Non-directional Overcurrent and Earth Fault Protection

7/23/2019 Non-directional Overcurrent and Earth Fault Protection

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Measurenents

ANALYSIS

AND

PROTMTION

OF

POWER SYSTEMS

NON-DIRECTIONAL

OVERCURRENT

A}ID

EAR]H FAULT

PROTBTION

BY

A

HASSALL



d

-1-

NON-DIRECTIONAL

OVERCURRENT

AND

EARTH

FAULT

PROTECTION

I. INTRODUCTION

This lecture is intended as a baslc

Lntroduction

to the

fundamental

principles

of non-dlrectional

overcurrent

and earth fault

protection.

This lecture

will

cover the different

types

of

overcurrent

relays

that

are

available, the

terminology

used,

deflnitions

of

particular

terxns and

certain

specific appllcations of

overcurrent

relaylng

to

particular

items of

plant.

2.

PRINCIPLE OF OVERCTTRRENT

PROTECTION

The

purpose

of

overcurrent

protectlon,

as wlth

other forms

of

protectlon

Ls to detect faults

on

a

polrer

system

and as

a

result

initiate

the

openlng of switchgear

in

order

to isolate

the

faulty

part

of

the system.

The protectlon

must

thus

be dlscrfuninative,

that

is

to say

lt shall,

as

far

as

posslble,

select

and isolate only

the

faulty

part

of the systen

leaving

alL

other

parts

in

normal

operatlon.

Discrinination

can be achieved

by overcurrent,

or by time, or by a

combinatlon

of overcurrent

and tlne.

2.I

DISCRIMINATION

BY CURRENT

Dlscrlnination

by

current

relles upon

the fact that the fault current

varies

wlth

the

position

of

the

fault.

This

variatlon

is

due

to

the

irnpedance

of

various ltems of

plant,

such as

cables

and transformers,



-2-

between the

source

and

the fault

and

the relays

throughout

the

system

are set

to

operate

at

sultable

values

such

that only

the

relay

nearest to

the fault operates.

There

are

a

number of

aEtemptLng

co

provide

z.L.L

The

fault

l-eveL either

substantially the

sane

$ouRcE

important

polnts

to

bear

in

rnind

when

discrlninatlon

by

thts nethod.

slde of a

clrcuit

breaker

wlll

be

i. e.

2.t.2

The

settlng

of

relay

at B

must be

such

that lt

wll-l

detect

a

fault

at

F2

but as the

fault level at

F2

and

Fl will

be

sinilar,

correct

discrininatlon

with the relay at A for

a fault

at

Fl wiLl

be nearly

inpossible.

In

practice

there

would

be

varlatl.ons

in

the source

fault

level,

typicall-y

a nax/nln

ratLo of as nuch

as

2/1.

Thus relays

set to

give

reasonable

discrinination

under

rnax.

fault

l-evel

conditions

nay

not. even

operate under

min.

fault level

conditions.

3.

DISCRIMINATION

BY TIME

If

the fault level

over a system

is reasonabl-y

constant

then

discrlnlnation by current

\ril1 not

be

possible.

An

alternative

ls

to

use

tine

discriml.nation

ln

whlch each overcurrent relay is

gl.ven

a

flxed

t,lrne

delay

wlth

the

relay farthest

away

from

the

source

havlng

the shortest

tlme del-ay.

Operating

tlne is

thus



- -

subsrantlally

lndependent

that

the

relay

nearest

the

and

thls

is

the

polnt

wlth

of

fault

level

but

the

nain

disadvantage

source

will

have

the

longest

tlme

delay

the

highesr

fault

level.

is

4.

DISCRIMINATION

BY

BOTH

TIME

AND

CURRENT

Due

to

the

llnttatlons

irnposed

by

the

lndependant

use

of

either

tlne

or

current'

the

inverse

tlme

overcurrent

characterlstic

has

been

developed.

I{tth

this

characteristlc

the

tftne

of

operation

is

lnversely

proportional

to

the

current

appried

t.e.

basicarly

the

higher

the

current

applted,

the

faster

the

reray

operates.

Thus,

the

actual

eharacteristlc

ls

a

functl.n

of

both

trne

and

current

settlngs,

thereby

gaining

the

advantages

of

the prevlous

mentioned

methods

and

elinlnating

some

of

the

disadvantages.

NOTE:

When

appl_ylng

definlte

tiue

overcurrent

taken

to

ensure

that

the

thermal

ratlng

measuring

element

is

not

exceeded.

PLUG

SETTING

MULTIPLER

AI.ID

TIME

MULTIPLIER

There

are

only

two

settlngs

or

adjustments

time

overcurrent

relay.

One

is

the

current

"tlne

nultiplier

settLng,,.

relays

care

must

be

of

the

current

SETTING

to

be

made

on

an

inverse

setting

and

the

other

the

4.L

The

current

settlng

rs

adJustable

by

means

of

a

plug

board

which

gives

normarly

seven

equally

spaced

steps.

when

the

prug

ls

removed

the

highest

tap

is

autonatlcar.ry

selected,

thus

enabling

on

load

adJustnent

without

"open

circuiting.'

the

C.T.rs.

Various

setting

ranges

are

available

and

it

is

convenient

to

refer

to

these

ranges

in

terns

of

a

percentage

of

c.T.

secondary

ratrng.

For

example 5o'2oo%

or

10-402

etc. rf

the

c.T.

has

a

nominal

secondary



rating

of

5

anp,

say,

then

a

settlng

range

of

2.5-10

anp.

current

range

of

0.5-2

anp.

ratlng,

then

a 50-2OO%

range

-4-

5O-2OO"A

setrlng

range

would

be

a

Likewise

a

L0-407"

range

would

be

If

the C.T.

had

a

1

amp

secondary

would

also

be

0.5-2

anp.

current

rf

the

c.T.

prinary

rating

is

eq'al

to

the

nornal

ful1

load

currenr

of

the

circuit

then

the percentage

setting

will

refer

directly

to

the

prinary

system.

Thls

is

an

important

point

as

if,

for

exampre,

the

nonnal

prlnary

full

load

current

nas

say

400

anp

but

the

c.T.

ratlo

was

500/5 then

a

relay

wlth settlng

range

50-2007.

of

5

anp

set

at

100%

would

not

represent

a

"full

10ad'.

settlng;

the

actual

settlng

would

in

fact

be

125"1

of

fulL

load

current.

rt

ls

convenlent

to

show

the

standard

lnverse

time

characteristlc

on

Log/Log

graph

paper

with

the

ryr

axis

scaled

in

seconds

and

the

'x,

axis

in

terms

of

'nultlples

of prug

settrng".

By

doing

this

the

characteristic

can be

applied

to

any

relay,

irrespective

of

setting

range

and

nominal

rating.

