Power System Protection [Vol 2 - Systems and Methods] 2nd Ed (IEEE, 1995) WW

8/18/2019 Power System Protection [Vol 2 - Systems and Methods] 2nd Ed (IEEE, 1995) WW

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ontents

F o r e w o r d

Chapter authors

Editor ia l Panel

Protec t ion symbol s used in c i rcu i t d iagrams

8.1

8.2

8.3

Overcurrent pro tec t ion J W H o d g k i s s

Introduction

Types of overcurrent system

8.2.1 Overcurrent and earth-fault protection systems

8.2.2 Grading of current settings

8.2.3 Grading of time settings: the definite-time system

8.2.4 Grading by both time and current inverse-time

overcurrent systems

8.2.4.1 Fuses

8.2.4.2 Delayed action trip coils

8.2.4.3 Fuse-shunted trip coils

8.2.4.4 Inverse-time overcurrent relays

Selection of settings

8.3.1 System analysis

8.3.2 Grading of relay settings

8.3.2.1 Grading for definite-time relays

8.3.2.2 Grading for inverse-time relays

8.3.2.3 Grading with 'very inverse' relays

8.3.2.4 Graphical method of grading

Current transformer requirements

8.3.3.1 Burdens

8.3.3.2 Variation of burden impedance

8.3.3.3 Additional burden

8.3.3.4 Significance of leads

8.3.3.5 Burden of earth-fault schemes

8.3.3.6 Effective setting

8.3.3

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8.4.

8.5

9

9.1

9.2

9.3

9.4

9.5

9.6

9.7

8.3.3.7 Time-grading of earth-fault relays

8.3.3.8 Phase-fault stability

8.3.4 Sensitive earth-fault protec tion

8.3.5 High-set instantaneous overcurrent relays

8.3.6 Relay co-ordination with fuses

Directional control

8.4.1 Directional relays

8.4.2 Connections for directional phase-fault relays

8.4.2.1 30 ° relay connection: m.t.a. = 0 °

8.4.2.2 60 ° relay connection: m.t.a. = 0 °

8.4.2.3 90 ° relay connection

8.4.3 Directional earth-fault relays

8.4.3.1 Polarisation by residual voltage

8.4.3.2 Polarisation by neutral current

8.4.3.3 Dual polarisation

8.4.4 Grading of ring mains

8.4.5 Multiple-fed ring mains

8.4.6 Parallel feeders

Bibliography

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F e e d e r p r o t e c t i o n : d i s t a n c e s y s t e m s

L J a c k so n

Introduction 66

Historical 66

Operating principles 67

9.3.1 Impedance measurement 67

9.3.2 Derivation of basic measuring quantities 69

Impedance-measuring elements comparators) and theft characteristics 71

9.4.1 Presentation of characteristics 71

9.4.2 Derivation of relay characteristics 73

9.4.3 Equivalent of amplitude and phase comparators 76

9.4.4 Basic range of impedance characteristics 78

9.4.5 Measuring characteristics of relay schemes 83

9.4.6 Mho characteristics 86

9.4.7 Practical polarised mho characterist ic 90

Development of comparators 92

9.5.1 Induction cup 92

9.5.2 Rectifier bridge moving coil 93

9.5.3 Electronic relays: introduct ion 94

9.5.4 Comparator development 95

9.5.5 Practical realisation of static phase comparators 100

More complex relaying characteristics 107

9.6.1 Basis for shaped polar characteristics 107

9.6.2 Change of angular criterion 108

9.6.3 Multicomparator schemes 108

9.6.4 Multi-input comparators 110

9.6.5 Alternative characteristics 113

Presentation of performance 113

9.7.1 Requirements 113

9.7.2 Display of measuring accuracy 115

9.7.3 Display of operating time 118

9.7.4 Application of contour timing curves 119

9.7.5 Alternative methods of presentation 123

9.7.6 Steady-state performance presentation 123

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9.8

9.9

9 . 1 0

9.11

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10.1

10.2

10.3

10.4

10.5

10.6

10.7

10.8

9 . 7 .7 D y n a m i c p o l a r c h a r a c t e r is t ic s

S w i t c h e d a n d p o l y p h a s e d i s t a n c e p r o t e c t i o n

9 . 8 . 1 I n t r o d u c t i o n

9 . 8 .2 S w i t c h e d d i s ta n c e p r o t e c t i o n

9 . 8 .3 P o l y p h a s e d i s ta n c e p r o t e c t i o n

D i s ta n c e p r o t e c t i o n s c h e m e s b a s ed o n i n f o r m a t i o n l i n k s

9 .9 .1 Genera l

9 . 9 .2 T r i p p i n g s c h e m e s

9 .9 .2 .1 Di rec t i n t e r t r ip

9 . 9 .2 . 2 P e rm i ss iv e i n t e r t r i p - u n d e r r e a c h i n g s c h e m e s

9 . 9 . 2 . 3 P e rmi s si ve i n t e r t r i p - o v e r e a c h i n g s y s t e ms

9 . 9 .3 B l o c k i ng s c h e m e s

9 . 9 .3 . 1 D i s t a n c e p r o t e c t i o n b l o c k i n g s c h e m e

9 . 9 . 3 .2 D i r e c t i o n a l c o m p a r i s o n

P r a c t i c a l c o n s i d e r a t i o n s i n t h e a p p l i c a t i o n o f d i s t a n c e p r o t e c t i o n

9 .10 .1 Fau l t r es i st ance

9 .10 .2 Measu r ing e r ro r s

9 . 1 0 . 3 H e a l t h y p h a s e r e la y s

9 . 10 . 4 L o a d e n c r o a c h m e n t

9 .1 0 .5 P o w e r s w in g e n c r o a c h m e n t

9 . 1 0 . 6 L i n e c h e c k

9 . 1 0 . 7 V o l t a g e t r a n s f o r m e r s u p e r v is i o n

T r e n d s in d i st a n ce p r o t e c t i o n d e v e l o p m e n t

B i b l i o g r a p h y

F e e d e r p r o t e c t i o n p i l o t w i r e and ca r r i e r - cu r ren t s y s t e m s

F.L. Hamilton L. Jackson and J. Rushton

G e n e r a l b a c k g r o u n d a n d i n t r o d u c t i o n

S o me b a s i c c o n c e p t s o f u n i t p r o t e c t i o n f o r f e e d e r s

B a s i c t y p e s o f p r o t e c t i o n i n f o r ma t i o n c h a n n e l s

10.3.1 Pi lot wires

1 0 . 3. 2 M a in c o n d u c t o r s

10 .3 .3 Ra d io l inks

T y p e s o f i n f o r m a t i o n u s ed

1 0.4 .1 C o m p l e t e i n f o r m a t i o n o n m a g n i t u d e a n d p h a s e o f

p r i ma r y c u r r e n t

1 0 .4 . 2 P h a se - an g le i n f o r m a t i o n o n l y

1 0 .4 . 3 S i mp l e t w o - s t a t e o f f / o n ) i n f o r m a t i o n

S ta r t ing r e l ays

C o n v e r s i o n o f p o l y p h a s e p r i ma r y q u a n t i t i e s t o a s in g le - ph a s e

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

1 0 .6 .1 G e n e r a l p h i l o s o p h y

1 0 .6 .2 I n t e r c o n n e c t i o n s o f c u r r e n t t r a n s fo r m e r s

1 0 .6 .3 S u m m a t i o n t r a n s f o rm e r s

1 0 6 4 P h a s e- s eq u e n c e c u r r e n t n e t w o r k s

E l e m e n t a r y t h e o r y o f lo n g i tu d i n a l d i f fe r e n ti a l p r o t e c t i o n

10 .7 .1 Lo ng i tud ina l d i f f e ren t i a l p ro tec t ion wi th b i ased r e l ays

10 .7 .2 Phas e-com par i son p r inc ip les

10 .7 .3 No n l inear d i f f e ren t i a l sy s t em s

1 0 . 7. 4 D i r e c t i o n a l c o m p a r i s o n s y s t e ms

10 .7 .5 Cu r ren t sou rces and vo l t age sou rces

10 .7 .6 N on l ine ar i ty and l imi t ing

P i lo t -w i r e p r o t e c t i o n

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10.8.1

10,8 .2

10.8 .3

10.8 .4

10.8 .5

10,8 .6

10.8 .7

10.8 .8

10.8 .9

Bas ic pr inc ip les

Prac t ica l re lay c i rcui ts

Su m m a t i o n c i r c u i t s

Bas ic d i sc r imina t ion fac to r

Typica l p i lo t c i rcu i t s

T y p i c a l s y s t e m s fo r p r i v a te l y o w n e d p i lo t s

Use o f ren ted p i lo t s

T y p i c a l s y s t e m s fo r u s e w i t h r e n t e d p i l o ts

V .F . p h a s e - c o m p a r i s o n p ro t e c t i o n (R e y ro l l e p ro t e c t i o n )

