Switchgear and Protection

Elements of Protection 5
4
There a r e
ous
occurringimportant
1 . 1I n t r o d u c t i o n :
c o m p l i c a t e d .

wide
and
Generating station and the load centre

supply
system
is
is every
p0ssibility of
ault in are connected through the transmission line.
Power
equipments
in it.
There
see that the
that
the fault
fault does not take place ( GENERATOR
substation and distribution lines.
The load ca but
see
and
costly
are
taken to
of fault, Th
Steps 11 KVN
O c c u r r e n c e

Ol
power
system.
p0ssibility f inot 11Ky STEPUPP Modern power supply system is not as
The
though
there 1s every laull 1s cleared.
faulty equipment 220 KV TRANSFORMER
even
and unlil the
simple as described above. Especially the link
unless that the damage
damage otherto FRIMARY
be given
be
power

isolated
from the system
$o

system is
healthy
employed for this
is emploved
TRANSMISSION joining the generating station and the load
should The protecuve centre is complicated. Single line diagram of
equipment
can be prevented.
information
regarding
the basic elements of protect In
element 220 KV
66 KV RECEVING power supply system is shown in figure 1.2.
e shall obtain
in the remaining nit STATION
this unit in details
shall d i s c u s s Power is generated at voltage of around
and then we
SECONDARY
system 400 KV are used in power
220 kV or

Voltagesof 11 kV. 132 kV.

thousand amperec
system. TRANSMISSION 11 kV. This voltage is stepped up to the
several
which becomes
66 K voltage of transmission with the help of step
Value of current is also large. These rela
11 KV
POwer system. SUBSTATION up transformer. Normally the voltage of
time of fault. Protective relays are emp10yed
For this protent
relays
curent. purpose transmission is 220 kV or 400 kV. Long
cannot be operated high voltage and hign
at ctive
PRIMARY
is stepped down to I10 V using the potenti. HT DISTRIBUTION distance transmission lines are run in
transformers are used. High voltage cONSUMERs POLE
value ol current is stepped
down to low value with the 11 K MOUNTED different areas. This is called high voltage
transformer while large he 415 V
SUB STATION
current transformer. In
this unit we shall also study the protective transinission or primary transmission. There
help of IIT||secoDARY
DISTRIBUTION is receiving station at the end of each line.
transformers.
Fig. 1.2 Single line diagram of In this the voltage is normally stepped down
The three-phase system can be used in two ways. In the first method, the
power system to 66 kV. From substation 66 kV lines are
neutral is not grounded. This is called the isolated neutral system or free neutra
run in to different areas. This is called
system. In another method the neutral is earthed directly or through resistance
secondary transmission. Substation is kept at the end of each 66 kV line. In
or reactor. The later method is popular. In this unil, we Snal aiso uy ferent
methods of earthing the neutral and the relative
dc csubstation 66 kV
voitage is stepped down to 11 kV with the help of step down
advantages and
alsadvantages transformer. High voltage consumers like factories, big industries are
given
1.2 Basic elements of a power system: supply using 11 kV feeders. This is called high voltage distribution or primary
Power system can be divided distribution. Some 11 kV feeders are taken in to different areas. There is
in to three main distribution transformer at the end of each 1l kV feeder. This transformer is
parts.
1. Generating station installed on the pole. This iranslormer has
LOAD vollage ratio of 11 kV / 415 V.
CENTRE
2. Load centre cONNECTING LINK Different 415 V distribution lines are run in different streets. From these lines
3. Link the L.T. domestic consumers
joining 1 and 2. Fig. 1.1 Basic elements of power system are given either 3-phase supply or l phase supply.
This is called low voltage
Declric
power is generated in generating station. This includes or secondary distribution.
Beneralor, prime In addition the modern
mover and necessary equipments to run the so that if trouble supply system is interconnected.
exciter, step
up transformer, prim mover urises in one substation. the
circuit breaker etc. power can be fed from other substation.
Load centre includes consumers Complications increase due to the interconnection but considering other
actory, Snops, which consume electric such
water supply. hospitals, powe advanlages of interconnection, the complication has to be accepted.
cinemas, domestic consumers
 Switchgear and Protectin

Elements of Protection
in to tlh main parts.
p r o t e c t i o n
d i v i d e d
4. Power now is in the desired direction.
be
6
S w i t c h g e a r
and
system
can ranstlormer.
high voltage and
high
5. Value of current is equal to the rated value or less than it.
uP
1.3 the
power
step d i s t r i b u t i o

saw
that g e n e r a t o r ,
transiormer.
eder, 1.4.2 Abnormal condition of a power system :
We
parts
include

step
down
are
to
l o be
be swilched
swilche
on and ofr.
thee lines,
When the power system is operating with the conditions as stated above.
lled switchgears. Important
d e v i c e s

These transnission s w i t c h g e

these called
All are

operation is said to be the normal operation. But when there is change in

voltage etc.
r c u i t b r e a k e r has its
c i r c u i t s

low translormer
nas to operate
ofl t h e
breaker

and
C r C u i t

in
voltage. current. frequency. power factor or temperature beyond some limit,
d i s t r i b u t i o n

on b r e a k e r .

switeh
sed to circuil
Devices

swilchgear
is
the the condition of the power system is called the abnormal condition. The
condition,
in
device
circuit
in
healthy characteristics indicating the abnormal condition are given below.
t w o s i t u a t i o n s .

and
olf the such
such a
as short
s s hort circuit
tion or
Overioading of equipment:
c o n d i t i o n

switches
on
1.
. It in
abnormal

circuit
breaks
the This happens when the load on the equipment exceeds beyond its rated
2. It control
relay,
laull. switch,
tuse,
1solators,

t
anel capacity. Due to this the current of the equipment increases. m o r e heat is
include ransformers in
in a
addiuon
d d i t i o n to the above
also
produced and the insulation over the conductor is damaged. This further
t r a r n s i o r m e r s

Switchgears
and potential
arrester.
current
increases the current and short circuit or ground fault results. This situation
lightening
mentioned devices.
should
not be damaged, M can be avoided by either reducing the load on the equipment or isolating the
these

equipments
are costly so
of some
important equ1 equipment.
All the due to the outage
of power to protect
t
area is deprived So it
becomes essential the 2. Unbalanced loading :
some loss.
results in monetary
employed lor this. Proterctive
also are
This relays Currents of all thhe three phases should be almost equal. The system losses
the fault.
Protective
the signal to ciro
equipment
against
the abnormal
condition and s e n d s
cuit| its stability when there is unbalancing of currents beyond some limit.
senses the fault or
circuit breaker opens the ciros.
Cuit
relay of the
to operate. Operating system This is called the 3. Failure of prime mover in power station
breaker
Irom the system.
section is isolated
breaker and the faulty Generator is driven by prime in station. When the
mover generating prime
protection system. mover stops running due to some reason, the generator is isolated from the
and faults in power
system: bus bars. So the load on other
1.4 Abnormalities generator connected to the bus bars increases
operating properly
it is called the normal or which results in decrease in frequency of the
When the power system is system. It results in unstable
does not work always in the normal operation when there is more drop in the frequency. To overcomne this problem,
healthy condition. But the power system times. This
diverted from its normal condition somne
either the standby generator is run or the load o n the
or healthy condilion. It is system is reduced.
is called its abnormal condition. We shall now obtain information regarding 4. Failure of exciter in power station
the fault and abnormalities of the power system.
Exciter ld. c. generator) is used to excite the field
1.4.1 Normal operation of power system: winding of the alternator.
The generator has to be isolated from the bus bars whern the exciter fails. Thi=
The following are the conditions when the operating in
power system is 1tS increases the load on other generators. So either the standby has to
generator
normal condition without any fault.
be run or the load has to be reduced to
overcome this problem.
1. Voltages and currents are balanced i. e. the three-phase voltages are 5. Loose contacts:
equal and the three-phase currents also almost
2.
are
equal. When the connections are loose. the arc is
PTequency of the
supply is equal to the rated produced on the contacts. Larg
little frequency. There is VEly
amount of heat is produced due to this. This increases the
voltage drop als
change in the
frequency. It is essential to check the contacts
There is and joints periodically which is the part
no
change in the voltage beyond some limit. the routine mainienance.
 Switchgear and Protection
9
Elements o f Protection

incr Defect in overhead line :

speed
s peed increases 3.
its
1 ts
line.
: suddenly. of overhead
surge
insulator. in
suddenly,