Thus

simpry'

to

obtain

the

relay

operatlng

tine

(neglecting

errors

etc)

at

this

stage)

one

wouLd

proceed

as

follows.

e.g.

C.T.

ratio

500/5.

Relay

serrlng

range

SO-ZOO%

I.e.

2.5_10

anp

Prlnary

fault

current

5000

anp.

Assume

relay

is

set

at

1O0Z

i.e.

5

amp

Now,

secondary

fault

current

=

5000

x

5

=

50

anp

56d-

This

value

of 50

aups

represents

r0

tines

the

actual

relay

setting

1.e. plug

setting

rnultiplier

=

A

=

10

5



-5-

As

can be

seen

fron

the

curve,

with

a secondary

current

equal-

to lO

tlmes

the relay settlng,

the

relay

will

operate

in 3

secs

(assurning

a tlne

rnultipller

settlng

of

unity).

If

the

primary

fault

current

was 2500

anp

(25

amp

secondary)

and

the

relay

was

set at 507" 7.e. 2.5

anp

then thls still

represents

a

plug

setting

nultiplier

of

10

(25/2.5)

and the relay

wlll

again

operate

in 3

secs

(TMS

=

1).

The

choice of current

settLng

thus

depends

on

the

load

current

and

the C.T.

ratio

and

l-s

nornally

close

to

but

above

the

naximum

load

current

-

assuming

of

course

the clrcuit

is

capable

of

carrying

the

maximum

foreseeable

load.

It

should be

stressed

at this

point

that

the relay

ls

neither

deslgned

nor

intended

to

be

used

as an overload

relay

but

as

a

protective

relay

to

protect

the

system

under

fault

conditlons.

rt

is

also inportant

to

consider

the resetting

of

the relay.

The

relay

wlll

reset when

the current

is

reduced

Eo

907"

of

the

setting

and

if

the

normal

load

current

is

above

thls value

the

relay

w111

not

reset

after

starting

to operate

under

through

fault

condltlons

whlch

are

cleared

by

other

swLtchgear.

The

"tine

nultlpLier

settlng"

1s a

mechanical

adJustnent

of

the

movlng

contact

backstop

and ls calibrated

fron 0.1

-

1.0.

The

scale

ls non-llnear

and

adjustment

sinply alters

the

dlstance

the dlsc

has

to

travel

to nake

the

contacts.

The

tine nultiplier

setting

gives

a dlrect

nulttplytng

factor

to

Ehe

reJ-ay

operatlng

time

when

quoted

at

10

tlnes

plug

setting. That

is

to say that if

the relay

operates

in

3

secs with 10

times

Ehe

plug

settlng

current

applied and

a

TMS

=

1,

then

wlth

a

TMS

=

0.5 with

the

same current

and

setting, the

relay

operating

time would

be

1.5

secs.

Sirnilarly with

a

TMS

=

0.1

the relay operatlng

tlme would

be 0.3

secs. Note

that thls direct

relationship

only

applies

at l0

tlmes

the

plug

settlng

current.

For other

values

of

plug

setting

nul-tlplier

t,he

relationship

is



-6-

not

dlrect

(although

not

very

far

out)

and

reference

should

be

uade

to

the

publlshed

characteristlcs

to

obtain

actual

operating

tines.

Note:

rn

the

prevlous

sinple

examples,

all-

error

conslderations

have

been

lgnored.

4.2

GRADING

INTERVAIS

OR

MARGIN

As

prevlousLy

mentloned,

to

obtaln

correct

discrimination

lt

is

necessary

to

have

a

tLme

interval

between

the

operation

of

two

adJacent

relays.

Thrs

tine

rnterval

or

grading

nargin

depends

upon

a

number

of

factors.

1.

The

circult

breaker

fault

interruptlng

tine.

2.

The

overshoot

tlme

of

the

relay.

3.

Errors.

4.

Flnal

rnargln

on

completlon

of

operation

(safety

nargin).

4'2'L

The

dlscrlninatlng relay

can

only

be de-energlsed

when

the circuLt

breaker

has

conpletely

interrupted

the

fault

current.

rt

is

now

normal

practice

to

use

a

value

of

100

ms

for

clrcult

breaker

overall

interruptlng

tine

but

obviously

if

lt

ls

known

that

the

swltchgear

is

slower

than

this

tiue,

thls

nust

be

taken

into

account.

4'2'2

operatlng

of

the

reLay

nay contlnue

for

a

short

tfune

after

the

relay

I's

de-energised

until

any

stored

energy

ls

dissipated.

For

example

an

inductlon

disc

element

wlll

have

stored

kinetLc

energy

(or

inertia)

and

a

statlc

relay

nay

have

stored

energy

in

capacLtors.

Although

these

factors

are

mLnLmised

by

design,

some

allowance

is

usualLy

necessary.

rt

is

common

to

use

a

flgure

of

50

ms.



-7

-

Note:

The overshoot

tine

ls not

the

actual tine

operation

takes

p1-ace

but

ls

the

tine that

to

travel-

the

same

dlstance

had

the

relay

during

which some

forward

the

relay

wouLd

have taken

remained

energised.

Texver

dvurs\d

l.ave)

.

7-

rolo-g

d.-one1giseA

-

aeiu.al

otergnool

f

rn'tL

-

Ooersh*f *rrne

Us.d

lnlt:.e

ea,lcnla,hon

o{

tt-t

a-r3

in

.

zf;l-----.-

f3.n

ll----

tt

l..r

-Ttrw

€

4.2.3

A11

rneasuring

devices

such as

relays

and

current

transformers

are

subject

to

some

degree

of error.

The time

characteristic

of either

or

both

of

the

relays

lnvolved

nay have

positive or

negatlv€

€rrof,so

Current

transformer

errors

are

nainly due to

the

nagnetlslng

characterlstic.

It shouLd

be

noted

that

C.T.

errors

do not

affect

definlte

time

overcurrent

relays'

4.2.4

A safety

nargln

of

100

ms

Ls

nornally

added

to

the

final

cal-cul'ated

margln

to

ensure

correct

discrlntnation.

Thts additlonal

tlne

ensures a

satisfactory

contact

gap

(or

equlvalent)

is

malntained.

5.

RECOMMENDED

MARGIN

In

the

past a

fixed

margln

of

0.5

secs

Itas

consLdered

adequate

for

correct

discrlnination.

With

faster

modern

switchgear

and

lower

overshoot

times

a flgure

of

0.4 secs

is

qulte

reasonable

and

under

the

best possible conditions 0.35

secs

may

be

feaslble.

t-e



8-

However,

rather

than

uslng

a

flxed

nargin

lt

is

better

to

adopt

a

fixed

time

for

circuit

breaker

operation

and

reray

overshoot

and

add

to

this

a

varlable

tine

value

which

takes

into

account

reray

and

c.T.

errors

and

the

safety

nargln.

