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Directional earth-faultovercurrent protection for solidlyearthed networks

ABB Network Partner AB

Version 121

1MDX80015-EN

Page 1

September 1997

Contents Page

1 DIRECTIONAL EARTH-FAULT OVER-

CURRENT PROTECTION FOR SOLIDLY

EARTHED NETWORKS

2

11 Application

2

12 Theory of operation

4

13 Setting

6

14 Testing

8

15 Technical data

11

16 Appendix

12

161 Terminal diagrams 12

162 Signal list 13163 Setting table 14

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Directional earth-faultovercurrent protection for solidlyearthed networks

September 1997

ABB Network Partner AB

In case of single-phase earth-faults the primary fault resistance will vary

with the network conditions and location of the fault In many cases thefault resistance is much higher than the resistance that can be covered by

an impedance measuring distance relay

Earth-faults with high fault resistances can be detected by measuring the

residual current (3I

0

) Directional earth-fault protection is obtained by

measuring the residual current and the angle between this current and the

zero-sequence voltage (3U

0

)

The current 3I

0

lags the polarising voltage (-3U

0

) by a phase angle that is

equal to the angle of the zero-sequence source impedance In solidly

earthed networks this angle will be in the range of 40

deg

to nearly 90

deg

where the high value refers to stations with direct earthed transformers

with delta winding To obtain maximum sensitivity under all conditions

the forward measuring element should have a characteristic angle of 65

deg

As a general rule selectivity is more easily obtained by using the direc-

tional instead of the non-directional earth-fault overcurrent protection

High resistive earth-faults can also be detected by the directional protec-

tion the limiting condition being that sufficient polarising voltage must be

available

Measurement of the distance to the fault can not be made using the zero-sequence components of the current and voltage since the zero-sequence

voltage is a product of the zero-sequence components of current and

source impedance Hence the necessary selectivity must be obtained by

current grading or time delay

The best selectivity is generally obtained by using inverse time delay all

relays having the same type of inverse characteristic An earth-fault in a

line will be selectively tripped if the difference between the fault current

in the line and the residual current (3I

0

) in the other lines gives a time dif-

ference of 03-04 seconds A logarithmic characteristic is generally the

most suitable for this purpose since the time difference is constant for a

given ratio between the currents see Fig 2

1 DIRECTIONAL EARTH-FAULT OVERCURRENT PROTECTION FOR SOLIDLYEARTHED NETWORKS

11 Application

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Directional earth-faultovercurrent protection for solidlyearthed networks

ABB Network Partner AB

Version 121

1MDX80015-EN

Page 3

September 1997

The directional earth-fault overcurrent protection module for relay

REL 5xx has available independent time delay plus four inverse time

characteristics viz

bull Normal inverse (NI) according to IEC 255-3bull Very inverse (VI) according to IEC 255-3

bull Extremely inverse (EI) according to IEC 255-3

bull Logarithmic inverse (IDG) according to the formula

t = 58 - 135 sdot l

n

I I

a (s) where I

a

is

the set characteristic value (3I

0

gt)

In some cases the selectivity can be improved by adding a settable mini-

mum operating current (IMin) and a minimum operating time (tMin) to

the inverse characteristic These functions are included in the protection

The residual inrush current can cause unwanted tripping of the earth-fault

overcurrent relay when energizing a directly earthed power transformer

The earth-fault overcurrent protection is therefore provided with second

harmonic restraint which blocks the operation if the residual current (3I

0

)

contains 20 per cent or more of the second harmonic component

A serial fault can be caused by broken phase conductor(s) with no contact

to earth or pole discrepancy in a circuit-breaker or a disconnector The

most common type of serial fault is pole discrepancy at breaker manoeu-

vring To minimise the operating time the earth-fault overcurrent protec-

tion module is provided with a switch-onto-fault logic which can be

activated at breaker closure which temporarily reduces the tripping timeto 300 ms

