Theory of Arc Interruption

Theory of Arc Interruption

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Dr. Bhavesh Bhalja, Dr. Bhavesh Bhalja, Dr. Bhavesh Bhalja, Dr. Bhavesh Bhalja, Senior Member IEEESenior Member IEEESenior Member IEEESenior Member IEEE

[email protected]@[email protected]@ieee.org

What is Electrical Switchgear?

All electrical devices used for

making and breaking the electrical

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making and breaking the electrical

circuits are grouped under the

term Electrical Switchgear

What comes under ES?

Relays

OFF/ON-load switches and Fuses

Isolators

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Isolators

Circuit Breakers (CBs)

Control Panels

Circuit Breaker (CB)

CB is a device that can open or close a

4

high voltage circuit in a fraction of a

second.

Functions of CB?

1. Capable to carry IFL at rated PF

continuously.

2. Capable to interrupt heavy s/ckt current

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at very low power factor.

3. Capable to perform making duty (closing

on to a circuit in which a fault exits and

immediately reopening to clear the fault.

Functions of CB?

4. Capable to carry currents of short-circuit

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magnitudes until the fault is cleared by

another CB or fuse (near to fault point).

Functions of CB?

5. During open condition of CB, the gap

must withstand the steady state power

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must withstand the steady state power

frequency system voltage continuously

and transient high frequency voltage for

a short duration of time.

Functions of CB?

7. Capable to withstand the effects of arcing

of the contacts and electromagnetic

8

of the contacts and electromagnetic

forces produced due to high currents.

Fundamentals of Circuit Breaking

When the movable contacts begin to separate, the

CB begins interrupting the current.

9

CB begins interrupting the current.

As a result, the contact area decreases.

This results in a high current density, which finally

vaporizes the metal, and an arc is generated

between the switching contacts.


In spite of the physical separation of switching

contacts, current flows continuously because of

10

contacts, current flows continuously because of

the sustained arc.


This arc plasma must be cooled and extinguished

in a systematic way so that the gap between the

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in a systematic way so that the gap between the

contacts can again withstand the voltage in the

circuit.


The separation of switching contacts of any CB

leads to the formation of gas and metal vapour

12

leads to the formation of gas and metal vapour

between them in its current carrying condition.

Any kind of gas or vapour always contains

positive and negative charge carriers.


Even when no potential is applied across the

electrode, the gas conducts and sets up a small

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electrode, the gas conducts and sets up a small

current due to natural ionization.

This current is known as leakage current.


When an electrical potential is applied across

the two electrodes, the charge carriers gain

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the two electrodes, the charge carriers gain

mobility, and their motion depends on the

applied electrical field intensity.


When the moving charges collide with the

electrode (ions move towards the cathode and

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electrode (ions move towards the cathode and

electrons move towards the anode), they

disperse their charges, and a current flows

between the electrodes.


This process of current conduction in a gaseous

medium is due to an ionization process such as

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medium is due to an ionization process such as

photoelectric or thermionic emission and

remains continuous till a potential is applied.

V-I relationship during electric discharge for different values of applied voltage

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1. Linear current limit:- OP

Linear relationship between I and V for a small


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Linear relationship between I and V for a small

value of applied voltage.

2. Saturation current limit:- PQ

When an equilibrium is reached (the production


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When an equilibrium is reached (the production

of charge carriers is equal to the charge

carriers received by the electrodes), there is no

increase in significant current.

2. Saturation current limit:- PQ

This phenomenon depends on


20

This phenomenon depends on

(a) the intensity of ionization

(b) the quantity of gas between the electrodes

(c) the gas pressure

3. High current :- QR

If the electrical potential across the electrode


21

If the electrical potential across the electrode

increases to a high level, ionization occurs

freely and free positive charges gain a high

velocity.


In this situation, they strike the cathodes with


22

In this situation, they strike the cathodes with

enough force to knock out a number of free

electrons that maintain the discharge.


Such discharge remains self-sustained because

it does not require any external excitation.


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it does not require any external excitation.

This process rises exponentially, and the

current continues to increase between two

electrodes even when the applied voltage

remains constant.


The voltage that forces such a high current


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The voltage that forces such a high current

density (million charges) through a gas medium

is known as breakdown voltage.

