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CIRCUIT BREAKER

Abdullah Asuhaimi Mohd.Zin

INSPIRING CREATIVE AND INNOVATIVEMINDS


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Introduction

A power system network needs a switchgearfor its switching and or isolation undernormal or abnormal operation conditions.

Two functions of switchgear :

1. To permit plant and distributors (transmission lines)

to be conveniently put into and taken out of service.2. To enable the same plant and lines when thesebecome faultyto be rapidly and safely isolated byautomatic means.



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Introduction

The function of a circuit breaker is to isolate

the faulty part of the power system in case of

abnormal conditions.A protective relay detects abnormal

conditions and sends a tripping signal to the

circuit breaker. After receiving the trip

command from the relay, the circuit breakerisolates the faulty part of the power system



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Introduction

A circuit breaker has two contacts afixed contact and a moving contact.

Under normal conditions these twocontacts remain in closed position.

When the circuit breaker is required to

isolate the faulty part, the movingcontact moves to interrupt the circuit.



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Introduction

On the separation of the contacts, the

flow of current is interrupted, resulting in

the formation of arc between thecontacts, as shown in Figure 5.1. The

contacts are placed in a closed

chamber containing some insulatingmedium (liquid or gas) which

extinguishes the arc.



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Introduction


Figure 5.1 Separation of the contacts of circuit breaker


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Arc Voltage

Figure 5.2 shows the wave shape of arc voltage. Thevoltage drop across the arc is called arc voltage.

As the arc path is purely resistive, the arc voltage isin phase with the arc current.

The magnitude of the arc voltage is very low,

amounting to only a few per cent of the rated voltage.A typical value may be about 3 per cent of the ratedvoltage.



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Arc Voltage


Figure 5.2 Short circuit current and arc voltage


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Arc

Interruption

There are two methods of arc

interruption:

1. High Resistance Interruption

2. Current Zero Interruption



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1. High Resistance

Interruption

In this method of arc interruption, itsresistance is increased so as to

reduce the current to a valueinsufficient to maintain the arc.

The arc resistance can be increasedby cooling, lengthening, constrainingand splitting the arc.



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1. High Resistance

Interruption

When current is interrupted the energyassociated with its magnetic field

appears in the form of electrostaticenergy.

A high voltage appears across thecontacts of the circuit breaker.



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1. High Resistance

Interruption

If this voltage is very high and more than

the withstanding capacity of the gap

between the contacts, the arc will strikeagain.

Therefore, this method is not suitable for a

large-current interruption. This can be

employed for low power arc and dc circuit

breakers.



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2. Current Zero

Interruption

This method is applicable only in caseof ac circuit breakers.

In case of ac supply, the current wavepasses through a zero point, 100

times per second at the supplyfrequency of 50 Hz.



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2. Current Zero

Interruption

This feature of ac is utilised for arc

interruption. The current is not interruptedat any point other than the zero current

instant, otherwise a high transient voltage

will occur across the contact gap. The

current is not allowed to rise again after azero current occurs.



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2. Current Zero

Interruption

There are two theories to explain the

zero current interruption of the arc.

1. Recovery rate theory (SlepainsTheory)

2. Energy balance theory (CassiesTheory)



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2. Current Zero

Interruption

Recovery rate theory

The arc is a column of ionised gases.

To extinguish the arc, the electrons and ions are tobe removed from the gap immediately after thecurrent reaches a natural zero.

Ions and electrons can be removed either byrecombining them into neutral molecules or bysweeping them away by inserting insulatingmedium (gas or liquid) into the gap.



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2. Current Zero

Interruption

Recovery rate theory (cont.)

The arc is interrupted if ions are removed from thegap at a rate faster than the rate of ionisation.

In this method , the rate at which the gap recoversits dielectric strength is compared with the rate atwhich the restriking voltage (transient voltage)

across the gap rises.

.



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2. Current Zero

Interruption


If the dielectric strength increases more rapidlythan the restriking voltage, the arc is extinguished.

If the restriking voltage rises more rapidly than thedielectric strength, the ionization persists andbreakdown of the gap occurs, resulting in an arcfor another half cycle.

Figure 5.3 explains the principle of recovery ratetheory.



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2. Current Zero

Interruption



Figure 5.3 Recovery rate theory


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2. Current Zero

Interruption

Energy balance theory

The space between the contacts contains some

ionised gas immediately after current zero andhence, it has a finite post-zero resistance.

At the current zero moment, power is zerobecause restriking voltage is zero.

When the arc is finally extinguished the poweragain becomes zero, the gap is fully de-ionisedand its resistance is infinitely high.



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2. Current Zero

Interruption

Energy balance theory (cont.)

In between these two limits, first the

power increases reaches a maximumvalue, then decreases and finallyreaches zero value as shown in Fig. 5.4.

Due to the rise of restriking voltage andassociated current, energy is generatedin the space between the contacts.



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2. Current Zero

Interruption



Figure 5.4 Energy balance theory


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2. Current Zero

Interruption


The energy appears in the form of heat. Thecircuit breaker is designed to remove thisgenerated heat as early as possible by cooling thegap, giving a blast of air or flow of oil at highvelocity and pressure.

If the rate of removal of heat is faster than the rate

of heat generation the arc is extinguished. If therate of heat generation is more than the rate ofheat dissipation, the space breaks down againresulting in an arc for another half cycle.



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5.3 Restriking Voltage and Recovery Voltage

The voltage across the contacts of the circuit breaker is arcvoltage when the arc persists.

This voltage becomes the system voltage when the arc is

extinguished. The arc is extinguished at the instant of currentzero.

After the arc has been extinguished, the voltage across thebreaker terminals does not normalise instantaneously but itoscillates and there is a transient condition.

The transient voltage which appears across the breakercontacts at the instant of arc being extinguished is known asrestriking voltage.



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5.3 Restriking Voltage and Recovery Voltage (cont.)

The power frequency rms voltage,

which appears across the breaker

contacts after the arc is finallyextinguished and transient oscillations

die out is called recovery voltage.



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The instantaneous recovery voltage at the instant ofarc extinction is called active recovery voltage.

The instantaneous recovery voltage is given by :

Var= kVm sin

Where k=1 if the three-phase fault is grounded andk=1.5 if the three-phase fault is isolated. Vm is peak

recovery voltage and is the power factor angle.

Figure 5.5 shows the restriking and recovery voltage.



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Figure 5.5 Restriking and recovery voltage


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5.3.1 Expression for Restriking Voltage and RRRV

Figure 5.6(a) shows a short circuit on a

feeder beyond the location of the circuit

breaker.

Figure 5.6(b) shows an equivalent electrical

circuit where L and C are the inductance and

capacitance per phase of the system up tothe point of circuit breaker location,

respectively.



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5.3.1 Expression for Restriking Voltage and RRRV


Figure 5.6 (a) Fault on a feeder near circuit breaker

(b) Equivalent electrical circuit for analysis of

restriking voltage


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5.3.1 Expression for Restriking Voltage and RRRV (cont.)

