lD km

M n

ab

'=

+

4

0 0

044 10=

s

km

= 658f

= Depth of penetration in groundf = Frequency in Hz

= Specific resistance in/ mDab= Spacing in meters between the two conductors

For a typical value of the specific earth resistanceof= 100/m, a system frequency of 50 Hz,a conductor spacing of 20 m and an earth-fault ofIa= 1000 A, the following result is arrived at.

ZM j 0.24 km' .= +0 05 /

Then the induced voltage in the parallel conduc-tor can be calculated withUb= ZM

Ia, and 250 V

per km is obtained.On a 100 km parallel line, this would give an in-duced voltage in the conductor of 25 kV.

3. Calculation of the measuring error of the

distance protection caused by a parallel

line in the event of an earth fault

Fig. 5 Parallel lines with separate infeed

Fig. 6 Inductive coupling of parallel lines

Fig. 7 Tower diagrams


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Earth resistance100/m

Positive impedance (/ km) 0.032 + j 0.254Zero impedance (/ km) 0.139 + j 0.906Coupling impedance (/ km) 0.107 + j 0.488

R1 = 0.032/ kmX1 = 0.254/kmR0 = 0.139/ km

X0 = 0.906/ kmR0M= 0.107/ km

X0M= 0.488/ km

Z

Z

Z Z

ZE

L

=

=0 113

0 86.

Z

Z

Z Z

ZM

L= =

0 1

130 65.

Phase current :ILA= IA1+IA2+IA0 ILB= IB1+IB2+IB0

Earth current:IEA= 3IA0 IEB= 3IB0

K0 = ( ZLo- ZL1) / 3 ZL1K0M = Z0M/ 3 ZL1IC0/IA0 = x / 2-x

For measured impedance ZAfor the distance relay

Z1is

Z x

lZ

x

lZ

Z

Z

x

l x

Z

Z

1

2

1

= +

+L L

0M

L

E

L

3

The measured impedanceZBfor the distance relayZ2is

Z l x Z

x Z

L2 2= +

( )

0M

L

E

L

3 Z

1+Z

Z

By placing the values in the equations we can cal-culate the measuring errors for this double-circuitline with single-end infeed.The results are shown in the diagram below:

Fig. 8 Parallel line with an infeed without parallel linecompensation

Measuring error

Fig. 9 Double-circuit line with single-end infeed

Measuring error

Fig. 10 Distance measuring error on a double-circuit line withsingle-end infeed


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The greatest measuring deviation (35 %) occurs inthe event of a fault at the end of the line, becausethe coupled length up to the fault position is at

maximum.

This example shows that the zone reach needs tobe reduced to 70 % to avoid overreaching in theevent of earth faults.

3.1 Result

The fault is proportional to K0M= Z0M/ 3 ZL1 The fault increases with the ratio of the earth

current of the parallel line IEPto the earth-faultcurrent of the relay

The relay has an underreach when the earth-fault current of the parallel line and the earth

current of the relay are in phase (same direc-tion)

The relay has an overreach when the earth-faultcurrent of the parallel line and the earth currentof the relay have opposite phase (opposite direc-tion).

The measuring error of the relay on the faulty linewith two-end infeed is shown in the above dia-gram. It can be seen that the fault becomes nega-

tive in the case of faults in the first 50 % of the lineunder the same infeed conditions. This is exactlythe reach where the earth current on the parallelline flows in the opposite direction.

The following figures show that the parallel lineinfluence changes strongly with the switching stateof the parallel line. The reason is the differentearth-current distribution.

ZU

I k I k I ph E

ph E

ph E E E EM Ep

=+ +

withk

Z

ZEM0M

1L= 3

= kk

EM

E

L1+

Z 24 % vonZL

Fault at the line end (Fig. 12):Infeed sources for positive-sequence and zero-sequence system at the line end

= k

kEM

E

L1+

Z 24 % vonZL

Fault at the line end (Fig. 13):One switch open, star-point earthing and relay atopposite ends.

Fig. 11 Earth fault on a double-circuit linewith two-endinfeed

Fig. 12 Faultat the line end

Fig. 13 Fault at the line end with open circuit-breaker


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Z =k

3

1+ ZEM

E

L0M

0

L

=

kZ

ZZ 40 % ofZL

Fault at line end (Fig. 14):

Z = -ZLEM

0M

0L

E

+

k Z

Z

k1 -10 % ofZL

Fault at line end (Fig. 15):

4. Parallel line compensation

In order for the distance protection to be able tooperate with parallel line compensation, it is as-sumed that it receives IEPof the parallel line as ameasuring variable.

