Phylosophy of Protection

8/16/2019 Phylosophy of Protection

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Chap2-4-15 21/06/02 10:42 Page 5

Introduction to Protective

relaying:

About protective relaying, Shunt &

Series Faults, causes and Effects of

faults, Importance of protective

relaying, Protective zones, primary

& Bac!up protection, Bac!up

protection by time grading principle, desirable "ualities of

protective relaying, some terms in

protective relaying, #istinction

bet$een relay unit, protective

scheme and Protective system,

Actuating "uantities, %hermal

elays Electromechanical relays

and static relays, Po$er line carrier

channel, programmable relays,

system security, role of engineers'

Different Principles of protection! (ver current& earth fault )non!

directional & directional types* ,

differential protection, distance

protection )+oring Principle of

Impedance relay, auses and

remedies of (ver reach!under

reach, eactance and -ho relay,

Po$er s$ing blocing relay*'


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• 2 • Fu n

d a m e n t

a l s

of Protection

Practice

2 . 1 I N T R OD U C T I O N

The purpose of

an electrical

power system is

to generate and

supply electrical

energy to

consumers. The

system should be

designed andmanaged to

deliver this

energy to the

utilisation points

with both

reliabili ty and

economy. Severe

disruption to the

normal routine of

modern society is

likely if power

outages arefrequent or

prolonged,

placing an

increasing

emphasis on

reliabili ty and

security of

supply. As the

requirements of

reliabili ty and

economy are

largely opposed,

power system

design is

inevitably a

compromise.

A power

system

comprises

many diverse

items of

equipment.

Figure 2.2

shows ahypothetical

power system


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2.! illustrates

the diversity of

equipment that

is found.

Figure 2.1: Modern power station

N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e

• 5 •


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Chap2-4-15 21/06/02 10:42 Page 6

"ydro power station

G G2

R R2

T T 2

F u n d a m e

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

•

380kV

110kV

Steam power station

R3

220kV

L

T

T

T

33kV

D

A

L 1A

L

1B

3

8

0

k

V

B

L3

L4

T 3

T 4


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33kV B

##$T power station

G5 G6 G 7

R5 R6 R

7

T 7 T 8 T 9


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L 7A 380kV E

T 14

L6

$rid 380kV G

substation L5

F

T 16

T 17

L8

$rid 110kV G

380kV F

e 2.Figure 2.2: Example power system

Figur

• 6 • N e t w o r k P r o t e c t i o n &

A u t o m a t i o n G u i d e


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Chap2-4-15 21/06/02 10:42 Page 7

Figure 2.3: Onset of an overheadline fault

%any items of

equipment arevery e&pensive,

and so the

complete power

system

represents a

very large

capital

investment. To

ma&imise the

return on this

outlay, the

system must be

utilised as much

as possible

within the

applicable

constraints of

security and

reliability of

supply. %ore

fundamental,

however, is that

the power

system should

operate in asafe manner at

all times 'o

matter how

well designed,

faults will

always occur

on a power

system, and

these faults

may represent

a risk to l ife

and(or

property.

Figure 2.)shows the

onset of a

fault on an

overhead line.

The

destructive

power of a

fault arc

carrying a

high current is

very great it

can burnthrough

copper

conductors or

weld together

core

laminations in

a transformer

or machine in

a very short

time * some

tens or

hundreds of

milliseconds.

+ven away

from the fault

arc itself,

heavy fault

currents can

cause

damage to

plant if they

continue for

more than a

few seconds.

The provision

of adequate

protection to

detect and

disconnect

elements of

the power

system in the

event of fault

is therefore an

integral part of

power system

design. nlyby so doing

can the

t

h

T


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Figure 2.4:ossi!le"onse#uen"e ofinade#uate

prote"tion

2 . 2 P R OTE C T I O N

E Q U I P M EN T

The

definitions

that follow are

generally

used in

relation to

power system

protection

5888 /rotec

tion

Systema

complete

arrange

ment of

protectio

n

equipme

nt and

other

devices

required

toachieve

a

specified

function

based on

a

protectio

n

principal

01+#

32445

2365889 /rotect

ion

+quipm

ent a

collectio

n of

protecti

on

devices

0relays,

fuses,

etc.6.

+&clude

d aredevices

such as

#T7s,

#87s,

#ontact

ors, etc.