Serial faults can be correctly detected if the voltage transformers feeding

the directional earth-fault protection are situated on the bus side of the

breaker

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Directional earth-faultovercurrent protection for solidlyearthed networks

ABB Network Partner AB

Version 121

1MDX80015-EN

Page 5

September 1997

The current 3I

0

lags the polarising voltage (-3U

0

) by a phase angle that isequal to the angle of the zero sequence source impedance The forward

measuring element operates when

where

ϕ

is the angle between 3I

0

and -3U

0 (positive if 3I

0

lags -3U

0

)

3I0D is the set operate value

The change in operate value is small when the phase angle deviates moder-ately from 65

deg

A deviation of 20

deg

will increase the operate value by only

65

The polarising voltage normally obtained from the broken delta windings

of the VTs can have a high content of harmonics relative the fundamentalfrequency when the output voltage is low particularly when capacitive VTsare used To secure a correct measurement the directional function must

have an effective bandpass filtering of the voltage In the module the filter-

ing secures a correct function for fundamental frequency polarising volt-

ages down to 1 of rated voltage

In case of an external fault the capacitive current generated on the line will

decrease the current to the earth-fault relay situated at the line end towardsthe fault The reverse direction comparator should therefore have an

increased sensitivity to secure reliable blocking in case of external faults

when a directional comparison or a blocking communication scheme is

used The operate current of the reverse direction measuring element in themodule is as a fixed ratio set at 06 sdot 3I0D

The independent time delay function is activated by setting CurveType= Def

The timer t1 starts when the current 3I

0

to the relay is equal to or higher

than the set operating value for IMin and the content of the second har-

monic in 3I

0

is less than 20

The inverse time calculation starts when 3I

0

is equal to or higher than the

set operating value for IMin and the content of the second harmonic in 3I

0

is less than 20 The inverse time delay is determined by the selection of

the characteristic (NI VI etc) under setting ldquoCurveType =rdquo and the set-ting of the characteristic current 3I

0

gt The timer t1 starts when both the

inverse time characteristic and the timer tMin operate Timer t1 is nor-

mally set at zero It can be used to add a constant time to the inverse time

delay

The effect of the settings IMin and tMin on the inverse characteristic is

shown in Fig 2

3I0 65deg ϕndash( ) 3I0Dgecossdot

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September 1997

ABB Network Partner AB

Fig 2 Normal inverse and logarithmic inverse time characteristics

To detect high resistive earth-faults a low operating current is required

On the other hand a low setting will increase the risk for unwanted opera-

tion due to unbalance in the network and the current transformer circuits

The minimum operating current (IMin) of the earth-fault overcurrent pro-

tection must be set higher than the maximum false earth-fault current

The unbalance in the network that causes false earth-fault currents is

caused mainly by untransposed or not fully transposed parallel lines with

strong zero-sequence mutual coupling This false earth-fault current is

directly proportional to the load current

In a well transposed system the false earth-fault current is normally lower

than 5 of the line current except for extremely short parallel lines (less

than 5 kilometres) where a higher false earth-fault current may be found

In case of extremely short or not fully transposed parallel lines the false

earth-fault current must be measured or calculated when maximum sensi-

tivity is desired Generally 80 A is recommended as a minimum primary

operating value for the earth-fault overcurrent protection

The choice of time delay characteristic independent (definite time) nor-

mal inverse very inverse extremely inverse or logarithmic inverse

depends on the network

(X80056-22)

x 3Iogt

Logarithmic Inverse

1

mint

1

2

52 3

3

4

5

t [s] I min

7 10 20 30 50

Normal Inverse(K=04)

13 Setting

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Directional earth-faultovercurrent protection for solidlyearthed networks

ABB Network Partner AB

Version 121

1MDX80015-EN

Page 7

September 1997

To achieve optimum selectivity the same type of characteristic should be

used for all earth-fault overcurrent protections in the network Therefore

in networks already equipped with earth-fault overcurrent relays the best

selectivity will normally be achieved by using the same type of character-

istic as that in the existing relays

The following formulas for the operating time in seconds apply to the

characteristics used within the REL 5xx line protection terminal

Characteristic Operating time (s)

