Theory of Arc Interruption
1
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
2
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
3
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
5
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
6
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
7
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.
Fundamentals of Circuit Breaking
In spite of the physical separation of switching
contacts, current flows continuously because of
10
contacts, current flows continuously because of
the sustained arc.
Fundamentals of Circuit Breaking
This arc plasma must be cooled and extinguished
in a systematic way so that the gap between the
11
in a systematic way so that the gap between the
contacts can again withstand the voltage in the
circuit.
Fundamentals of Circuit Breaking
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.
Fundamentals of Circuit Breaking
Even when no potential is applied across the
electrode, the gas conducts and sets up a small
13
electrode, the gas conducts and sets up a small
current due to natural ionization.
This current is known as leakage current.
Fundamentals of Circuit Breaking
When an electrical potential is applied across
the two electrodes, the charge carriers gain
14
the two electrodes, the charge carriers gain
mobility, and their motion depends on the
applied electrical field intensity.
Fundamentals of Circuit Breaking
When the moving charges collide with the
electrode (ions move towards the cathode and
15
electrode (ions move towards the cathode and
electrons move towards the anode), they
disperse their charges, and a current flows
between the electrodes.
Fundamentals of Circuit Breaking
This process of current conduction in a gaseous
medium is due to an ionization process such as
16
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
17
1. Linear current limit:- OP
Linear relationship between I and V for a small
V-I relationship during electric discharge for different values of applied voltage
18
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
V-I relationship during electric discharge for different values of applied voltage
19
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
V-I relationship during electric discharge for different values of applied voltage
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
V-I relationship during electric discharge for different values of applied voltage
21
If the electrical potential across the electrode
increases to a high level, ionization occurs
freely and free positive charges gain a high
velocity.
3. High current :- QR
In this situation, they strike the cathodes with
V-I relationship during electric discharge for different values of applied voltage
22
In this situation, they strike the cathodes with
enough force to knock out a number of free
electrons that maintain the discharge.
3. High current :- QR
Such discharge remains self-sustained because
it does not require any external excitation.
V-I relationship during electric discharge for different values of applied voltage
23
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.
3. High current :- QR
The voltage that forces such a high current
V-I relationship during electric discharge for different values of applied voltage
24
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.
V-I relationship during electric discharge for different values of applied voltage
25
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.
V-I relationship during electric discharge for different values of applied voltage
26
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.
Types of Gaseous Discharges
28
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),
Types of Gaseous Discharges
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.
Types of Gaseous Discharges
30
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
Types of Gaseous Discharges
31
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
Types of Gaseous Discharges
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
Ionization Process in a Gaseous Insulating Medium
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
Ionization Process in a Gaseous Insulating Medium
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
Ionization Process in a Gaseous Insulating Medium
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.
Ionization Process in a Gaseous Insulating Medium
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.
Ionization Process in a Gaseous Insulating Medium
38
energy called energy of liberation.
This energy is different for different electrode
materials.
4. Ionization on the surface of electrodes
The energy can be imparted by
(a) heating the cathode (called thermionic
Ionization Process in a Gaseous Insulating Medium
39
(a) heating the cathode (called thermionic
emission)
(b) bombardment on the surface of the metal by
particles
4. Ionization on the surface of electrodes
The energy can be imparted by
(c) irradiating the cathode surface by short wave
Ionization Process in a Gaseous Insulating Medium
40
(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.
1. Reduction of Velocity of Charged Particles
Hence, collision takes place before the charged
Decay Process (Deionization)
42
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.
1. Reduction of Velocity of Charged Particles
Chances of recombination are greatly reduced and
Decay Process (Deionization)
43
Chances of recombination are greatly reduced and
the gas gets de-ionized.
1. Reduction of Velocity of Charged Particles
Chances of recombination are greatly reduced and
Decay Process (Deionization)
44
Chances of recombination are greatly reduced and
the gas gets de-ionized.
1. Voltage across the electrode and its variation with
time
Factors responsible for the formation of an arc
45
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
Theory of Arc Quenching
LRt
m ewLE
is known as transient component or dc offset.
If fault occurs at an instantaneous voltage
49
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
Theory of Arc Quenching
When such a fault current is interrupted, high voltageis developed across the contacts of CB when the arcdue to this fault current is quenched.
50
Theory of Arc Quenching
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51
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Theory of Arc Quenching
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52
Theory of Arc Quenching
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53
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Theory of Arc Quenching
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54
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Theory of Arc Quenching
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55
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Theory of Arc Quenching
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56
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Theory of Arc Quenching
'
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57
)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,
Theory of Arc Quenching
LCf n
pi21
=
The maximum value of this transient voltage (Vcmax) is given by,
58
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
59
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
Definitions related to quenching of an Arc
60
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
Definitions related to quenching of an Arc
61
contacts of CB immediately after the instant of arc
extinction is known as TRV.