The gases between electrodes no more remain

insulators but provide a current conducting path.


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insulators but provide a current conducting path.

Hence, a continuous arc is formed between

electrodes, which are surrounded by hot ionized

gases.

The quenching or extinction of high current is

done externally.


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done externally.

Thus, it is very important to decide the

breakdown voltage and insulating medium to

quench the arc while designing the CB.

1. Decay Discharge: (exp: tube light and lamps)

When potential is applied in a gas having small

Types of Gaseous Discharges

27

pressure (less than atmospheric), number of

atoms or molecules/unit volume are less.

Hence, discharge current is less at the time of

break down.

2. Spark Discharge: (exp: switches)

When potential is applied to the gap intermittently

in a gas having high pressure (more than

atmospheric), spark discharge occurs.


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atmospheric), spark discharge occurs.

Such discharge will occur between two contacts of

a simple switch or in a CB when it interrupts a

rated load current at higher PF.

3. Arc Discharge: (exp: switches)

When high potential is applied to the gap in a gas

having high pressure (more than atmospheric),


29

having high pressure (more than atmospheric),

number of charges are quite large and collision

ionisations do occur.

Temp is of the order of 20000 to 90000 C.

3. Arc Discharge: (exp: opening of CB)

The arc is self-sustaining unless quenched by

external means.


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external means.

4. Corona Discharge: (exp: opening of CB)

This discharge occurs across the two contacts

when an electric field is not uniform but dense


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when an electric field is not uniform but dense

near one electrode and scarce near another

electrode.

Breakdown is limited to a small area (around one

of the electrode).

4. Corona Discharge: (exp: opening of CB)

This discharge mechanism is useful to decide

(a) the material of the electrode


32

(a) the material of the electrode

(b) the smoothness of the electrode

(c) the profile of the electrode

Ionization is the process of removing an electron

from the neutral molecule by externally applying

Ionization Process in a Gaseous Insulating Medium

33

from the neutral molecule by externally applying

finite amount of energy.

1. Ionization by Collision

If a particle of mass m and velocity v collides with

a neutral atom, kinetic energy of the moving


34

a neutral atom, kinetic energy of the moving

particle can create ionization state if

. m.v2 > wi

wi is energy of ionization

2. Photo Ionization

When energy is imparted to a neutral atom, an

electron may jump from lower orbit to a higher


35

electron may jump from lower orbit to a higher

orbit.

The atom can remain in such a meta-stable state

for a small period (micro/nano-second).

2. Photo Ionization

When it returns to its original stable state, it

releases energy in the form of a quantum of light


36

releases energy in the form of a quantum of light

or in the form of a wave of different frequencies.

Such energy is termed as photon.

hf > wi

3. Thermal Ionization

Molecules of gases move with all possible

velocities.


37

velocities.

At higher temperature, velocity increases and

probability of ionization is more.

4. Ionization on the surface of electrodes

Liberation of electron needs a certain amount of

energy called energy of liberation.


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energy called energy of liberation.

This energy is different for different electrode

materials.


The energy can be imparted by

(a) heating the cathode (called thermionic


39

(a) heating the cathode (called thermionic

emission)

(b) bombardment on the surface of the metal by

particles


The energy can be imparted by

(c) irradiating the cathode surface by short wave


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(c) irradiating the cathode surface by short wave

radiation

(d) superposition of strong electric field

1. Reduction of Velocity of Charged Particles

Gas pressure , the mean free path (distance of

Decay Process (Deionization)

41

Gas pressure , the mean free path (distance of

travel of a charged particle before collision) of a

charge particle gets reduced.


Hence, collision takes place before the charged


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Hence, collision takes place before the charged

particle can acquire a velocity enough to gather

kinetic energy more than the ionization energy.

Hence, collision can not result in ionization.


Chances of recombination are greatly reduced and


43


the gas gets de-ionized.




44


the gas gets de-ionized.

1. Voltage across the electrode and its variation with

time

Factors responsible for the formation of an arc

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2. The nature, shape and separation of electrodes

3. The nature and pressure of the medium

4. Presence of external ionizing and de-ionizing

agents

1. Arc characteristic is the curve between the

instantaneous voltage across the electrode and

Characteristic of Arc

46

the corresponding current through the arc.