This

is

partlcularly

so

when

grading

at

10w

values

of

plug

settlng

nultlplier

where

the

relay

operating

time

is

10nger

and

a

flxed

total

nargrn

may

be

of

the

same

order

as

the

relay

tlmLng

error.

A

fixed

value

0.25

secs

is

chosen

whlch

is

clrcuit

breaker

operatlng

tlme,

0.05

secs

and

0.1

sec

for

safety

nargln.

rnade

up

of

0.1

secs

for

for

relay

overshoot

tine

rn

considerlng

the

variable

tine

value

it

is

assuned

that

each

rDMT

relay

complles

with

error

class

E7.5

which

ts

deflned

as

normal

Brrtish

practlce

in

BS142zrg66.

The

errors

for

an

E7.5

relay

are

7

'5i(

but

all0wance

should

be

nade

for

the

effects

of

tenperature,

frequency

and

departure from

the

reference condrtlons

as

laid

down

ln

the

B's'

A

more

practlcal

approxinatron

's

to

assume

a

total

effective

error

of

2

x

7.5

i-e.

L57r

and

this

is

to

apply

to

the

reray

nearest

the

fault

which

is

consldered

slov.

To

thls

total

effectlve

relay

error

a

further

10%

ls

added

to

a110w

for

overall

c.T.

error.

Thus

lt

is

proposed

to

gradlng

nargin

between

t

=

0.25t

+

0.25

secs

where

t

=

normal

operatlng

tlne

of

relay

nearest

the

fault

As

far

as

deflnite

tlne

overcurrent

relays

are

concerned

the

fixed

value

will

renain

the

same

but

the

relays

are

assumed

to

conply

with

error

class

E10

1'e'

*

102.

For

the

reasons

stated

prevlously,

a

practlcal

approxlmatron

is to

assume

a

total

effectlve

error

of

2oZ

wlth

the

relay

nearest

the

fault

consldered

s1ow.

As previously

adopt

the

foll0wrng

equatl0n

to

deternlne

the

IDMT

relays

:



-9-

tt

=

0.2t

*

0.25

secs

For the

rnaJority

of

systems

an

overcurrent

grading

exercise

can

be

perforrned

quite

adequately

using

a

fixed

nargin

of

0.4

secs.

It

is

only

when

a

number

of

stages

are

involved

and

difficulties

are

belng

encountered

that

it

rnay

become

necessary

to

investlgate

nargin

tines

Ln

more

detail.

To

sumuarlse,

each

system

is

dlfferent

and

shourd

be

treated

as

so,

it

is

not

possible

to

lay

down

rigid

rules

regarding

grading

nargins

and

every gradlng exerc'se

w111

ultinately

be

a

compromr'se

of

some

form.

rt

should

also

be

noted

that

GECM

rDMT

relays

have

errors

less

than

7.52.

6.

TYPES

OF

RELAYS

6.1

INVERSE

TII.{E

OVERCURRENT

RELAYS

These

relays are

of

the

inductlon

disc

type

and

several

characterlstlcs

are

avail_able

as

fol_lolrs:_

6 .1

.1

NORMAL

IWERSE

TIME

-

CDGll

The

cDGll

characterlstic

conforns

to

BS142

and

is

commonry

known

as

che

3/10

characterrstrc

i.e.

At

ten

tines

plug

setting

current

and

TMS

of

I

the

relay

will

0perate

rn

3

secs.

The

relay

rs

fltted

wrth

a

slngle

dr.sc

contact

lrhich

wrll

'ake

and

carry

for

0.5

secs,

250ovA

with

rnaxina

of

10A

and

660v.

An

auxlllary

unlt

can

be

fltted

ln

the

same

case

which

has

two

electrLcally

separate

contacts

rated

to

make

and

carry

for

0.5

secs,

7500vA

wlth

naxlma

of

30A

and

660v.

stated,

C.T.

errors

Ehus

lt

ls

proposed

If

requlred

the

relay

can

be

electrlcally

separate).

The

contact

ls

used

for

tripplng

will

have

little

effect

on

the

operatlng

time,

to

adopt

the

equatlon

:

fitted

with

two

disc

contacts

(which

are

relay

is

then

deslgnated

CDG16.

The

top

and

the

bottom

contact

for

alarn.



-10-

The

cDGll

is

wrdely

used

to

all

system

voltages

-

as

back

up

protection

on EIIV

systems

and

as

the

main

protection

on

lrv

and

MV

distributlon

systerns.

An

alternative

characterlstlc

is

avallable

in

the

cDGll

range

and

is

referred

to

as

the

I .3/Lo

characterlstic.

The

characteristic

has

a

sinil.ar

shape

to

the

3/10

characteristlc

but

the

operatlng

rime

ar

10

tines

plug

setting

is

1.3

secs

(TMS

_

1).

general,

the

nornal

inverse

characterlstics

are

used

when

:

There

are no

co-ordinatr.on

requlrements

with

other

types

of

protective

equipment

farther

out

on

the

systen

e.g.

fuses,

thermal

characterrstrcs

of

transformers,

motors

etc.

In

(a)

(b)

The

fault

level

at

the

near

and

vary

significantly.

far

ends

of

the

system

does

not

(c)

There

is

nininal

inrush

on

cold

load

piek

up.

cold

load

inrush

ls

that

current

which

occurs

when

a

feeder

is

energlsed

after

a

prol0nged

outage. rn

general

the

relay

cannot

be

set

above

thls

value

but

the

current

should

decrease

below

the

relay

settlng

before

the

relay

contacts

close.

6.I.2

VERY

IWERSE

TIME

-

CDG13

rf

there

Ls

a

substantial

reductlon

ln

fault

Level

as

the

dlstance

fron

the

source Lncreases,

there

rnay

be an

advantage

ln

uslng

the

very

Lnverse

characteristlc

rather

than

the

normal

Lnverse

characteristic.

The

cDG13

operatlng

tlne

ls

approximately

doubled

for

a reductLon

in

current

froo 7

to

4

times

the

relay

settlng

and

this

enables

the

use

of

a

common

time

nultlpller

setting

for

a

number

of

relays

in

seri.es.



-

11

-

6.1.3

EXTREMELY

INVERSE

TIME

.

CDG14

I,lith

thls

characteristic

the

operating

ttne

is

approximately

inversery

proportional

to

the

square

of

the

current.