10 .9 Som e aspec t s o f app l i ca t ion o f p i lo t-wi re feede r p ro tec t io n

10.9 .1 Genera l

1 0 .9 .2 C u r r e n t t r a n s fo rm e r r e q u i r e m e n t s

10 .9 .3 Op era t ing t imes

10 .9 .4 Fau l t s e t tings

10 .9 .5 Pro tec t ion cha rac te r i s t i c s

1 0 .1 0 Po w e r -l in e c a r ri e r p h a s e - c o m p a r i s o n p ro t e c t i o n

1 0 ,1 0 .1 In t ro d u c t i o n

1 0 .1 0 .2 T y p e s o f i n f o r m a t i o n t r a n s m i t t e d

10 10 .3 Bas ic p r inc ip le s o f phase -co m par i son p ro tec t i on

1 0 .1 0 .4 S u m m a t i o n n e t w o r k s

10 .10 .5 M odu la t ion o f h . f. s igna l

1 0 ,1 0 .6 J u n c t i o n b e t w e e n t r a n s m i t t e d a n d r e ce iv e d s ig n als

10 .10 .7 Rece ive r

10 .10 .8 T r ipp ing c i rcu i t

10 .10 .9 S ta r t ing c i rcu it s

1 0 .1 0 .1 0 T e l e p h a s e T 3

10 .10 .11 Co nt raph ase P 10

10 ,10 .12 Marg inal gua rd

10 .10 .13 Check ing and t e s t ing

1 0,1 1 P ro b l e m s o f a p p l i c a t i o n o f p h a s e - c o m p a r i s o n f e e d e r

p r o t e c t i o n

10 ,11 .1 Genera l

1 0 .1 1 .2 A t t e n u a t i o n o v e r t h e l in e l e n g t h

10.1 1.3 Tr ipp ing and s tabil is ing angles

10 ,11 .4 Fau l t s e t t ings re la ted to capac i t ance cur re n t

10,1 1 .5 C.T , req uir em en ts

1 0 .12 D i r e c t io n a l c o m p a r i s o n p ro t e c t i o n

10 ,12 .1 Genera l

10,12.2 Bas ic pr inc ip les

10.12.3 Bas ic un i ts

10 .12 .4 Di rec t iona l re lays

10 .12 .5 Fau l t de tec t in g

10 .12 .6 Change o f fau l t d i rec t ion

10 .13 Pow er supp l ie s

I 0 .13 .1 Ge nera l

1 0 .1 3 .2 S t a t i o n b a t t e r y s u p p l y

10 .13 .3 Sepa ra te ba t t e r i e s

10 .14 B ib l iography

11

11.1

O v e rv o lt ag e p ro t e c t i o n

L C s u r o s

O v e rv o l t a g e p h e n o m e n a i n p o w e r s y s t e m s

1 1 .1 .1 Ex tern a l overvo l tages ( l ightning)

11 .1 .2 In te rn a l ove rvo l tages

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11.2

11.3

11.4

11.5

11.6

11.7

11.8

11.9

ontents

Travelling waves

11.2.1 Wave propagation along a transmission line without losses

11.2.2 Reflections at the end of the line

11.2.3 Discontinuities in surge impedance and junctions with

infinitely long lines

11.2.4 Effect of waveshape and of finite length of lines

Insulation co-ordination

11.3.1 Fundamental principles of surge pro tec tion and

insulation co-ordination

11.3.2 Basic requirements

11.3.3 Insulation and protective levels

11.3.4 Relation between overvoltage tests and service condit ions

11.3.5 Practical choice of insulation levels

Protection against external overvoltages

11.4.1 Shielding of overhead lines and substations

11.4.2 Surge protec tion by effective system layout

11.4.3 Voltage limiting devices

Protection against internal overvoltages

11.5.1 Protection against switching transients

11.5.2 Protection against sustained internal overvoltages

11.5.3 Protection against internal temporary overvoltages

Practical aspects and some special problems of insulation

co-ordination and surge protection

1 1 6 1

11.6.2

11.6.3

11.6.4

11.6.5

Probabilistic or statistical approach in insulation co-ordination

11.7.1 Statistical aspects of overvoltages and insulation strength

11.7.2 Application of statistical distribution to insulation

co-ordination

Economic aspects

Bibliography

285

285

286

286

288

288

288

289

289

289

291

292

292

295

296

302

302

305

305

306

Effect of system neutral earthing on insulation requirements 306

Choice of surge arresters and derivation of basic impulse

insulation levels 308

Clearances to earth between phases and across isolating gaps 309

Standard insulation levels clearances with recommended

co-ordinating gap settings or surge arrester ratings or both 310

Effect of rain humidi ty and atmospheric pollution 311

312

312

315

316

317

Index

3 2 2

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6/330

Overcurrent protect ion

by J.W. Hodgkiss

m

C f i a p t e r 8

8.1 In t roduc t ion

When the f i rs t small power systems were se t up, the need to add automatic protec-

t ion was soo n real ised. Equ ipm en t responsive to excess curren t ( in the f i rs t place by

fuses) was the obvious solut ion to the dif f icul t ies which had ar isen and st i l l today,

by far the major i ty of a l l c ircui ts are protected by this means. Select ive act ion was

soo n need ed an d the graded overcu rrent system has evolved to give discriminat ive

faul t pro tec t ion .

Overcur rent pro tec t ion should not be confused wi th over load protec t ion which

is re la ted to the therm al capabil i ty of plant or c ircui ts , whereas overcurren t protec-

t ion is pr imari ly pro vided for the correct c learance of faul ts. V ery of ten , how ever ,

se tt ings a re a do pted which make some com promise in order to cover bo th of these

objectives.

O vercurrent pro tect io n is achieved by the use of fuses, by direct-act ing tr ip

mechanisms on c ircui t breakers or by re lays.

8 .2 Typ es of overcur rent sys tem

Where a source of electr ical energy feeds directly to a single load, l i t t le complica-

t ion in the c i rcui t pro tec t ion i s requi red bey on d the provis ion of an overcur rent de-

vice wh ich is suitable in op erat ing characteris t ics for the lo ad in que st ion, i .e. appro-

pr ia te current se t t ing possibly with a t ime-lag to permit harmless shor t t ime over-

loads to be supplied.

De velopm ent o f the pow er-system to one such as is sho w n in Fig. 8 .2, in wh ich

the pow er source A feeds thro ugh a num ber of subs ta t ions B , C , D and E, f rom

each of the busbars of which load is taken, necessi ta tes a more se lect ive t reatment .

I t i s usua l ly not good enough to shut down the whole sys tem for every faul t a long

the line.

The system requires discr iminat ive protect ion designed to disconnect the mini-

m um am ou nt of c ircui t and load tha t will isola te the faul t . Circuit breakers are


7/330

2 Overcurrent pro tect ion

ins talled at the feeding end of each l ine sect ion and a graded scheme of pro tect ion

is applied.

A B C D E [

M e d i u m

I

n

f e e d ~ v o l t a g e

h ) a d

F u s e

p r o t e c t i o n

F ig . 8 . 2

. _ • i f

N/

L o a d s

Radial distribution system

F - - " I

S u p p l y --- ~ L o a d

J ' L

I '

M

' L

r - "

(a )

Fuses

C i r c u i t

b r e a k e r

0

I

I

I

I

O I C

1 - -

I

I

I

I

I

I

I A . C t r i p

I co i l

( b ) D i r e c t a c t i n g a . c . t r i p

F ig. 8 . 2 . 1 A Types of overcurrent protect ion

C i r c u i t

b r e a k e r

0 0 '

-

I '

I

0 1 0

I

I

0 1 0

' t . " T -

I

I

i l , i

k S

nn~ab: t

( c) O v e r c u r r e n t r e l ay s


8/330

Overcurrent prote ction 3

8 . 2 . 1 O v e r c u r re n t a n d e a r th - f au l t p r o t e c t i o n s y s t e m s

Ov ercurrent pro tect io n involves the inclusion o f a sui table device in each phase,

s ince the object is to detect faul ts which may af fect only one or two phases . Where

relays are used, the y will usual ly be energised via curren t- t ransform ers , w hich lat ter

are included in the above s tatement . Typical ar rangements are shown in Fig . 8 .2 .1A

The m agni tude of an in terphase f au l t cur ren t wi ll norma l ly be governed by the

known impedances o f the power p lan t and t r ansmiss ion l ines ; such cur ren ts a re

usually large.