When the circuit breaker is closed, the shortcircuit current flows through R, L and thecontacts of the circuit breaker, the

capacitance C being short-circuited by thefault.

When the circuit breaker contacts are openedand the arc is extinguished, the current isdiverted through the capacitance C, resultingin a transient condition.



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The inductance and the capacitance form a

series oscillatory circuit. The voltage across

the capacitance which is restriking voltage,rises and oscillates, as shown in Fig. 5.5.

The natural frequency of oscillation is given

by


LC2

1fn


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The voltage across the capacitance

which is the voltage across the contacts

of the circuit breaker can be calculatedin terms of L, C, fnand system voltage.

The mathematical expression for

transient condition (neglectingresistance) is as follows.


EdtiC1

dt

diL


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Where E is the system voltage at the

instant of arc interruption. As the

transient oscillation is a fastphenomenon, E can be regarded as a

constant for a short duration.



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Where vc= voltage across the capacitor

Therefore


dt

)d(cv

dt

dqi c


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Substituting these values in the equationgiven above, we get

Taking Laplace Transform of both sides ofthe equation, we get

Where (S) is the Laplace Transform of


Evtd

vd

LC c2c

2

S

E)(v)(SLC c

2 SSvc

where vc(S) is the Laplace Transform of vc


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Other terms are zero as initially q = 0 at t = 0,

or

, therefore


LCwn

1 2

nLC

1


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or


)(S

(S

)S(S

E

(s)V 2n2

nn

2n

2

2

n

c

E

Taking the inverse Laplace, we get

t

0 nnc tsinE(t)V

t

0n

nnc

tcosE(t)V


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As = 0 at t = 0, constant = 0

(t) = E(1cos wnt ) , or

= Restriking voltage

The maximum value of restriking voltage = 2 Epeak= 2 x peak value of the system voltage

The Rate of Rise of Restriking voltage (RRRV)

= (1 - cos wnt) = wn E sin wnt

The maximum value of RRRV = wnE

= wnEpeak


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Assignment



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Example 5.1


Example 5.1 For a 132 kV system, the reactance andcapacitance up to the location of the circuit breaker is 3 ohmsand 0.015 F, respectively. Calculate the following:

a. The frequency of transient oscillation.

b. The maximum value of restriking voltage across

the contacts of the circuit breaker

c. The maximum value of RRRV


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Example 5.1


Solution

(a)The frequency of transient oscillation

L = , f = 50, the system frequency

= 0.00954 H=

= =

=

= 13.291 kHz=


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Example 5.1


(b) The restriking voltage

= E [ 1 - cos wnt ]The maximum value of the restriking voltage = 2 Epeak

= 2 x

= 215.56 kV(c) maximum value of RRRV = wnEpeak

100023

1322 xxxf

n

V/sx10002x3

132x013.291x1002x

kV/9.01045V/s10x9010.45 6 sec


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Example 5.2


Example 5.2 : In a short circuit test on a 132kV 3-phase system, the breaker gave thefollowing result : p.f. of the fault 0.4, recoveryvoltage 0.95 of full line value, the breakingcurrent is symmetrical and the restriking

transient had a natural frequency of 16 kHz.Determine the rate of rise of restriking voltage.Assume the fault is grounded.


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Example 5.2


Solution :

The peak value of line to neutral voltage =

kV107.7523

132

kV93.850.916x102.4

0.4cossinxx107.750.95V 1ar


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Example 5.2


The maximum restriking voltage = 2 x 93.85

= 187.7 kV

sec16x2

10

2f

1twheret

VRRRV3

n

kV/V/6.0

kV/sec10x187.7x32

10

16x2x187.7RRRV 3

3-

sec


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Determinat ion of Recovery Vol tage

The recovery voltage is an indication ofthe severity of the test.

Less than the normal system voltagedue to voltage drop.

A typical recovery voltage oscillogram is

shown in the figure


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Determination of Recovery voltage

YELLOW PHASE V


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In this oscillogram, Line LL is drawn through the pointof final arc extinction, this point being determinedfrom an examination of the three corresponding

current waveforms. Line MM and NN are drawn at half-cycle intervalsfrom LL.

The peak to peak recovery voltage (V1,V2, etc) aremeasured during the second complete half-cycle

after final arc extinction, that is between the lines MMand NN.

If a peak to peak voltage occurs exactly on theselines, it is measured on NN, as shown.


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The above measurements are made during thesecond half-cycle so as to allow a reasonable time for

the high frequency restriking transients to disappear

and for the voltage recovery to commence. The rms recovery voltage for any phase is obtained

by dividing the peak to peak value by 22.

Average recovery line voltage for a three-phase

breaker is1 2 3( ) 3

2 2 3

V V V

x


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5.4 Resistance Switching


To reduce the restriking voltage, RRRV and severity of

the transient oscillations, a resistance is connected

across the contacts of the circuit breaker.

This is known as resistance switching. The resistance is

in parallel with the arc.

A part of the arc current flows through this resistance

resulting in a decrease in the arc current and increase in

the deionization of the arc path and resistance of the

arc.


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This process continues and the current through the

shunt resistance increases and arc current decreases.

Due to the decrease in the arc current, restriking voltage

and RRRV are reduced.

The resistance may be automatically switched in with

the help of a sphere gap as shown in Fig. 5.7.

The resistance switching is of great help in switching out

capacitive current or low inductive current.


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Figure 5.7 Resistance switching


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The analysis of resistance switching can be made to find

out the critical value of the shunt resistance to obtain

complete damping of transient oscillations. Figure 5.8

shows the equivalent electrical circuit for such ananalysis.

Figure 5.8 Circuitfor analysis of

resistance switching


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The voltage equation is given by

RCC iiiandEdtiC

1

dt

diL

Therefore, the above equation become

Edt

)id(iL C

RC

Edt

diLdt

diL CRC

dt

)d(C

dt

dqi cC


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Therefore,

2

c

2

2

c

2

c

dt

dC

dt

)(Cd

dt

di

dt

d

R

1

dt

/R)(d

dt

di ccR

Substituting these values in the main equation, we get

Edt

d

R

L

dt

dLC C

c

2

C2


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Taking Laplace Transform, we get

Other terms are zero, as


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For no transient oscillation, all the root of the equation should

be real. One root is zero, i.e. S = 0 which is real. For the

other two roots to be real, the roots of the quadratic equation

in the denominator should be real. For this, the followingcondition should be satisfied.


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CL /

Therefore, if the value of the resistance connected across the

contacts of the circuit breaker is equal to or less than there will be no transient oscillation.

If R > there will be oscillation. R = is known as critical resistance.

Figure 5.9 shows the transient conditions for three different values of

R. The frequency of damped oscillation is given by


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Figure 5.9 Transient oscillations for different value of R


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Example 5.3 In a 220 kV system, the reactance and

capacitance up to the location of circuit breaker is 8

and 0.025 F, respectively. A resistance of 600 ohms

is connected across the contacts of the circuit breaker.