The

rong

operating time

of

the

relay

at

peak

values

of

load

current

nake

the

relay

particularly

suitable

for

grading

with

fuses

and

also

for

protection

of

feeders

whlch

are

subject

to peak

currents

on

switchlng

in,

such

as

feeders

suppryrng

refrrgerators,

pumps,

water

heaters

etc.'

which

remain

connected

even

after

a

prolonged

interruptlon

of

supply.

Another

appllcation

for

this

relay

is

with

auto

reclosers

in

low

voltage

dlstribution

circuits.

As

the

najority

of

faults

are

of

a

transLent

nature,

the relay

is

set

to

operate

before

the

normal

operatlng

tine

of

the

fuse

thus preventlng

perhaps

unnecessary

blowlng

of

the fuse.

upon

reclosure,

if

the

fault

persists

the recloser

locks

itself

ln

the

closed

posltion

and

allows

the

fuse

to

blow

to

crear

the

fault.

6.L.4

LONG

TIME

IDMT

-

CDG12

The

CDG12

is

a

heavily

dauped

lnductlon

disc

unlt

with

a

long

tine

characteristlc

and

1s

used

for

protectlon

of

neutral

earthing

resistors

(which

nornally

have

a

30

sec.

rating).

The

relay

has

two

fixed

settlngs

of

L5t

ar.d

207".

A

tapped

version

Ls

avaLlable

with

a

setting

range

of

80-2402

of.

5

anp

and

this

can

be

applied

to

give

a

measure

of

overload

protectlon

to

motors

and

generators.

The

relay

operating

tlue

at

5

tlnes

current

settlng

ls

30

secs.

for

a TMS

of l.



-12_

6.2

DEFINITE

TI}M

OVERCURRENT

RELAYS

The

GECM

electromechanical

definlte

tlme

overcurrent

phase

or

earth

fault

relay

type

cAu

comprises

one,

two

or

three

attracted

amature

lnstantaneous

overcurrent reray

elements

(type

cAGlg)

conbined

in

the

same

case

wlth

a

single

mechanicat

deflnite

tlme

delay

elemenr.

Various

tine

important

to

when

choosing

settlng

ranges

are

available

take

lnto

account

the

thermal

a

tine

setting.

up

to

60

seconds,

but

it

is

ratlng

of

the

CAG19

element

Relay

thermal

rating

Ls twice

settlng

current

ratlng

ls

20

times

maximum

settlng

current

for

varles

rnverser-y

to

the

square

of

the

current,

12

seconds.

contlnuously.

Short

tine

3

seconds.

The

rating

l.e.

10

times

current

for

are

avallable.

These

ratl.o

and

cover

the

Statle

deflnite

tine

have

a

low

VA

burden

usual overcurrent

and

overcurrent

relays

type

CTU

and

a

hlgh

drop

offlpick

up

earthfault

ranges.

6.3

OVERCURRENT

RELAY

TYPE

MCGG

IN

THE

NEI^I

MIDOS

RANGE

The

MCGG

relay

ls

a

static

overcurrent

relay

in

the

new

MrDos

(Modular

rntegrated

Drawout

systen)

range

of

relays.

The

MrDos

system

is

the

latest

GECM

relay

housing

system

wlth

a

prrne

obJective

to

better

space

utilizatlon

on

racks

and

panels.

Each

relay

unlt is self sufflcient,

contains

all

lnput/output'

polter

supply

and

other

circuitry

to

fulfil

its

duty'

The

drawout

phllosophy

ls

naintar.ned

for

ease

of

maintenance

and

fault

findlng.

The

MCGG

relay

uses

sol1d

static

technlques,

the

base

element

being

a

microcomputer'

current

measurement

ls performed

by

an

analogue-to-digltal

converter

and

the

tlne/current

characterLstic

of

the

relay

is

detennined

by

a program

selected

by

swltches

on

the

relay

nameplate.



-13-

The

followrng

tine/current

characteristics

can

be

selected:

l.

Standard

Inverse (or

Norrnal

Inverse)

2.

Very

Inverse

3.

Extremely

Inverse

4.

Long

Tine

Inverse

5.

Definite

Tine

D2

(0.1

_

2.0

sec)

6.

Definire

Tine

D4

(O.Z

-

4.0

sec)

7.

Definire

Tine

Dg

(0.4

-

g.O

sec)

\g

with

all

the

characterlstics

available

on

one

relay,

a

standard

relay

can

be

ordered

before

detailed

co-ordlnation

studles

are

carrled

out

-

a

distinct

advantage

for

conplex

systems

where

a detalled

study

could

take

some

tLme

and

could

delay

the

ordering

of

relays.

Also,

changes

ln

system

eonfiguration

can

be

easily

accommodated.

An

Lnstantaneous

overcurrent

element

can

be

lncorporated

with

the

tine-de1_ayed

element

and

has

a settlng

range

of

1

to

31

tines

the

current

settlng

of

the

tine

deLayed

element'

A

single

poLe

tlne

delayed

overcurrent

relay

is

known

as

McGGl1'

wlth

an

addltlonal

lnstantaneous

element

it

becomes

MCGG21.



6.4

-14-

SENSITIVE

EARTH

FAULT

RELAYS

I'Ihere

the

earth

path

resistrvrty

is

high

whtch

ray

be

the

case

on

systems

that

do

not

utrlise

earth

conductors,

the

earth

fauLt

current

may

be

limlted

Eo

such

an

extent

that

nornal

earth

fault

protectl0n

may

not

be

sensitrve

enough.

To

overcome

these

problens

a

very

sensitr.ve

relay

is

required

but

also

the

relay

uust

have

a

very

10w

burden

ln

order

that

the

effective

setting

is

not

increased

due

to

reasons

described

earlier.

GECM

has

developed

a

statrc

relay

type

crulsB

which has

a

settlng

range

of

L-r67"

of

rated

eurrent

and

a

burden

of

the

order

0.005

_

0'012

vA

at

settlng.

This

very

sensltlve

protectron

cannot

be

graded

wrth

other

conventronal

systems

and

it

ls

normal

to

apply

thrs

protectr.on

wlth

a

definite

tlme

delay

of

up

to

10

0r

15

secs.

This

tlne

delay

will

prevent

unwanted

operatlon

due

to

transient

unbalance

under

phase

fault

condltions.

care

must

be

taken

to

ensure

that

the

relay

setting

is

above

any

resrduar

current

that

nay

be present

under

nornal

load

conditlons

due

to

sltght

dlfferences

ln

C.T.

characteristics

or

unbalanced

leakage

or

capacitive

currents

in

the

prlnary

system,

and

also

to

ensure

that

the

relay

wirr

reset

after

the

translent

operation

of

the

current

measurlng

unit.

(Note:

p.u.

/d.o. ratLo

approx.

ggTt)

.