Phase- to-ear th faul t cur rent may be l imited also by such features as :

( a ) the m etho d of ear th ing the sys tem neu t ra l

(b) the character is tics of cer tain types of p lant , e.g. faul ts on del ta-con nected

transformer windings

(c) res is tance in the ear th-pa th .

In conseq uence , ear th -fau l t cur ren t m ay be o f low or m odera te va lue and a lso

of ten r a ther uncer ta in in mag ni tude par t icu lar ly on accou nt o f i t em (c) above .

The protect ion is of ten required to have a high sensi t iv i ty to ear th faul ts , i .e .

ear th- faul t set t ings are of ten required to be lower than system rat ing. Response t o

an e ar th faul t a t a lower curre nt value th an system rat ing or loading is achieved by

the r es idual connect ion shown in F ig . 8 .2 .1B. Three cur ren t - t r ansformers , one in

each phase, have their secondary windings connected in paral le l and the group

con nec ted to a protect ive device, e i ther ci rcui t -breaker t r ipping coi l or a relay.

- - t

1 I ' L , .

F i g .

8 2 1 B

Residua l c i rc u i t and ea r th- fau l t re lay

With normal load current , the output of such a group is zero , and th is is a lso

the case with a system phase- to-phase shor t c i rcui t . Only when current f lows to

ear th is there a res idual component which wil l then energise the protect ive de-

vice.

Since the protect ion is not energised by three-phase load current , the set t ing can

be low, giving the desired sensit ive response to earth-fault current. Phase-fault and

ear th- faul t protect ion can be combined, as shown in Fig . 8 .2 .1C.

The pro tec t ion scheme, invo lv ing equ ipm ents a t a sere s o f subs ta t ions , can be

graded in var ious ways, the s ignif icance o f which is exam ined below.


9/330

Overcurrent rotection

M

. _ _ _ _ _ . _ . ~ " L

- P h a s e

f a u l t

r e l a y

l J

F i g . 8 2 1C

( a) T h r e e - p h a s e a n d

e a r th - f a u lt s c h e m e

( b) T w o - p h a s e a n d

e a r t h - f a u l t s c h e m e

Com bined phase and earth- fau l t pro tec t ion

8 . 2 . 2 G r a d i n g o f c u r r e n t s e t t i n g s

I f protect ion is given to the system shown in Fig. 8 .2 by simple instantaneous t r ip-

ping devices, se t so that those fur thest f rom the power source operate with the

low est curren t values and progressively higher se t tings apply to each stage back

towards the source , then if the current were to increase through the range of

set t ings, the device with the lowest se t t ing of those affected would operate f i rs t

and disconnect the over load a t the nearest point . Faults , however , rare ly occur in

this wa y; a sho r t c ircui t on the syste m will imm ediate ly establ ish a large curren t of

many t imes the t r ip se t t ings l ikely to be adopted, and would cause a l l the t r ipping

devices to operate s imultaneously.

The posi t ion would appear to be bet ter when the feeder sect ions have suff ic ient

impedance to cause the prospect ive shor t-c ircui t current to vary substant ia l ly over

the length of th e radia l sy stem , as indicate d in Fig. 8 .2.2. One m ight a t te m pt to se t

the c i rcui t breaker t r ips to jus t opera te wi th the expec ted faul t cur rent a t the end

of the associa ted feeder sect ion, bu t this wo uld n ot be successful s ince:

( i ) I t is no t prac ticable to d is tinguish in mag ni tude be tween faul ts a t FI and F2,

s ince these two poin ts may, in the l imi t , be separa ted by no more than the

path through the c ircui t breaker . The faul t current a t the a l ternat ive faul t

locat ions wil l then dif fer by only an insignif icant amount (e .g. 0 .1% or less)

necessi ta ting an unreal is t ic se t t ing accuracy.

( ii ) In the diagram, the faul t curre nt a t F I is given as 88 00 A when the source

busbar faul t ra t ing is 250MVA. In pract ice the source faul t power may vary


10/330

F a u l t c u r r e n t

M a x . 1 3 1 0 0 8 8 0 0 2 9 0 0 1 2 0 0

M in . 6 8 5 0 5 4 0 0 2 4 0 0 1 1 0 0

A B (" D

Overcurrent protection 5

6 3 0 ) E q u i v a l e n t

6 0 0 ) h . v . c u r r e n t s

" , - j

,,, . .

N /

Loads

F i g. 8 . 2 . 2 Rad ia l sys tem w i th va r ia t ion i n f au l t cu r ren t due to f eede r impedance

over a range of a lmost 2 :1 by , for exam ple , the swi tching o ut of one of two

supply t ransformers; in some cases a bigger range is possible . A minimum

source power of 130MVA may be assumed for i l lus t ra t ion in th is example ,

for which the cor respond ing fault cur rent a t F I w ould be only 540 0 A and

for a faul t c lose up to S ta t ion A the cu r rent w ould b e 68 50 A. A t r ip se t a t

880 0 A wo uld there fore no t pro tec t any of the re levant cable und er the

reduced infeed condi t ions .

I t is , therefo re , c lear that discr iminat ion by curre nt se t t ing is no t in general

feasible.

A s ingle except ion to the above dedu c t ion occurs when there is a lum ped impedan ce

in the system . Where , as in the last sect ion of Fig. 8 .2.2, the l ine feeds a t ransfo rm er

direct ly, without other interconnect ions, there wil l be a s ignif icant dif ference in the

faul t current f lowing in the feeder for faul ts , respect ively, on the pr imary and on

the secondary s ide of the t ransformer . In many cases, i t is possible to choose a

current se t t ing which wil l inhibi t operat ion for a l l secondary s ide faul ts , while

ensur ing ope rat ion for a ll pr im ary side faul ts und er a ll ant ic ipated infeed cond it ions.

The t ransformer i s not necessa r i ly adequa te ly pro tec ted by such a h igh se t pro tec -

t ion; the pro tec t ion of the t ransformer wi ll be d iscussed la te r.

In special cases, instantaneous relays with high settings are used as a supplemen-

tary feature to other protect ion systems. This subject is descr ibed in more deta i l

in Sect ion 8.3 .5.

8 . 2 . 3 G r a d in g o f t i m e s e tt in g s : t h e d e f i n i te - t im e s y s t e m

The problem discussed in the preceding Sect ion is resolved by arranging for the

equipment which t r ips the c i rcui t breaker most remote f rom the power source , to


11/330

6 O vercu rrent rotect ion

operate in the shor test t ime, each successive c ircui t breaker back towards the

supp ly s ta t ion being tr ipped in progressively longer t imes; the t ime interval be-

tween any two adjacent c i rcui t breakers is kno w n as the 'g rading margin '.

In this ar rangement, instantaneous overcurrent re lays are used in the role of

star ters or faul t detectors . They could, in pr inciple , have identical se t t ings but are

bet ter with graded current se t t ings increasing towards the supply s ta t ion. All se t-

tings are subject to a tolerance; if all relays were given the same nominal setting,

some would in fact operate a t a lower current value than others . I f the current were

to be ra ised into the tolerance band, the re lay with the lowest operat ing current

would operate f i rs t . This re lay might be located a t the supply end of the feeder and

would shut down the ent i re sys tem.

These faul t-detector re lays ini t ia te the operat ion of d.c . def ini te t ime re lays,

wh ich are se t to provide the required t ime grading. This system is therefore k no w n

as 'def ini te- t ime overcurrent protect ion ' .

Referring to the radial system of Fig. 8.2, busbar E feeds separate circuits

throu gh fuses . A re lay a t D might have ins tantaneou s op era t ion w i th a h igh cur rent

se t t ing wh ich will no t p erm it op erat ion with a faul t a t E. Alternat ively, i f this is no t

feasible on account of the range of possible current , a lower current se t t ing, but one

above the maximum load cur rent , may be used wi th a t ime se t t ing chosen to d is -

cr iminate with the fuse blowing.

Usually a t ime-lag of 0.2 s is sufficient, although it is desirable to check the

sui tabi l i ty of this value for a faul t current equivalent to the overcurrent e lement

setting.

The re lay a t C may be se t to operate in 0-5 s longer than th at a t D, i .e . in 0 .7 s,

and those a t B and A wil l be progressively s lower by the same amount , giving an

operat ing t ime for re lay A o f 1.7 s.