5890 /rote

ction

Scheme

a

collectio

n of

protectio

n

equipme

nt

providin

g a

defined

function

and

includin

g all

equipme

nt

required

to make

the

scheme

work

0i.e.

relays,

#T7s,

#87s,

batteries, etc.6

1n order to

fulfil the

requirements

of protection

with the

optimum

speed for the

many different

configurations

, operating

conditions and

construction

f

e

9

1 electr om

echanical

2 sta

tic

3 digital

4 n

umer ical

T

h


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F u n d a m e

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

t i c e

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Chap2-4-15 21/06/02 10:42 Page 8

F u n d a m e

n t a l s o f P

r o t e c t i o n P r a c t i c e

•

1n many

cases, it is

not feasible

to protect

against all

ha:ards with

a relay that

responds toa single

power

system

quantity. An

arrangement

using

several

quantities

may be

required. 1n

this case,

either

several

relays, each

responding

to a single

quantity, or,

more

commonly, a

single relay

containing

several

elements,

each

respondingindependentl

y to a

different

quantity may

be used.

The

terminology

used in

describing

protection

systems and

relays is

given in

Appendi& !.

;ifferent

symbols for

describing

relay

functions in

diagrams of

protection

schemes are

used, the two

most

common

methods 01+#

and

1+++(A'S16

are provided in

Appendi& 2.

2 . 3 Z O N ES O F P ROT E C T I ON

To limit the

e&tent of the

power system

that is

disconnected

when a fault

occurs,

protection is

arranged in

:ones. The

principle is

shown in Figure

2.4. 1deally, the

:ones of

protection

should overlap,

so that no part

of the power

system is left

unprotected.

This is shown in

Figure 2.0a6,

the circuit

breaker being

included in both

:ones.


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into prote"tion&ones

F

o 8usbarprotection

Feeder protection

0a

6#T?sonbothsidesof circuit

br eaker

A

8usbar protection

F

Feeder protection0b6#T?soncir cuitsideofcir cuitbr eaker

Figure 2.': () *o"ations

the circuit

breaker A

that is not

completely

protected

against

faults. 1n

Figure 2.0b6

a fault at Fwould cause

the busbar

protection to

operate and

open the circuit

breaker but the

fault may

continue to be

fed through the

feeder. The

feeder

protection, if of

the unit type

0see section

2.4.26, would

not operate,

since the fault

is outside its

:one. This

problem is

dealt with by

intertripping or

some form of

:one

e&tension, to

ensure that the

remote end of the feeder is

tripped also.

The point of

connection of

the protection

with the power

system usually

defines the

:one and

corresponds to

the location of

the current

transformers.

@nit type

protection will

result in the

boundary being

a clearly

defined closed

loop. Figure

2.> illustrates a

typical

arrangement of

overlapping:ones.

Figure 2.+: Overlapping &ones


13/29

be

unrestricted

the start will

be defined

but the

e&tent 0or

reach76 will

depend on

measureme

nt of the

system

quantities

and will

therefore be

sub-ect to

variation,

owing to

changes in

system

conditions

and

measureme

nt errors.


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• 8 • N e t w o r k P r o t e c t i o n &



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Chap2-4-15 21/06/02 10:42 Page 9

2 . 4 R E L IA B I L I T Y

The need for a

high degree of

reliability is

discussed in

Section 2.!.1ncorrect

operation can

be attributed

to one of the

following

classifications

1 incorrectdesign(settings

2 incorrect

installation(testing

3 deterioration inservice

2.4.1 Des!"

The design of a

protection

scheme is of

paramount

importance.

This is to

ensure that the

system will

operate under

all required

conditions, and

0equally

important6

refrain from

operating when

so required

0including,

where

appropriate,

being restrained

from operating

for faults

e&ternal to the

:one being

protected6. ;ue

consideration

must be given

to the nature,

frequency and

faults likely to

be

e&perienced,

all relevant

parameters of

the power

system

0including the

characteristics

of the supply

source, and

methods of

operation6 and

the type of

protection

equipment

used. f

course, no

amount of

effort at this

stage can

make up for

the use of protection

equipment

that has not

itself been

sub-ect to

proper design.

2.4.2Se##"!s

1t is essential

to ensure thatsettings are

chosen for

protection

relays and

systems

which take

into account

the

parameters of

the primary

system,

including fault

and load

levels, and

dynamic

performance

requirements

etc. The

characteristics

of power

systems

change with

time, due to

changes in

loads,

location type

a

n

2

T

h


16/29

2.4.4 Tes#"!

#omprehensive

testing is -ust as

important, and

this testing

should cover all

aspects of the

protection

scheme, as well

as reproducing

operational and

environmental

conditions as

closely as

possible. Type

testing of

protection

equipment to

recognised

standards fulfils

many of these

requirements,

but it may still

be necessary to

test the

complete

protection

scheme 0relays,

current

transformers

and other

ancillary items6

and the tests

must simulate

fault conditions

realistically.