Voltage distribution across an arc column

Arc column

Anode Cathode

+VA

Arc length

47

VC

Varc

Theory of Arc Quenching

)]sin([ += wteEiRt

m

When a sudden short circuit occurs in a power system,the fault current is given by,

48

)]sin([ += wtewLE

i Lm


LRt

m ewLE

is known as transient component or dc offset.

If fault occurs at an instantaneous voltage

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If fault occurs at an instantaneous voltageequal to Vmax , there would be no dc offset.

The steady state value of fault current is given by

)sin( = wtwLE

i m


When such a fault current is interrupted, high voltageis developed across the contacts of CB when the arcdue to this fault current is quenched.

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functionarycomplementtakingBywtSinEv

idtCdt

diLiRv

m=

++= )(

1

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LCjD

iC

LD

iCdt

idL

functionarycomplementtakingBy

=

+=

++=

]1[0

100

2

2

2


LCjt

LCjt

BeAei

bewillsolutiontheHence

+=

,

52


BABA

iextinctionarcoftinstheattAt

=

+=

==

00,)tan(0

53

LCk

dtdi

tAt

LCt

LCk

dtdi

LCtkeeAi LC

jtLCjt

==

=

==

,0

)(cos

)(sin][


breaacrossvoltageVWhere

dtiC

VtBut

dtiCdt

diLv

itAt

C

=

===

+=

==

ker,

01,0

10,0

54

LC

vk

Lv

LCk

Lv

dtdi

vdtdiL

breaacrossvoltageVWhere C

=

=

==

= ker,


breaacrossvoltageVWhere

dtiC

VtBut

dtiCdt

diLv

itAt

C

=

===

+=

==

ker,

01,0

10,0

55

LC

vk

Lv

LCk

Lv

dtdi

vdtdiL

breaacrossvoltageVWhere C

=

=

==

= ker,


(sin11

)(sin

dttLC

vC

dtiC

V

LCt

LC

vi

c ==

=

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')(cos

')](cos[1

(sin

kLCt

vV

kLCtLC

LC

vC

V

dtLCL

vC

dtiC

V

c

c

c

+=

+=

==


'

'00,0,)tan(0

vkkv

VhenceandiextinctionarcoftinstheattAt c

=

+=

===

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)cos1(LCt

vVc =

Hence, a transient voltage (Vc) is superimposed on steady-state phase to neutral Voltage. The frequency of this transient voltage is known as natural frequency (fn) and given by,


LCf n

pi21

=

The maximum value of this transient voltage (Vcmax) is given by,

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cmax

is given by,

LCtatvVc pi== 2max

Hence, the insulation of CB should be able to withstandtwo times the peak of phase to neutral voltage whichwill appear across the contacts of CB.

1. Re-striking Voltage:-

The peak of voltage at the time of re-striking of

Definitions related to quenching of an Arc

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an arc when the dielectric strength of the

insulating medium between contacts of CB has

not bulid up and breaks down.

2. Arc Voltage:-

The voltage across the contacts of CB


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immediately after the instant of separation of

contacts of CB.

It is in phase with arc current.

3. Transient Re-striking Voltage (TRV):-

The high frequency voltage appear across the


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contacts of CB immediately after the instant of arc

extinction is known as TRV.

4. Recovery Voltage:-

The power frequency steady-state voltage appear


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across the contacts of CB after arc extinction is

known as recovery voltage.

5. Active Recovery Voltage:-

The instantaneous value of recovery voltage at


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the instant of arc extinction is known as active

recovery voltage.

6. Rate of Rise of Restriking Voltage:-

The slope of the steepest tangent to the restriking


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voltage curve is defined as the rate of rise of

restriking voltage (RRRV).

It is expressed in kV/ s.

6. Rate of Rise of Restriking Voltage:-


LCt

dtd

vdt

dVRRRV C ))cos(1( ==

65

LCvRRRV

LCtAt

LCt

LCvRRRV

LCdtdt

12

)sin(1

=

=

=

pi

1. High Resistance Interruption:-

The arc is restricted by increasing its effective

Arc Interruption Theories

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resistance with respect to time.

