Circuit Breaker: Part 2

LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT -I
CIRCUIT BREAKERS (PART II)
Presented by,
Dr. Rohit Babu, Associate Professor
Department of Electrical and Electronics Engineering

Syllabus
Miniature Circuit Breaker (MCB)– Elementary principles of arc interruption–
Restriking Voltage and Recovery voltages– Restriking phenomenon - RRRV–
Average and Max. RRRV– Current chopping and Resistance switching–
Introduction to oil circuit breakers– Description and operation of Air Blast–
Vacuum and SF6 circuit breakers– CB ratings and specifications– Concept of
Auto reclosing.

Classification of Circuit Breakers
• Classification Based on Voltage
i. Low Voltage Circuit Breaker (less than 1 kV)
ii. Medium Voltage Circuit Breaker (1 kV to 52 kV)
iii. High Voltage Circuit Breakers (66 kV to 220 kV)
iv. Extra High Voltage (EHV) Circuit Breaker (300 kV to 765 kV)
v. Ultra High Voltage (UHV) Circuit Breaker (above 765 kV)
• Classification Based on Location
Circuit breakers based on their location are classified as
i. Indoor type
ii. Outdoor type

• Classification Based on External Design
Circuit breakers can be classified into following categories depending on their external design.
i. Dead tank type
ii. Live-tank type
• Classification Based on Medium Used for Arc Quenching
Depending on the arc quenching medium employed, the following are important types of circuit
breakers
i. Air-break circuit breakers:
ii. Oil circuit breakers
iii. Air blast circuit breakers
iv. Sulphur hexafluoride (SF6) circuit breakers
v. Vacuum circuit breakers

Substation Circuit Breaker Testing Video 1

Introduction to oil circuit breakers
Oil circuit breaker is such type of circuit breaker which
used oil as a dielectric or insulating medium for arc
extinction.
The oil circuit breaker is divided into two categories
•Bulk Oil Circuit Breaker
•Low Oil Circuit Breaker

Construction of oil circuit breakers
• Oil circuit breaker is very easy in
construction.
• It consists of current carrying contacts
enclosed in a strong, weather-tight earth
metal tank and the tank is filled with
transformer oil.
• The oil is both acts as an arc extinguishing
medium and as an insulator between the
live part and earth.

Working Principle of oil circuit breakers
• During the normal operating
conditions, the contact of the oil
circuit breaker is closed and carry
the current.
• When the fault occurs in the
system, the contacts of the
breaker are moving apart, and an
arc is struck between the contacts.

Advantage and Disadvantage of oil circuit breakers
Advantages of Oil as an Arc Quenching
• The oil has a high dielectric strength
and provides insulation between the
contact after the arc has been
extinguished.
• The oil used in circuit breaker provides
a small clearance between the
conductors and the earth components.
• The hydrogen gas is formed in the
tank which has a high diffusion rate
and good cooling properties.
Disadvantages of Oil as an Arc Quenching
• The oil used in oil circuit breaker is
inflammable and hence, cause a fire
hazard.
• There is a risk of formation of explosive
mixture with air.
• Due to decomposition of oil in the arc,
the carbon particles is generated which
polluted the oil and hence the dielectric
strength of the oil decreases.

Bulk Oil Circuit Breaker
A breaker which uses a large quantity of oil for arc extinction is called a bulk oil circuit breaker.
• Such type of circuit breaker is also known as dead
tank-type circuit breaker because their tank is held at
ground potential.
• The quantity of oil requires in bulk oil circuit
breaker depends on the system voltage.
• If the output rating of the voltage is 110 KV, then it
requires 8 to 10 thousand kg of oil, and if their output
rating is 220 KV, then breakers need 50 thousand Kg of
oil.