1/ced the case

Over
voltage
is
reduced
ation ol the
of the armature Bare conductors are placed over
6.
i n s u l a t i o n

the when the insulator

Conductor is grounded and line to ground fault results
generator

on damage
load
the may to prevent

Birds build nest o n thhe

When which i m m e d i a t e l y

generated
punctured or there is crack on the insulator.
is
gets
d e c r e a s c d

m. I. be
piece of metal wire along
more
e. should
sometimes they bring a
cross a r m o n the pole and
e x c i t a t i o n

The
winding.

power
system: be in the desired dir.
tion with the straw so the conductor gets shorted to ground and the line to ground
in
fall on the
shoula
Fault
1.4.3
flow of
curtent
is diverted fro fault results. Short circuit also results when branches of trees
the tlow ol
current the
system of
In power
The
direction
as follows. Fault means .
line.
desired path. can be
delined

the shore. This

and in The lault l o w of c u r r e n the insulators of the line near the sea
the fault. which the t t is Salt gets deposited over
due to due to
desired path system
in
electrical
the power s y s t e m d ues to results in leakage and the fault occurs.
developed in
the defect produced
Fault is
its
desired path. accident. excessive stw.
stress Defect in the bus bars of control panel of the substation:
diverted from insulator, 4.
conductor. damage to the as there is deres..
panels of the substation. Sometimes
value
of high
the breaking current
increase to
Bus bars are provided in the control
short circuit
etc. Value of the
lizard or chameleon enters the control panel and shorts the bus bars which
in the impedance results in fault.
1.4.4 Causes of fault:
n u m b e r s of
reasons for
occurrence of tault. Some probable 1.4.5 Consequences of fault:
There may be The following consequences may result due to the occurrence of the fault.
discussed below.
reasons are
electrical equipment: 1. Arc may be produced due to the fault.
Defect in insulation of the winding of
1.
of all the three phases of the 2. There is possibility of fire when the arc is. produced in the equipment
conductors
Insulation is provided o v e r due lo the fault. This may also affect the nearby healthy equipments.
isolated from one another and from the
phases are
winding. So. all the three
level decreases beyond some limit, 3. Large current flows due to short. circuit. This reduces the voltage of
body of the equipment. When the insulation
to ground fault. It is not the system so the consumers are affected.
it results either in phase to phase fault or phase
result in short circuit or
necessary that the decrease in insulation level should 4. Motors take increased current due to the reduced voltage so the motors
in the insulation the
ground fault. The following are the causes of defect of are over heated.
winding 5. No volt coil of starter gets tripped when the voltage,reduces beyond
Ageing : Quality of insulation deteriorates as the time passes. Deiect
(a some value. The motor stops and it has to be restarted when the
is produced after some
years. This process is called ageing. normal voltage is restored. This results in monetary loss.

(0 Voltage surge: Insulation of the 6. Pressure coil of some electromagnetic relay cannot operate due to the
But
winding can bear normal voltaBc
sometimes the insulation reduced voltage.
gets punctured due to the switching
surge or ightening
surge which results in fault. 7. Large electromagnetic forces are produced due to large value of the
2. Defect in
underground cable : short circuit current. This damages the equipment.
In
underground cable also, fault
can 1.4.6 Statistics of various types of faults
surges as
discussed above. occur due to the aging or Itage
vo Percentage of faults occurring in dilferent equipmnents in a power syslem
are given in table 1.1

SGPEng.\ 2015 12
 Switchgear and Protection
Elements of Protection 11
10 Table 1.1

different equipments 4. Line to line to line (LLL) fault :

in
occurring
of faults
Percentage
% of fault In this all the three line wires get connected together.
Equipment
In figure 1.3 the diagrams of the types of fault and their percentage are

Over head line

50
shown.

Underground cable 10
1.4.8 Symmetrical and unsymmetrical faults
15 The faults can be divided mainly in to two categories.
Switch gear
12 faults and (b) Unsymmetrical faults.
Transformer (a) Symmetrical
2 in the three
CT and PT symmetrical type of faults the value of the fault current
In
two
3 phases a r e equal. And there
is phase displacement of 120° between the
Control equipment
of fault is of this type.
6 phases. Normally the LLL type
Miscellaneous does not remain equal
The value of the fault current in the three phases
of the faults occur in
From the above table
it is s e e n that the majority in unsymmetrical type
the line is in the open s o the
and the phase displacement also does not remain equal
obvious. as
overhead lines. The r e a s o n is of fault. LG, LL o r LLG faults a r e such type of faults.
type
possibility of occurrence of fault is much. of unsymmetrical types of faults.
In power system mostly the faults are

in overhead lines
1.4.7 Types of fault 1.4.9 Difference between abnormality
and fault :

take place in overhead lines are as shown faults.

The types of fauli which can
of the power system and
We discussed about abnormal operation
is the
below. that the fault is the c a u s e and the abnormality
From this it is found
current
1. Line to ground fault (LG): and then due to the fault large
result. Because. first the fault o c c u r s
above. It should be
conditions discussed a s
When there is breakdown of insulation flows which results in abnormal
only due to the faults for
a r e not produced
between one of the three phase noted that abnormal conditions
G 85%
conductors and ground, it is said that the example overloading of the equipment.
line to ground (LG) fault has taken place. of protective system:
2. Line to line fault (LL) :
1.5 Function a
system, the faulty
in power
abnormal condition is produced
When fault or
a s quickly
ass
isolated or should be isolated
This type of fault occurs when there is 3% immediately
component should be in the system
because otherwise the fault spreads
breakdown of insulation between the two
possible from the system
line conductors. This o c c u r s to the system.
type of fault occurs and more damage
LG
especially when the two open wires
get
57%
Primary function: isolated from
joined together. faulty component is
detects the fault and the function of the
Protective relay This is the primary
3.
Line to line to ground fault
(LLG): the system with the help
of the switchgear.
cannot predict the
occurrence

In this two line wires L 2% that the relay

It should be noted or after the
are
joined together protective relay. of the fault
and get connected to after the occurrence

used in
Relay works only relay
of fault is called
the LLG
ground. This type Fig. 1.3 Faults in overhead lines
Or the fault.
conditions are developed.
However the Buchholz

which may
take plac
fault. abnormal
the occurrence of fault
about
ransiormer gives warning
in transformer.
 Switchgear and rotection
Elements of Protection 13

12 oil. When the condition

coll. is normal the current
Nowing through the relay coil is
the protective
Secondary function:
is the
primary
lunction
ol
location
relay. less so the armature cannot be attracted. When the feeder current exceeds.
and its
The
function
shown
above

information
about the
type of lault This the secondary current of CT also increases so the current flowing through the
ol
Some relays also give protective
relay. relay coil increases to attract the armature. The contacts close and circuit
of the
the trip coil is completed and the circuit breaker trips. Thus the feeder
function is
s the secondary
system
protective isolated from the system.
1.5.1 Basic
elements of a are shown
in the block diagran
of
of a protective system
Basic elements -BUSBAR
figure 1.4.