The

sensltive

earthfault

relay

type

MCSU

in

the

MrDos

range

has

a

setting

range

of

0.5

-

5.2i1

of

rated

current

and

a

burden

of

0.001

vA

at

any

settlng

for

1A

rer-ays

and

0.006

vA

at

any

settrng

for

5A

relays'

The

reray

's

tuned

to

reJect

thtrd

har-rnonlc

currents.



7.

-15-

EARTH

FAULT

PROTECTION

Earth

faur-ts,

which

are

by

far

the

nost

frequent

type

of

fault,

will

be

detected

by phase

overcurrent

unlts

as previously

described

but

1t

is

posslble

to

obtaln

more

sensitlve protection

by

utillsing

a

relay

which

responds

only

to

the

residuar

current

in

a

system.

Residual

(or

zero

sequence)

current

only

exists

when

a

current

f10ws

to

earth.

The

residual

current

can

be

detected

erther

by

connectlng

a

c.T.

in

an

available

neutral

to

earth

connection

or

by

connecting

llne

c.T.rs

ln

paral1er.

By

using

this

pararler

connection

the

earth

faurt

relay

ls

completely

unaffected

by

load

currents

whether

baLanced

or

unbalanced.

The

pararlel

connection

can

be

extended

to

include

either

two

or

three

overcurrent

unlts

wlthout

any

effect

on

the

earth

fault

reray.

Two

elements

are

often

consldered

sufflcrent

as

any

interphase

fault

must

affect

at

least

one

of

the

relays,

however

eonsideratr.on

must

be

glven

to

the

posslbility

of

2-1-1

current

distributlon

Ln

the

systen

(refer delta/star

transformer

protectlon).

rt

should

be

noted

that

on

an

L.v.

4

wrre

dlstrlbution

system,

4

c'T'fs

wir-l

be

requrred

to

ensure

stabillty

under

all

10ad

conditlons,

the

4th

c.T.

being

placed

ln

the

neutral

connection.

Thls

fourth

c.T.

can

be

omrtted

lf

the

earth

fault

relay settlng is

above

the

naxinun

spll1

current

caused

by

unbalanced

loads,

but

as

the

degree

of

unbalance

ls

not

norrnally

known

(accurately)

the

incluslon

of

the

4th

C.T.

Ls

recommended.



-

16

-

7.I

EFFECTIVE

SETTING

The

prinary

settlng

of

an

overcurrent

relay

can

usually

be

taken

as

the

relay

setting

nurtlprled

by

the

c.T.

ratio.

The

c.T.

can

be

assurned

to

maintain

a

sufficiently

accurate

ratio

for

this

to

be

so.

An

earth

fault

relay

wlll

nornally

have

a

much

lower

settlng

than

an

overcurrent

relay

but

will

have

a

slnllar

VA

consumption

at

settlng

current'

However'

at

nominal

or

rated

current

the

vA

burden

will

be

much

hlgher

due

to

the

lower

setting.

For

example

:

Assume

3VA

relay,

2OZ

(1

anp

basis).

At

setting,

relay

lmpedance,

Z

=

\IA

=

3

=

75

ohms

?ffi

Assume

similar,

relay

wtth

1002

setting,

At

setting,

reJ_ay

impedance

=

T?

=

3

ohns

Thus

a relay

wlth

a

20%

settrng

will

have

an

lmpedance

25

tlnes

that

of

a slnilar

relay

wiEh

a

l00Z

settlng.

Now,

the

burden

inposed

by

a reray

wlth

a

207t

settlng

at

rated

current

w111

be

t2z = t2 ls =

75

vA

(assumlng

no

saturation)

(Thls

figure

would

normally

be

much

rower

than

thls

due

to

nagnetic

saturatlon

whlch

reduces

the

effectlve

inpedance.

Actual

burdens

at

various

uultiples

of

settrng

current

are

avalrable

on

request).



(As

the

settlng

ls

l0wered,

so

the

number

turns

must

increase

Eo

naintain

the

ampere_turns

at

the

operate

level.

As

the

number

of

turns

ls

r.ncreased

so

the

wlre

dlaneter

decreases

thus

increaslng

the

resistanee).

Thus,

as

can

be

seen,

an

earth

fault

relay

with

a

very

sensltrve

setting

will

present

a

very

r.arge

burden

to

a

c.T.

whlch

ls

attemptlng

to pass

many

tines

setting

current

through

the

relay.

rt

nay

be

thought

therefore

that

correspondlngl_y

larger

c.T.rs

would

be

required

ln

order

to provide

thls

extra

output

but

this is

not in

fact

the

case.

rt

can

be

assuned

that

above

20

tines

setting

current

the

relay

n'gnetic

circult

goes

into

comprete

saturation

and

the

burden

remainS

effectively

constant

r.e.

the

lnpedance

falls

more

rapidly

with

lncreaslng

current

(T27

=

constant).

Typical

flgures

:

Relay

L0

-

4OZ

of

1

aop

Current

0.1

anp

VA

burden

at t x

setting

current

=

(

Impedance

)

2.3

vA

(23OJL

)

-L7-

=

14

VA

(155

Jt-

)

=

90

vA

(90JL

)

=

250

vA

(62.atL)

Thus

to

pass

20

tr.nes

settlng

current

through

the

relay

a

voltage

of

20

x

0

'L

x

62'5

=

L25

volts

is

required

and

l5vA

10p10

c.T.

which

ls

nornally

recommended

for

overcurrent

and

earth

fault

protectron

would

3x

10x

I'

rr

20x



-18-

be

qulte

adequate.

Ilowever,

thrs

voltage

drop

across

the

relay

will

also

be

inpressed

on

all

the

c.T.rs

that

are

in

the

paralle1

group whether

or

not

they

are

carrylng

prlnary

current.

Due

to

this

voltage

the

c.T.rs

wilr

draw

a

nagnetlslng

current

the

value

of

which

wtll

be

dependant

on

the

magnetlsing

characteristic.

The

total

magnetrsing

current

courd

be

appreciable

in

comparison

wlth

the

relay

setting

current

and

in

extreme

cases

where

the

settr.ng

current

ls

very

10w

and

the

c.T.rs

are

of

10w

performance,

may

even

exceed

the

settlng

current.

Thus,

for

exanple,

under

slngle

phase

to

earth

fault

conditlons

the

energising

c.T.

does

not

only

have

to

supply

the

relay

operatlng

current

but

also

the

nagnet.isatlon

loss

for

all

of

the

connected

c.T.fs.

The

"effective

settlng

current,,

in

terms

of

secondary

amps

is

therefore

the

relay

setting

current

plus

the

total

magnetlslng

current

10ss.

