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During fault behavior of circuit breaker
• A circuit breaker has two contacts – a fixed contact and a moving contact.
Under normal conditions these two contacts remain in closed position.
• When the circuit breaker is required to isolate the faulty part, the moving
contact moves to interrupt the circuit.
• On the separation of the contacts, the flow of current is interrupted,
resulting in the formation of arc between the contacts. The contacts are
placed in a closed chamber containing some insulating medium (liquid or
gas) which extinguishes the arc
• Making current: It is the peak (maximum) value of current during the first
cycle of current wave after the closure of circuit breaker contacts
• Breaking current: It is RMS value of current at the instant of contact
separation
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The voltage drop across the arc is called arc voltage. As the arc path is purely
resistive, the arc voltage is in 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 rated voltage.
There are two methods of arc interruption:
1. High Resistance Interruption
2. Current Zero Interruption
1. High Resistance Interruption
In this method of arc interruption, its resistance is increased so as to reduce the
current to a value insufficient to maintain the arc.
The arc resistance can be increased by cooling, lengthening, constraining and
splitting the arc.
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High Resistance Interruption
When current is interrupted the energy
associated with its magnetic field appears in
the form of electrostatic energy.
A high voltage appears across the contacts of
the circuit breaker.
If this voltage is very high and more than the
withstanding capacity of the gap between the
contacts, the arc will strike again.
Therefore, this method is not suitable for a
large-current interruption. This can be
employed for low power arc and dc circuit
breakers.
Current Zero Interruption
• This method is applicable only in case of ac circuit
breakers.
• In case of ac supply, the current wave passes through
a zero point, 120 times per second at the supply
frequency of 60 Hz.
• This feature of ac is utilized for arc interruption. The
current is not interrupted at 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 a zero current
occurs.
• Restriking voltage: It is the resultant transient voltage which appears
across circuit breaker contact at the instant of arc extinction
• Recovery voltage: It is power frequency RMS voltage which appears
across the circuit breaker contacts after transient oscillation die out and
final extinction of arc has resulted in all the poles
• Amplitude(crest) factor: It is the ratio of peak of transient voltage to the
peak system frequency voltage
• Rate of Rise of Restriking Voltage (RRRV):It is defined as the slope of
steepest tangent to the restriking voltage curve.it is expressed in kilovolts
per microsecond.
• Arc voltage: It is the voltage that appears across the circuit breaker
contacts during he arcing period.arc voltage becomes system voltage
when the arc is extinguished.
Expression for Restriking Voltage and RRRV
• Figure (a) shows a short circuit on a feeder
beyond the location of the circuit breaker.
• Figure (b) shows an equivalent electrical
circuit where L and C are the inductance and
capacitance per phase of the system up to the
point of circuit breaker location, respectively.
• When the circuit breaker is closed, the short
circuit current flows through R, L and the
contacts of the circuit breaker, the capacitance
C being short-circuited by the fault.
• When the circuit breaker contacts are opened
and the arc is extinguished, the current is
diverted through the capacitance C, resulting
in a transient condition.
• The voltage across the capacitance which is
the voltage across the contacts of the circuit
breaker can be calculated in terms of L, C, fn
and system voltage.
• The mathematical expression for transient
condition (neglecting resistance) is as follows.
• Where E is the system voltage at the instant of
arc interruption. As the transient oscillation is
a fast phenomenon, E can be regarded as a
constant for a short duration.
Example: For a 132 kV system, the reactance and capacitance up to the
location of the circuit breaker is 3 ohms and 0.015 μ 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
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.
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.
The resistance switching is of great help in switching out
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 its natural zero instant.
In such a situation, the energy stored in the
magnetic field appears in the form of high
voltage across the stray capacitance, which will
cause restriking of the arc.
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
Interruption of 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 shows an equivalent electrical circuit of a
simple power system. C represents stray
capacitance of the circuit breaker. CL represents
line capacitance. The value of CL is much more
than C.
Rating of Circuit Breakers
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
Breaking Capacity
The breaking capacity of a circuit breaker is of
two types
(i) Symmetrical breaking capacity
(ii) Asymmetrical breaking capacity
Symmetrical breaking capacity
It is the rms value of the ac component of the
fault current that the circuit breaker is capable
of breaking under specified conditions of
recovery voltage.
Asymmetrical breaking capacity
It is the rms value of the total current
comprising of both ac and dc components of the
fault current that the circuit breaker can break
under specified conditions of recovery voltage.
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
• 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 the rms 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-circuit current at this instant. Therefore,
asymmetrical breaking current is given by
• The breaking capacity of a circuit breaker is
generally expressed in MVA. For a three-phase
circuit breaker, it is given 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 rated
current is asymmetrical (American practice).
The rated breaking current is taken by
designer as 1.6 times the rated symmetrical
current.
Making Capacity
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 dc
component) in the first cycle at which a circuit
breaker can be closed onto a short-circuit.
Ip in Fig. is the making current.
The capacity of a circuit breaker to be closed
onto a short-circuit depends upon its ability to
withstand the effects of electromagnetic forces
Making current = √2 x 1.8 x symmetrical
breaking current. The multiplication by √2 is to
obtain the peak value and again by 1.8 to take
the dc component into account.
Making capacity = √2 x 1.8 x symmetrical
breaking capacity
=2.55 x symmetrical breaking capacity
Short-time Current Rating
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 circuit breaker (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 that the circuit breaker can carry
safely for a specified short period.
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 a period longer than 1 second but not more
than 4 seconds.
Three phase Fault calculations
Three phase fault calculations
Eg is generator per phase prefault voltage