Determine the following:

1. Natural frequency of oscillation

2. Damped frequency of oscillation

3. Critical value of resistance which will give no

transient oscillation4. The value of resistance which will give damped

frequency of oscillation, one-fourth of the natural

frequency of oscillation.


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f = 3.413 kHz

f =

(ii) Frequency of damped oscillation is given by

=

kHz413.3

9

10

36.6

10

2

1 1010

=


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(iii) The value of critical resistance

35.50410x0.025

0.02544

2

1

C

L

2

1R

6-


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5.5 Current Chopping


When low inductive current is being interrupted and

the arc quenching force of the circuit breaker is more

than necessary to interrupt a low magnitude of

current, the current will be interrupted before itsnatural zero instant.

In such a situation, the energy stored in the magnetic

field appears in the form of high voltage across thestray capacitance, which will cause restriking of the

arc.


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The energy stored in the magnetic field is Li2, if i is the

instantaneous value of the current which is interrupted. This

will appear in the form of electrostatic energy equal to cv2.

As these two energies are equal, they can be related as

follows.


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Figure 5.10 shows the current chopping phenomenon.

If the value of v is more than the withstanding capacity of the

gap between the contacts, the arc appears again.

Since the quenching force is more, the current is again

chopped.

The phenomenon continues till the value of v becomes less

than the withstanding capacity of the gap.

The theoretical value of v is called the prospective value of the

voltage.


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Figure 5.10 Current chopping


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Example 5.4A circuit breaker interrupts the magnetising

current of a 100 MVA transformer at 220 kV. The

magnetising current of the transformer is 5% of the full load

current. Determine the maximum voltage which may appearacross the gap of the breaker when the magnetising current

is interrupted at 53% of its peak value. The stray

capacitance is 2500 F. The inductance is 30H.


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Solution

The full load current of the transformer

=

= 262.44 A

Magnetising current = x 262.44 = 34.44 A

Current chopping occurs at 0.53 x 34.44 = 25.83 A


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5.6 Interruption Capacitive Current


The interruption of capacitive current produces high voltage

transients across the gap of the circuit breaker.

This occurs when an unloaded long transmission line or a

capacitor bank is switched off.

Figure 5.11(a) shows an equivalent electrical circuit of a

simple power system. C represents stray capacitance of the

circuit breaker. CLrepresents line capacitance. The value ofCLis much more than C.

Figure 5.11(b) shows transient voltage across the gap of the

circuit breaker when capacitive current is interrupted


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(a)Electrical circuit of a simple power system


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Figure 5.11 Interruption of capacitive current


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At the instant M, the capacitance current is zero and the

system voltage is maximum. If an interruption occurs, the

capacitors CL remains charged at the maximum value of the

system voltage. After instantM

, the voltage across the breakergap is the difference of Vcand VCL. At instant N, i.e. half-cycle

from A, the voltage across the gap is twice the maximum value

of Vc. At this moment, the breaker may restrike.

If the arc restrikes, the voltage across the gap becomespractically zero. Thus, the voltage across the gap falls from

2Vcmaxto zero. A severe high frequency oscillation occurs.


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The voltage oscillates about points between R and N, i.e.

between -3emaxand emax.

When restriking current reaches zero, the arc may be

interrupted again. At this stage, the capacitor CL remainscharged at the voltage -3emax.

At instant P, the system voltage reaches its positive

maximum shown by the point T in the figure, and at thismoment the voltage across the gap becomes 4emax. The

capacitive current reaches zero again and there may be an

interruption.


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If the interruption occurs at this moment, the transient

voltage oscillates between P (-3emax) and Q (+5emax).

In this way, the voltage across the gap goes on increasing.

But in practice, it is limited to 4 times the peak value of thesystem voltage.

Thus, it is seen that there is a problem of high transient of

high transient voltage while interrupting a capacitivecurrent.


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5.7 Classification of Circuit Breaker


The classification of circuit breakers has been made

on the basis of insulating medium employed in the

circuit breakers to extinguish the arc. Depending

on the arc quenching medium employed, thefollowing are important types of circuit breakers.

(i) Oil Circuit Breakers

(ii) Air Blast Circuit Breakers

(iii) Sulphur Hexafluoride (SF6)Circuit Breakers

(iv) Vacuum Circuit Breakers


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5.8 Oil Circuit Breaker


Mineral oil has better insulating properties

than air. Due to this very reason it is

employed in many electrical equipment

including circuit breakers.

Oil has also good cooling property. In a

circuit breaker when arc is formed, itdecomposes oil into gases. Hence, the arc

energy is absorbed in decomposing the oil.


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5.8 Oil Circuit Breaker


The main disadvantage of oil is that it isinflammable and may pose a fire hazard.

Other disadvantages include the possibility of

forming explosive mixture with air and the

production of carbon particles in the oil due to

heating, which reduces its dielectric strength.

There are various types of oil circuit breakers

developed for use in different situations. Some

important types are discussed below.


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5.8.1 Plain-break Oil Circuit Breakers


In a plain-break oil circuit breaker there is a fixed and a

moving contact immersed in oil. The metal tank is strong,

weather tight and earthed.

Figure 5.12 shows a double break plain oil circuit breaker.

When contacts separate there is a severe arc which

decomposes the oil into gases. The gas obtained from

the oil is mainly hydrogen. The volume of gasesproduced is about one thousand times that of the oil

decomposed. Hence, the oil is pushed away from arc and

the gaseous medium surrounds the arc.


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Figure 5.12 Plain-break oil circuit breaker


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The arc quenching factors are as follows:

(i) Elongation of the arc

(ii) Formation of gaseous medium in between the

fixed and moving contacts. This has a high

heat conductivity and high dielectric strength.

(iii) Turbulent motion of the oil, resulting from the

gases passing through it.


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A large gaseous pressure is developed

because a large amount of energy is dissipated

within the tank.

Therefore, the tank of the circuit breaker is

made strong to withstand such a large

pressure.

When gas is formed around the arc, the oil is

displaced.


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To accommodate the displaced oil, an aircushion between the oil surface and the tank is

essential.

The air cushion also absorbs the mechanicalshock produced due to upward oil movement.

It necessary to provide some form of vent fitted

in the tank cover for the gas outlet.

A sufficient level of oil above the contacts is

required to provide substantial oil pressure at

the arc


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Certain gap between the contacts must be createdbefore the arc interruption occurs.

To achieve this, the speed of the break should be as

high as possible.

The two breaks in series provide rapid arc elongation

without the need for a specially fast contact. The

double break also provides ample gap distance before

arc interruption.

But this arrangement has the disadvantage of unequal

voltage distribution across the breaks.


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Figure 5.13 shows voltage distribution acrossbreaks. C is the capacitance between the fixed

contact and moving contact, C is thecapacitance between the moving contact and

earth. V1is the voltage across the first contact

and V2 across the second contact. Suppose

the fault current is i.