With the increase of arc resistance, the current is

reduced to a value inadequate to sustain the arc

across the contacts of CB.

1. High Resistance Interruption:-

How to increases arc resistance?


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(i) lengthening the arc column

(ii) Cooling

(iii) Splitting the arc in many sub-segments.

Such a high resistance interruption method is not

appropriate for a high power AC CB because of


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high energy losses at the time of arc interruption.

Thus, the use of this method is limited to low

power AC and DC CBs.

2. Low Resistance Interruption:-

Mainly used for the interruption of an arc in an AC


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circuit as the arc current passes through zero

twice in a cycle (100 times per second for a 50 Hz

system).

2. Low Resistance Interruption:-

The arc tries to either die out at every current


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zero or reignite with rising current.

(a) Slepians Theory

(b) Cassies Theory

2. Slepians Theory:- Background

1. During the separation of contacts of the CB,


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the medium is ionized because of high field

intensity, and it sets up a very hot gaseous path.


2.To stop this ionization process, it is necessary


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to remove the ionized gases by

(a) prohibiting electron generation

(b) advancing the recombination process.


3.The rate of ionization is very low in the zero


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current region.

Hence, it is easier to increase the resistance of

the arc in this region and build up high dielectric

strength across the contacts of CB.

2. Slepians Theory:- Actual Theory

At each current zero, there is a race between the


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RRRV and the rate at which the insulating

medium recovers its dielectric strength.

2. Slepians Theory:- Actual Theory

If the rate at which the dielectric strength


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progress is faster than the rate at which the

voltage rises (RRRV), the arc will be quenched;

otherwise, the arc restrikes and is not interrupted.

2. Slepians Theory:-


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Representation of Slepians theory (a) Arc interruption (b) Dielectric failure

2. Slepians Theory:- Assumptions

1.Build-up of the restriking voltage and dielectric


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strength during the interruption are totally

different processes.

2. Slepians Theory:- Assumptions

This assumption is not valid as it does not


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calculate the rate at which the dielectric strength

recovers.

Moreover, it does not consider the energy relation

at the time of the interruption.

2. Cassies Theory:-

The interruption of arc is a process of energy

balance.

At current zero, if the rate at which the energy


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input to the arc column is higher than the rate at

which maximum energy is lost from the arc

column, the arc restrikes; if not, the arc can be

interrupted.

2. Cassies Theory:- Assumptions:-

1. The temperature of the arc is assumed to be

constant during this high current period.


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However, the change in current creates the

necessary change in the diameter of the arc,

maintaining almost constant temperature at the

centre of the arc column.


2. When current decays towards the zero current

regions, the cross section of the arc is a small


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fraction of a millimetre, still maintaining high

temperature.


3. This high temperature can reignite the arc with a

bigger cross-section if the electric field intensity


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reappears across the contacts of CB.

It means interruption of small inductive currents.

During interruption of small inductive currents

(such as no-load current of transformer or shunt

Current Chopping

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reactors), the arc path which is ionized by the low

current may reach a zero value before natural

zero.

This phenomena is known as current chopping.

The electromagnetic energy of inductance of the

circuit is converted into electrostatic energy of the

capacitance of the system.

Current Chopping

CVLI = 2221

21

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This voltage is impressed on a power frequency

voltage and can damage insulation of transformer or

other equipments.

ICLV =

Current Chopping

33/110 kV

85

33/110 kV20 MVA

C=5000 pF

No-load current is 2% of the full load current of transformer.What will be the voltage appear on the transformer insulationon HV side?

Current Chopping

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Arc is de-ionized before natural current zero

comes.

The prospective value of this voltage may reach a

dangerous level, even higher than the dielectric

Current Chopping

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dangerous level, even higher than the dielectric

strength, and leads to the next current conduction

by restriking.

If de-ionizing force is still sufficient, second re-

strike occurs. This time chopped current and

RRRV are lower than previous re-strike.

Current Chopping

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Finally, when enough dielectric strength

recovers after selective current chop, the

current is concealed without restrike.

Self Blast Breaker:- gas pressure current to

interrupted.