Since the t iming is not re la ted to faul t current , but is based only on posi t ion,

the dif f icul ty that was discussed for current grading does not exist here . A faul t a t

any poin t wi l l be removed by t r ipping the neares t c i rcui t breaker on the supply

side , which wil l occur before any of the others which carry the faul t current have

t ime to opera te . The m inimu m am ou nt o f pow er sys tem is the reby iso la ted ,

a l though for a faul t on any but the las t sec t ion , some disconnec t ion of unfaul ted

sect ions an d loss o f load is inevitable .

The only disadvantage with this method of discr iminat ion is that faul ts c lose to

the power source , which wil l cause the largest faul t current are c leared in the

longest t ime.

At subs ta t ions B , C and D, loads a re connec ted through t ransformers . The

time set t ing on these c ircui ts are se lected in the same way as that a t D on the feed

to E and should never be grea te r than tha t on the outgoing feeder f rom the same

busbars .

8 . 2 . 4 G r a d i n g b y b o t h t i m e a n d c u r r en t : i n v e r s e - t im e o v e r c u r r e n t s y s t e m s

The d isadvantage of the def in i te t ime sys tem ment ioned above , i s reduced by the


12/330

Overcu r ren t p ro tec t i on 7

use o f p ro tec t ive dev ices wi th an inver se t ime-cu r ren t charac te r i s t i c in a g raded

sys tem. F ig . 8 .2 .4 shows two t ime-cu r ren t cu rves in wh ich the opera t ing t ime

is in inver se r a t io to the excess o f cu r re n t above se t t ing . An exac t inver se r a t io has

been used in th i s d iag ram in o rder tha t i t ma y be seen tha t the e f f ec t s ob ta ined in

the g rad ing a re genera l and no t r e la ted to any spec i f ic dev ice o ther than tha t the

device has an inverse type of character is t ic .

2 . 0 -

| . 9 -

| . 8 -

1 . 7 - ,

1 . 6 -

1.5

1.4

1.3

1.2

m 1.1

l.O

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.

B A

,

I I

. I

" ' J

I

I

I

I

I

I

-J. 3

I

I

I

I

I

I

I

I

I

I

, i i i i

5

Plug sett ing multiplier

F ~. 11.2.4 Principle o f inverse time-grading

The tw o cu rves co r resp ond to p ro tec t ive dev ices hav ing the same cu r re n t se t t ing ,

wh i l s t the cu rve A show s twice the opera t ing t im e as does cu rve B fo r any cu r re n t

va lue . The cu r re n t sca le is in m u l t ip les o f se t ting c u r ren t .


13/330

8 Overcurrent pro tect ion

Tw o sect ions o f a radia l s ystem A-B- e tc . prote cted a t A and B by the corres-

po nd ing devices, is show n w ith a faul t in a l ternat ive posi t ions im m ediate ly af ter

sta t ions A and B.

Fault F~ fol lowing sta t ion B produces 5 t imes se t t ing current in both devices

which tend to operate in 0-5 and 1.0 s , respect ively. There is therefore a 'grading

margin ' of 0 .5 s wh ereby the faul t can be expec ted to be c lea red a t B and p rotec tive

device A will reset w ith ou t co m plet ing i ts ful l operat io n.

Alternat ively, i f faul t F2 occu rred, the faul t curre nt throu gh A is no w 8.5 times

set t ing, and device A wil l op erate in 0-59 s which is l i t tle s lower th an device B with

close-up faul t F~. Had the current s tep been greater , the best operat ion of A might

have been even faster than that for B, a l though such a gain in speed is unusual in

pract ice . This gain can be made a t every s tage of grading for a mult isect ion feeder

so that the t r ipping t ime, for a faul t c lose up to the power source , may be very

much shor ter than would have been possible with a def ini te- t ime system.

Inverse- time o vercu rrent system s include:

(a) fuses

(b) delayed act ion a .c . t r ip coils

(c) fuse sh un ted a.c. tr ip coils

(d) inverse-time relays.

8.2.4.1 Fuses: The fuse, the f irst pro tective device, is an inverse-time grad ed

protect ion. Although fuses are graded by the use of dif ferent current ra t ings in

ser ies , discr iminat ive operat ion with large faul t currents is obta ined because the

fuses then operate in dif ferent t imes.

This continu es to be effective even w hen the c learance t im es are very sho r t , be-

cause the fuse const i tutes both the measur ing system and the c ircui t breaker ,

whereas with a re lay system, a f ixed t ime margin must be provided to a l low for

c i rcui t-breaker clearance t i m e .

Fuses are deal t w ith ful ly in C hapter 5.

8.2.4.2

D e l a y e d a c t i o n

tr ip coils: G rading can be achieved by fit t ing delay ing

devices direct to the c ircui t-breaker t r ip mechanism. Dash-pots have been used for

this purpose but unfor tunate ly these devices are not suff ic ient ly accurate or consis-

tent to permit more than re la t ively crude grading.

8.2.4.3 F use - shunted t r ip coils : This form o f pro tect io n consists of an a .c . t r ip

coil on the c ircui t breaker , shunted by a fuse . The c ircui ts for overcurrent and

com bined overcur rent and ea r th faul t are show n in F ig . 8 .2 .4 .3A.

The res is tance of the fuse and i t s connec t ions i s low compared wi th the impe-

dance of the t r ip coi l , and consequent ly the major i ty of the cur rent , suppl ied by

the current t ransformer , passes through the fuse . I f the current is high enough when


14/330

Overcurrent pro tection 9

a fa ul t o ccurs , the fuse b low s and the cu rren t is then t ransfe rred to the t r ip coil .

The t ime t aken , f rom the ins t an t when the fau l t occurs , fo r the c i rcu i t b reake r to

t r ip i s t hus ma in ly dependen t on the t ime /cur ren t cha rac te r i s t i c s o f the fuse .

T r i p

coil

[ 1 _

( a ) . Overcur ren t

i i i ........

T r ip

coil

( b ) O ve r c u r r e n t a nd e a r th - f a u l t

Fig. 8.2.4.3A

Arrangements for fuse-shunted t r ip coi ls

So m e e r r o r is i n t r o d u c e d b y t h e c u r r e n t wh i c h is sh u n t e d f r o m t h e fu se th r o u g h

the t r ip coi l , but th is can be m ade smal l. The coi l and i t s assoc ia ted c i rcui t sho uld


15/330

10 Ov ercu rrent rotection

have an impedan ce o f m any t imes t ha t o f t he s ma l le s t f us e t h a t is t o be us ed . The

i mpedan ce o f t he co il m us t be l ow enough , howeve r , to no t u ndu ly ove rburden t he

cur rent t rans former , when the fuse b lows .

I t i s now s tandard prac t i ce to employ 2 A t r ip co i l s wi th cur rent t rans formers of

5 A secon dary ra t ing an d to use 2.5, 5 .0 , 7 .5 or 10 A fuses as requ ired. The ra t ing

of a t ime- l imi t fuse is genera l ly spec i fi ed in te rm s of the m inim um cur ren t requi red

to cause the fuse to blow, whereas in the usual appl icat ion of a fuse , i t s ra t ing is the

max imum cur r en t wh ich can be ca r r i ed con t i nuous l y w i thou t de t e r i o ra t i on .

In the design of the c i rcu i t, the fuses should be con nec ted d i rec t ly , no t throu gh

iso la t ing p lugs , to the cur rent t rans formers , to avoid the inc lus ion of addi t iona l

res is tance in the fuse c i rcui t .

F ig. 8 .2 .4 .3B shows typica l t im e/cu r ren t charac te r is t ic s for t ime- l imi t fuses. I t

wi ll be no ted tha t the d i f fe rence in opera t ing t im e be tw een a ny two ra tings of fuse

1 0 m i n

1 m i n

0 s e c

1 s e c

._=

0 . l s e c

0 . 0 1 s e c

0 , 0 0 1 s e c

- r - T A

I I

i t I t t \ . , ~ L , - - 1 2 . 5 A

size 18 S.W .G .