Z U

I k IA

A

ph E E

=+

=+ +

+

Z I Z I Z I

k I

E1

0L ph

L

1L

EM

1L

Ep

ph E E

Z 3 Z

I

As can bee seen from the equation the fault im-pedance is measured correctly when we add the

term Z

ZI0

13M

L

EP in the denominator. With the

normal setting kE=ZE/ZLthe denominator is thenreduced against the bracketed expression in thecounter andZ1Lis produced as measured result.

The distance protection has another measuring

input to which the earth current of the parallelline is connected. The addition is numeric. Itshould be noted that the relay on the healthy linesees the fault at too short a distance due to cou-pling of the earth current of the parallel line. Ifzone 1 of the line without a fault is set to 85 %, thedistance protection would lead to an overreachthrough the fed parallel fault. The distance protec-tion would still see faults on the parallel line at upto 55 % line length in zone 1.

Fig. 14 Infeed sources of the positive and zero-sequence systems at oppositeline ends

Fig. 15 Parallel line disconnected and earthed at both ends

Fig. 16 Connection of the parallel line compensation


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Example:

Z ZE

L

M

LZ 3 Z=

=

0 86 0 650. / .

ZL = line impedance

The so-called earth-current balance is used to pre-vent this overfunction. It compares the earth cur-rents of both line systems and blocks the parallelline compensation when the earth current of theparallel line exceeds the earth current of the ownline by a settable factor.

I l x

x

x

lx

l

E

E2

1 2 2

I=

=

At a setting ofx/lof 85 %, the parallel line com-pensation is effective for faults on the own lineand for a further 15 % into the parallel line. Thisresults in a factor ofIE1/IE2= 1.35 as a standardvalue for the earth-current balance.

Setting instructions for the parallel line compen-sation

The compensation is only possible where bothlines end in the same station.

In the distance protection the compensation isonly used where no sufficient backup zone ispossible without compensation (When the dou-

ble-circuit line is followed by short lines).

Fig. 17 Distance measurement with parallel linecompensation

Fig. 18 Effect of the parallel line compensation

Fig. 19Example of adouble-circuit linestation

S"SC = 10000 MVA

Z

Z0

1

1=

ZVA1 = 16 PN = 1000MVAS"SC = 2500MVA

Z

Z0

1

1=

ZVB1 = 64

S"SC = 20000 MVA

Z

Z0

1

1=

ZVC1 = 8


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5. Calculation examples

The procedure for setting a normal single-circuitline is explained in the available manuals. Specialapplications are dealt with here.

5.1 Double-circuit line in earthed system

The coupling in the zero-sequence system requiresdetailed consideration of the zone setting for earthfaults.

5.2 General procedure

It is recommended to first determine the gradingof the distance zones for phase faults without tak-ing into account the parallel line coupling. In thesecond step the zone reaches are then checked for

earth faults and a suitable earth-current compen-sation factor selected.The use of parallel line compensation must beconsidered so that an adequate remote backupprotection can be ensured in the event of earthfaults.

5.3 Grading of the distance zones for

phase-to-phase short-circuits

The zones must be set according to the basic rulesof grading plans. For the backup zones, the para-bolic course of the impedance dependent on thefault location is important.When double-circuit lines are connected in series

there are also different reaches of the backupzones dependent on the switching state and theinfeed at the opposite end.Theoretically speaking, this results in relativelyhigh effort for creating the grading plan of dou-ble-circuit lines.The procedure is usually simpler in practice.Half the impedance of the following parallel linecan be used for practical grading of the secondzone (double-circuit line follows single-circuitline). This gives:

Z GF Z Z2A A B B C= + 2 0 5( . )

In the third zone, the grading should be per-formed according to the backup protection strat-egy. A selective grading for all switching statesleads to relatively short third stages which hardlyget any longer than the corresponding secondstage. In the high and extra-high voltage system,an attempt will be made for the third stage tocover the following double-circuit line in normalparallel line mode. In this case the following stepsetting is derived:

( )Z Z Z3A A B B C= + 11.