Minimum Oil Circuit Breaker
• In this type of circuit breaker minimum oil is used as
an arc quenching medium and it is mounted on a
porcelain insulator to insulate it from the earth.
• The arc chamber of such type of circuit breaker is
enclosed in a bakelised paper.
• The lower portion of this breaker is supported by the
porcelain and the upper porcelain enclosed the
contacts.
• There are two different designs of the arcing
chambers (i) axial and (ii) radial venting.

Air Circuit Breaker (or Air Blast Circuit Breaker)
What is Air Circuit Breaker?
• An Air Circuit Breaker (also known as an Air Blast Circuit Breaker or ACB) is an automatically
operated electrical switch that uses air to protect an electrical circuit from damage caused by
excess current from an overload or short circuit.
• Its primary function is to interrupt current flow after a fault is detected.
• When this happens, an arc will appear between the contacts that have broken the circuit.
• Air circuit breakers use compressed air to blow out the arc, or alternatively, the contacts are
rapidly swung into a small sealed chamber, the escaping of the displaced air, thus blowing out
the arc.

Working Principle of Air Circuit Breaker
This circuit breaker increases the arc voltage by mainly three different ways
i. It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is
decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is
required to maintain the arc.
ii. It may increase the arc voltage by lengthening the arc path. As the length of arc path is
increased, the resistance of the path is increased, and hence to maintain the same arc current
more voltage is required to be applied across the arc path. That means arc voltage is increased.
iii. Splitting up the arc into a number of series arcs also increases the arc voltage.

Operation of Air Circuit Breaker
The operation of an ACB can be broken down into three steps:
1. The first objective is usually achieved by forcing the arc
into contact with as large an area as possible of insulating
material. Every air circuit breaker is fitted with a chamber
surrounding the contact. This chamber is called ‘arc chute’.
The arc is driven into it. If inside of the arc chute is suitably
shaped, and if the arc can be made conform to the shape,
the arc chute wall will help to achieve cooling. This type of
arc chute should be made from some kind of refractory
material. High temperature plastics reinforced with glass
fiber and ceramics are preferable materials for making arc
chute.

How Air Circuit Breaker Work? Video 2

Operation of Air Circuit Breaker contd.
2. The second objective that is lengthening the arc path, is achieved concurrently with fist objective.
If the inner walls of the arc chute is shaped in such a way that the arc is not only forced into close
proximity with it but also driven into a serpentine channel projected on the arc chute wall. The
lengthening of the arc path increases the arc resistance.
3. The third technique is achieved by using metal arc slitter inside the arc chute. The main arc chute
is divided into numbers of small compartments by using metallic separation plates. These
metallic separation plates are actually the arc splitters and each of the small compartments
behaves as individual mini arc chute. In this system the initial arc is split into a number of series
arcs, each of which will have its own mini arc chute. So each of the split arcs has its own cooling
and lengthening effect due to its own mini arc chute and hence individual split arc voltage
becomes high. These collectively, make the overall arc voltage, much higher than the
system voltage.

Air Circuit Breaker
Advantages
1. There is no chance of fire hazard caused by oil.
2. The breaking speed of circuit breaker is much
higher during operation of air blast circuit breaker.
3. Arc quenching is much faster during operation of
air blast circuit breaker.
4. The duration of arc is same for all values of small
as well as high currents interruptions.
5. As the duration of arc is smaller, so lesser amount
of heat realized from arc to current carrying
contacts hence the service life of the contacts
becomes longer.
6. The stability of the system can be well maintained
as it depends on the speed of operation of circuit
breaker.
7. Requires much less maintenance compared to oil
circuit breaker.
Disadvantages
1. In order to have frequent operations, it is
necessary to have sufficiently high capacity
air compressor.
2. Frequent maintenance of compressor,
associated air pipes and automatic control
equipment's is also required.
3. Due to high speed current interruption
there is always a chance of high rate of rise
of re-striking voltage and current
chopping.
4. There also a chance of air pressure leakage
from air pipes junctions.

Air Circuit Breaker
It can be sub divided further into three different categories.
1. Axial Blast ACB.
2. Axial Blast ACB with side moving contact.
3. Cross Blast ACB.