- ' - - - -

1 0 A s i z e 1 9 S . W . G ,

O O

C u r r e n t ( A m p e r e s )

F i g . 8 . 2 . 4 . 3 B Time/ fus ing cur ren t characteris tics o f t im e- l im i t fuses

decreases as the cur rent inc reases . These d i f fe rences cons t i tu te the d i sc r imina t ion

t ime . Unl ike main fuse pro tec t ion , th i s m argin m us t no t be redu ced to a ve ry low

value , s ince i t mus t cover the t r ipping t ime of the c i rcu i t b reaker . Thus for 0 .5 s

marg in t he f au l t cu r ren t m us t no t exceed a c e rt a in l imi t . F o r ex am ple , fo r a cu r r en t

of 37 A there is 0 -5 s be tw een the t im e for b lowing a 5 A and a 7 .5 A fuse . I f these

fus e s we re us ed i n con junc t i on w i t h 3 0 0 /5 A cu r r en t t r ans fo rmer s , f o r t he p ro -

t ec t i on o f tw o s ec ti ons o f a rad ia l f e ede r, t he m ax im um fau l t cu r r en t a t t he be -


16/330

Overcurrent pro tec tion 11

ginn ing o f the second sec t ion , tha t is a t t he send ing end o f the second feede r , m us t

n o t e x c e e d

3 7 x 3 0 0

= 2 2 2 0 A

5

If the maximum fau l t cu r ren t a t t h i s loca t ion cou ld be h ighe r , i t would be necessa ry

to use a la rger ra t ing for the f i rs t sec t ion fuse . I t would then be necessary to dec ide

w he the r th is i s admiss ib le und e r cons ide ra t ions o f min im um fau l t cu r ren t , o r o f

des ired ove r load p ro tec t ion .

F u s e r a t i n g : 1 2 . 5 1 0 . 0 7 . 5 5 . 0 2 . 5 A m p e r e s

13oo/5

M a x i m u m c u r r e n t

300/5

B

3 6 0 0

J 3oo/5

r ' - l ~

I I _ J l ' i

(2

2 9 0 0

3 o o / s

D

I

3 o o 1 5 l

2 2 0 0 1 3 0 0 A m p e r e s

3 0 "

l O -

v )

- , . j

I . O -

u 'J

. , . . ,

O . i -

0 . 0 1 -

1 0 0

F ig. 8 . 2 . 4 . 3 C

E D

\

B A

\

5 0 0 1 0 0 0

P r i m a r y c u r r e n t ( A m p e r e s )

I i ' : I o i c I B

I | I I

X ~ I I I

I I I

I I I

~

,~ I I

,


17/330

12 Ove rcurrent pr o tec t ion

not proved to be entirely sat isfactory; their characterist ics can be changed by

through-fault current and also by ageing. In view of this , al l fuses that have carried

faul t current even i f appa rent ly sound , should be replaced w i th new fusel inks .

1oo

6O

, o \ \

' 0

A

. 0

"t3

o 4.0

t~

t~

E

g 2 .o

1 . 0

0 . 6

0 . 4

I I I

C u r v e A - S t a n d a r d ( B S 1 4 2 )

B -

V e r y i n v e r s e

C - E x t r e m e l y in v e r s e

\

\ \

B

~ C

0.2

0.

2 4 6 0

20 40 60 1 0 0

F i g . 8 . 2 . 4 . 4

P lu g se t t i n g m u l t i p l i e r

Typ ica l t ime curves fo r ID M TL relays

8 . 2 . 4 . 4 I n v e r s e - t i m e o v e r c u r r e n t r e la y s : Relays designed to provide an inverse

characterist ic are both more rel iable and provide more scope for precise grading

than the above techniques . The relays are usual ly , a l though not inevi tably, of the

induction disc type and are adjustable over a wide range in set t ing current and

operating t ime. A description of relay design detai ls for this application is given in

Volume I , Chapter 6 , Sect ion 6.2 .3 .

Typical current-t ime characterist ics are shown in Fig. 8.2.4.4. Curve A is the most

commonly used characterist ic and is standardised by BS 142: Parts 1-4, 1990 (1993).


18/330

Overcurrent

protection

13

I t wil l be noted that the curve, a l though nominally inversive , depar ts f rom an exact

ra t io a t each end du e to the effects of mech anical restra int and sa tura t ion of the e lectro-

magne t . S teeper charac te r i s t ics curves B and C a re known as 've ry inverse ' and

'extremely inverse ' character is t ics , respect ively, and have specia l applicat ions.

I t will be observed that the characteristic curves are plotted with an abscissa

of 'plug se t t ing mult ipl ier ' . Relays can have many dif ferent se t t ings for which

there are as m an y t im e-current curves. This cum bersom e sta te o f affa irs is simplified

by real is ing th at a re lay of a given typ e a lways has the same amp ere- turn loading o f

i ts winding a t se t t ing and fo r each operat ing t ime po int , the coil turns b eing in

inverse ra t io to the se t t ing. I t fol lows th at a curve plo t ted in terms of mu lt iples of

set t ing curren t is applicable to a ll re lays of that typ e and a t a ll plug se t t ings. Tak ing

in to acco unt the cu r rent t ransform er w i th which the re lay is used we can wr i te :

pr imary cur rent

plug se t t ing mu lt ipl ier (p.s .m .) =

pr imary se t t ing cur rent

The ordina te i s the opera t ing t ime sca le for maximum disc movement . Res t r ic t ing

the movement by se t t ing fo rward the back s top p ropor t iona te ly r educes the ope r -

a t ing t ime, the ra t io o f such redu ct ion apply ing a t al l curre nt levels and hence being

known as the t ime mult ipl ier se t t ing ( t .m.s . ) . When the curve is plot ted with

logar i thmic scales , the effect is to displace the basic curve downwards, mainta ining

the or iginal curve shape. Because o f cer ta in small er rors in the con stanc y o f the

t ime mult ipl ier over the current range, a family of curves with t .m.s , f rom 0.1 to

1.0 is som etimes given by the re lay man ufactu rer .

The pr inciple of grading is general ly s imilar to that of the def ini te- t ime system.

For any faul t , the re lays in a ser ies system are graded so that the one nearest to the

faul t t r ips the associa ted b reaker before the others have t ime to ope rate . A grading

margin of 0 .4 or 0 .5 s is usual ly suffic ient .

8.3 Select ion of settings

I t will be c lear f rom the general ised discussion above that a know ledge of the faul t

current that can f low is essential for correct relay application. Since large-scale

power-system tests are normally impract icable , faul t currents must be calcula ted.

I t is f i rs t necessary to col lect system data and then to calcula te maximum and

m inim um fault currents for each stage of grading. Time-grading calcula t ions are

made using the maximum value of faul t current ; the grading margin wil l be in-

creased with low er currents so tha t discr imin at ion, i f correct a t the highest current ,

is ens ured for all low er values.

M inimum faul t cur rents ( i.e . shor t -ci rcuit cur rent w i th minim um am ou nt of

feeding p lant or c i rcui t connec t ions) a re de te rmined in order to check tha t cur rent

se t t ings are sa t isfactory to ensure that correct operat ion wil l occur .


19/330

14 Overcurren t pro tection

8.3 .1 Sy stem analys is

Some or al l of the fol lowing data may be needed:

(a) A one- line diagram of the power system , showing the type of all protect ive

devices and the rat io of a l l protect ive current t ransformers .

(b) The impeda nces in ohm s, percen t or per uni t , of a ll pow er t ransform ers ,

rotat ing machines and feeder ci rcui ts .

I t i s general ly suf f icient to use machine t ransient reactance X c t , and to use

the symmetr ical current value; subtransient ef fects and of fset are usual ly of

too shor t dura t ion to a f fec t t ime-graded pro tec t ion .

(c) The s tar t ing curre nt requirem ents of large m otors and the s tar t ing and s tal ling

t imes o f induc t ion mo to r s may be o f impor t ance .

(d ) The ma xim um peak load cur ren t which is expe cted to f low th rough pro tec-

t ive devices . 'Peak load ' in th is context includes al l shor t - t ime over loads due

to m oto r s tar t ing or o the r causes; i t does no t refer to the peak of the curren t

waveform.

(d) Decrem ent curves showing the ra te o f decay of the f au l t cur ren t suppl ied by

generators .

( / ) Exc i tat ion curves of the cu rrent t ransform ers and detai ls of seco ndary wind-

ing resi stance , l ead burden and o ther co nnected burden .

Not al l the above data is necessary in every case; with discret ion some i tems may be

deemed to be i r relevant .

The maximum and min imum values o f shor t - c i r cu i t cur ren ts tha t a re expected to

f low through each protect ive device are calculated. Three-phase shor t-ci rcui t

calculat ions are adequ ate for phase- faul t s tudies and are relat ively s imple.