Normal inverse

Very inverse

Extremely inverse

Logarithmic inverse

where

I is a multiple of set current 3I

0

gt

k is a time multiplying factor settable in the range of 005 to 11

All inverse time characteristic settings will be a compromise between

short fault clearing time and selective operation in a large current range

The main determining factors are the maximum allowed fault clearing

time at the maximum fault resistance to be covered and the selectivity at

maximum fault current

The minimum operating current (IMin) of the earth-fault overcurrent

protection is settable one to four times the set characteristic quantity

(3I

0

gt) of the inverse time delay Hence an inverse characteristic with a

low set 3I

0

gt to get a short operating time at minimum fault current can be

combined with a higher set minimum operating current IMin in order toavoid unwanted operation due to false earth-fault currents

The minimum operate time is set independent of the inverse time charac-

teristic This time is normally set to be longer than the time delay of dis-

tance Zone 2 in REL 5xx in order to avoid interference with the

impedance measuring system in case of earth-faults with moderate fault

resistance within Zone 2

The polarising voltage for directional earth-fault overcurrent protection is

normally obtained from broken delta-connected secondary windings of

instrument voltage transformers or interposing voltage transformers The

voltage contains a certain amount of harmonics especially when the pro-tection is connected to CVTs

t0 14

I0 02

1ndash

--------------------- k sdot=

t13 5I 1ndash------------ k sdot=

t80

I2

1ndash

------------- k sdot=

t 5 8 1 35 Ilnsdotndash=

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Directional earth-faultovercurrent protection for solidlyearthed networks

September 1997

ABB Network Partner AB

Due to the bandpass filtering within REL 5xx a polarising voltage down

to 1 per cent of the rated voltage will provide correct directional function-

ing This is also valid when the protection is connected to CVTs

The minimum polarising voltage to the protection (U

min

) is calculated

from the formula

where

I

Fmin

is the minimum primary operating fault current

Z

0min

is the minimum zero-sequence impedance at the relay site

U

sec

U

prim

are the rated phase voltages of the open delta-connected

CVTs

Observe that when a blocking scheme or a permissive scheme with cur-

rent reversal or weak infeed logic is used I

Fmin

represents the primary

operating current of the reverse looking directional element

To secure operation in unfavourable cases as well U

min

should be equal to

at least 1 volt plus the maximum network frequency false voltage due to

measuring errors in the VT circuits

If not blocked the directional comparator will operate during the dead

time in case of a single-phase autoreclosure Therefore blocking input

EF---BLOCK should be activated during the single-phase autoreclosing

cycle

For testing of the directional protection and the directional comparison

logic functions a test set with a variable current and a variable voltage

output as well as a variable phase angle between the current and voltage

is required Normally the earth-fault overcurrent protection is tested in

conjunction with the testing of the distance protection functions using the

same multiphase test set Fig 3 below shows the connection of a three-

phase test set at the test of a directional relay

Umin

IF min

Z0 min

Usec

Uprim

--------------sdot sdot=

14 Testing

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Directional earth-faultovercurrent protection for solidlyearthed networks