The voltages V1and V2will be expressed as follows :


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Figure 5.13 Voltage

distribution across

breaks


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The capacitance between the moving contact and

earth Cis in parallel with C, the capacitance ofthe second break.

Taking C = 8 pF and C = 16 pF, we get

V1 + V2 = V = system voltage

V2 = 25% of the system voltage

and

V1 = 75% of the system voltage


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To equalise the voltage distribution across the breaks,

non-linear resistors are connected across each break.

The plain break circuit breakers are employed for breaking

of low current at comparatively lower voltages. They are

used on low voltage d.c. circuits and on low voltage a.c.

distribution circuits. Their size becomes unduly large for

higher voltages.

Also, they require large amount of transformer oil. They

are not suitable for autoreclosing. Their speed is slow.

They can be used up to 11 kV with an interrupting capacity

up to 250 MVA.


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5.8.2 Self-generated Oil Circuit Breakers


In this type of circuit-breakers, arc energy is

utilised to generate a high pressure in a

chamber known as explosion pot or pressure

chamber or arc controlling device.

The contacts are enclosed within the pot.

The pot is made of insulating material and it is

placed in the tank. Such breakers have high

interrupting capacity. The arcing time is

reduced.


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Since the pressure is developed by the arc itself, itdepends upon the magnitude of the current. Therefore,

the pressure will be low at low current and high at high

values of the current. This creates a problem in

designing a suitable explosion pot.

At low current, pressure generated should be sufficient

to extinguish the arc.

At heavy currents, the pressure should not be too high

so as to burst the pot. Various types of explosion pots

have been developed to suit various requirements. A

few of them have been discussed below.


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Figured 5.14 shows a plain explosion pot.

This is the simplest form of an explosion pot.

When the moving contact separates a severe arc is

formed. The oil is decomposed and gas is produced. It

generates a high pressure within the pot because there

is a close fitting throat at the lower end of the pot.

The high pressure developed causes turbulent flow of

streams of the gas into the arc resulting in arc-

extinction.

Plain explosion pot


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Plain explosion pot (cont.)

Figure 5.14 Plain explosion pot

8 2 S lf d Oil Ci i B k


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If the arc extinction does not occur within the pot, it occurs

immediately after the moving contact leaves the pot, due

to the high velocity axial blast of the gas which is released

through the throat.

Since the arc extinction in the plain explosion pot is

performed axially, it is also known as an axial extinction

pot. This type of a pot is not suitable for breaking of heavy

currents. The pot may burst due to very large pressure.

At low currents, the arcing time is more. Hence, type of an

explosion pot is suitable for the interruption of currents of

medium range.

Plain explosion pot (cont.)

5 8 2 S lf t d Oil Ci it B k


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Figure 5.15 shows a cross-jet explosion pot.

It is suitable for high current interruptions. Arc splitters

are used to obtain an increased arc length for a givenamount of contact travel. When the moving contact is

separated from the fixed contact, an arc is formed, as

shown in Fig. 5.15(a).

The arc is pushed into the arc splitters as shown in Fig.

5.15(b), and finally it is extinguished, as in Fig. 5.15 (c).

In this type of a pot, the oil blast is across the arc and

hence it is known as a cross-jet explosion pot.

Cross-jet explosion pot



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Figure 5.15 Cross-jet

explosion pot



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Self-compensated explosion pot

This type of a pot is a combination of a cross-jet explosion potand a plain explosion pot.

Figure 5.16 shows a self-compensated explosion pot. Its upper

portion is a cross-explosion pot, and the lower portion a plainexplosion pot.

On heavy currents the rate of gas generation is very high and

consequently, the pressure produced is also very high.

The arc extinction takes place when the first or second lateral

orifice of the arc splitter is uncovered by the moving contact.

The pot operates as a cross-jet explosion pot.



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Self-compensated explosion pot (cont)

Figure 5.16 Self-compensated explosion pot



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Self-compensated explosion pot (cont)

When the current is low, the pressure is also low in the

beginning. So the arc is not extinguished when the tip of the

moving contact is in the upper portion of the pot.

By the time the moving contact reaches the orifice at thebottom of the pot, sufficient pressure is developed.

The arc is extinguished by the plain explosion pot action.

Thus, it is seen that the pot is suitable for low as well as highcurrent interruptions.

5 8 3 D bl B k Oil Ci it B k


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5.8.3 Double Break Oil Circuit Breakers


To obtain high speed arc interruption, particularly

at low currents, various improved designs of the

explosion pot have been presented.

Double break oil circuit breaker is also one of

them. It employs an intermediate contact

between the fixed and moving contact. When

the moving contact separates, the intermediate

contact also follows it.

5 8 3 Double Break Oil Circuit Breakers


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The arc first appears between the fixed contactand the intermediate contact. Soon after, the

intermediate contact stops and a second arc

appears between the intermediate contact and

the moving contact.

The second arc is extinguished quickly

employing gas pressure and oil momentumdeveloped by the first arc. Figure 5.17 (a) shows

an axial blast pot and Fig. 5.17(b) shows a cross

blast pot.

5 8 3 Double Break Oil Circuit Breakers


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Figure 5.17 Double break oil circuit

breakers

5.8.4 Bulk Oil and Minimum Oil Circuit


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Breakers


In bulk oil circuit breakers, oil performs twofunctions.

It acts as an arc extinguishing medium and it also

serves as insulation between the live terminals and

earth. The tank of a bulk oil circuit breaker is

earthed.

Its main drawback is that it requires a huge amount

of oil at higher voltages. Due to this very reason it is

not used at higher voltages.



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Breakers


A minimum content oil circuit breaker does not employ asteel tank. Its container is made of porcelain or other

insulating material.

This type of a circuit breaker consists of two sections namelyan upper chamber and a lower chamber.

The upper chamber contains an arc control device, fixed and

a moving contact. The lower chamber acts as an insulating

support and it contains the operating mechanism.

These two chambers are filled with oil but they are physically

separated from each other.



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Breakers


The arc control device is placed in a resin bounded

glass fiber cylinder (or backelised paper encloser).

This cylinder is also filled with oil. The fibre glasscylinder is then placed in a porcelain cylinder. The

annular space between the fibre-glass cylinder and

the porcelain insulator is also filled with oil.

Minimum oil circuit breakers arc available in the

voltage range of 3.3 kV to 420 kV. Now-a-days they

are superseded by SF6circuit breakers



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Breakers


One of the important advantages that the bulk

oil circuit breaker has over both the low

content oil circuit breakers and air blast circuitbreakers is that the protective current

transformers can be accommodated on the

bushings instead of being supplied as aseparate piece of apparatus



105/231

Breakers


The number of interrupter units contained in a tankdepends upon the fault current to be interrupted,

and the system voltage. Up to 11kV voltage, the

minimum oil circuit breakers generally employ a

single interrupter per phase.