In the pickup zone the following lines should be inthe protected zone in the worst switching state(single line follows parallel line). The following

setting should be set for this:

( )Z Z Z+ = + AA A B B C11 2.As a rule infeeds are available in the intermediatestations of the double-circuit line which have tobe taken into account in the grading of the backupzones. This is demonstrated by the following ex-ample (see Fig. 19):

Double-circuit line.Setting of the distance zones for phase-to-phaseshort-circuits

Given:

100 kV double-circuit lineLine data:Configuration according tol1and l2= 150 km, l3and l4= 80 kmZ1L'= 0.0185 + j 0.3559/kmZ0L'= 0.2539 + j 1.1108/kmZ0M'= 0.2354 + j 0.6759/kmPnat.= 518 MW per lineCurrent transformer: 2000/1 AVoltage transformer: 400/0.1 kV

Task:Calculation of the zone setting for relay D1.

Solution:OnlyXvalues are used in the short-circuit calcula-tions, for the sake of simplicity:

XL1=XL2= 0.3559/km 150 km = 53.4XL3=XL4= 0.3559/km 80 km = 28.5We generally apply a grading factor (GF) of 85 %.The zone reaches are calculated as follows:

X1= 0.85 53.4For selective grading of the 2nd stage it is assumedthat the parallel line L2 is open but that always atleast half the short-circuit power is available fromthe intermediate infeed in B. Grading takes placeselectively to the end of the 1st zone of the distance

relays of the following lines 3 and 4. That meanswe can use about half the line impedance. Thisgives a simplified equivalent circuit. For a three-pole fault in C we calculate the short-circuit cur-rents drawn in the figure.


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Under consideration of the intermediate infeed ef-fect we get

X2 534 28 5

21

119

2190 85

64 120

= + +

=

=

. . .

..

% XL 1

According to the above recommendation we getfor zone 3

X X3 L= + = =( . . ) . . %534 28 5 11 901 169 1

and for the pickup zone:

X A+ = + = =( . . ) .534 2 285 11 121 226 % L1X

we adapt the reach of the zones in R direction tothe impedance of the natural power:

ZU

PNat.

Nat.

N2

= = =400

518309

2

We assume that a new line has to transmit doublethe power temporarily and allow an additionalsafety margin of 30 %. This gives the maximum Rreach of pickup as:

RA1 07 3092

108= =.

We further selectA= 50 andRA2 = 2 RA1 = 208

An R/X ratio of 1 offers adequate compensationfor fault resistance for the distance zones.

5.4 Zone reach during earth faults

The earth-current compensation factor kEis de-cisive in the Ph-E measuring systems. For single-circuit lines this is set to the corresponding ZF/ZLvalue. The protection then measures the same im-pedance for Ph-Ph and earth faults.On double-circuit lines the zero-sequence systemcoupling produces a measuring error for earthfaults. The measurement can be corrected with theparallel line compensation. This function is con-tained optionally in the relays 7SA. Only the earthcurrent of the parallel line needs to be connectedto the relay and the coupling impedance set. Theearth-current balance may be left at the standardvaluex/l= 85 %. The earth-current factor must beadapted to the single-circuit line in this case.

5.5 Setting of the kEfactor (operation without

parallel line compensation)

In the event that the parallel line compensation isnot used, a kEfactor must be found which ensuresadequate protection for the possible operatingstates of the double-circuit line (see Table 1).

Fig. 20 Protection setting in double-circuit lines: Systemdata for the calculation example


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a) This equation applies for x

l1

Forx

l>1it applies that:

GF k k XX

1 1( ) ''

+ + XER XEM0M

0L

XEL1+ k

b) k X

XXEL

EL

L Line

=

'

'1

c)k X

XXEM

M

0L Line

=

'

'0

3

Adapting the setting to a particular operating statecauses an overreach or underreach in the respec-

tive other states. GF1 in % is the selected gradingfactor for the 1st zone (reach for phase-to-phasefaults).x/lin % then specifies how far the zone 1(Ph- E loop) reaches in the event of earth faults,referred to the line length.

