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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
8/16/2019 Phylosophy of Protection
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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
• 2 •
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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 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
8/16/2019 Phylosophy of Protection
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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 &
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 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
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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&le,
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
• 2 •
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• 9 •
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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 &
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 10:45 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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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
• 1 1 •
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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.