ABB Network Partner AB

Version 121

1MDX80015-EN

Page 9

September 1997

Fig 3 Connection of the test set to the relay

Make the appropriate settings including the connections to the digital

inputs and outputs The logic diagram of the tested protection function is

suitably considered when performing the test

The impedance measuring zones may need to be blocked depending on

the zone settings to prevent operation of the impedance function when

checking the earth-fault protection

11 Set the polarising voltage to 2 of Ur and the phase angle between

voltage and current to 65deg the current lagging the voltage Check that

the operating current of the forward directional element is equal to the

setting 3I0D The function 3I0D activates digital output EF---DEFF

Check with angles ϕ = 20deg and 110deg that the measuring element

operates when 3I0 sdot cos (65deg-ϕ) ge 3I0D

12 Reverse the polarising voltage (ϕ = 180deg+65deg=245deg) and check that

the operating current of the reverse directional element is 06 sdot 3I0D

The function activates digital output EF---DEFR

13 activate the directional function by setting Dir = On

Set the polarising voltage to 2 of Ur and the phase angle between

voltage and current to 65deg

Check the operate current of the IMin function The function acti-

vates digital output EF---STEF

IL1

IL2

IL3

IN

U4

TRIP L1

TRIP L2

TRIP L3

L1I

L2I

L3I

NI

L1U

L2U

L3U

NU

R E L A Y

T E S T

S E T

R E L 5 x x

(X80057-4)

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Directional earth-faultovercurrent protection for solidlyearthed networks

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ABB Network Partner AB

Fig 4 Measuring characteristic of the directional element

14 When independent time delay is selected check the operate time of

timer t1 by injecting current two times the set operate value IMin

When inverse time delay is selected check the operate time at three

points on the inverse characteristic The formulas for operate time for

different types of inverse time delay curves are given in item 13 on

page 7

Check also the functions tMin (minimum operating time) and IMin

(minimum operating current)

15 Activate the input EF---BC to check the function of the switch-onto-

fault logic

Check that digital outputs EF---TRSOTF and EF---TREF are acti-

vated with time delay 300 ms when injecting current two times the set

operate value IMin

16 Check the blocking functions from digital inputs EF---BLOCK and

EF---BLKTEF

Input EF---BLKTEF blocks the output from the timers to digital out-

puts EF---TREF EF---TRSOTF and the tripping output

17 Set the phase angle of the polarising voltage to ϕ = 245deg and check that the directional current function and the switch-onto-fault logic

gives no operation when the current is in reverse direction

Upol = -3Uo

Iset

65

3Io Operation

ϕ

(X 80015-4 (2))

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Version 121

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September 1997

Table 1

Setting range Accuracy

Basic current 3I0 gt(5-300) Ir in stepsof 1

plusmn5 Iset

Operating value for direc-tional current measure-ment

forward 3I0 sdot cos (ϕ-65deg)

reverse

(5-35) Ir in stepsof 1

60 of the setting forforward operation

(5-10) Iset plusmn10(10-35) Iset plusmn5

(5-10) Iset plusmn15(10-35) Iset plusmn10

Independent time delay (0-60) s in steps of001

plusmn05 plusmn10 ms

Normal inverse charact k = (005-11) in stepsof 001

IEC 255-3 class 5

plusmn60 ms

Very inverse characteristic k = (005-11) in stepsof 001

IEC 255-3 class 75

plusmn60 ms

Extremely inverse charact k = (005-11) in stepsof 001

IEC 255-3 class 75

plusmn60 ms

Logarithmic characteristic See the formula below plusmn004 sdot [1 - I Iset]at I = (13-29) Iset

tMin for dependent charact (005-60) s in steps of001 s

plusmn05 plusmn10 ms

Start current for independ-ent and minimum operatecurrent for inverse timecharacteristic IMin (100-400) of 3I0 gt in

steps of 1plusmn5 Iset

Rated voltage 110radic3 V -

Minimum polarisingvoltage

1 Ur plusmn5 at 50 Hz

plusmn15 at 60 Hz

Characteristic angle 65deg lag - fixed plusmn5deg at 20 V and3I0D = 35

15 Technical data

t 5 8 1 35 lnsdotI

3I0set

---------------+=

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Directional earth-faultovercurrent protection for solidlyearthed networks

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ABB Network Partner AB

Fig 5 Simplified terminal diagram of the function

Fig 6 Terminal diagram of the function

16 Appendix

(X 80015-5 (2) )