Ear th- faul t cur re nt and i ts d is t ribut ion throug h the system should also be

examined. This wil l be necessary i f the system is ear thed through a l imit ing impe-

dance, in cases of mult ip le sol id ear thing, and also if the rat io of phase- fault cur re nt

to overcurrent set t ing is not large. Ear th- faul t calculat ions wil l be required

whenever ear th- faul t re lays are used, but should be made also in the above cases ,

even i f only overcurrent e lements are included, s ince in any case ear th faul ts wil l

have to be covered by the p ro tec t ion .

The calculat ion of faul t cur rent is covered in Volume 1, Chapter 3 .

8.3 .2 Grading of re lay se tt ings

Certain guiding pr inciples apply to the design of a graded system of protect ion;

(a) W herever possible , in any graded sequence , use relays having the same oper-

at ing charac ter is t ic a t a ll points .

This is no t an absolute rule b ut i f th is pract ice is no t fo l lowed, ex tra

care is necessary to ensure that d iscr iminat ion is maintained at a l l cur rent

values.

(b) Choo se curre nt sett ings for all relays. This choice is arb itra ry to a large

ex te n t , bu t mu s t t ake in to acco unt m axim um load cur ren ts and leg i timate


20/330

( c )

( d )

Overcurrent pro tectio n 15

shor t- t ime over loads caused b y, for exam ple, the s tar t ing o f large moto rs .

Set t ing currents as above are related to pr imary system currents . Most

re lays , however , a re c onnec ted to the sys tem through cur ren t t r ans formers ,

and the com bina t ion is to be regarded as a s ingle ent i ty . Thus a relay having

a ra ted cur ren t o f 5 A and a se t ting o f 150% would have a cur ren t se t ting of

7.5 A. I f the relay were operated f rom a current- t ransformer having a rat io

of 300•5 A, i t wo uld have a pr imary set t ing of

3 0 0

7.5 x - - - = 4 5 0 A

5

Provided the rated current of the relay is ident ical with the secondary rated

curre nt of the c urren t t ransfo rme r , the pe rcentage sett ing can be appl ied

direct ly to the pr imary rat ing of the c . t . ; hence, a lso in the above example,

150

pr imary se t ting = ~ x 30 0 = 450 A

1 0 0

Where the choice o f relay set t ing is refer red to below, i t i s the pr im ary set t ing

which is imp lied.

Pr imary set t ing currents should be graded so that the relay fur thest f rom

the power source has the lowest set t ing and each preceding relay back

towards the source has a h igher set t ing than that fo l lowing. Not only does

this ensure th at relay and c . t . er rors do no t produ ce a range of curre nt value

within which m aldiscr imina t ion m ay occur , b ut a lso i t a llows for load which

may be taken f rom the in termedia te subs ta t ions .

Time grading for inverse t ime-current systems should be calculated at the

highest possible faul t cur rent for each grading s tage. The grading margin wil l

be greater a t lower currents .

The 'grading ma rgin ' is required to cover :

( i ) Circui t -breaker clearance t ime

( ii ) Re lay oversho ot

( ii i) R elay and curren t- t ransform er er rors

0v ) An al lowance to ensure a f inal con tact gap for the discriminat ing relay

See Volume 1, Chapter 6 , Sect ion 6.2 .3 for fur ther d iscussion.

A margin of 0 .4 s is suf f icient with modern switchgear and relays . Old,

s lower and less accura te equ ipm ent m ay need a l it t le more .

8 . 3 . 2 . 1

Grading for def in i te - t ime re lays:

The overcur ren t s ta rt ing e lemen ts mus t

be given set t ings , as s tated above, which are higher than the greates t peak loads .

Moreover , i f the element has a returning rat io ( rat io of reset t ing to operat ing

curre nt) which is apprec iably below u ni ty , the reset t ing value should exceed the

peak load. This is because the relay is l iable to be operated by t ransformer inrush

cu rrent o r quickly cleared throug h-faul t cur re nt ; i f fo l lowing the curre nt surge the

relay holds in with sustained load current , the associated t iming relay wil l cont inue


21/330


22/330

Overcurrent prote ct ion 17

t ime for se t t ing current being inf ini te . The re lay, however , cannot operate

inf ini te ly s lowly since f r ic t ion, ignored in the basic theory, would become

dominant , in this condit ion. I t is therefore accepted that the re lay wil l not

operate with se t t ing current .

British Standard 142, Parts I-4, for relays specifies that ' the relay shall not

operate a t a current equal to or less than the se t t ing; the minimum operat ing

cur rent sha ll not exceed 130% o f the se tt ing .* Typica l des igns of mo dem re lays

will operate with about 115% of the setting in a fair ly long time.

Relays with the s teeper character is t ics ( 'very inverse ' and 'extremely

inverse ') will in general requ ire a larger margin, fo r exam ple 130% o f setting

curren t , to operate . This is because these re lays usual ly have a low er operat ing

torqu e, so th at pivot f r ic t ion is of m ore s ignificance.

(e) The restr ic t ions on m inim um cu rrent se t t ing are a l levia ted, as com pared with

def ini te- t ime re lays, by the fact that induct ion re lays have a fa ir ly high

re turn ing ra t io . In fac t , i f fo l lowing p ar tia l ope ra t ion on th rou gh faul t , the load

remained high and approaching re lay se t t ing so as to inhibi t reset t ing of the

relay disc, discrim ination w ou ld stil l no t be lost since the indu ctio n relay has

to be dr iven forward by current above i ts se t t ing value to complete operat ion.

Grading can now be considered for the radia l system shown in Fig. 8 .3 .2.2, in

which power is supplied f rom a source point A to substa t ions B, C, D and E a t each

of which loads a re fed th roug h s tep-dow n t ransformers .

The range of faul t current must f i rs t be establ ished. Maximum fault current for a

three-phase shor t c ircui t a t each sta t ion in turn, with the system ful ly connected, is

calcula ted. In this examp le i t is conv enient to express the 3 3 kV system and the

33 /1 lk V t ransformers as percent im pedance on a base of 100 M VA to obta in the

faul t pow er a t A. The equiva lent s tar impedance behind A is de te rmined , to which

value the impe dance of the fol lowing feeder sect ions is add ed and the faul t curren t

calcula ted a t each subsequent s ta t ion.

M inimum fault cur rent i s ca lcula ted for the condi t ion of one 10 M VA supply

t ransformer be ing d isconnec ted . A reduc t ion of faul t M VA in the 33 kV feed might

a lso have been co nsidered b ut this has no t be en d on e in this case s ince:

(a ) A s imul taneous outage in the 33 kV and l lkV sys tems is a double cont ingen cy

w ith the faul t in the 1 l kV system m aking a tr iple con tingen cy. This shou ld

be covered , a l though some loss of pe r formance may be to le ra ted , depending

upo n the prob abi l i ty of th is event .

(b) The effect is re la t ively un im po rtan t in this exam ple . Sw itching ou t one of the

two para llel 33 kV fines w ould not reduce the fault MV A on the 33 kV busbar

to half i ts normal value , i .e . would not double the source impedance, s ince

some impedance must ex is t beyond these l ines . The minimum faul t cur rent

a t busbar A w ould be reduced by perhaps 5% wi th less reduc t ion for more

rem ote fault s. This produces only u nim po r tant changes in the per form ance .

In other instances such a condit ion might be more s ignif icant and should then be

assessed. The prospective fault currents are shown in Table 1.

*Qu oted fr om BS14 2:1953 ; the 19 66 edition contain s the sam e da ta in a different presenta-

tion.


23/330

18 Overcurrent pro tection

33 kV

33/11 kV A

10 MVA

-crKEr-cr-

1 0 MVA

X = 7 %

0.SF~ 11 kV /4 15 V

I 1 M VA

1

600

A

8 0 A S 0 A X = 4

Fig. 8.3.2.2 1 Ik V radial system; data for grading calculation

M aximum load cur rent taken f rom each s ta t ion is show n on F ig . 8 .3 .2 .2 ; making

the safe assumption that such peak loads are coincident and of s imilar power factor ,

the values are sum m ated to give the feeder sect ion loads. Cu rrent- t ransform er ra t ios

have been previously chosen. Relay se t t ings are now chosen so that pr imary se t t ings

have a suff ic ient margin over normal peak loads to permit some abnormal var ia t ion

of load , whils t s t il l ensur ing that a degree of over load pro tect io n is provided. These

values are all included in Table 1.