TRIP COIL
MEASURING AMPLIFIER ACTUATOR
SENSING (C.B.)
ELEMENT
SELECTOR ELEMENT
C.B.
CT& PT)

Fig. 1.4 Block diagram of protective system

-FEEDER
elements are as shown below.
The

1. Sensing element (CT and PT) ARMATURE

2. Selector (Summation transformer)
RELAY
COIL
3. Measuring element (Comparator and relay)
4. Amplifier

5. Actuator LRELAY
The value of current increases and the value of voltage decreases due to
LOAD
the fault or the abnormality. This is sensed by the current transformer (CT)
Fig. 1.5 Basic arrangement of protection system
and the potential transformer (PT).
The summation transformer
1.5.2 Desired functional charaeteristics of protection system:
decides whether the fault is in its own zone
or outside its zone. The protective system should posses certain characteristics so that it

When functions as desired and can clear the fault in minimum time. These
it is confirmed that the fault is in its own zone, the comparator/
relay operates and gives
signal. characteristics are as shown below.
If the signal given by the relay is of low magnitude, it is to
1. Reliability
amplify it which is done by the necessary
amplifier. 2. Sensitivity
Circuit breaker is
trilpped with the
help of
Basie amplilied signal. 3. Selectivity
arrangement of protectiorn
help of single line system is shown in figure 1.5 with the 4. Discrimination
diagram.
Feeder is taken out of the 5. Speed
bus bar
feeds power Lo the load. through
the circuit
the feeder.
Current
Current transformer senses the current breaker. Feeder 6. Simplicity
lowing through the secondary of CT is flowing througn 7. Economy
given to the relay
 Suitchgear and Protection Elements o f Protection 15

should be the
The relay eapable to dlscrtminate between the fault and
14 system shoula
should not power translormer is switched on tt takes five t) seven tines
When a
7eans
fhe system
ovrrverload.
that
current whlch is called the in rush magnetizlng current. Now this
. R e l i a b i l i t y

hold be
relable
c h a r a c t e j i s t i e
ecause, r
the he fll
load
rstem
important
not the tault cirrent so the over current relay should not operate. Relkay
Frotrrtin
providing
ow7
This is an
Is o
tiSe o1 the ne
ifs there

te
orkini n the
eleve:17th hour sa1d the ion possth sho 1ottld have disertminating power to distinguish between these two.
at can De
frils wn ikine other
words
T'ate
Thus
rliabtlttv
in
the
lailure ol the
protective svei.
stem It should be noted that the terms selectivity and discrimination are used
potenti..
vatem

protertir
behind

of the
sstrm
The
reasons

irrent
translormer,
tlal alternatively.
of f a i l u r e wiring.
breakrr.
defrct in
rela.
fail also
wnen the operating taff
stat
be the can
5. Speed
the follout
ar
The system
tTansforr. baten etc
be
increascd by taking ing
misiake
Thc reliability
can
The faulty component should be isolated from the system as qulckly as
me
rommits
nossible when the fault occurs. This is because.
Teasure
svstem 1. The equipment ís damaged if the fault current persists for long period.
1Good design of the rrlay
Selertio7 of proper rrlav 2. The system voltage reduces when the fault current lows. If the reduced
2
of z0od
material and cquipment no volt coil of starter of the
voltage persists for longer period. the
Seirction
3
healthy motor operates and the motor stops.
ma!ntenance of the svstem
4Proper

checking 3. If the fault is not cleared, it may spread in the healthy section.
5. Periodic testing and

The time taken for the relay from the instant the actuating quantity of the
Sensitivity:
the ability of the relay to operate from the relay reaches to the pickup value to the closure of the relay contacts is called
Sensitity of the reiay means

relay Sensitivity of the

is indicated by volt the operating time of the relay. The operating time should not be less than
minimum magnitude.
signal of the
to operate it. Suppose relay
(a) requires 2 VA to operate certain value because if it is too less. sometimes the selectivity is lost. For
amperes (VA) requtred
then the sensitivity of relay (a) is example. some time is required for the lightening surge to divert it to ground
whlle relay (b) requires 5 VA to operate.
hence relay should not operate. Moreover definite time is required for the relay
more than that of relay (b).
in the time grading system.
3. Selectivity:
It is the ability of the relay to sense the fault in its own zone and to 6. Simplicity:
operate the circuit breaker to isolate the faulty component without affecting The protecting system should not be complicated. The maintenance can
other healthy sections which are not in its zone. be done easily when the system is simple. Reliability increases when the system
When the system senses the fault in its own zone and isolates the faulty is simple.
section, it 1s called the absolute
selectivity and the system Is called the unit 7. Economy:
system. And when the selectivity 1s obtained by
grading of varlous zones, it is Protecting system should not be costly. Normally the cost of protection
called the relative
selectivity and the system is called the non unit system.
4. Discrimination should be within 5% of the total cost. However it is obvious that the cost of

The relay should have

protection system is more with the costly equipments.
the power of discrimination which
means that it
should operate 1.5.3 Zones of protection
only when the fault occurs in its own zone
operate when there is and should not
through fault. Thus it should be Protection system is divided in different zones to give selectivity feature to
the fault is in its
own zone or il is
capable to sense whether
the protecting system. These zones are as under.
the through laull.
 Switchgear and Protection Elements of Protection
16
then
o breaker will
operae when the
17

-GENERATOR Thus
'blind spot 18 produced when fault occurs between
the the two
regions.
PROTECTION there is no
overlap.
1.6 Back up protection
y s are
pelays used in
protection system
for the
LOW
SWITCH
VOLTAGE
GEAR rolection scheme is called the protection of the equipmernt.
This primary protection. Great loss results when
PROTECTION
the orotection system 1ails. To
prevent this back up
to the primary protection. protection is provided in
addition
when Lhe fault occurs in the system the
primary protection system starlss
POWER TRANSFORMER ing and the relay operates in the
working specified period and trips the circuit
PROTECTION
-eaker. The backup protection system also starts working as soon as the
Caslt Occurs. But the operating time of the backup protection 1s kept more
HIGH VOLTAGE than that of the primary operating system. So when the fault is not cleared by
SWITCH GEAR
the
PROTECTION the primary relay. backup relay operates after some time and clears the

fault
TRANSMISsION UNE
1.6.1 Necessity of back up protection

PROTECTON The reasons of providing back up protection are as follows.

fault when the
1. The back up protection system operates and clears the
fails.
LOW VOLTAGE primary protection system
SWITCH GEAR
in maintenance
primary protection system is under repairs
or
PROTECTION 2. When the
protection systerm.
the back up protection system works a s the primary
are as follows.
system
Reasons of failure of the primary protection
to the relay.
and current supplied
- -

1. Due to the failure of the voltage

does not get d. c. supply.
coil of the circuit breaker
2. When the trip
Fig. 1.6 Zones of protection relay.
in the protective
When the defect
is developed
3. mechanism.
1. Generator breaker
defect in the
4. Due to the
2. Low voltage switch gear circuit
breaker.
defect in the
5. Due to the
Power transformer
3 protection:
4 High voltage switchgear 1.6.2 Types of back up
the following
three ways.