2.4.5De#e$%$%"" Se$'(e

Subsequent to

installation in

perfect

condition,

deterioration of

equipment will

take place and

may eventually

interfere with

correct

functioning. For

e&ample,

contacts may

become rough

or burnt owing

to frequent

operation, or

t i h d i

to

atmospheric

contamination

coils and

other circuits

may become

open5

circuited,

electronic

components

and au&iliary

devices mayfail, and

mechanical

parts may

sei:e up.

The time

between

operations of

protection

relays may

be years

rather than

days. ;uringthis period

defects may

have

developed

unnoticed

until revealed

by the failure

of the

protection to

respond to a

power

system fault.For this

reason,

relays should

be regularly

tested in

order to

check for

correct

functioning.

Testing

should

preferably be

carried out

without

disturbing

permanent

connections.

This can be

achieved by

the provision

of test blocks

or switches.

The quality of testing

personnel is

f

e

1

2

/

r


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F u n d a m e

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

t i c e

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Chap2-4-15 21/06/02 10:45 Page 10

F u n d a m e

n t a l s o f P

r o t e c t i o n P r a c t i c e

•

as an

incident and

only those

that are

cleared by

the tripping

of the

correctcircuit

breakers are

classed as

?correct?. The

percentage

of correct

clearances

can then be

determined.

This principle

of

assessmentgives an

accurate

evaluation of

the

protection of

the system

as a whole,

but it is

severe in its

-udgement of

relay

performance.

%any relays

are called

into

operation for

each system

fault, and all

must behave

correctly for

a correct

clearance to

be recorded.

#omplete

reliability is

unlikely ever

to be achieved

by further

improvements

in

construction. 1f

the level of

reliability

achieved by a

single device

is not

acceptable,improvement

can be

through

redundancy, e.g.

duplication of

equipment. Two

complete,

independent,

main protection

systems are

provided, and

arranged so that

either by itself

can carry out therequired

function. 1f the

probabil ity of

each equipment

failing is &(unit,

the resultant

probabil ity of

both equipments

failing

simultaneously,

allowing for

redundancy, is

&2 . Bhere & is

small the

resultant risk

0&2 6 may be

negligible.

Bhere multiple

protection

systems are

used, the

tripping signal

can be

provided in anumber of

different ways.

The two most

common

methods are

1all

protect

ion

system

s must

operat

e for atrippin

g

operati

on to

occur

0e.g.

two5

out5of5

two7

arrang

ement6

2only oneprotecti

on


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sys

te

m

ne

ed

op

era

te

to

ca

us

e atrip

0e.

g.

on

e5

out

5of

two

7

arr

an

ge

me

nt6

T

1

t

2

B

h

2.5.1 T)e*$&+"!

/rotection

systems in

successive

:ones are

arranged to

operate in

times that are

graded

through the

sequence of

equipments

so that upon

the

occurrence of

a fault,

although a

number of

protection

equipmentsrespond, only

those relevant

to the faulty

:one

complete the

tripping

function. The

others make

incomplete

operations

and then

reset. Thespeed of

response will

often depend

on the

severity of the

fault, and will

generally be

slower than

for a unit

system.

2.5.2 U"#S,s#e)s

1t is possible to

design

protection

systems that

respond only

to fault

conditions

occurring

within a clearly

defined :one.

This type of

protection

system is

known as ?unit

protection?.

#ertain types of

unit protection

are known by

specific names,

e.g. restricted

earth fault and

differential

protection. @nit

protection can

be applied

throughout a

power system

and, since it

does not involve

time grading, is

relatively fast in

operation. The

speed of

response is

substantially

independent of

fault severity.

@nit protection

usually involves

comparison of

quantities at the

boundaries of

the protected

:one as defined

by the locations

of the current

transformers.