T IM E G R A D I N G

Grading is begun a t the mo st remo te s ta t ion by choosing a su itable relay opera t ing

t ime . N o da ta i s g iven of the a r rangemen ts bey on d the 1 M VA t ransformer except

that fuse protection is used. Assuming that a single low-voltage main fuse is used

having a ra t ing equivalent to a bo ut 60 A in the 1 lk V l ine , (e .g. i f the t ransform er

has a ra t io of 11000 /415 V, a fuse ra ted a t 1600 A)a shor t c i rcui t immedia te ly

fol lowing w il l melt this fuse in app rox im ately 0 .05 s . Allowing an equal time for

arcing, the fault will be cleared in 0-1 s. Since only relay overshoot has to be added,

a re lay operat ing t ime of 0 .15 s is suff ic ient for discr iminat ion. Note that a small

m argin is permissible, since w ith on ly a single fuse, l i t t le loss occurs if the brea ker

actual ly fails to discr iminate . I f , how ever , the load is divided into tw o c ircui ts , each

separate ly fused a t half the above ra t ing, a faul t on e i ther wo uld be c leared in 0 .03 s ,

so that more than suff ic ient margin is provided by the above re lay t ime to ensure

full d iscr iminat ion,

The ma x imu m 1 lk V f au lt cu r r en t f o r the f au l t beyond the t r ans fo rmer is 626 A ,

corresponding to a p.s .m, equal to

6 2 6

s = 5 .008 ( say 5 .01)

125


24/330

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25/330

20 Ove rcurrent rotect ion

Refer r ing to Fig . 8 .2 .44 g ives a t ime for fu l l d isc t ravel o f 4 .3s . The smal les t t ime

m ul t ip l ie r se t t ing w h ich i t i s w ise to u se in no rm al c i r cum stances i s 0 -05 , wh ich

gives an actual t ime of

4 .3 x 04)5 - 0-215s

This value is g rea ter th an the value cho sen above (0 .15 s ) and is the refo re sa t is fa cto ry

to d i sc r imina te wi th the fuse .

A fau l t on sec t ion DE c lose to D wi l l r esu l t in a max imum cur ren t o f 1410A,

corresponding to a p . s .m, of 11 .3 and a fu l l t ravel t ime

(T c)

of 2 .82s . The t .m. s .

is 0 . 05 as be fo re , so tha t the ac tua l t ime (T A) is Tc x t .m.s . = 2 .82 x 0 .05 =0.141 s .

The nex t r e lay neare r to the sou rce a t C wi ll ca r ry the f au l t cu r r en t and mu s t

d iscr iminate . Al lowing 0-5 s grad ing margin , th is re lay should have a prospect ive

1 4 1 0

opera t ing t ime o f 0 .641 s. R e lay C has a p . s .m. - - 9 .4 und er th i s con d i t ion ,

150

for whic h the curve t im e is 3 .1 s . R elay C m us t the refo re be g iven a t ime m ult ip l ier

0 .641

s e t t i n g = - - - - - - = 0 . 2 0 7 .

3.1

W hen a f au l t i s p laced c lose to S ta t ion C the f au l t cu r r en t beco m es 2882 A equ iva-

len t to a p . s .m, o f 19 .2 and a cu rve t ime o f 2 .23 s . The t .m. s , has a l r eady been

chosen above to be 0 -207 in o rder to g rade wi th r e lay D , so tha t the opera t ing t ime

is now

TA = 2 .23 x 0 .207 = 0-462 s .

The process o f grad ing re lays B and A is s imilar to the s teps se t ou t above ; the

comple te g rad ing ca lcu la t ion i s se t ou t in Tab le 2 . I t i s we l l wor thwhi le , in any

grad ing ca lcu la t ion , to a r r ange the work ing in a t ab le o f th i s fo rm so tha t the r esu l ts

can be eas i ly rev iewed.

Th is exam ple shows tha t the opera t ing t ime fo r r e lay A is no t the sum o f the

g rad ing marg ins . A l though th ree s teps o f g rad ing , each wi th 0 .5 s marg in a re p rov ided ,

r e lay A opera tes fo r a c lo se -up f au l t in 0 .86 s ; i f a de f in i te t ime sys tem wi th a

s imi la r marg in had been used the op era t ing t ime fo r a f au l t c lose to the supp ly

s ta t ion A would have been 1-65 s , ( i . e . 0 .15 + 3 x 0 .5 = 1 .65 s ) .

The gain is due to the red uc t ion in operat in g t im e fo r each re lay as the fau l t is

moved f rom the fo l lowing s ta t ion where i t s t .m . s , fo r g rad ing i s ca lcu la ted , to the

close-up pos i t ion .

T h e c a l c u l a t io n is p e r f o r m e d f o r t h e m a x im u m f a u lt c u r r e n t r e le v a n t t o e a c h

grading s tep ; e .g . re lay C is g raded with D with a fau l t c lose to D, no t fur ther down

the sys te m a t E o r F . I f g rad ing is pe r fo rm ed a t the h ighes t poss ib le cu r ren t , even

grea te r m arg ins wi ll ex i s t un der low er cu r ren t cond i t ions . Th is f ac t is i l lu s t r a ted by


26/330

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29/330

24 Overcurren t pro tection

ca lculat ing the per formance of each re lay under the m inimu m faul t cur rents q uo ted

in Table 1. In this case, the t .m.s, values are those det erm ined in Table 2. The

results are sho w n in Table 3 . I t will be observed that reasonable o perat ing t imes are

stil l obtained and that each grading margin is slightly increased above 0.5 s.

8 .3 .2 .3 Grading wi th 'very inverse ' re lays: Reference is mad e in Sect ion 8.2.4.4

to a l ternat ive re lay character is t ics. T he above exam ple of grading is perfo rm ed using

the s tandard (BS142) charac te r i s t ic , which i s the one most commonly used . By

steepening the character is t ic , the recovery in tota l t im e, which is achieved by the

speeding up of each re lay as the faul t po si t ion is m oved thro ug h the re levant sect ion

of the system, is increased. In a typical case there may be li t t le increase in total

tr ipping time for the successive relays, back to the power source, whilst in an

extrem e case the t r ipping t ime m ay actual ly decrease in successive sect ions countin g

back to the supply s ta t ion .

A grading calcula t ion for the system of Fig. 8 .3 .2.2 and the maximum fault

currents l is ted in Table 1 is se t ou t in T able 4. R elays C and D have the same c urrent

se t t ings as previously but the t ime mult ipl ier se t t ing of re lay D has been increased

to 0 .1 in orde r to discr iminate with the med ium voltage fuse. The grading calcula t ion

then proceed s by similar s teps to those in Table 2; i t is fou nd to be necessary to

increase the cu rrent se t t ings of re lays B and A to 3 75 a nd 80 0 A , respect ively, in

order to p rovide a grading margin of 0 .5 s .

I t will be seen th at the t r ipping t ime for a faul t c lose to the supply s ta t ion A is

very substant ia l ly reduced, compared with that for the ' s tandard ' re lay. This

reduc t ion in t ime could be a very im po r tant ga in in the opera t ion o f the pow er

sys tem.

I t may well be asked why the very inverse re lay is not used universal ly. One

reason is that the s teep character is t ic is obta ined a t the expense of a considerable

reduction in operat ing torque. The re lay e lement is therefore more del ica te in a l l

respects , having a lower main contact ra t ing, reset t ing more s lowly, and obviously

being m ore susceptible to fa ilure when in a less than perfect s ta te of mechanical

condition and cleanliness.

The above comparison refers to e lectromechanical re lays. I f pract ice changes to

the universal use o f e lectronic re lays, the posi t ion wil l require reassessment , taking

into account a l l the features of the equivalent e lectronic re lays.

8.3 .2.4 Graphical

m e t h o d o f

grading: Fig. 8.3 .2.4 A illustrates the resu lt o f

applying the se t t ings calcula ted. Provided logar i thmic scales are used, the effect of

increasing the curre nt se t t ing is to shif t the curve hor izo nta l ly , and a l ter ing the t ime

setting shifts the curve vertically. In neither case is there a change in shape. I t is

possible therefore to determine the required se t t ings by using a t ransparent

template cut to the shape of the re lay t ime/current character is t ic . Such a template

is i l lustrated in Fig. 8.3.2.4B. The curve is plotted to the same scale as will be used


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Overcurrent prote ction 25

IO.0

6.0

4.0

2.0

C

.~ 1.0

E

0.6

0.4

0.2

0.1

\ 0 '

Vertical lines corr esp, md to

maximum fault currents at

each substat ion.

Vert ical separat ion of curves

gives grading margin.

;c ' i

. , - fA

i ,

0.5 second

grading margin

t i

Maximum fault current

at supply point.