can be given by
protection
Transmission ine The back up
5.
Low voltage switch gear back up
6 fault has
1. Relay
in which zone the
The circuit breaker of that zone operates 2.
Breaker back up
in some
1.6 it is that there are some overlaps
Occurred. From figure seen
Remote back up
than one circuit breakers of that 3.
adjacent zones. With this arrangement more

in that But when overlap is not provided

zone operate when fault occurs zone.
SGP_Eng.\ 2015 13
 lements of Protection
Switchgear and Protection
19
nose the
Suppose the lault occurs at A on
feeder of station B. Then the circuit
18 al station B should operate
back up rela.. within 0.1 second. If due to some
up: relay
and
ay DT not operate. the circuit breaker at station C reason
1. Relay back relay-primary
two tr
In this
arrangement
two separate
circuit
breaker.
For this
trip If this breaker also cannot operate the
should operate after
0.
same
the Dy the primar. circuit breaker at the
are provded
whlch can operate
One (rtp
cofl operales y
secon

will operate after second.

generating
breaker.
the circuit relay. station
cofls are provded In the back up
by
relay and
the other trip
coils operates
Principle and working of protective transformers
Before studying the protective transformers it is necessary to understand
CB
and working.
BARELAYUP their principle
PRIMARY 1.7.1 Current transformer:
RELAY
Basic construction of current transformer is shown in figure 1.9 in order to

Fig. 1.7 Relay back up

understand its working principle.
breaker is used but there
one
circuit
as only
fault is developed in the
arrangement
This is cheap when the
back up protection
Is no use of providing
cireuit breaker lsell.
RELAY
2. Beaker back up: COIL
circult breaker operates
two breakers are provided. One Np
In this system from the back
circuit breaker operates
Irom the primary relay and the second

up relay. This system is costly.

LAMINATED
3. Remote back up: LOAD
time CORE
be obtained by grading. The operaling
the time P
Remotr back up can
leeder to the generaling p
of the relays are kept higher as we proceed from the
1.8. A.C.
station. This is shown in figure SUPPLY
current transformer
Basic construction of current
Fig. 1.10 Symbol of
Fig. 1.9
transformer
0.6 0.16 S
the laminated core. This winding comprises
Primary winding is wound over
load.
series with the
is connected in
1 S of thick wire of few turns. This winding Relay coil is
of turns of thin wire.
of more number
Decondary winding comprises transformer is used for measurement

Connected a c r o s s this winding.

If the current connected in
meter is
of watt mete or energy
coil
a m m e t e r or current
transformer.
0.6 S nen the the instrument
transformer is
called
coil. Then this flowing through
place of the relay
in the core
due to the
current
Ip
STATION C STATION B e. m . f. is
ne alternating flux is produced the secondary winding and
flux links with winding
This the secondary
ne
primary winding. Current I
flows through
Fig. 1.8 Remote back up
winding. in the r e v e r s e
flux in the
c o r e
c e d in the secondary
This produces
t 1 s shorled by the
relay coil.
 Switchgear and Proteetu
glements f Protection 41
40 connection:

delta
S.
Star-open
c o n n e c i i o n
the primary
29
1 . 2 5
Current limiting reactor :
ol iss Current
this Vpe
lts
neutral
limiting reactors are also known as the series reactors.
In
in star. Current
are
connected

is the inductive coil with low resistance and high inductive

windings
ing reactor
or between
solidlr earthed.
is C O n n e c t e d in series with the feeder and tie. generator
are
connected in s e r i e s
a ctance. Tt short circuit current at
Secondary
Windings
coil. b u s sections. It is used to limit the magnitude of the
connected uith the relay short circuit.
and
this t i m e ot
the applications of the reactor:
The following
are

29.1 Primary functions or advantages of current limiting

Vpe of conneciion A1 current becomes high when the

1. The magnitude of the short circuit

ol
the voltage to the restraining If the magnitude
1.To supply reactor is not joined in the circuit.
fault
current limiting
and earth of thhe
coil in over current
circuit current is more than the breaking capacity
the short
circuit curTent
connection. The value of the short
circuit breaker, it cannot operate.
connection. c a n trip at the
time of fault.
2 In resiricted earth fault i s reduced so the circuit breaker

3. In directional rarth fault protection. circuit breaker of lower

breaking capacity
When the system is expanded, Instead it
2. capacity.
that of the higher breaking
4 ln distance earth fault protection. has to be replaced by current c a n be
short circuit
2 suitable value is used, the
5. In earth fault alarm relay. the reactor of so the
the original circuit breaker
of
limited to the breaking capacity
1.28Difference between CT and PT: TAGE be employed.
COIL ODF old circuit breaker can
The difference between Ci and ri RELAY reduced due to
the c u r r e n t
i disturbance voltage is
below in the form of table. o*Fig. 1.32 Star-open delta
connection
The magnitude of the
limiting reactor.
in the
CT so there is decrease
PT current is reduced
4. Value of the fault reduced.
.The primary overheating is
current does
not|1. The primary current depends mechanical s t r e s s e s on the apparatus and
the
depend on the
secondary current upon| feeder is stopped that means

the secondary burden. current fed by other healthy of the

2. Fault
Primary winding of CT is connected 5. the possibility
of the continuity
in series |2. Primary winding is connected fault stops from spreading. So
with the line so
less there is across the line so it supply is increased.
voltage drop across the gets the full | reactor:
primary winding. supply voltage. 1.29.2 Drawbacks of current limiting reactance.
3. The percentage
in the total
secondary winding should I. There is increase
due to the
increase in the
be not |3. The voltage drop
opened while the secondary winding increase in the
reactive

winding carries current. primary There is

e

opened even when the ca

2. becomes poor.
regulation
So the voltage
primary reactance.

factor.
4.
Winding is connected to the decrease in the power
The primary current Supply. 3. There is
of current limiting reactor
there is
change in the changes
and types
when |4. Construction

load current. Normally the line is

1.29.3

types of
current limiting
reactor..

constant and the VO There are

two

burden is also |
Constant so there is 1. Iron core type
the
current of the
no
change Air core type
primary windinë 2.
SGP_Eng.1 2015 16
 A2
In tron
Switehgear and Prote
roir type reactor, the winding Is wOund
the iron. over
f rrar t r , he
gets siaturatrd al the Ume ol tshort creult
Iron core
core
In this tyetlon glements of Protection
creranr Tu prrvent thin, alr gap in provded in the core and
the
of theco nereased. Thin inerranrs llhe cost. MoreoVer
in