This

comparison

may be

achieved by

direct hard5

wired

connections or

may be

achieved via a

communication

s link. "owever

certain

protection

systems derive

their ?restricted?property from

the

configuration of

the power

system and

may be classed

as unit

protection, e.g.

earth fault

protection

applied to the

high voltagedelta winding of

a power


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B

h2

T

h2

T


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• 1 0 • N e t w o r k P r o t e c t i o n &



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Chap2-4-15 21/06/02 10:45 Page 11

As the loading

on a power

system

increases, the

phase shift

between

voltages at

different

busbars on the

system also

increases, and

therefore so

does the

probability that

synchronism

will be lost

when the

system is

disturbed by a

fault. The

shorter the

time a fault is

allowed to

remain in the

system, the

greater can be

the loading of

the system.

Figure 2.C

shows typical

relations

betweensystem loading

and fault

clearance

times for

various types

of fault. 1t will

be noted that

phase faults

have a more

marked effect

on the stability

of the systemthan a simple

earth fault and

therefore

require faster

clearance.

Figure 2.C

p o w e r

D o a d

Figure 2.,:)ypi"al

power-timerelationship

forvarious fault types

System

stability is not,

however, the

only

consideration.

9apid

operation of

protection

ensures that

fault damage

is minimised,

as energy

liberated

during a fault

is proportional

to the square

of the fault

current times

the duration of

the fault.

/rotection

must thus

operate as

quickly as

possible but

speed of

operation must

be weighed

against

economy.

;istribution

circuits, which

do not

normally

require a fast

fault

clearance, are

usually

protected by

time5graded

systems.

$enerating

plant and +"E

systems

require

protection gear

of the highest

attainable

speed the

only limiting

factor will be

th it

f

o

2

S

eB

i


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2 . 9 P R I M A R Y A N D B A C - U P P R OT E C T IO N

The reliability of

a power system

has been

discussed

earlier,

including the

use of more

than one

primary 0or

main76

protection

system

operating in

parallel. 1n the

event of failure

or non5

availability of

the primaryprotection some

other means of

ensuring that

the fault is

isolated must

be provided.

These

secondary

systems are

referred to as

back5up

protection7.8ack5up

protection may

be considered

as either being

local7 or

remote7. Docal

back5up

protection is

achieved by

protection

which detects

an un5clearedprimary system

fault at its own

location and

which then trips

its own circuit

breakers, e.g.

time graded

overcurrent

relays. 9emote

back5up

protection is

provided by

protection that

detects an un5

l d i

system fault

at a remote

location and

then issues a

local trip

command,

e.g. the

second or

third :ones of

a distance

relay. 1n both

cases the

main and

back5up

protection

systems

detect a fault

simultaneousl

y, operation of

the back5up

protection

being delayed

to ensure that

the primary

protectionclears the

fault if

possible.

'ormally

being unit

protection,

operation of

the primary

protection will

be fast and

will result in

the minimumamount of the

power system

being

disconnected.

peration of

the back5up

protection will

be, of

necessity,

slower and

will result in a

greater

proportion of

the primary

system being

lost.

The e&tent and

type of back5

up protection

applied will

naturally be

related to the

failure risks

and relative

economic

importance of

F

o

8

a

sepa

r

a

t

e

c

u

r

r

e

n

t

t

r

a

n

s

f o

r

m

e

r

s

0

c

o

r

e

sa

n

d

s

e

c

o

n

d

a

r y

w

i

n

d

i

n

g

s

o

n

l

y

6


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Chap2-4-15 21/06/02 10:45 Page 12


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F

u n d a m e

n t a l s o f P r o t e c t

i o n P r a c t i c e

• 2 •


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common current transformers that would have to be larger because of the combined burden. This

practice is becoming less common when digital or numerical relays are used, because of the

e&tremely low input burden of these relay types

0 voltage transformers are not duplicated because of cost and space considerations. +ach protection

relay supply is separately protected 0fuse or %#86 and continuously supervised to ensure security

of the ET output. An alarm is given on failure of the supply and, where appropriate, prevent anunwanted operation of the protection

1 trip supplies to the two protections should be separately protected 0fuse or %#86. ;uplication of

tripping batteries and of circuit breaker tripping coils may be provided. Trip circuits should be

continuously supervised

2 it is desirable that the main and back5up protections 0or duplicate main protections6 should operate on different

principles, so that unusual events that may cause failure of the one will be less likely to affect the other

;igital and numerical relays may incorporate suitable back5up protection functions 0e.g. a distance relay may

also incorporate time5delayed overcurrent protection elements as well6. A reduction in the hardware required to

provide back5up protection is obtained, but at the risk that a common relay element failure 0e.g. the power

supply6 will result in simultaneous loss of both main and back5up protection. The acceptability of this situation

must be evaluated on a case5by5case basis.

Documents

Phylosophy of Protection