The typical figures for higher voltages are, two per

phase at 132 kV, and six per phase at 275 kV. Eachinterrupter has a provision of resistance switching (a

typical value being 1200 , linear wire wound

resistor) to damp restriking voltage.

5 9 Air Blast Circuit Breakers


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5.9 Air Blast Circuit Breakers


In air blast circuit breakers, compressed air at a pressure of10-30 kg/cm2is employed as an arc quenching medium.

Air blast circuit breakers are suitable for operating voltages of

132 kV and above. They have also been used in 11kV-33 kVrange for certain applications.

At present, SF6 circuit breakers are preferred for 132 kV and

above. Vacuum circuit breakers are preferred for 11 kV33kV range.

Therefore, the air blast circuit breakers are becoming

obsolete.



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The advantages of air blast circuit breakers over oil circuit

breakers are:

1. Cheapness and free availability of the interrupting

medium, chemical stability and inertness of air.2. High speed operation

3. Elimination of fire hazard

4. Short and consistent arcing time and therefore, less

burning of contacts.5. Less maintenance

6. Suitability for frequent operation

7. Facility for high speed reclosure



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The disadvantages of an air blast circuit breaker are as

follows.

1. An air compressor plant has to be installed andmaintained.

2. Upon arc interruption the air blast circuit breaker

produces a high-level noise when air is discharged to the

open atmosphere. In residential areas, silencers need to

be provided to reduce the noise level to an acceptablelevel.

3. Problem of current chopping

4. Problem of restriking voltage



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Switching resistors and equalising capacitors aregenerally connected across the interrupters. The

switching resistors reduce transient overvoltages

and help arc interruption. Capacitors are employed

to equalise the voltage across the breaks.

The number of breaks depends upon the system

voltages. For example, there are 2 for 66 kV, 2 to 4for 132 kV, 2 to 6 for 220 kV, 4 to 12 for 400 kV, 8 to

12 for 750 kV.



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The breaking capacities are, 5000 MVA at 66

kV, 10,000 MVA at 132 kV, 20,000 MVA at

220 kV; 35000 MVA at 400 kV, 40,000 MVA at

500 kV; 60,000 MVA at 750 kV.

Circuit breakers for higher interruptingcapacity have also been designed for 1000

kV and 1100 kV systems.



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An air-blast circuit breaker may be either of

the following two types.

1. Cross-blast Circuit Breakers

2. Axial-blast Circuit Breakers

5 9 1 Cross-blast Circuit Breakers


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5.9.1 Cross blast Circuit Breakers


In a cross-blast type circuit breaker, a high-pressure blast ofair is directed perpendicularly to the arc for its interruption.

Figure 5.18 (a) shows a schematic diagram of a cross-blast

type circuit breaker.

The arc is forced into a suitable chute. Sufficient lengthening

of the arc is obtained, resulting in the introduction of

appreciable resistance in the arc itself. Therefore, resistance

switching is not common in this type of circuit breakers.

Cross-blast circuit breakers are suitable for interrupting high

current (up to 100 kA) at comparatively lower voltages.

5.9.1 Cross-blast Circuit Breakers


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5.9.1 Cross blast Circuit Breakers


Figure 5.18 (a) Cross-blast circuit breaker (b) Single blast type axial-blast

circuit breaker (c) Double blast type ( or radial-blast type) axial-blast circuit

breaker

5.9.1 Axial-blast Circuit Breakers


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5.9.1 Axial blast Circuit Breakers


In an axial-blast type breaker, a high-pressure

blast of air is directed longitudinally, i.e. in line

with the arc. Figure 5.18 (b) and (c) show

axial-blast type circuit breakers.

Figure 5.18(b) shows a single blast type.

Whereas Fig. 5.18(c) shows a double blasttype or radial blast type.



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Figure 5.18(d) : axial-

blast type

circuit

breakers.



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Axial blast circuit breakers are suitable for EHV and

super high voltage application. This is because

interrupting chambers can be fully enclosed in

porcelain tubes. Resistance switching is employedto reduce the transient overvoltages.

The number of breaks depends upon the system

voltage, for example, 4 at 220 kV and 8 at 750 kV.

Air-blast circuit breakers have also been

commissioned for 1100 kV system.

5.10 Air-break Circuit Breakers


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5.10 Air break Circuit Breakers


Oil circuit breakers are not suitable for heavy

current interruption at low voltages due to

carbonisation of oil, whereas air-break circuit

breakers are quite suitable for high currentinterruption at low voltages.

In this type of a circuit breaker, air atatmospheric pressure is used as an arc

extinguishing medium.



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Figure 5.19 shows an air-break circuit breaker. Itemploys two pairs of contacts main contacts andarcing contacts.

The main contacts carry current when the breaker is inclosed position. They have low contact resistance.

When contacts are opened, the main contacts separate

first, the arcing contacts still remain closed. Therefore,the current is shifted from the main contacts to the

arcing contacts. The arcing contacts separate later on

and the arc is drawn between them.



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Figure 5.19 Air-break

circuit breaker



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Figure 5.19 Air-break

circuit breaker



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In air-break circuit breakers, the principle ofhigh resistance is employed for arc

interruption. The arc resistance is increased

by lengthening, splitting and cooling the arc.The arc length is rapidly increased employing

arc runners and arc chutes. The arc moves

upward by both electromagnetic and thermal

effects. It moves along the arc runner and

then it is forced into a chute. It is splitted by

arc splitters.



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A blow-out coil is employed to provide

magnetic field to speed up arc movement and

to direct the arc into arc splitters. The blow-

out coil is not connected in the circuitpermanently. It comes in the circuit by the arc

automatically during the breaking process.

The arc interruption is assisted by currentzero in case of ac air break circuit breakers.

High resistance is obtained near current zero.



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AC air-break circuit breakers are available in thevoltage range 400 to 12 kV. They are widely

used in low and medium voltage system.

They are extensively used with electric furnaces,with large motors requiring frequent starting, in a

place where chances of fire hazard exist, etc.

Air-break circuit breakers are also used in dc

circuit up to 12 kV.

5.11 SF6Circuit Breakers


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6


Suphur hexafluoride (SF6) has good dielectricstrength and excellent arc quenching property.

It is an inert, nontoxic, nonflammable and heavy

gas.

At atmospheric pressure, its dielectric strength is

about 2.5 times that of air.

At 3 atmospheric pressure its dielectric strength is

equal to that of transformer oil.



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6


It is an electronegative gas, i.e. it has high affinity for

electrons.

When a free electron comes in collision with a neutralgas molecule, the electron is absorbed by the neutral

gas molecule and a negative ion is formed.

As the negative ions so formed are heavy they do notattain sufficient energy to contribute to ionization of the

gas. The property gives a good dielectric property.



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Besides good dielectric strength, the gas has an

excellent property of recombination after the

removal of the source which energizes the arc.

This gives an excellent arc quenching property.