and the hysterrsin lusn Inerra". Thin loss Is ahoul 4

the: cdd
eddy
linne
1s
In

placed
ion-rapieLCany
in an
alurminur
snielded ol
Innerned reactor, 3
% to s % Currenn
kVA l In shield:d ty l
tube. t is
tank iled
alr core
kept coil is used. it
in a
In alr r(rtypr Iron core s nol used
n t i e r e 1% T1o
(lcereas in 154:d. Sngle dlse
irnrnersed reactwr, core 1s not
with o.
at thr time ol nhort cirrult.
.
wndinK, 1s kept on the used or core with
Coll asembly 1 placed central air gap
Therr iIe two of lhe alr corr re1clor,
in oil core of the
nagnetic circuit.
LyJrr 29.4 Location of current
1. 5are or dry tyjpe limiting reactor:
The following are tfie
rnethods of
2. Ol Immersrd lyp 1. Generalor reactor lucating reactor in the power systen.
In barr
dry typr reactors. cireular coll or
or
2. Fccder reactor
embedded in thr slranded coDDer
specially deslgned ronerete slabs. Due lo this Are
3. Bus bar reactor
protecon in avallable against the mechanlcal force al constrs
the tume of (a) Ring systen
Conling of uch typr of reactor may be of short etr
natural alr clrculation ult. (b) Tie bar
nlr cireulation
tyjpr. Rractor I» kept on type or of farM system
by the the conerete slab and Lhe slab
Is supporia
porrelatn Insulators no It is Isolated from the 1. Generator reactor:
carth. The
diudvantagen ol this type of constructlon. followlng are Lhe In thiS Syslem reactor is connecled in series with each
reactor Individually prOvides protertion generator. This
1 I oeupleH more to the generator.
space.
2. The following are the
Covling oflarge coll Is dificult. disadvantages of
Lhis syslen.
3 I s not sultable for the outdoor
4. ls Is
servlce . When a lault occurs on lecder lhere REACTOR
Use
Jimited up to 33 kV. Is much voltage drop across the
Ol mnersed type reactors reactor. Due to this, the bus bar
lled are
kept in the tank lIk« C.B C.8.
n the tank. The
advantages of tlhe oll
turansfornmer. Oil Is voltage also drops so the generator
follows. Inmersed ype reactor are as
on the bus bar falls out of
synchronism. This affects the
There is
prolection agalnst the lashover. stability of the system.
2. The size Is
reduced. 2. Full load current lows through the
BUS BAR

3.
HIgh thermal capacity. reaclor in the heallhy condition also
4. the FEEDERSs
Useful above 33 kV. so there is vollage drop a c r o s s c B A
too
there is power loss
There r e two reactor and
1.33 G e n e r a t o r reactor
of ype construc tlon of oil in the reactor
Fig.
(1) Non iimmersed reactor. are designed with
magnellcally shleld oll immersed reactor This system is normally not used. Moclern generators
But when the old
(2) is nol requirecd.
Shlelded oil immersecd reactor inherent
reactance so external reaclor
are used.
more reactors
exlernal
used in the system,
Benerators are
 n t s o f Protection
lements .

45

44
Switchgear and Protection

2. Feeder reactor
C.B.
In this system reactor is connected in
series with each feeder.
C.B.
Advantages:
1. The voltage drop occurs in that ()
feeder only on which the fault
has occurred and there is no CB. CB.]CE GEN.
BUS
effect on the bus bar. REACTOR
2. Fault on feeder does not affect FEEDERS
BUS BAR
the other feeder this means the B A
effect of fault is localized. Fig. 1.35 Bus bar reactor-Ring system

Disadvantages C C.B.
system:
1. Generator does not get Tie bar
(b)
REACTORSs is the modification of the ring system. In this the generator
protection when the fault occurs The tie bar system
on the bus bar. the common bus bar through reactor but the feeder is fed
connected to
FEEDERS
side. Operation is similar to the ring system.
2. There is voltage drop and power through the generator
loss in reactor in the normal Fig. 1.34 Feeder reactor
condition also.

3. Reactor for each feeder becomes

costly so it is used in the group
(e) ) GEN.
BUS
feeder.
3. Bus bar reactor

Bus bar reactors are used to prevent continuous voltage drop and power
loss in reactor. There are two types. REACTORS
FEEDERS
(a) Ring system
TIE BA
(b) Tie bar system
(a) Ring system:
BUS
FE
reactor-tie
bar system
In this
system. the bus bars are in sections.. Generator and eeder are 1.36 Bus
bar
Fig.
connected on each section. Different while
sections are connected the fault
In normal condition
through feeders. gene:ator
feeds
fault
then that So the
no
much current flows through the reactor so there is on a
feeder.
through
the
reactor.

no
voltage drop and power loss. So the
reactor can be wnen
fault
occurs

feed the
fault

resistance so its cost designed for higher the other generat erators
have to
becomes low. When fault occurs on
generator feeds the fault current any feeder, that current is reduced.
whuile the other
current through the reactors. So its generators feed the fault
magnitude decreases.
 mente
f P r o t e

diagram of the health un grounde

Soitchgear and Protectios 17
stem. Va
pite
p l h ns e r

eutral voltas Curreents

value ot the tault current doe he natse to

lce x Vy and
and V are
There is hnother advantage that the
secttons are increased
does not p h a s e
voltage vector by 90°. nd
len are
head of thelr
numbers of bius and
exCeed the certain valne when the
desighed on that value can .
sitchgrars ey CB =p
depends upon the reactor So the work
modilcatton.
with the additionnl sections and
there is no nred for CR
110
Thts vtem is used when modification of
the system has to be dom
dome by Xc
1s
the ne to
line loground capacitance. All these
Where,
through the earth. The currents are
potential of the neutral 1s balancedto
1 0 Current lows

adding adedittonal grnerator

equal
1.30Isolated neutral system: theground potenat
tial. Voltage of each phase is V,= . and are equal.
In the solatrd neutral system the
neutral
is not earthed. Such system

ahown in fhgre 137

ne
130.1 S i n g l e l i n e to ground fault in an lsolated neutral
system :

The situatlon described

R
above
changes greatly when a single line
CRYS to ground fault occurs in the
CpR m isolated neutral system.
CY N
In figure 1.39 It Is shown that
B

an earth fault occurs on phase B

There are two components of the
IOR
Ce CR C IBR current of phase B:

Fig. 1.37 Isolated neutral system

to ground fault in an
Single line
Fig. 1.39 isolated neutral system
We know that a capacitor is formed between the two line conductors and
through the pathphase B.
Which lows
between each conductor and ground. Capacitors formed between conductors IBR and to phase R.
are represented by Cpr Cyp and CBR while the line-to ground capacitors are fault, capacitance CR
the path phase B,
represented by Cp Cy and Cp in the grounding characteristic of the system. which flows through -

(1) IBy phase Y.

the eflect of line to line CY and to
capacitance is negligible fault, capacitance VY
the voltages
so we shall not consider them in the discussion VRN currents flow due to
Both these
that follows. of thelr driving
Actually the
line to ground ICY and these are
ahead

capacitance is distributed type but we shall treat BR and VBy diagram is

shown in figure

by 90°. Vector 1s
them as he lumped capacitances. voltages and current Br
voltage
is the line
Value of each line lcR 1.40. VBR Vsyis
alsoline

to ground capacítance 1s
ahead of it by 90°. Similarly.
is ahead
ol t
equal if the conductors are
placed symmetrically. current
vector Igy Fig. 1.40
the when ground
So the values of the VBN voltage and
diagram
charging currents ICR CY Ice Fig. 1.40
Vector
system.
and oy 9 0 . neutral
IcB are
equal. Each current is ahead of its in
isolated

phase B
respective phase voltage by 90. Fig. 1.38 Phaser diagram of faultoccurs on

Figure 1.38 shows healthy un ground system

 Switchgear and clements f Protection 49

48
Protection 2
oltage
earthing.
of the system during earth fault depends upon the neutral

_v3 Vph upon the

BR
VBR Xc The type of earth
fault protection to be provided depends
3 of earthing of the neutral.
3 Vph .. 1.12 method
And BY phase B
phase B
is the vector sum or
vect
.32 Disadv dvantages of ungrounded system
result when the neutral is not grounded.
through
following disadvantages
current
lowing
The
The resulting
equal to p neutral is not grounded and if
the single line to ground
and it is When the
BR and gy
becomes
Vp 1.
the potential from earth
of the healthy line
faull o c c u r s ,
C 1.13 than that before the
So the insulation
occurrence of the fault.
conclusion can
be
be drawn.
d. 3times
connected to the line may
be damaged.
tlhe foilowing
the above
discussion of the equipment then
From occurs in u n ground ound neutral
nes.

grounded and if
the earth fault
occurs

1. When single line

to ground
lault

was the ground poten. system, 2.