Determining of the relay setting value kERis dem-onstrated by the example of double-circuit lineoperation. For a fault at the distancex/l, the volt-age at the relay location is:

U x

lZ I

x

lZ I

x

l

ZIMPh E L Ph E E EP = + +

0

3

Whereby with single-end infeed:

I I

x

lx

l

IPh E EP Eand= =

I2

For the measurement on the Ph-E loop the resultis

Z U

I k I

x

l

Z Z

x

lx

lkP

Ph EPh E

h ER E

L E0M

ER

Z

3

=

+ =

+ +

+

2

1

kERis the complex earth-current compensationfactor set at the relay.X and R are calculated separately for the numeri-cal relays 7SA. Simplified equations apply for thisif phases and earth currents have the same phaserelation.

This results in:

Initially, only the X value measured is of interestfor the reach.

Withk X

Xk

XLXE

E

L

XEMM

L

and3

= =

0

Xthis gives:

X x

lX

k k

x

lx

lk

Ph E L

XEL XEM

XER

=

+ +

+

1

2

1

The Ph-E measuring system and the Ph-Ph mea-suring system have the same impedance pickupvalue (common setting value Z1).Therefore: ZPh-E=ZPh-Ph=Z1 = GF1 ZL, applies,whereby GF1 is the grading factor of the first zone.The following equation is finally arrived at for theearth-current compensation factor which must beset at the relay:

k

k k

x

lx

l

GF

x

lXER

XEL XEM

=

+ +

1

2

1

1

We can vary the reach of the Ph-E measuring sys-tems of a given zone reach for phase faults (GF1 in% of ZL) by adjusting the kXERfactor.We can also solve the previous equation accordingto x/l and then arrive at the reach for a given kXERsetting.

In the same way we derive the equations specified

in Table 1 for the cases parallel line open andparallel line open and earthed at both ends.

x

l

GF k k k k=

+ + + + [ ( ) ( )] [..] ( ) (1 1 2 1 8 12XER XEL XEL XEM 1 1+

k F

k

XER

XEL XEM

G

2 (1+

)

)k

X U

X

XI

Ph EPh E SC

PhE

L R

E

sin

=

+

I

=

+

+

x

lX

XX

x

lx

l

X

X

L

E

L

M

L

E

L R

1+3

XX0

2

1

R U

R

RI

Ph EPh E SC

PhE

L R

E

=

+

cos

I

=

+ +

+

x

lR

R R

R

x

lx

l

R

R

L

E

L

M

L

E

L

13

2

1

0

R


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The choice of the setting of kXERrequires a com-promise which takes all three cases of operationinto account (Table 1)1). At a grading factor ofGF1 = 85 %, adaptation to the single-circuit lineusually offers an acceptable solution. The two-enddisconnection of a line with earthing at both endsonly occurs in maintenance work, so the briefoverreach of 8 % is only rarely effective becauseoverreaching is usually reduced by intermediateinfeeds.

In operation with single-pole auto-reclosure, the

overreach would only lead to excessive auto-reclosure and not to final disconnection, providedthat a transient short-circuit is concerned (about90 % of faults).

Alternatively, the reach can be reduced slightly forearth faults by setting a lower kXERfactor. If it werereduced from kXER= 0.71 to kXER= 0.5, overreachwould just about be avoided in this example. Thereach with both lines in service would then onlybe 64 %, taking into consideration that the paral-lel line coupling only takes full effect in the worstcase of single-end infeed. In the normal case oftwo-end infeed the earth current on the parallel

line is much lower in the event of faults close tothe middle of the line, and the zone reach corre-sponds almost to the single-circuit line. In addi-tion, the parallel line coupling at the other end of

the line always has the opposite direction, i.e. thezone reach is increased. Reliable fast disconnec-tion can always be ensured by intertripping. How-ever, in reducing the kXERfactor it must be takeninto account that the reach of the backup zones isalso reduced accordingly in the event of earthfaults. Zone reach reduction (e.g. GF1 = 0.8)should therefore also be considered instead ofonly reduction of the kXERfactor.

5.6 Setting the overreach zone

Zone Z1Bshould be set to 120 130 % ZL. Thisreach would also apply for earth faults in the caseof operation with parallel line compensation.

1) The numeric values in Table 1 were calculated with theline layers of the previous example. For the sake of sim-

plicity the complex factors kEL= 0.71 - j0.18 andkEM= 0.64 - j0.18 were only taken into account withtheir real components, which correspond to the valueskXEL=XE/XLand kXEm =XM/(3 XL) in the first approxi-mation. This gives sufficient accuracy for the extrahigh-voltage system.