The gas has also an excellent heat transferproperty. Its thermal time constant is about 1000

times shorter than that of air.



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Under normal conditions, SF6 is chemically inert and it doesnot attack metals or glass. However, it decomposes to SF4,

SF2, S2, F2, S and F at temperatures of the order of 1000oC.

After arc extinction, the products of decomposition recombine

in a short time, within about 1 microsecond.

In the presence of moisture, the decomposition products can

attack contacts, metal parts and rubber sealings in SF6circuit

breakers. Therefore, the gas in the breaker must bemoisture-free. To absorb decomposition products, a mixture

of soda line (NaOH + CaO) and activated alumina can be

placed in the arcing chamber.



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One major disadvantage of SF6 is its condensation at low

temperature.

The temperature at which SF6changes to liquid depends onthe pressure. At 15 atm. Pressure, the gas liquefies at a

temperature of about 100C.

Hence, SF6 breakers are equipped with thermostaticallycontrolled heaters wherever such low ambient temperatures

are encountered.



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SF6 circuit breakers are manufactured in the voltage range

3.6 kV to 765 kV. However, they are preferred for voltages

132 kV and above.

The dielectric strength of SF6gas increases rapidly after final

current zero. SF6 circuit breakers can withstand severe

RRRV and are capable of breaking capacitive current without

restriking. Problems of current chopping are minimised.

Electrical clearances are very much reduced due to high

dielectric strength of SF6.

5.11.1 Types of SF6Circuit Breakers


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This is the early design of SF6circuit breakers. Its operating

principle is similar to that of air-blast circuit breakers.

In this type of a circuit breaker, the gas from a high-pressurecompartment is released to the low pressure compartment to

extinguish the arc.

Because of its complicated design and construction, and itsneed for various auxiliaries such as gas compressors, filters

and control devices, this type of circuit breakers have

become obsolete.

Double-pressure SF6circuit breakers



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This type of circuit breakers are also sometimes

called single-pressure or impulse type SF6circuit

breakers.

In this type of breakers, gas is compressed by a

moving cylinder system and is released througha nozzle to quench the arc. This type is

available in the voltage range 3.6 kV to 765 kV.

Puffer-type SF6circuit breakers



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Figure 5.20 (a) shows a puffer-type breaker in

closed position. The moving cylinder and the

moving contact are coupled together.

When the contacts separate and the moving

cylinder moves, the trapped gas is compressed.

The trapped gas is released through a nozzle andflows axially to quench the arc as shown in Fig.

5.20(b).

Puffer-type SF6circuit breakers (cont.)



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Figure 5.20 Puffer-type SF6circuit breaker



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There are two types of tank designs. Live

tank design and dead tank design.

In live tank design, interrupters are supported

on porcelain insulators. In the dead tank

design, interrupters are placed in SF6 filled-tank which is at earth potential. Live tank

design is preferred for outdoor substations.




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A number of interrupters (connected in series) on

insulating supports are employed for EHV systems up to

765 kV. Two interrupters are used in a 420 kV system.Breaking time of 2 to 3 cycles can be achieved.

In the circuit breaker the steady pressure of the gas is

kept at 5 kg/cm2. The gas pressure in the interruptercompartment increases rapidly to a level much above its

steady value to quench the arc.


5.12 Vacuum Circuit Breakers


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The dielectric strength and arc interrupting ability of high

vacuum is superior to those of porcelain, oil, air and SF6at

atmospheric pressure.

SF6 at 7 atm. pressure and air at 25 atm. pressure have

dielectric strengths higher than that of high vacuum. The

pressure of 10-5mm of mercury and below is considered to

be high vacuum.

Low pressures are generally measured in terms of torr; 1 torr

being equal to 1 mm of mercury. It has now become

possible to achieve pressures as low as 10-8torr.



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When contacts separate in a gas, arc is formed due to the

ionized molecules of the gas.

The mean free path of the gas molecules is small and the

ionization process multiplies the number of electrons to form

an electron avalanche. In high vacuum, of the order of 10-5

mm of mercury, the mean free path of the residual gasmolecules becomes very large. It is of the order of a few

meters.



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Therefore, when contacts are separated by a few

mm in high vacuum, an electron travels in the gap

without collision. The formation of arc in high

vacuum is not possible due to the formation of

electron avalanche.

In vacuum arc electrons and ions do not come fromthe medium in which the arc is drawn but they come

from the electrodes due to the evaporation of their

surface material.



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The breakdown strength is independent of gas

density. It depends only on the gap length and

surface condition and the material of the electrode.

The breakdown strength of highly polished and

thoroughly degassed electrodes is higher. Copper-

bismuth, silver-bismuth, silver-lead and copper-leadare good materials for making contacts of the

breaker



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The breakdown strength is independent of gas

density. It depends only on the gap length and

surface condition and the material of the

electrode.

The breakdown strength of highly polished and

thoroughly degassed electrodes is higher.Copper-bismuth, silver-bismuth, silver-lead and

copper-lead are good materials for making

contacts of the breaker



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When contacts are separated in high vacuum,

an arc is drawn between them. The arc does

not take place on the entire surface of thecontacts but only on a few spots.

The contact surface is not perfectly smooth.

It has certain microprojections. At the time of

contact separation, these projections form the

last points of separation.



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The current flows through these points of

separation resulting in the formation of a few

hot spots.

These hot sports emit electrons and act as

cathode spots. In addition to thermalemission, electrons emission may be due to

field emission and secondary emission.



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INSPIRING CREATIVE AND INNOVATIVE MINDS

Figure 5.21 shows the schematic diagram of a vacuumcircuit breaker. Its enclosure is made of insulating

material such as glass, porcelain or glass fibre

reinforced plastic.

The vapour condensing shield is made of synthetic

resin. This shield provided to prevent metal vapour

reaching the insulating envelope.

As the interrupter has a sealed construction a stainless

metallic bellows is used to allow the movement of the

lower contact.



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Figure 5.21 Vacuum

circuit breaker



145/231


One of its end is welded to the moving contact.

Its other end is welded to the lower end flange.

Its contacts have large disc-shaped faces.

These faces contain spiral segments so that the

arc current produces axial magnetic field.

This geometry helps the arc to move over the

contact surface.



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The movement of arc over the contact surface

minimises metal evaporation, and hence erosion

of the contact due to arc.

Two metal end flanges are provided. They

support the fixed contact, outer insulating

enclosure, vapour condensing shield and themetallic bellows. The sealing technique is

similar to that used in electronic valves



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The vacuum circuit breaker is very simple inconstruction compared to other types of circuit breaker.

The contact separation is about 1 cm which is adequate

for current interruption in vacuum. As the breaker isvery compact, power required to close and open its

contacts is much less compared to other types of

breaker.

It is capable of interrupting capactive and small inductive

current, without producing excessive transient

overvoltages.



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Vacuum circuit breakers have other

advantages like suitability for repeatedoperations, least maintenance, silent

operation, long life, high speed of

dielectric recovery, less weight of movingparts, etc..