(Due

to

the

slnllarlty

of

power

factors

for

electromagnetlc

rel-ays

at

least,

it

is

consl.dered

sufficient

to

take

the

algebralc

sum

of

the

currents).

under

heavy

earth

fault

condltions

many

tlmes

the

setting

current

wll1

be

applied

to

the

relay

and

thls

can have

a

considerable

heatlng

effect.

rn

thls

respect

it

should

be

noted

that

the

heating

effect

on

a

relay

wlth

a

20-80%

range

set

at

say

2o"l

will

be

less

than

that

on

a

relay

wlth

a

L0-40"1

range

wlth

the

same

setting.

This

is

due

to

the

fact

that

the

lower

range

relay

has

twLce

the

number

of

turns

than

the

higher

range.

Consequently

the

wLre

dlameter

ls

smaller

and



-19-

thus

the

reslstance

is

hlgher.

Thus,

unless

neutral

earthlng

inpedance

or

very

sensltlve

relay

wirh

a

2O-BO%

setrlng

range

is

nornally

the

system

has

some

settlngs

are

requlred,

recommended.

7.2

TIME

GRADING

The proeedure

for gradlng

is

slnilar

to

that

for

phase

faurt

relays

but

as

already

descrrbed

the prirnary

tine

current

characterlstlc

cannot

be

kept

proportlonate

to

the

secondary

(relay)

characterlstic

with

anything

lLke

the

accuracy.

At

the

relay

setting

current,

the

C.T.

mag.

current

nay

be

apprecrable

in

comparison

wlth

the

reray

current

(resulting

ln

htgh

effective

settings)

but

at

hlgher

values

of

current,

the

c.T.

mg.

current

becomes

relatlvely

snaller

thus

reducing

the

effective

setting

to

a varue

nearer

to

the

ideal.

At

sttlr

higher

values

the

c.T.

output

ceases

to

increase

substantlally

and

due

to

saturatr.on

the

output

waveform

becomes

distorted

which

further

conpl_rcates

the

situation.

Thus

when

gradrng

earth

faurt

relays,

either

the

errors

must

be

accurately

ealculated

for

the

actual

current

levels

or

larger

narglns

must

be

allowed.

It

is

luportant

to

appreciate

phase

faults

and

earth

faul_ts

relays (which

have

relatively

posslble.

that

fuses

cannot

dlscrlmlnate

between

and

therefore

grading

of

earth

fault

sensLtive

settlngs)

with

fuses

is

not

when

the

system

contalns

sooe

neutrar-

earthing

inpedance,

fault

1eve1

rs

practically

constant

over

the

whole

system

is

carried

out

at

thls

fault

r.ever.

As

the

faurt

rever

rs

there

ls

no

partlcular

advantage

in

uslng

rDl"fr

earth

faurt

over

deflnLte

tfune

earth

fault

relays.

the

earth

and

grading

constant

relays



-20-

8.

INTERCONNECTED

SYSTEMS

The

foregoing

has basicarry

rooked

at

gradlng

procedure

as

appried

to

radlal

feeders.

rf

the

system

ls

Lnterconnected

and

involves

paral-lel

paths

and

rings,

the

gradlng

can

become

increasingly

rnore

complex.

For

example,

the

operation

of

a

particurar

cr.rcult

breaker

rnay

not

itself

result

in

the

isoration

of

the

faulty

plant,

but

nay

affect

the

fault

current

dlstrlbution

in

the

other

circuits.

The

affect

of

thls

rnay

be

to start

other relays

operatlng

or to

change

the

operating

parameters

of relays

that

have

already

started.

on

such

interconnected

systems

the

fault

lever

does

not

tend

to

vary

very

much

and

it

nay

be

found

inpossible

to

obtain

correct

dlscriminatlon

for

all

faults.

The

systern

nust

be

looked

at

ln

detall

under

max.

and

nLn.

faurt

condltlons

and

the

best

compromise

reached.

very

often

directlonal

overcurrent

relaylng

can

help

to

overcome

the

problens

sllghtly.



-2L-

9.

PROTECTION

OF

DELTA/STAR

TRANSFORMERS

There

are

t\ro

particular

polnts

that

must

be

appreciated

when

consLdering

overcurrent

protectlon

of

delta/star

transforners.

under

phase-phase

fault

conditions

on

the

star

slde

of

the

transformer,

the

current

distributlon

on

the

delta

side

will

appear

as

2-1-1

'

As

previously

stated

overcurrent

protectlon

can

be

applied

in

either

two

or

three

phases

to

cover

alL

types

of

fault

but

in

thls

case

the

relay

operatl'ng

time

would

be

reduced

if

the

relay

elements

happened

to

be

in

the

phases

carrying

the

slngLe

unit

of

current.

As

a

general

rule,

if

the

ratlo

current

is greater

than

4,

then

This

ensures

that

at

least

twice

delta

side

in

the

relays.

of

ninimun

faul.t

current

and

trro

overcurrent

elements

can

full

load

current

appears

on

load

be

used.

the

The

other

point

to

note

is

that

for

a

three

phase

fault

on

the

star

slde the

prrnary

and

secondary

rine

currents

are

equar

(assunrng

unity

voltage

ratro)

but

for

a

phase-phase

fault,

the

secondary

current

ts

0.956

tlnes

the

value

of

the

prlnary

current.

Thus

lf

grading

is

done

at

the

three

phase

fault

level,

the

nargin

nay

be

insufficrent

under

phase-phase

fault

condltl0ns.

see

Appendtx

1.

rt

is

worth

notlng

that

lt

is

not

always

necessary

to

grade

the

relaying

across

a

transformer

and

this

can

sometlmes

help

lf

there

are

a

10t

of

stages

to

grade

on

the

system.

rf

the

10ss

of

the

prlnary

circur.t

breaker

results

ln

unwanted

interruptlon

of

other

supplies

or

lf

the

prinary

circuit

breaker

is

under

the

control

0f

a

supply

authorlty

then

obviously

grading

across

the

transformer

is

necessary

and

lmportant.



-22-

10.

HIGH

SET

OVERCURRENT

where

the

source

inpedance

is

srnall

in

couparison

with

the protected

circuit

impedance,

the

use

of

high

set

lnstantaneous overcurrent

unlts

can

be

advantageous

(for

exanple

on

long

transmission

lines

or

transformer

feeders).

The

appllcation

of

an instantaneous

unlt

makes

possible

a reduction

in

the

tripping

tine

at

hlgh

faur-t

levels

and

also

a110ws

the

dlscrinlnating

curves

behlnd

the

high

set

unlt

to

be

lowered

thereby

lmprovlng

overall

system

grading.

(See

Fig.