Reachx/lat Ph-E short-circuits

x

lF

k= +G1+

XER

XEL

1 1

k

x

l=See equation on

previous page

x

l

k F

k k X

a= +

+

( )

' )

1 1

1

XER

XEL XEM0M

0L

G

X

kXER-setting with:

x

l=0 85.

GF1 = 0.85

kXEL = 0.71

b)kXEM= 0.64

c) X'0M= 0.72/kmX'0L= 1.11/kmX'1L= 0.356 /km

k k x

lER

EL

GF1= + =1 1 0 71 0 5. ( . )

85 %(75 %)

71 %(64 %)

108 %(98 %)

k

k k x l

x lF

XER

XEL XEM

G=

+ +

1

21

1

/

/

kXER= 1.18

108 % 85 % 132 %

k

k kx

lXER

XEL XEM0M

0L

'

'

G 1=

+ + =

1

1 0 31

X

X

F.

65 % 56 % 85 %

Distance measurement in the event of earth faults: Reach (in X direction) dependent on the relay setting

kX

XXER

E

L Relay

=

and the switching state


11/14



Adapting the setting to an operating state causesan overreach or underreach in the respective otherstates. GF1 in % is the selected grading factor for

the 1st zone (reach for Ph-Ph faults). x/lin % thenspecifies how far zone 1 (Ph-E loop) reaches in theevent of earth faults, referred to the line length.

Determining of the relay setting value kERis dem-onstrated by the example of double-circuit lineoperation:

The voltage at the relay location for a fault at thedistancex/lis:

U x

lZ I

x

lZ I

X

l

ZILPh E Ph E E

MEP = + +

0

3

with single-end infeed:

I I I

x

lx

l

IPh E EP Eand= =

2

for the measurement of the Ph-E loop the follow-ing is derived:

Z U

I k I

x

l

Z Z Z

x

lx

lk

Ph EPh E

Ph ER E

L EM

ER

3

=

+ =

+ +

+

0

2

1

whereby kERis the complex earth current compen-sation factor set on the relay. X and R are calcu-lated separately in the numerical relays 7SA.Simplified equations apply for this when phasesand earth currents have the same phase angle.This results in

Only the measured value is initially for the reach:

With k X

Xk

X

XXEL

E

L

XEMM

L

and= =

0

3this gives:

X x

lX

k k

x

lx

lk

Ph E L

XEL XEM

XER

=

+ +

+

1

2

1

The Ph-E measuring system and the Ph-Ph mea-suring system have the same impedance pickupvalue (common setting value Z1).Therefore: ZPh-E=ZPh-Ph=Z1= GF1 ZL, applies,whereby GF1 is the grading factor of the first zone:

The following equation finally results for theearth-current compensation factor which must beset in the relay:

k

k k

x

lx

lGF

x

l

XEL XEM

XER=

+ +

1

2

11

Without parallel line compensation the 120 %reach must be dimensioned for the case of parallelline operation under consideration of the previ-ously defined kXERfactor.

GF

k k

x

lx

l x

l=

+ +

1

2XEL XEM

XER1+ k

For a fault at the end of the line (x/l= 100 %) anda safety margin of 20 %, the following equationfor the overreach zone is produced:

X B GF X

k k

kX

1 120

100

1

11 2

100=

= + +

+

%

%

.

L

XEL XEM

XER

L

The selected kXER= 0.71 producesX1B = 165 % XL.

Without parallel line compensation, theoverreach zone must therefore be set veryhigh, so that a safety margin of 20 % is en-sured in double-circuit line operation.

5.7 Reach of the backup zones for earth

faults

We observe the behavior of the distancemeasurement with and without parallel linecompensation.