Air Circuit Breaker- Video 3

Axial Blast Air Circuit Breaker
• In axial blast ACB the moving contact is in
contact with fixed contact with the help of a
spring pressure as shown in the figure.
• There is a nozzle orifice in the fixed contact
which is blocked by tip of the moving contact
at normal closed condition of the breaker.
• When fault occurs, the high pressure air is
introduced into the arcing chamber.

Axial Blast ACB with Side Moving Contact
• In this type of axial blast air circuit breaker
the moving contact is fitted over a piston
supported over a spring.
• In order to open the circuit breaker the air
is admitted into the arcing chamber when
pressure reaches to a predetermined value,
it presses down the moving contact; an arc
is drawn between the fixed and moving
contacts.

Cross Blast Air Circuit Breaker
• The working principle of cross blast air circuit breaker is
quite simple.
• In this system of air blast circuit breaker the blast pipe is
fixed in perpendicular to the movement of moving
contact in the arcing chamber and on the opposite side of
the arcing chamber one exhaust chamber is also fitted at
the same alignment of blast pipe, so that the air comes
from blast pipe can straightly enter into exhaust chamber
through the contact gap of the breaker.

Vacuum Circuit Breaker or VCB and Vacuum
Interrupter
• A vacuum circuit breaker is such kind of circuit
breaker where the arc quenching takes place in
vacuum.
• The technology is suitable for mainly medium
voltage application. For higher voltage vacuum
technology has been developed but not
commercially viable.
• The operation of opening and closing of current
carrying contacts and associated arc interruption
take place in a vacuum chamber in the breaker
which is called vacuum interrupter.

Advantages of Vacuum Circuit Breaker or VCB
Service life of vacuum circuit breaker is much longer than other types of circuit breakers.
There is no chance of fire hazard like oil circuit breaker.
It is much environment friendly than SF6 Circuit breaker.
Beside that contraction of VCB is user-friendly.
Replacement of vacuum interrupter (VI) is much convenient.

Vacuum Circuit Breaker- Video 4

Operation of Vacuum Circuit Breaker
The main aim of any circuit breaker is to quench arc during
current zero crossing, by establishing high dielectric strength
in between the contacts so that reestablishment of arc after
current zero becomes impossible.
The dielectric strength of vacuum is eight times greater than
that of air and four times greater than that of SF6 gas. This
high dielectric strength makes it possible to quench a
vacuum arc within very small contact gap. For short contact
gap, low contact mass and no compression of the medium
the drive energy required in vacuum circuit breaker is
minimum.

Vacuum Circuit Breaker- Video 5

SF6 Circuit Breaker Types and Operation of SF6
Circuit Breaker
A circuit breaker in which sulphur hexafluoride gas or SF6 gas is used as the arc quenching
medium, known as an SF6 circuit breaker.
• Sulphur hexafluoride (SF6) has good dielectric strength and excellent arc quenching property.
• It is an inert, nontoxic, nonflammable and heavy gas.
• At atmospheric pressure, its dielectric strength is about 2.35 times that of air.
• At 3 atmospheric pressure its dielectric strength is more than that of transformer oil.
• It is an electronegative gas, i.e. it has high affinity for electrons.
• When a free electron comes in collision with a neutral gas 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 not attain sufficient energy to contribute to
ionisation of the gas.

SF6 Circuit Breaker Types and Operation of SF6
Circuit Breaker
• Under normal conditions, SF6 is chemically inert and it does not attack metals or glass.
• However, it decomposes to SF4, SF2, S2, F2, S and F at temperatures of the order of 1000°C.
• One major disadvantage of SF6 is its condensation at low temperature.
• SF6 gas because of its excellent insulating and arc-quenching properties has revolutionized the
design of high and extra high voltage (EHV) circuit breakers.
• SF6 gas because of its excellent insulating and arc-quenching properties has revolutionized the
design of high and extra high voltage (EHV) circuit breakers.