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In a.c., when the current decreases, vapour emission

decreases. Near current zero, the rate of vapour emission

tends to zero.

Immediately after current zero, the remaining vapour

condenses and the dielectric strength increases rapidly.

At current zero, cathode spots extinguish within 10-8

second.The rate of dielectric recovery is many times higher than that

obtained in other types of circuit breakers. Its typical value

may be as high as 20 kV/s.



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Vacuum circuit breakers have nowbecome popular for voltage ratings

up to 36 kV. Up to 36 kV they

employ a single interrupter.

5.13 Operating Mechanism


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To open and close the contacts of a

circuit breaker, one of the

following mechanisms isemployed

(i) Spring(ii) Solenoid

(iii) Compressed Air



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Opening and closing of a circuit breaker should be quick

and reliable. Closing is slow compared to opening.

For reliability, both operations should be independent

of the main supply.

The force for the opening of a circuit breaker may be

applied by spring or compressed air. When spring is

used for opening, it is precharged, i.e. compressed .The precharging may be done by hand or a motor or

by the closing mechanism. The closing mechanism

may be spring, solenoid or compressed air.



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If the system uses spring for both opening as well as closing,the closing spring has a higher energy level and is

charged by motor-driven gears. Two separate springs

are employed, one for closing and the other for opening.

The force of the closing spring is utilized for closing the

breaker and also for charging the spring which is used for

opening. The closing mechanism also latches the moving

contact in the closed position.

When the tripping signal is received, a small solenoid is

energised, which releases the latch and permits the

spring to exert its force to open the contact.



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For large oil circuit breakers, solenoid is used for

closing and a spring is used for opening.

For medium size breakers, where provision ofbattery for solenoid is uneconomical, spring is

employed for closing as well as opening.

In EHV oil circuit breakers, compressed air is used

for closing and a spring is used for opening. The

spring is charged during the closing stroke.



155/231


In air blast circuit breakers, compressed air is used for both

closing and opening.

In some cases of air blast circuit breakers, the moving

contact is held in closed position by a spring.

When compressed air enters the arc chamber and its

pressure exceeds the spring force, the contacts

automatically come in the closed position by the springaction when the supply of compressed air is stopped.

Therefore, the supply of compressed air must be

maintained till the auxiliary circuit breaker switch is

opened.



156/231


In SF6 circuit breaker, compressed air may be used for

closing and a spring for opening or compressed air for

both closing as well as opening.

In vacuum circuit breakers, a solenoid or spring mechanism

is fixed to the lower end to move a metallic bellows

upward or downward inside the chamber during closing

or opening the contacts.

The closing mechanism must be tripfree, i.e. if a tripping

signal comes during the closing operation, the circuit

breaker must trip immediately



157/231


Spring are very good for opening as their force is large in the

beginning and gradually decreases as the distance of

travel of the moving contact increases. But it is not

suitable for closing because the closing operation mustbe a smart one. However, in circuit breakers of small

capacity, it is also used for closing.

The spring which is used for closing is charged by hand or a

motor. In small circuit breakers, the spring can becharged by hand. For large ones, it is charged by means

of a motor. The spring which is used for opening is

charged by the closing mechanism.



158/231


Solenoids are very good for closing. The force of attraction

increases when the distance between the contacts

decreases. They are not suitable for opening because

they are slow in action.

Compressed air is suitable for both closing as well as

opening as its force remains almost constant even when

the distance between the contacts increases or

decreases.

Figure 5.22 shows the characteristics of the opening and

closing mechanisms.



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Figure 5.22 Characteristics of operating mechanism

5.14 Selection of Circuit Breakers


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Table 5.1 shows the summary of

various types of circuit breakers,

their voltage ranges and arcquenching medium they employ.

Table 5.2 shows the choice of circuitbreakers for various voltage

ranges.



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Type Arc Quenching Medium Voltage Range and BreakingCapacity

Miniature circuit breaker Air at atmospheric pressure 400-600 V; for small currentrating

Air-break circuit breaker Air at atmospheric pressure 400 V11 kV; 5-750 MVA

Minimum oil circuit breaker Transformer oil 3.3 kV220 kV;15025000 MVA

Vacuum circuit breaker Vacuum 3.3 kV33 kV;2502000 MVA

SF6circuit breaker SF6at 5kg/cm2pressure 3.3765 kV;

100050,000 MVA

Air blast circuit breaker Compressed air at high pressure(20-30 kg/cm2)

6.6 kV1100 kV;250060,000 MVA

TABLE 5.1 Types of circuit breaker



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Rated Voltage Choice of Circuit Breakers Remark

Below 1 kV Air-break CB

3.3 kV33 kV Vacuum CB, SF6CB, minimum oil CB Vacuum preferred

132 kV220 kV SF6CB, air blast CB, minimum oil CB SF6preferred

400 kV760 kV SF6CB, air blast CB SF6is preferred

TABLE 5.2 Selection of Circuit Breakers



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Earlier oil circuit breakers were preferred in the

voltage range of 3.3 kV - 66kV.

Between 132 kV and 220 kV, either oil circuitbreakers or air blast circuit breakers were

recommended.

For voltages 400 kV and above, air blast circuit

breakers were preferred.



164/231


The present trend is to recommend vacuum or SF6circuit breakers in the voltage range 3.3 kv 33kV.

For 132 kV and above, SF6 circuit breakers arepreferred. Up to kV, air break circuit breakers are

used.

Air blast circuit breakers are becoming obsolete and

oil circuit breakers are being superseded by SF6

and vacuum circuit breakers.

5.15 High Voltage DC Circuit Breakers


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At present, HVDC transmission lines are used for point

to point transmission of large power over long

distance.

Such lines have many advantages over ac transmission

lines such as lower cost, less stability problems, less

corona loss and less radio interference, etc.

HVDC circuit breakers are not essential for single HVDC

transmission lines which are used for point to point

transmission.



166/231


The current in HVDC lines is controlled by controlling

the firing circuits of the thyristors employed in

rectifiers and inverters.

Switching operations are performed from ac side with

the help of ac circuit breakers.

If HVDC circuit breakers are available, parallel HVDClines, HVDC lines with a tap-off line, closed loop

circuit, etc. can also be planned and designed.



167/231


The additional circuit creates artificial current zeros which

are utilised for arc interruption as shown in Fig. 5.23.

Figure 5.23 Artificial current zeros in DC



168/231


Figure 5.24 shows the schematic diagram of a

HVDC circuit breaker.

It is consists of a main circuit breaker MCB

and a circuit to produce artificial current zero

and to suppress transient voltage.

The main circuit breaker MCB may either be

an SF6or vacuum circuit breaker.



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Figure 5.24 HVDC circuit breaker



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R and C are connected in parallel with the main circuitbreaker to reduce dv/dt after the final current zero.

L is a saturable reactor in series with the main circuit

breaker. It is used to reduce di/dt before current zero.