2).

rt

is

important

to

note

that

when

gradlng

with

the

relay

lmnedlately

behind

the

hlgh

set

units,

the

gradr.ng

intervar

shourd

be

establlshed

at

the

current

settlng

of

the

high

set

unlt

and

not

at

the

maximum

fault

level

that

would

normarr-y

be

used

for grading

rDMT

rerays.

when

uslng

hlgh

set

unlts lt ls

important

to

ensure

that

the

relay

does

not

operate

for

faults

outside

the

protected

sectlon.

The

relays

are

nornally

set

at 1.2

-

L.3

tines

the

naxrmun

faurt

revel

at

the

remote

end

of

the

protected

sectlon.

This

partlcularly

applles

when

using

instantaneous

units

on

the

HV

slde

of

a

transformer

when

the

instantaneous

unit

should

not

operate

for faults

on

the

LV

side.

The

1.2

-

1.3

factor

allows

for

transr.ent

overreach,

c.T.

errors

and

sllght

errors

ln

transformer

lmpedance

and

line

length.

Translent

overreach

occurs

when

the

curreot

wave

contalns

a d.c.

component.

Although

a relay

nay

have

a

setting

above

the

r.m.s.

value

of

current,

the

lnitial

peak

value

of

current

due

to

the

d.c.

offset

may

be

sufficient

to

operate

the

relay.



-23-

Percentage

transient

overreach

is

defined

at

rt-T2x1oo

when

t2

relay

pick

up current

in

steady

state

r.m.s.

amps

r.m.s.

value

of current, that

when

fully offset will just

pick

up

the

relay

I1

l2

The

CAG17

instantaneous

overcurrent

circuit

has

a transient

overreach

of

up

to 80o.

The

relay which

has hlgh

protection

of

transformers

and long

relay,

whlch

has

a

partly

tuned

less

than

57"

f.ot

sysEem

angles

setting

ranges

is

used

for

the

feeders.

See

Appendix

2.

11.

The

cAG19

overcurrent

relay

has

lower

setting

ranges

and

is used

in

conjunetlon

with

rDMT

relays

(same

setting)

in

auto

reclose

schemes.

This

relay

also

has

a

high

drop

off/pick

up

ratio

and low

translent

ove rreach.

The

cAGl3

overcurrent

relay

is applied

where

hlgh

drop

off/pick

up

ratio

and

low

translent

overreach

are not partieularly

important.

INTERLOCKED

OVERCURRENT

rf

it is

only

possible

to

mount

c.T.ts

on

one

side

of

a

circult

breaker

(e.g'

air

blast swLtchgear)

then

lnevltably

there will

be

a

"blind

spot"

between

the

c.T.fs

and the

breaker.

Faults

occurrLng

in

this

bllnd

spot

although

detected

by

some

forn

of

protection

w111

not

result ln

complete

fault

clearance

and

thls this

reason

lnterlocked

overcurrent

protectlon

is

often

applled.

For

example

F

CtRCwr

;-->'

AFFEFENrUL

'

P&oGffitoif.

EusgAp

<_

Pnjr€crotu

I

NT€PLOCKEO

PnoTECltOil.

I

I

_>

lrlrFPlatP



-24-

rn

thls case,

a

fault

at

F

w111

be detected

by

the busbar

protectlon

which

will

operate.

Ilowever,

the feeder protection

wlll

not detect

the

fault

and

fault

current

wlLl

continue

t,o

flow

fron

the

remote

end.

The

lnterlocked overcurrent

relay

(GECM

type PDl)

is

an induction

disc relay

that

has

its

lower

coll

brought,

out

to the

case

terminals.

The relay

cannot

develop

torque untll

this

coll l_s

shorted

and

a

contact

from the busbar

protection

is

arranged

to

do thls.

I,lhen

the

relay

operates

an

intertrip

signal

is

sent

to

the

remote

end.

If

the C.T.fs

are

mounted

on the busbar

slde of

the

switchgear

then

the feeder

protectlon

is used

to

initlate

the lnterlocked

overcurrent

relay which

then

operates Lnto the

busbar

protectLon

to clear

the

busbar.



-25-

APPENDIX 1

COMPARISON

OF

TIIROUGH

A

D-A

tf

axo

g-fi

rmlrs sEEN

TRANSFORMER

a)

:/

rmlrs

vpntu

=

vsnc

rpntu

=

rsuc

IDELTA

=

tt

JT

Isrc

b)

i-fl

rm:.rs

rn----

Ea-t

q

/\-n'

..'_----_T_

*r',

---

/

----+

r

---)

r

,/r

E6-N

=

0.966I

E

ISE6=

$

Efi-N=

$

Zxr

-->

0.86€

T

Eg-n

2xt

rsrc

=

IPRIM

=

Ed-tt

X1

rsrc

?

I

Eg-r't

-Tr

=f

rsnc

=

IDELTA

=

Eg-g

=

2xr

LI

J3

This

shows that

for

a

prinary

and secondary

current

is

0.866

the

39 fauLt.

currents

vaLue

of

Ipntu

=

2

IDELTO

=

2

Eli-N

=

I

2xt

on a transformer

of

unlty voltage

ratio,

the

are

equal.

For

a

A-Q

fault,

the

secondary

the

prinary

currenL.

F+-.,

.c



-26-

APPENDTX

1 (coNTINrrED)

Therefore

if

the

grading

is

done

at

max.

nay

be

insufficient

for

a

f:_F

fault.

Pr,rrra."y

r.lq-j

tirne

a.l

See

e do.rg rd%l

i-

;

me

ol,

Mn*-'34

F*'IL

level

Ma,z.

Q$faullta*

e'*o'*lar3

r"lY

+irre

.,t'fuA..-'4

{a,,lL

b,x

3d

fault

level

the

tlme

lnterval

-

3+

Acirdrr.rc

i;rlrgR.VAL

trmS, lareRvAL FoR

f-*

Tau"rS



-27-

APPENDIX

2

TRANSIENT

OVERREACH

TYPICAL

FAULT

I4IAVEFORM

D.C.

T

Offser

To

flnd

how

the

relay

w111

0perate

under

this

fault

condltion

it

's

ubjected

to

trro

tests

:-

1.

The

relay

pick

up

current

under

steady

state

R.M.s.

conditions

is

deternlned.

F

B

F

The

relay

pick

up

current

with

the

detemined.

The

pereentage

transient

overreach

ls

defined

as

R.M.S.

current

fully

offset

is

100

(A-B)

B

2)

2.



-28-

\-

APPENDIX

2

(continued)

The

r.m.s.

value

of

the

pick

up

current

of

the

reray

under

fully

offset

conditlons

(B)

w111

be

less

than

the

ptck

up

level

under

steady

srate

conditrons

because

of

the

snaLl

effect of

the

d.c.

offset.

The

GEC

Measurements

relay

cAG17

has

a percentage

transient

overreach

value

typical_ly

3

to

5

per

cent.



-

zq-

Fr

e,

l.