X U

I X

XI

Ph EPh E SC

Ph

E

L RE

=

+

sin=

+ +

+

x

lX

X

X

X

X

x

lx

l

X

X

L

E

L

M

L

E

L R

13

2

1

0

R U

I R

RI

x

lR

R

R

Ph EPh E SC

PhE

L R

E

L

E

L

=

+

=

+ +

cos

1 R

R

x

lx

l

R

R

0

32

1

M

L

E

L R

+


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5.8 Distance measurement without parallel line

compensation

For the simple case that the parallel line is fol-lowed by a single-circuit line (Fig. 21, line 4 dis-connected), the measured impedance can bedetermined as follows:

Voltage at the relay location:

U Z I Z I Z

I

x

lZ I

x

l

Ph E L Ph E1 EM

E

L2 Ph3

3

= + +

+ +

1 1 10 1 2

2

2 2

Z IE2 E3

WithIPh1= IE1= IE2= ISCand IPh3= IE3= 2 ISCwe get for the relay reactance:

X UI k I

k k

k

Ph EPh E SC

Ph XER E1

XEL XEM1 2

= +

=

+ ++

sin1

11

1 XERL1

XEL3

XER

L2 + +

+ X

x

l

k

kX2

1

12

If we setx/l= 0 we arrive at the measuredreactance in the event of a fault at the oppositestation. For the calculated example this gives thevalue

X XPh E L = + +

= 1 0 71 0 64

0 711 37 1

. .

..

This shows that in the case at hand the backupzones only reach beyond the next station whenthey are set greater than 137 % ZL1. This does notapply for the 2nd zone at the selected setting.

At the grading (120 %) selected on the basis of thephase-to-phase short-circuits, the 2nd zone wouldonly reach up to 91 % ZL1in the parallel line statein the event of an earth fault.

This problem is particularly pronounced when thedownstream line, according to which the secondzone has to be graded, is substantially shorter thanthe protected line, and when only a short interme-diate infeed is present.

Fig. 21 Distance measurement on double-circuit lines: Fault on a following line


13/14



WithIPh1= IE1= IE2= ISCand

IPh3= IE3= 22

x

lISCand IE4=

x

l2 ISC

we get:

Resolution according tox/l2produces again theequation for the reaches of the zones:

with

For zone 3 (169 %XL1) we getx/l2= 33 %, i.e. onlyslightly more than for the single-circuit line. For thepickup zone (226 %XL1) the expression under theroot is negative because the zone only reaches to

just past the next substation.The limit (root = 0) is at 223 %XL1.

5.9 Distance measurement with parallel line

compensation

With parallel line compensation, faults on theown line are measured in the correct distance.

For faults beyond the next substation the zonesare extended by the factor:

k k k

k=

+ ++

1

1XER XEMR

XER

according to the compensation factors set at therelay.The term 1 + kXERmust be replaced in the equa-tions on pages 12 and 13 by 1 + kXER+ kXEMR.

This gives us a reach of up to 71 % ZL2for the 2nd

zone, i.e. the zone reaches up to just before theend of the first zone of the following line which is

set to 85 % ZL2.

U Z I Z Z

I x

lZ I

x

lPh E L1 Ph1 E E1

ME L2 Ph3

3

= + + + +10 1 2

2

2 2

I Z I x

l

ZIE2 E3

0M3 4E4 +

2 3

X k k

kX

x

l

x

lPh E

XEL1 XEM1 2

XER

L1=

+ ++

+

+

1

1

2 12 2

( k x

lk

kX

XEL2 XEM3 4

XER

L2

) +

+

2

2

1

x

l

k k k k

2

22 1 4 1 4 1=

+ + +

( ) ( ) ( )XEL2 XEL2 XEL2 XEM3 4

2 (

1+ XEL2 XEM3 4k k )

= + + +

X

Xk

X

Xk kL1

L2

XERZone

L1

XEL1 XEM1 2( ) ( )1 1


14/14



Taking account of the intermediate infeed in sta-tion B, there will be a further reduction of the 2nd

stage so that the safety margin is increased.

The 3rd zone (169 %ZL1) just reaches with parallelline compensation and the pickup zone (226 %ZL1) comfortably reaches over the next station butone ( C ) (A fault in C would correspond to 162 %ZL1). For the final definition of the setting, the in-termediate infeed again has to be taken into ac-count.

6. Summary

The zone setting can be estimated for the double-circuit lines based on the arithmetic proceduresand the derived equations shown here. In practicethe intermediate infeeds must be taken into ac-

count, so that the second zone can be gradedsafely past the next station (whilst retaining the se-lectivity and reliably detecting busbar faults).

If the line lengths do not differ, an acceptablecompromise for the relay setting can usually befound without parallel line compensation. Forshort following lines, parallel compensation musthowever be taken into account.

Computer programs are nowadays available forthe relatively complex testing of the backup zonesand the pickup.

Documents

Appl 15 Distance Prot at Parallel Lines En