4. Recovery Voltage:-
The power frequency steady-state voltage appear
Definitions related to quenching of an Arc
62
across the contacts of CB after arc extinction is
known as recovery voltage.
5. Active Recovery Voltage:-
The instantaneous value of recovery voltage at
Definitions related to quenching of an Arc
63
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
Definitions related to quenching of an Arc
64
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:-
Definitions related to quenching of an Arc
LCt
dtd
vdt
dVRRRV C ))cos(1( ==
65
LCvRRRV
LCtAt
LCt
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12
)sin(1
=
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pi
1. High Resistance Interruption:-
The arc is restricted by increasing its effective
Arc Interruption Theories
66
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?
Arc Interruption Theories
67
(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
Arc Interruption Theories
68
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
Arc Interruption Theories
69
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
Arc Interruption Theories
70
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,
Arc Interruption Theories
71
the medium is ionized because of high field
intensity, and it sets up a very hot gaseous path.
2. Slepians Theory:- Background
2.To stop this ionization process, it is necessary
Arc Interruption Theories
72
to remove the ionized gases by
(a) prohibiting electron generation
(b) advancing the recombination process.
2. Slepians Theory:- Background
3.The rate of ionization is very low in the zero
Arc Interruption Theories
73
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
Arc Interruption Theories
74
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
Arc Interruption Theories
75
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:-
Arc Interruption Theories
76
Representation of Slepians theory (a) Arc interruption (b) Dielectric failure
2. Slepians Theory:- Assumptions
1.Build-up of the restriking voltage and dielectric
Arc Interruption Theories
77
strength during the interruption are totally
different processes.
2. Slepians Theory:- Assumptions
This assumption is not valid as it does not
Arc Interruption Theories
78
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
Arc Interruption Theories
79
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.
Arc Interruption Theories
80
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. Cassies Theory:- Assumptions:-
2. When current decays towards the zero current
regions, the cross section of the arc is a small
Arc Interruption Theories
81
fraction of a millimetre, still maintaining high
temperature.
2. Cassies Theory:- Assumptions:-
3. This high temperature can reignite the arc with a
bigger cross-section if the electric field intensity
Arc Interruption Theories
82
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
83
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
84
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
86
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
87
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
88
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
89
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.
Interruption of Capacitive Current
91
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.
Interruption of Capacitive Current
92
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.
Interruption of Capacitive Current
93
of CB, which may damage the equipment.
Interruption of Capacitive Current
94
After point P, VCB = VU- VC =
At point Q (after half a cycle), VU reverses. Hence,
Interruption of Capacitive Current
95
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.
Interruption of Capacitive Current
96
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
97
resistance switching.
Utilization of shunt resistance across the contactsof CB leads to reduction of
1. transient restriking voltage (TRV)
2. RRRV
Resistance Switching
98
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.
Resistance Switching
99
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
Resistance Switching
100
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
Resistance Switching
101
frequency oscillation of the circuit can be easilysuppressed.
2
2
41
21
LR
LCf n =
pi
Resistance Switching
102
RRRV is directly proportional to the naturalfrequency of the circuit, and it mainly depends on
Resistance Switching
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.
Resistance Switching
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
106
For points between A & B, the available supplyvoltage is greater than the arc voltage and hence,arc is maintained.
Quenching of DC Arc
107
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
108
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
109
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
110
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
111
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.
Terms related to Circuit Breaker
112
3.Rated Breaking Capacity :
The CB starts to open its switching contacts aftersome time (t), starting from the inception of fault.
Terms related to Circuit Breaker
113
The real value of fault current to be interrupted bya CB is quite less than the initial value at the timeof fault inception.
3.Rated Breaking Capacity :
Therefore, the highest value of fault current thatflows through the switching contacts at the instantof contact separation is known as the breaking
Terms related to Circuit Breaker
114
of contact separation is known as the breakingcurrent of the CB.
3.Rated Breaking Capacity :
1. Symmetrical breaking current :
It is the RMS value of the AC component of currentflowing through the CB at the instant of contact
Terms related to Circuit Breaker
115
flowing through the CB at the instant of contactseparation.
symmetrical = 2xyI
3.Rated Breaking Capacity :
2. Asymmetrical breaking current:
It is the RMS value of current (including both ACand DC components of the current) flowing through
Terms related to Circuit Breaker
116
and DC components of the current) flowing throughthe CB at the instant of contact separation..
22
asymmetrical = + ( )2xyI yz
3.Rated Breaking Capacity :
Breaking capacity is expressed in MVA by takinginto account the rated breaking current and therated system voltage.
Terms related to Circuit Breaker
117
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.
Terms related to Circuit Breaker
118
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
Terms related to Circuit Breaker
119
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.
Terms related to Circuit Breaker
120
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
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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.
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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.
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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.