When the
neutral is not

arcing ground
occurs. Due to this
the system

the potential
of neutral
which

the s y s t e m nho-.
hase voltage.
before the phenomenon of
normal value.
This c a u s e s
damage
becomes equal to becomes 5 to 5.5 times the
occurrence
of the fault voltage connected to the line. The temporary
becomes equal to 3 that it
insulation of the equipment
of the healthy phase ecomes to the
fault due to the arcing ground.
2. Voltage r e s u l t in the of
problem of insulation
ine.. in permanent
voltage. This may fault results neutral is not
to the line when the
equal cannot be
detected easily
fault
breakdown. 3. The earth
current of the healthy phase becomes grounded ungrounded
neutral

in the
3. Value of the capacitive not get earth path
Lightening urges do
the previous value. 4.
becomes three times its norm al system..
of the faulty phase
4. Capacitive current neutral earthing
of
value. 1.33 Advantages the neutral.
of earthig of
earth. This produces arc in tho the advantages increases

current fNows through the The following

are the neutral
5. Capacitive potential of in the
even i1 the cause of the system the while
the fault. and the are persists neutral
ionized path of
occurs
ungrounded
earth fault
1. In an
when the equal
is called the arcing ground. Charging
.. remains
potential
fault is removed. This condition
neutral
from the ground of the
the potential
and dischargingof capacitance is continued through the fault.
system grounded
neutral system,
time of fault.
even at the line
oscillations and there is increase in the potential of the
healthy
This results in high frequency to the
ground
the voltage ground
system line to
becomes 5 to 5.5 times the normal neutral
system voltage. The system voltage an
ungrounded
voltage
when single
is
In ground
neutral

value. This may result in breakdown of the insulation of the equipment 2. phase to when
the
the arise
becomes 3 does not
difficulty
connected to the line. fault
occurs.
This
earthed.
This
neutral is
1.31 Importance of neutral earthing: grounded.
avoided
when the
equal
to the
ground is
become
lines
two
the ol arcing of the voltages
Earthing of the system neutral is important which will be clear from Problem over
currents the
3. the charging As a
result

following points. is
because

from
neutral to ground.
1lowing
current

Neutral, earthing is important to provide protection against unbalanCed produced

in the
system.

not
voltage, lightening etc. with reference to arcing ground
are

SGP_Eng.1201517
 Switchgear and p Elements of Protection
51

i n c r e a s e s
as
the
problem
of ectlon
arcing ground omes
ero but there is no change in potential of the healthy phases R and
50 system
bee
Y. There are three components of the current of phase B:
of the
Rellability

earthing the
e a r t h i r

the
dors not arise.

are
suppressed
d u e to
sed due
on
the
the
insulatins
n
So the eulral. Thts mm INR

cost
surges
occurríng
Voltage
s t r e s s e s

elc. 1s reduced and and there is of N

the
m
crease
reduces repairing
maintenanee.

breakdown
IF
system.

of the as the re is m
decrease In
reliability equipment

the the
in life of INR
in the of the
is
increase
lo the absorption voltag INY
6.
Therc

S t r e s s e s
on
i n s u l a t i o n
due

are hed.
earthe surges. NR INY
the in
atmosphere

produced
surges when the neutral is grounded
nen

7.
Lightening
be given
can easily
protection lows. IF
Earth fault current
8. earth
fault
suficient INR+IY=F
because so there is safety of
be provided
fault
protection
can persons Fig. 1.41 Single line to ground in solid earthing
Earth TO
9. CR
which flows from phase B to fault to capacitance
and the equipment.
to the desired value by providing
valhse
() Current INR -

can be kept
earth fault
current
phase R.
10. The earthing. to
reactance type fault to capacitor Cy
the
resistance
or
Current yy
-

which flows from phase B to

so the disturbance is not caused in (1)
earthed
Induced static
charge is phase Y.
11.
to fault to ground.
limes.
which flows from phase B
communication

the nearby
(1i) Fault current Ip- on
c u r r e n t depends
Value of the
fault
earthing:
1.34 Methods of neutral by of the follou. VRN impedance
form the phase

Earthing of neutral
in power system
can be provided one
ollowing the z e r o sequence

conductor of the power

source up to
the fault

component
of
resistive
methods. F the
point. Normally
impedance is negligible.
.Solidearthing the zero sequence
behind the faulty
So the fault
current
Ir lags
2. Resistance earthing NR diagram is
90°. Vector
phase voltage VeN by and Iyy
3. Reactance earthing 1.42.
Current Iyr
shown in figure andNY
4.
Peterson coil or arc suppression coil earthing to the voltages VNR
flow due obtained by
is
5. Voltage transformer earthing
Vector VNr
VeN INY CF respectively.
Current
vector NR
reversing
the vector VRN Similarly
1.34.1 Solid earthing vector VNR:
of voltage
VNR 1s
anead
the vector
reversing
In this method the neutral Is earthed using the pipe type or plate type diagram of
solid
is
obtained by
Fig. 1.42 Vector
the fault
when
vector Vy of voltage Vyy by
earthing. One or two earth electrodes are employed depending upon the need. is a h e a d
earthing system Current Iwy out of
Occurs on phase B Vyn which is 180°
Figure 1.4l shows the solid type earthing. is current Icp
currents NR and Iyy
sum of
We shall now discuss what 90°.Vector

happens when a single line to ground 1aui fault current Ip

OCCurs. Lel us
suppose that the earth fault phase by
occurs on phase B. Potential ol B
 Switchgear and Protection
glemeennt
isSo
offP
Prro
o tt
ection

53
52
but the
the
suppose that the fault
Advantages
phase
b e c o m e s
zero

potential o
L e t
1S
us

B. There are three

VRN

Potential
of the
faulty
the
same.
the rs on phase

current of phase
. remain nents of the
healthy
phases rent become out of
current
ase by 180 FR
charging comp
are p BR and BY
and the the .

reduces ' h e s e

or
ault
current
prevents of between
2.
i.e. they get
neutralize.
This

hea.
arcing nce
Resistance

neutral
R is connected

and the ground. So

given a s the
ground.
type
protection
can be
fault current the resistance also in the path of
t h e r e .