Current Chopping

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Forced Blast CB/SF6 CB:- gas pressure is

independent of current to interrupted.

Improve power factor.

The application of shunt capacitor banks hasbecome a special tool for improving the power

Interruption of Capacitive Current

90

become a special tool for improving the powerfactor.

It is common practice for utilities to

switch on /off the shunt capacitors as per daily loadvariations.


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variations.

Switching on /off of unloaded transmission line.

This creates a challenge since the voltage acrossthe capacitor cannot change instantaneously.

This switching causes unwanted high frequencyvoltage and current transients across the contactsof CB, which may damage the equipment.


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of CB, which may damage the equipment.

This switching causes unwanted high frequencyvoltage and current transients across the contactsof CB, which may damage the equipment.


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of CB, which may damage the equipment.


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After point P, VCB = VU- VC =

At point Q (after half a cycle), VU reverses. Hence,


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U

VCB = -Vm- Vm =-2Vm

This high voltage across CB contact leads torestriking of an arc. Hence, the circuit will reclose inan oscillating manner by developing the voltage

-2Vm-Vm=-3Vm

Thus, the line is charged at a voltage of 3 Vmafter the interruption of a restriking current.


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Afterwards, the voltage across the contacts of CBcontinues to increase, and at point S, the valuebecomes 4 Vm. If the breaker restrikes at thisinstant, high frequency oscillation of VC will occur ata voltage of 5 Vm.

Intentional insertion of resistance across thecontacts of CB after its separation is known asresistance switching.

Resistance Switching

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resistance switching.

Utilization of shunt resistance across the contactsof CB leads to reduction of

1. transient restriking voltage (TRV)

2. RRRV


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3. Arc intensity (due to diversion of part of arc

current)

4. Breaking capacity (MVA) of CB also increases.

In case of ABCB and SF6 CB, the pressure of thearc quenching medium is independent of normalcurrent or fault current.


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Hence, low current interruption is achieved withhigh pressure of insulating medium.

In this situation, it chops the current before naturalzeros.

This creates a very severe TRV across the contactgap of CB.

Consequently, these TRV can cause flashover on


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Consequently, these TRV can cause flashover onthe insulation.

Therefore, resistance switching is used to damp outsuch extreme high voltages.

If the value of the added resistance is higher thantwice the surge impedance of the line, the naturalfrequency oscillation of the circuit can be easily


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frequency oscillation of the circuit can be easilysuppressed.

2

2

41

21

LR

LCf n =

pi


102

RRRV is directly proportional to the naturalfrequency of the circuit, and it mainly depends on


103

frequency of the circuit, and it mainly depends onthe value of the inserted shunt resistance.

Thus, insertion of deliberate shunt resistanceacross the contact of CB increases the rupturingcapacity of the breaker.


104

Find out the natural frequency of TRV when CB isopened on fault assuming L=0.5 H and C=5000pF.

Examples on Resistance Switching

105

opened on fault assuming L=0.5 H and C=5000pF.What will be the natural frequency if a deliberateresistance of 10 k is added across CB contacts?

Quenching of DC Arc

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For points between A & B, the available supplyvoltage is greater than the arc voltage and hence,arc is maintained.

Quenching of DC Arc

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arc is maintained.

A is a stable point.

B is an unstable point.

When the available voltage is not sufficient tomaintain the arc current then it is quenched.

Quenching of DC Arc

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This can be obtained by

(i) Increasing the arc length so that arccharacteristic can be shifted upward and thereis no intersection of arc characteristic and V-iRline.

This can be obtained by

(ii) By increasing the external resistance in thecircuit so that V-iR characteristic can be shifted

Quenching of DC Arc

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circuit so that V-iR characteristic can be shifteddownward (V-i1R) and there is no intersectionof arc characteristic and V-iR line.

A 50 Hz, 13.8 kV, three-phase generator withgrounded neutral has an inductance of 15mH/phase and is connected to a busbar through aCB. The capacitance to earth between the

Examples

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CB. The capacitance to earth between thegenerator and the CB is 0.05 F/phase. Neglectingthe resistance of generator winding, determine :

1. Maximum restriking voltage

2. Time for maximum RRRV

3. Average RRRV up to the first peak

4. Frequency of oscillations

1. Rated current :

It is defined as the highest RMS current-carryingcapability of a CB without exceeding the limit oftemperature rise.