Cp and Lp are connected in parallel to produce artificial

current zero after the separation of the contacts in the

main circuit breaker MCB.

A non-linear resistor is used to suppress the transient

overvoltage which may be produced across the contacts

of the main circuit breaker.


Switch S which is a triggered vacuum gap is switched


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Switch S, which is a triggered vacuum gap, is switched

immediately after the opening of the contacts of the main

circuit breaker.

The capacitor Cpis precharged in the direction as shown

in the figure.

When S is closed, the precharged capacitor Cp

discharges through the main circuit breaker and sends a

current in opposition to the main to the main circuit

current.

This will force the main circuit current to become zero

with a few oscillations. The arc is interrupted at a current

zero

5.16 Rating of Circuit Breakers


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A circuit breaker has to perform the

following major duties under short-circuit

conditions.

(i) To open the contacts to clear the fault

(ii) To close the contacts onto a fault

(iii) To carry fault current for a short time

while another circuit breaker is clearingthe fault

5.16 Rating of Circuit Breakers


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Therefore, in addition to the rated voltage,current and frequency, circuit breakers

have the following important ratings.

(i) Breaking Capacity

(ii) Making Capacity

(iii) Short-time Capacity

5.16.1 Breaking Capacity


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The breaking capacity of a circuit

breaker is of two types

(i) Symmetrical breaking capacity

(ii) Asymmetrical breaking capacity



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Symmetrical breaking capacity

It is the rms value of the accomponent of the fault current that the

circuit breaker is capable of breaking

under specified conditions of recoveryvoltage.



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Asymmetrical breaking capacity

It is the rms value of the total current

comprising of both ac and dccomponents of the fault current that

the circuit breaker can break under

specified conditions of recoveryvoltage.



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Figure 5.25 shows a short-circuit current wave.

The short-circuit current contains a dc

component which dies out gradually as shown in

the figure.

In the beginning, the short-circuit current is

asymmetrical due to the dc component.

When dc dies out completely, the short-current

becomes symmetrical.



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Figure 5.25

Short-circuitcurrent

waveform



179/231


The line X-X indicates the instant of contact separation.

AB is the peak value of the ac component of the current

at this instant.

Therefore, the symmetrical breaking current which is therms value of the ac component of the current at the

instant of contact separation is equal to current AB/2.

The section BC is the dc component of the short-circuitcurrent at this instant. Therefore, asymmetrical breaking

current is given by 2

2

asym. BC2

ABI


Th b ki it f i it b k i ll


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The breaking capacity of a circuit breaker is generally

expressed in MVA. For a three-phase circuit breaker, it isgiven by

Breaking capacity = 3 x rated voltage in kV x rated

current in kA.The breaking capacity will be symmetrical if the rated

current in the above expression is symmetrical (British

practice).

The breaking capacity will be asymmetrical if the ratedcurrent is asymmetrical (American practice). The rated

breaking current is taken by designer as 1.6 times the

rated symmetrical current.

5.16.2 Making Capacity


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The possibility of a circuit breaker to be closed on short-circuit is also considered.

The rated making current is defined as the peak value of

the current (including the component) in the first cycle atwhich a circuit breaker can be closed onto a short-circuit.

Ipin Fig. 5.25 is the making current.

The capacity of a circuit breaker to be closed onto ashort-circuit depends upon its ability to withstand the

effects of electromagnetic forces.

5.16.2 Making Capacity


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Making current = 2 x 1.8 x symmetrical breakingcurrent.

The multiplication by 2 is to obtain the peakvalue and again by 1.8 to take the dc component

into account.

Making capacity = 2 x 1.8 x symmetrical breakingcapacity=2.55 x symmetrical breaking capacity

5.16.3 Short-time Current Rating


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INSPIRING CREATIVE ANDINNOVATIVE

MINDS

The short-time current rating is based on thermal and

mechanical limitations.

The circuit breaker must be capable of carrying short-

circuit current for a short period while another circuitbreaker (in series) is clearing the fault.

The rated short-time current is the rms value (total

current, both a.c and d.c components) of the current thatthe circuit breaker can carry safely for a specified short

period.

5.16.3 Short-time Current Rating


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MINDS

According to British standard, the time is 3 seconds if the

ratio of symmetrical breaking current to rated normal

current is equal to or less than 40 and 1 second if this

ratio is more than 40.

According to ASA there are two short-time ratings; one is

the current which the circuit breaker can withstand for 1

second or less. Another is rated 4-second current which

is the current that the circuit breaker can withstand for aperiod longer than 1 second but not more than 4 seconds.

5.16.4 Rated Voltage, Current and

Frequency

In a power system the voltage level at all points


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MINDS

In a power system, the voltage level at all points

is not the same. It varies, depending upon thesystem operating conditions. Due to this reason

manufacturers have specified a rated maximum

voltage at which the operation of the circuit

breaker is guaranteed. The specified voltage is

somewhat higher than the rated nominal voltage.

The rated current is the rms value of the currentthat a circuit breaker can carry continuously

without any temperature rise in excess of its

specified limit.

5.16.4 Rated Voltage, Current and

Frequency


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The rated frequency is also mentioned bythe manufacturer. It is the frequency at

which the circuit breaker has been

designed to operate.The standard frequency is 50 Hz.

If a circuit breaker is to be used at a

frequency other than its rated frequency, its

effects should be taken into consideration.

Example 5 5


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Example 5.5The rating of a circuit breaker are as follows : 3-phase, 1500amp, 100 MVA, 3.3 kV, 3 seconds. Calculate symmetricalbreaking current, making current and short-time current rating.

Solution :

Given Rated voltage = 3.3 kV Rated normal current = 1500 ampSo, Rated symmetrical breaking current (rms) = = 17.5 kA

Rated making current = 17.5 x 1.8 x 2 = 44.62 kA Short-time current rating is 17.5 kA for 3 seconds.

3.33

100

x

Example 5 6


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Example 5.6

The portion of the test oscillograms which wereobtained during a make-break test on a circuitbreaker is shown in the figure. Given A = 20kV, B =30kA, X = 21kA and Y = 12kA. Determine:-

a) The line voltage rating of the breakerb) The peak making current for the red phase only.

c) The AC and DC components of breaking current atthe instant of contact separation for yellow phase

only. Hence determine whether the test issymmetrical or asymmetrical, and estimate thebreaking current.


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Solution


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a) Peak to peak phase system voltage = A = 20 kV.rms phase system voltage = 20/(22) = 7.07 kV

line voltage rating of breaker = 3 * 7.07 = 12.25kV

b) Peak making current = B = 30 kA.

c) AC component of making current = 21 kA.

DC component of making current = 12 kA.

Y/X = 12/21 = 0.578, therefore the test isasymmetrical. (Note : greater than 0.5)

asymmetrical breaking current :

5.16.5 Rated Operating Duty

The operating duty of a circuit breaker prescribes its


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IN

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