EART

l{F

AULT

&ELAV

_€_+,rllcTror.rS

5

Frfls€

6.trf,<,uRe.Grs?

3

hfSl

€

rtr€-{AGcrrr

lrrr5

tetrxg;r1La

tt{Aoa

ovtilcvllf}rf

r..D

Slr"ttFr$?

I

t

C

N

t-l|lTt(

rinJcr

r.l

.t

lllr€t

tYf"lli



X

z

q

U

,.{,

o

ul

{

F

-

3o'

ooltA

tooltt l

FL

L.

ltooA

too\

I

I

I

I

I

I

t

I

Rai

FAut,J-

aueeENT

(,./{"€es>

SouBcE

aSoAAVA

ftl<v

IP/vlT RE

IJAY

5€TTIN6S

R

l

sEf

AT

{'ooA

,,

c,'tz-sTlt^'3

3

2

sET AT

t ZsA

,

6't

5

'r,{>-

Rg s€T AT 1L5A,

o'to1Ms

4ocl6

z30d\

F.L.

tS,oooA

HICrI{

S€T

INS'T

RELAY

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sET

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3oooA

ser

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l4oaA

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AT SDoA

RI

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R.t

/V\BIi{ED

IDMT

AND

I N

STANTANEOU

S

OYERcI/R.RENT

RE

LAY



OVERCUNEE|T GB-U}IXG

EX.IXPIE

Iu this era.ople,

tJre

11

kV

A

buebare are

fed via

tvo

grid

traosfoncrs

rrhlcb

are connected to an

EEY

aysteo.

[he

eupply

rubetetiou

A le

.boua

feedin3

subgtatiou l, C,

D

ead

t

throngh a rgdlrl

ifutrlbntLon

ryrtcr.

Ioeds

r.n

tuppllcd

fron

each

subctetlon,

tbe sur8ted

lord

cnrrcntr

flovbg

fr

tha

feeder

circuits

behg shova.

lhe

narinun

fault

level

at

ceoh

rubrtrtlon

1r

eho

shovn.

lto

date

arc

analysed

ia TabLc

1.

hor

tbe date

of

furlt

lcvrlr

and lord orncate, suJ'tlbh

onrrrnt

trrnrfclrr

retloa

and overcunrnt

relay

scttingr

ere

celcct

d. It

rbotld

br notcl

tbrt

tL

prinary

currest

aettln6

sbouLd

bc

safrly

lbovo tbc lerhr

crtlltd

ld

crrrrrnt

ln ordcr to

allov

eoEc

aargln

for load

grovth,

uncrlrotcd

h{Gb

lc.alr

trrnrhat

pcak

loade aad

thr conplete rrsettlng

of

tb

.rrlay

eftcr

tbrorr6h

fultll

ultb

tL

clrcult

oarrTing

the oarbnr

prospcctlvr

load,

currrnt.

Overcu-rraut

relaya are lntended

to

provlde

e

dlacrirstlvc

prottotloa

rerlt|tt

tyatcr faulto,

aad do not

gl.vt preclse

ovcrload

protectloa.

trrvrrthrbrrt

tbr rcartrn of

overload

protectlon

vblch 1r obtrlned

ls

oftcn

tlougbt

to

ba

of

valuc

ln

protectrng

eablee

agalnst

rbaorrel loadLng.

It

1s

for thlr

ree^aon the,t tbe

pr{ner.17

relay acttln6a

arc

not

rraurtly

redc

r

h{Ch

r. rould

bc

posalble

lf

only

feult

cunsnta

viaDt

cor,sldcred.

Sterting vlth

the

rclay

la

gubetatlon

D

fartbcat

fro

tba

Porls

rouroor

tbr

rcleif

plugl

sctttng

oultlpllar la cslcnlatcd

frol

a

loovlcdtr

of

t[

Hrlrrr

farlt

currcat

flovlog

t.brou€b

thls

polat

aDd

the rclay

sranrat

rettin3f

Subetetioa

D

C ratlo

1@/5L

8,elay

CDC11

(atandard

I.D.U.T.)

currcot

cott{'6

-

1M

t{rrtrur feult levcl

et

eubatatloa

D

b,sban

-

1;t:

llhcrcforc

rela,y

PS'[

=

92

-

1r.95

100

Xov, froo

tJre

relay

tlne/cu:rcut

charaotcrletlcs,

thc o1:retr"8

ttD

of

tha

gtaudard

I.I). 1. .

ralay

-at

1).95

tl,noa

thc rclay

plug

rcttlng.Dd

l.O

lS

b

2.6 secouds.

No relan

folloue

D,

but n

oa,ll

tLn

detey

1g

rtlll

nqlrtlad

to

pennlt

dlscri.El63tlon

vltJr

the

lovgr

voltago

eyetol

lrottatlog.

A

tt-le

lqltlpllcr

eettlog

of

0.05

ls

Eolact

d.

Ernorl .D

.ctnd

H.DDlrS

tLrc

for

rolsy

D 1g

obtalnsdr

O.OJ-2.6

E

O.1)e

A

6grad{ng

lrgia of

0.4g

le

adopted

i-n

thls

e-aaple,

so

t'bat

t'hr

lclqt

rt

gubetatloa

C

ebould have an

operating

tLne,

for e

fautt

rt

aubatetlo

Dl lr

follove:



a

o\

il

o

I

{

o

.{

6

o

o

*

.(,

o

&

(

s

90

|4

t

Z

9

l--

5

m

(.

t-

v)

A

)

{

A

0(

V

=

{.

a

o

I

o

d

I

\t

h

In(

x

{

;

J

o

r<

F

t/.

s=

{r-

0't

v

,dg

Y6

3r

v)o

3[

{)

F

6

q\)

1<

'j

t-

lr*

u

ru

q.

tn

o

o

t

9

Fl-

UJ

v)

o

a

t)

Irl

o

o

s

SZ

oU

r

6Y

:

r\t

gF

J-

{

t-,i

-Jd.

i

'i5

z

o

F

\J

o

-J



Subetatlon

C

CT

ratio

ZAO/jL

Reley

CDG11

(etandard

I.I).U. .)

current

eettlng

-

10016

tfsrlru

faul.t

level

for

gradlng

reJ.ay

ltcnfore

ratqy

rcr

-

lU#,

E

6.g7i

f,ov'

fron

the

r1lay

tln/current

aheracterleti,or,

tbo

operetln8

tb

of

tbr

i$H"i;l;I.r.

nlay

"tZ.its

tinee

-irc;il

prrry

acttrry

rd

t.o

t8

lequircd

rolaya

dlgcrlolnagrng

tLno .

0.1]

+

0.4

-

O.j)t

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Documents

Non-directional Overcurrent and Earth Fault Protection