VBY
D i s c r i m i n a t i o n

ault current in addition to the VON

3. the lau VyN
flows.
equipment
of the equipment conner impedance. As a
on he
the insulation
insulation
sequence
ill effect health zero

of the Ip lags less

4. There is no
increase
in voltage hases. result the fault
current

there is
no
the line as 90°
behind the faulty phase
han
Disadvantages and limitations
solid
voltage. (In sol
earthing this angle
as t hee value of the
value of the fault
become
unstable
curren+
nt there is no resistance).
earthing
The system may
as of the resistance
1. is 90
1s shown in figure rig. 1.44 Vector diagram o c c u r s on phase B
diagram when the fault
becomes large. Wector
handle the
circuit breaker as
breaker a s the hea.
heavy current
difficult to
2 It becomes 1.44. is ahead ol
iS ahead of voltage Var by 90° and current IBy
has to be
interrupted. Current BR is current Icp There
on the nearby communicatior
9 0 . Vector sum of currents IgR and Igy
disturbances are produced
voltage VBY
by which is in phase
3. The One component is Ipp
fault current. components
ol current
Ip which is lagging 90°
due to the heavy are two
Another component
1s Ie,
disadvantages this type
oi earthing is used only here with the laulty
voltage VBN that the components
Due to the above Value of resistance R' is so adjusted and
is employed up to 33 kV
circuit is more. This system behind voltage VBN As a result the problems
of arcing ground
the impedance of the cancelled. R is m o r e ,
8et
Igl, and CF
resistance
of
avoided. But if the value
1.34.2 Resistance earthing: are oscillations
similar to ungrounded
transient condition
In this method the neutral and the
cannot cancel lcp
then
current p transient over voltage.
m is earthed through resistance R. which results in
neutral is produced
This method is employed when
N
mY- Advantages not produced.
are
the fault current has to be
arising
due to arcing ground
Problems
1.
limited. Resistance is of metal or current is
reduced.

Value of the fault are

reduced.
liquid column. When the 2. c o m m u n i c a t i o n
line
IBR to the nearby
R resistance is made of metal then 3
Disturbances

be employed.
CR inductance also. So fault protection
can

it has some
4. Earth

the insulation of the equipment

Disadvantages
not so

when there is costly. it is

is damaged 1. The system some
value
however

to equipment
Fig. 1.43 Resistance earthing on the over
increases

of the
lightening surge phase
voltage
to the
insulation

Healthy damage
head line. This difficulty does not arise with iquid resistance. Figure 1.45 2. cause

as to
so
shows high line.
system using this type of earthing. C o n n e c t e d
to that
54
Switchgear and Prot. nents
ofPro
tection

lor the system havin. ion blem of arcing

ground 55
method is used ving is avoided
This type of earthing

2.2 kV to 33 kV and where

the capacity
ol power source is more
voltages o
than 5 MV
4
charging current due o the line
or

capacitance
is reduced
is
as

reduced.
the ellect of the

1.34.3 Reactance carthing: pisadvantages:

is connected between
belween the neutral
neutroi
and eansient over voltages are not
In this method. a reactance
suppressed if the value of the reactance
more. So the insulation ot the
as shown in figure 1.45. is
equipment connected to the line may
be damaged.

m
Value of the surge voltage increases due to the reactance at the
switching operation.
m
Fault current and capacitive charging current are not neutralized
.
So effect of arcing ground remains.
m8 properly. some

LBR LBY Peterson coll or are suppression coil earthing:

1.34.4
In all the methods of
earthing discussed above.
RECTANCE m the other healthy phases
when
cannot supply power
IF IpR IEY N fault o c c u r s on
the ground
Fig. 1.45 Reactance earthing fault is
one line until the
m
Peterson coil
cleared. In
the external reactance connected the
Value of the fault current depends on
PETERSON type earthing
system

ol the power source and the COlL C can

also in addition to the zero sequence impedance other healthy phases
when a
can be to the required
impedance of the faulty phase. The fault current adjusted supply power
occurs on one
method is employed to fault
value by adjusting the value of the reactance. This IF IeR LEY ground
phase. Other advantages
this method, the value
obtain the characteristic of the solid earth system. In method.
Peterson coil earthing in this
1.46 also result
of the earth fault current should be one fourth of the three-phase fault current. Fig. that
adjusted
coil is so
of the
This value is more than the fault current of the resistance earthing. reactor.
Reactance
Arcing
is

a n iron
cored
healthy phase.
coil is of the power
where the value of the Peterson capacitance
to supply
type of earthing method can be employed
the
This with continue
r e s o n a n c e

phases the are

produces healthy called
cable etc. it other two also
charging current is more e.g. transmission line. underground due to
this. The
phase.
Peterson
coil is
obtained by this
prevented one
on
occurs grounding
fault The
Advantages when an
earth
1ault
n e u t r a l i z e r .

or
ground
coil
. Earth fault protection can be given. suppression grounding.
resonant

called
2. Value of the fault current is reduced so the disturbances to the nearny method
is
in figure
1.47.
shown
is
diagram
communication lines are reduced. Phaser

3. Cireuit breaker has to break less value of the lault

comparatively
current.
 of Protection
glements o f .
57
Switchgear and prot. ssary to change the inductance of the coil when the

ahead oteofctitohne
is neceSsary

capacitance
Current vector BR is ahea o ft h e
e t work changes. For this tapings are given on the coil.
network

56 voltage
vector BR by 90.
90 Similarly Current rating of the coil can be found from Ip= 3Vph When the arc
vector gy 1s ahead of
current
vector B1 by 990 yollageo
Vector Sum suppr
ce
ssion coil is working. lat the time of fault). voltage to earth of the healthy
ana By 1s
CP which times the phase to neutral voltage. So ratings of the lightning
currents gR is hecomes v3
with current out phase

should be equal to the total voltage.

of phase by 180 arrester

method is useful when the

IcF3aR=V3 1,14
This
line is fed from a
m
transmission
because there is increase in
ransformer. N mY
3 3 pht =3 . . 1.15 voltage of the healthy phase at the
the

fault. If the line were fed from an

of
3 x line to neutral chara ime
insulation has to bne
rging alternator,
the e x t r a
which
AUTOMATIc
BYPASS
current
1.l6 on the alternator winding SWITCH
Drovided PETERSON
this can easily be COIL | R IF REQUIRED
while
diagram
of
is
difficult
1.47 Vector Are suppression
Fig.
Peterson coil earthing on
transformer.
faulty phase co
omes across provided
of the
the voltage take continuous current
oí fault. and is in phase
rated to
coil is
an.

During the
condition
about
90 osition about five
rating of
arc
l2gs switch with
coil and
cuTent p
during the fault and c of the
short time
Fig. 1.48
Automatic

the arc
suppression
does n o t persist fault or
automatic
switch is provided
suppression switch
arc An
So the of the coil
with the current
e not ilow
even when
inductance minutes.
coil which shorts
current
ilows
do time rating result the
earth
capacitive
current
resonant current lou
ws. with short As a
the neutral directly.
current or so some
balance exactly and grounds
capacitance
do not
the coil
system a s follows. operates.
can be made earth fault relay
Calculauon of the
inductance
and the
neutralized
are
Advantages the time of fault
currents flowing at
Capacitive
charging coil.
Peterson
1. of the the
reactance
zero so
the
inductive
equal to
.. 1.17 by kept
practically
Vp fault
current can
be
So over
voltages
are

And X Value of the does not

occur.

2. ground
phenomenon
of arcing
At the tirne of resonance, Ip = lCF in the
there is change
not produced.
coil so if of
value
the
Peterson
then the
in line.
3Vplt 3.
There
are
tapings
the
capacitance
of the
in taping.
help of
line or
of with the
length the fault
can
be adjusted power
when
to supply
X1 3 Xc
inductance
continue

phases
healthy
other
two
4. The
phase.
one
on
O C c u r s
employed.
be
30C faull relay
can

carth
The
5.
. 1.18
18
3 C SGPEng.
12015
 of Protection

ements 59
Switchgear and Protection
Secondary winding pf the potential
2. VRN
transformer can be used to drive the
suppressed.

58
stroke
is
relay or for the fault indication .
the voltage of
l i g h t n i n g

kept more
to be
The
6.
p i s a d v a n t a g e s

has

Disadvantages: e q u i p m e n t

the
normal
volta

ltage. Neutral becomes the rellecting point

ofthe
l i m e s

insulation
3 .