1

m t ., 4

Faul t cur ren t (Amperes)

Fig. 8.3.2.4A

Display o f grading calculat ion fo r radial system (Table 2)

fo r dep ic t ing the ac tua l g rad ing . In add i t ion , a ho r izon ta l l ine X i s d rawn th rough

the value 1 .0 on the t ime scale to serve as an ind icat or line for the t ime mu lt ip l ier ,

and the ver t ica l l ine Y i s d rawn th rough the po in t p . s .m. = 1 to ind ica te the p r imary

cur ren t se t t ing . The g rad ing ope ra t ion is pe r fo rm ed on a g raph shee t w i th a s imi lar

s ized ru l ing to th at used fo r p lo t t ing the c harac ter is t ic curve, bu t the abscisca is

sca led in p r imary cu r ren t over a su f f ic ien t r ange to accommodate the sys tem fau l t -

cur ren t values .

T h e m a x i m u m v a lu e s o f fa u l t c u r r e n t a t e a c h r e l ay p o s i t i o n a r e c a l c u l a te d , a n d

ver tica l lines a re d raw n th roug h the co r respon d ing scale va lues . S ta r t ing wi th the

re lay wh ich i s requ i r ed to o pera te in the sho r tes t t ime and a t the lowe s t cu r ren t

( r e lay D in th is ex am ple ) , p lace the tem pla te over the sca les and ad jus t i t s pos i t ion

so tha t the ver t ica l l ine Y co r resp onds to the cu r ren t se t t ing chosen w i th r egard to

load , and s l ide the templa te un t i l the t ime a t the cu r ren t va lue fo r a f au l t a t F i s

su i tab le to d i sc r imina te wi th the fuse p ro tec t ion a t tha t po in t ; tha t is , w i th 626 A ,

the t im e m us t be no t le ss than 0 .15 s . The ho r iz on ta l l ine X then ind ica tes the

t .m.s , on the t ime scale . In th is exam ple , the t .m.s , ind ic ated is 0 .03 5 , which is


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26 O vercu rrent rotection

ra ther low, so the template is moved up t i l l l ine X indicates 0 .05. The re lay t r ipping

time is then 0 .21. With the template in this posi t ion a l ine is drawn to represent the

characteristic as far as the vertical l ine D, where the time is 0.14.

A point is marked 0 .5 s higher a long l ine D, i .e . a t 0 .64 s , and the template is

moved so that the curve passes through this point a t the same t ime making the l ine

Y pass thro ug h the chosen curre nt se t t ing for re lay C. The cu rrent se t t ing m ay be

chosen a t this s tage by sui tably moving up l ine Y, but the same considerat ions of

load current , and available plug se t tings are required.

Line X indicates a t .m.s , for re lay C of 0 .21 .

The curve is no w draw n as far as the ver t ica l l ine C, show ing a m inim um t ime for

re lay C of 0 .46 s. Th e se t t ings for a ll re lays back to the s upp ly s ta t ion are determ ined

by proceeding in a s imilar manner .

The graphica l method i s convenient where many gradings have to be made , and

where drawing board faci l i t ies are avai lable and standard templates have been cut .

In o ther cases, the tabular m eth od is perhaps s impler and w ith the a id of a

calcula tor can be about as quick.

When an a t tempt is made to grade devices having dif ferent character is t ics , e .g.

' s tandard ' and 've ry inverse ' the graphica l method may he lp to show poss ib le

dangers , i .e . unexpected encroachment of character is t ics a t some level of current .

Whether tabular or graphical method is used, the concise presenta t ion is c lear and

suitable for f i ling as a perm an en t reco rd.

The n um be r" of s tages with sa t isfactory discr iminat ion betw een re lays is

cont ro l led by the maximum faul t cur rents and the maximum permiss ib le t ime for

faul t c learance a t the infeed posi t ion, to grade with other protect ion in the higher

voltage system. Where the dif ference in maximum fault current between the re lay

points is appreciable , the number of s tages can be increased within a permit ted

infeed clearance time, but where these differences are small the relays tend to

operate as definite-time devices, whatever their basic characteristics, and the

number of s tages is more l imited. When a l imit ing ear thing resis tor is used, there is

l ikely to be co m parat ively l i t tle v ar ia t ion o f ear th-faul t current thr ou gh ou t a radia l

sys tem such as the above example . For tuna te ly , ze ro-sequence components do not

pass be yo nd the usual del ta-star t ransform er so that ear th-faul t cu rrent in a

distr ibut ion system wil l produce only phase-phase currents in the higher vol tage

F i g . 8 . 3 . 2 . 4 B

Y k Y - Vertica l da tu m line to

ind icate pr imary se t t ing

X - H o r i z o n t a l d a t u m l in e to

Template fo r graphical grading m etho d f or ID M T relays


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Overcurrent pro tectio n 27

system, which being of l imited magnitude are less l ikely to involve the re levant

protect ion. The t ime l imita t ion which is imposed is l ikely to be less severe .

Never theless , the number of ear th-faul t re lay grading stages needs to be careful ly

examined.

8 .3 .3 Cur rent t ransformer requi remen ts

I t is obvious tha t an inverse t im e/cu rren t re lay can no t give i ts correct perform ance

if i t is not energised with the current for which i ts operat ing t ime is ca lcula ted. A

relay and energising current t ransformer operate as a s ingle ent i ty , but the

cal ibra t ion of the re lay is lost i f the t ransformer does not fa i thful ly reproduce the

input current on the reduced scale . I t is not easy to calcula te an accurate correct ion

for c .t . e rrors . An indu ction re lay energised f rom a c . t. o f inade qua te ou tp ut

operates too s lowly, par t ly because of ra t io error in the c . t . and par t ly because of

waveform dis tor t ion of the secondary cu r rent .

Where t im e/cur rent grading is requi red the accuracy o f the cur rent t ransformer

must be mainta ined up to many t imes the pr imary ra t ing . This i s poss ib le wi th

protec t ive cur rent t ransformers provided the to ta l burden in VA at ra ted cur rent o f

the seco nd ary c ircui t, including re lays, instrum ents i f any and wir ing is suff ic ient ly

below the ou tp ut ca pabil i ty o f the t ransform er . B S39 38 c lassif ies protect ive curre nt

t ransformers as 5P or 10P, cor respond ing to m axim um er rors of 5% and 10%,

respec t ive ly , a t the maximum cur rent for measurement . The maximum cur rent i s

spec if ied in te rms of an ' accuracy l imi t c ur ren t ' and the ra t io of the accuracy l imi t

curren t to the ra ted curren t is the 'accuracy l imit fac to r ' . Protect ive current

transformers are specif ied in terms of VA at ra ted current , c lass and accuracy l imit

fac tor , e.g . 10 VA /5P /15.

The burden VA and the accuracy l imi t fac tor a re approximate ly mutua l ly

inverse . I f the se con dary ra t ing in the abo ve is 5 A, then the vo ltage across the ra ted

burd en a t ra ted c urren t will be 2V , and wil l be 3 0V a t the acc uracy l imit current .

The la t te r va lue must be wi th in the sa tura t ion l imi t o f the c . t. secondary w inding, f f

the bu rde n is halved, the accuracy limit curren t can increase near ly bu t no t qui te to

double the form er f igure, the lack o f s t ric t inverse prop or t ion a l i ty be ing on acc oun t

of the res is tance of the secondary winding, which must be a l lowed for i f an

accurate assessment is required.

The fol lowing example i l lustra tes the procedure :

impedan ce of bu rden (1 OVA a t ra ted cur rent )

10

52

0 . 4~ 2


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28 Overcurrent protec tion

Suppose res is tance of c . t . secondary winding is 0 .15 ~2 ,

To ta l secondary impedance = 0 .55 ~2 .

(Ar i thmet ica l add i t ion i s u sua l ly good enough)

Seco ndary e .m. f , a t accu racy l imi t cu r r e n t is 0 -55 x 5 x 15 = 41 -25 V ,

which the c . t . mus t be ab le to deve lop .

I f the bu rd en is halved ( i .e . to 0 -2 S2),

Seco ndary imp edan ce = 0 -2 + 0 .15 = 0 .35 ~2

41-25

Ne w a c c u r a c y l im i t c u r r e n t = - - - - - - - = 1 1 7 .8 A

0 . 3 5

Ne w a c c u r a c y l im i t f a c to r = 2 3 -5

Fo

Power System Protection [Vol 2 - Systems and Methods] 2nd Ed (IEEE, 1995) WW

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Transcript of Power System Protection [Vol 2 - Systems and Methods] 2nd Ed (IEEE, 1995) WW