EF---BLKTEF

EF---BLOCK

DIRECTIONAL EARTH-FAULT OVERCURRENT PROTECTION

EF---BC

EF---TRSOTF

EF---DEFF

EF---DEFR

EF---STEF

EF---TREF

161 Terminal diagrams

t

t

2fn+-

amp

2fn

Directional

3Uo

EF---BLOCK

EF---BLKTEF

001 Un

EF3IoSTD

3 Io x

50 ms

t

60 REVERSE

100 FORWARD

1

NI

3Io

IDG

EI

VI

Def

EF---BC

3Iogt

EFK

amp

5

Imin

tmin

t

300 ms

amp

1000 ms

amp amp

amp

amp amp

EF---DEFR

EF---DEFF

1

amp

tamp

t1EF

amp

EF---STEF

1 EF---TREF

EF---TRSOTF

cos( -65)φ

(X 80015-62)

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162 Signal list

C O N N E C T I O N S

T O

S E T

T I N G

D E S C R I P T I O N

E F - - - B L O C K

B I

B l o c k o v e r c u r r e n t E F p r o t e c t i o n

E F - - - B L K T E F

B I

B l o c k o f t r i p t i m e r

E F - - - B C

B I

B r e a k e r c l o s i n g

P R O D U C T I O N

T O

S E T

T I N G

D E S C R I P T I O N

E F - - - S T E F

B O

E F P i c k - u p

E F - - - T R E F

B O

T r i p E F

E F - - - T R S O T F

B O

T r i p E F f o r s w i t c h - o n t o - f a u l t c o n d i t i o n

E F - - - D E F F

B O

D i r e c t i o n a l E F F o r w a r d o p e r a t i o

n

E F - - - D E F R

B O

D i r e c t i o n a l E F R e v e r s e o p e r a t i o

n

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Directional earth-faultovercurrent protection for solidlyearthed networks

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ABB Network Partner AB

163 Setting table

P A R A M

E T E R

S E T T I N G R

A N G E

S E T T I N G

A C T U A L

G

r o u p 1

G r o u p 2

G r o u p 3

G r o u p 4

D E S C R I P T I O N

O p e r a t i o

n

O n O f f

A c t i v a t i o n o f t h e p r o t e c t i o n f u n c t i o n

C u r v e T y

p e

D e f N I V I E I I D G

D e f = I n d e p e n d e n t ( d e fi n

i t e ) t i m e d e l a y N I = N o r m a l

i n v e r s e

V I = V e r y i n v e r s e

E I = E x t r e m e l y i n v e r s e

I D G = L o g a r i t h m i c i n v e r s

e

3 I 0 gt

( 5 - 3 0 0 ) o

f I r

S e t t i n g v a l u e o f t h e c h a

r a c t e r i s t i c q u a n t i t y f o r t h e

i n v e r s e t i m e d e l a y

I M i n

( 1 0 0 - 4 0 0 ) o

f 3 I 0 gt

S t a r t i n g c u r r e n t f o r i n d e

p e n d e n t t i m e d e l a y a n d m i n

o p e r a t i n g c u r r e n t f o r i n v

e r s e t i m e d e l a y

t 1

0 0 0 - 6 0 0 s

D e fi n i t e t i m e d e l a y s e t t i n g a l t e r n a t i v e l y s e t t i n g o f a

fi x e d t i m e i n a d d i t i o n t o

t h e i n v e r s e t i m e d e l a y

K

0 0 5 - 1 1 0

T i m e m u l t i p l i e r f o r i n v e r s e t i m e d e l a y

t M i n

0 0 5 - 6 0 0 s

M i n i m u m

o p e r a t e t i m e f o r i n v e r s e t i m e d e l a y

D i r e c t i o n

N o n D i r

D i r e c t i o n a l

S e l e c t i o n o f d i r e c t i o n a l o r n o n - d i r e c t i o n a l E F p r o t e c -

t i o n f u n c t i o n

3 I 0 D gt

( 5 - 3 5 ) o

f I r

O p e r a t i n g c u r r e n t o f t h e

f o r w a r d d i r e c t i o n a l e l e m e n t

O p e r a t i n g c u r r e n t o f t h e r e v e r s e d i r e c t i o n a l e l e m e n t i s

0 6 x 3 I 0 D