.
The
phase
b e c o m e s

ol
the
lightnin
rester has to for the travelling waves so there is
the
healthy the
rating
be elffect of high voltage on the winding.
VBY
r e a s o n

above
the VEN
voltage.
transformer
transfor VYN
Voltage induced in the secondary is
to
2.
Due
to
the
full
line
ted by it is not
kept
equal
useful
for
the
alternator tor a s
it becomes
ficult to provid proportional to the fault current. So LeR
is
current is less, there is
if the fault
method

the
3.
This
line
fed by alternator
winding.

lor the in operation of the relay

used for the difficulty
i n s u l a t i o n

is damaged if the
and the relay CF
i n c r e a s e d

earthing:

is earth rthed through the primar current is

value of the fault
transformer
more.
system
of the voltage
1.34.5
Voltage
of the 1.49, Fig. 1.50 Vector diagram
in figure
neutral

method
the
as
shown
This system is less efficient. transformer type earthing
system
In this t r a n s f o r m e r

3.
voltage
of the 1.35 Earthing transformer
winding
or in many
in delta
is connected
m

available when the system

is not it. In such
Neutral to connect
Y available but it is not possible
N m
the n e u t r a l is Earthing
SYstems to be made.
sys artificial neutral has
arrangement of the
L m
circumstances
IeR is used for this. compensator
transformer
neutral earthing
VOLTAGE This is called the
TRANSFORMER- core in earthing
three-limbed
also. There is a
on each
are provided
transformer. Two windings star and
connected in
is
imb. One set of winding winding
this. This
out of
neutral lead is taken such that
the other set
transformer earthing of
1.49 Voltage to the winding limb
Fig. o n each
connected
is kept
that of the isolated two windings
is similar to the currents of the the
normal
în
c h a r a c t e r i s t i c of
this type of earthing directions.
So
The transtormer provides high the opposite
in
winding of the
voltage are
current
flows.

neutral type. Primary the indication of fault less exciting is

addition to this
conditions
transformer

neutral to earth. In earthing

impedance to the method is of the
the secondary voltage.
Difficulty in this The impedance fault
current is high.
can be given by measuring
of the current
reflection for the travelling
wave
less so the magnitude current,

that the earthed neutral becomes

the point of of the fault
the value neutral

of high voltage on the reduce the

So to either in
machine. So there is efect
through the winding of the
connected
is shows the
resistor 1.52
transtormer limiting Figure
1.51 earthing from
uransformer.

winding. Fig.
of the
earthing t r a n s m i s s i o n
line fed
winding to
in the
connected

Advantages: circuit or
earthing
translorner

neutral can of the

method the characteristics similar to isolated
C o n n e c t i o n s

With this

De achieved.
 Switchgear and
60
limitlng resistor R' ic
is
Protection
a star delta connected
transformer.
CurTent

onnected in Elements of Protection

61
the neutral circuit.
Ground lault
protection system used for generator.
3. The magnitude of the possible fault current.
4. Number of generators connected in
m
parallel.
Generator in
power station is connected in two ways. In the first method
all the generators are connected in parallel and connected to the bus bars.
2F/ IF This is known as the classical connection. Another modern method of connecting
the generator is the unit system. In this each generator, its unit transformer
POMER TRANSFORVER
and unit auxiliary transformer make one unit. This is called the unit system.
Neutral grounding for classical generator connection:
F
When more number of generators are connected to the bus bar, neutral
of only one generator is grounded. This is because if more number of
neutrals are grounded, the zero sequence component of the circulating
current causes disturbances.

(1) Arrangement is made in generating station for grounding ot two

generators. In normal condition neutral of one generator is grounded.
EARTHING TRANSFORMER When one generator is not in operation. then neutral of the other

generator is grounded.
Fig. 1.52 Earthing transformer connected to transmission line fed from the star-delta
connected transformer (ii) When there are more number of generators, arrangement of the fourth
bus bar (neutral bus bar) is also made. Neutral bus bar is grounded
1.36 Neutral grounding systema: through the reactance. Neutral of one generator is connected to the

After discussing the grounding of neutral in the system shall neutral bus bar through the circuit breaker.
we
now see
the points where the neutral is
grounded in the system. For this the following Neutral grounding for unit system of connection of generator
points should be kept in mind.
(1) In this neutral of each generator is grounded.
1. There are diflerent voltage levels in the
system. So for each voltage neutral is done through resistor or through the voltage
level (ii) Grounding of
one neutral ground is essential.
transformer.
2. Neutral grounding done in the source and not in the
is
load. Neutral of the star connected high voltage side of step up transformer

3.
(iii)
The neuiral should be grounded in each
important bus section. is grounded through resistor
1.36.1 connected
generator side is
Generator neutral grounding (iv) In the unit auxiliary transformer the the
Neutral os the
The side is connected in star.
general neutral in delta and the auxiliary
gronding depends upon the
Ciection of the following factors. L. V. side is earthed.
generator to the
connected to the common bus bars such is
of power
transformer

bus bars or to
as
whetne 1.36.2 Grounding of
neutral
transformer is grounded.
the unit
translormel The neutral of the
star connected side of power
 Elements Qf Protection

62 Switchgear and Protect al Defect in the bus bar of the

63
control panel of the
transformer: substation
1.36.3 Grounding of neutral of protective state the types of fault
connecled side is
4. occurring in the overhead
line and state their
In CT and T also the neutral on star grounded percentage.

Short Questions and

Answers Ans. (i) Line to ground fault (LG)-85 %
(11) Line to line fault (LL)-8 %
1. State three main components of the power system and state
state their stem and their main (ii) Line to line to ground fault (LLG)-5 %
equipments.
(iv) Line to line to line fault (LLL)- 2 %
Ans. Threr main components of the power system are as lollows.

5. State the difference between the symmetrical and unsymmetrical fault.

(0 Grnrrating statton
Ans. In symmetrical lault, the value of the fault currents in all the three phases
() Load center
are equal and the phase angle between the adjacent phases is 120. In
f) Transmission and distribution system.
unsymmetrical fault, the values of the fault current in all the three phases
In generating station. generator. prime mover, exciter, step up transforr are not equal and the phase angle also does not remain to 120°
rmer
r c u t breakrt are included.
3. State the elements of the protective system
In igad enter, low and high tmsion consumers are included.
Ans. i) Sensing element
Intransmission and distribution systen, transformer, transmisslon ant
(ii) Selector (summation transformer)
distribution ines, cireuit breakers etc. are included.
(i11) Measuring element (comparator and relay)
What is meant by the abnormal condition of the power system? State
the characteristics for them. (1v) Amplifier
Ans. When the (v) Actuator
voltage, current. poweT factor, frequency or
temperature exceed
Tatn level it 1s ralled theabnormal endition of the power system. State the desirable characteristics of
the protection system.
7.
Charatteristir.s shTung
abriH nal condition of the system are as follows, Ans. (1) Reliability
rinadrng of the nutprnert (1) Sensitivity
rbalanr ed ading (11) Selectivity
Paliure prirn nsue (1v) Discrimination

(v) Speed
vi) Simplicity
( e r vltagr urgr
(vil) Econony
9tate the causes of back up
protection.
occurrence of fault in State the types of
power 8.
Ans. Drfert nystem.
in
th Inaulation sf the eltriral cqutpment Ans. (1) Relay back up
(11) Dfu:t in th undergfound line Brcaker
back up
(11)
(1) Des1 in the
erhead ine (1)
Remole
back up