Terms related to Circuit Breaker

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temperature rise.

2. Rated Voltage:

It is defined as the maximum RMS voltage of a CBabove the nominal system voltage for which the CBis designed.

3.Rated Breaking Capacity :

It is defined as the highest RMS current-carryingcapability of a CB without exceeding the limit oftemperature rise.


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The CB starts to open its switching contacts aftersome time (t), starting from the inception of fault.


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The real value of fault current to be interrupted bya CB is quite less than the initial value at the timeof fault inception.


Therefore, the highest value of fault current thatflows through the switching contacts at the instantof contact separation is known as the breaking


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of contact separation is known as the breakingcurrent of the CB.


1. Symmetrical breaking current :

It is the RMS value of the AC component of currentflowing through the CB at the instant of contact


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flowing through the CB at the instant of contactseparation.

symmetrical = 2xyI


2. Asymmetrical breaking current:

It is the RMS value of current (including both ACand DC components of the current) flowing through


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and DC components of the current) flowing throughthe CB at the instant of contact separation..

22

asymmetrical = + ( )2xyI yz


Breaking capacity is expressed in MVA by takinginto account the rated breaking current and therated system voltage.


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rated system voltage.

Thus, if I is the rated breaking current in kA and Vis the rated system voltage in kV, then for a three-phase circuit,

the breaking capacity = MVA3 V I

4.Rated Making Capacity :

The rated making capacity of a CB is related to itsability to withstand maximum current, closed underthe fault condition.


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the fault condition.

The electromagnetic forces produced during thistime are proportional to the square of peakinstantaneous current.

4.Rated Making Capacity :

The peak RMS value of short circuit currentmeasured for the first cycle of current wave afterthe closure of CB under fault condition is known as


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the closure of CB under fault condition is known asthe making capacity.

The peak of making capacity is given by ,

Where, is the maximum asymmetry.

2 symmetrical breaking capacity

5. Short time rating :

It is the duration for which the CB can carrymaximum fault current under its fully closedcondition without any damage.


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condition without any damage.

Short time rating depends on the electromagneticforce withstanding capacity of the CBs and thermallimitations.

6. Rated Standard Duty Cycle

The standard duty cycle is estimated as thefrequent operation of CB for its particularapplication.


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application.

The CBs are usually able to follow an open-close-open cycle with an energized spring chargemechanism without any new manual or electricalcharging.

6. Rated Standard Duty Cycle

The standard operating duty of a CB is as follows:

O t CO t' CO where,


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where,

O = Open

CO = Close-open

t = 3 min

t = 15 s for CBs not rated for rapid reclosing

t = 0.3 s for CBs rated for rapid reclosing

Used for low voltage to medium voltage distribution

Systems.

In order to increase the effective resistance of an arc

Air Break Circuit Breaker

123

In order to increase the effective resistance of an arc

in an ACB, the following methods are used.

Arc lengthening

Arc cooling

Arc splitting

Arc constraining

Contact separation and arc extinction take place at

atmospheric pressure.

Construction of Air Break Circuit Breaker

124

When the contacts are opened, an arc is drawn

between them.

The arc resistance is increased to such an extent that

the system cannot maintain the arc, and thus, the arc

is finally extinguished.

It consists of two contacts, namely, main contact and arcing contact.

1. Main contact:


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1. Main contact:

It conducts current in the closed position.

It has low contact resistance and is silver plated.

The main contact consists of a moving contact anda fixed contact assembly.


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2. Arcing contact

The arcing contact is hard, heat resistant, andmade of copper alloy.


127

While opening, the main contact is dislodged firstand the current is shifted to arcing contacts, andthen they are dislodged.

Thus, the arc is drawn between the arcingcontacts.

The arc is now forced by electromagnetic forcesand thermal action.

The ends of the arc move along the arc runners,and they are divided by the arc splitter plates in thearc chute.


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The dimension of the arc chute depends on thenumber of arcing contacts.

Hence, the arc is extinguished by lengthening,cooling, and splitting.

Theory of Arc Interruption

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