SF6 Circuit Breaker Video 6

Properties of SF6 Gas
The properties of SF6 gas can be divided as
(i) Physical properties
(ii) Chemical properties
(iii) Electrical properties
1. Physical Properties of SF6 Gas
The physical properties of SF6 gas are as follows:
(i) It is a colourless, odourless, non-toxic and non-inflammable gas.
(ii) Pure gas is not harmful to health.
(iii) It is in gas state at normal temperature and pressure.
(iv) It is heavy gas having density 5 times that of air at 20°C and atmospheric pressure.

2. Chemical Properties of SF6 Gas
(i) It is chemically stable at atmospheric pressure and at temperatures up to 500°C.
(ii) It is a chemically inert gas.
(iii) Moisture is very harmful to the properties of this gas.
(iv) It is non-corrosive on all metals at ambient temperatures.
(v) It is an electronegative gas.
(vi) The products of decomposition of SF6 recombine in a short time after arc extinction.

3. Electrical Properties of SF6 Gas
(i) Dielectric properties
Its dielectric strength at atmospheric pressure is 2.35 times that of air and 30% less than that of
dielectric oil used in oil circuit breakers. The excellent dielectric strength of SF6 gas is because of
electronegativity (electron attachment) property of SF6 molecules. In the attachment process, free
electrons collide with the neutral gas molecules to form negative ions by the following processes.
SF6 gas maintains high dielectric strength even after mixing with air. A mixture of 30% SF6 and 70%
air by volume has a dielectric strength twice that of air at the same pressure. Below 30% of SF6 by
volume, the dielectric strength falls sharply.

Breakdown voltages of SF6,
air and dielectric oil as a
function of pressure

(ii) Corona inception voltage
Corona inception voltage for SF6 in a non-uniform
electric field is also considerably higher than that for
air.
(iii) Dielectric constant
Because of being non-polar (i.e., dipole moment is
zero), the dielectric constant of SF6 is independent of
the frequency of the applied voltage. Further, the
dielectric constant changes by only 7% over a pressure
range of 0 to 22 atmospheres.
(iv) Arc-interrupting capacity
Current interrupting capacity of SF6, air and a
mixture of both gases

Type of SF6 Circuit Breaker
The following are two principal types of SF6 circuit breakers:
(i) Double Pressure Type SF6 Circuit Breaker
This type of circuit breaker employs a double pressure system in which the gas from a high-pressure
compartment is released into the low-pressure compartment through a nozzle during the arc
extinction process.
(ii) Puffer-type (Single-pressure Type) SF6 Circuit Breaker
In this type of circuit breaker the SF6 gas is compressed by the moving cylinder system and is
released through a nozzle during arc extinction.

Type of SF6 Circuit Breaker
Puffer-type SF6 circuit breaker

Advantages of SF6 Circuit Breakers
(i) Low gas velocities and pressures employed in the SF6 circuit breakers prevent current chopping
and capacitive currents are interrupted without restriking.
(ii) These circuit breakers are compact, and have smaller overall dimensions and shorter contact gaps.
They have less number of interrupters and require less mantenance.
(iii) Since the gas is non-inflammable, and chemically stable and the products of decomposition are
not explosive, there is no danger of fire or explosion.
(iv) Since the same gas is recirculated in the circuit, the requirement of SF6 gas is small.
(v) The operation of the circuit breaker is noiseless because there is no exhaust to atmosphere as in
case of air blast circuit breakers
(vi) Because of excellent arc quenching properties of SF6, the arcing time is very short and hence the
contact erosion is less. The contacts can be run at higher temperatures without deterioration.

Advantages of SF6 Circuit Breakers
(vii) Because of inertness of the SF6 gas, the contact corrosion is very small. Hence contacts do not
suffer oxidation.
(viii) The sealed construction of the circuit breaker avoids the contamination by moisture, dust, sand
etc. Hence the performance of the circuit breaker is not affected by the atmospheric conditions.
(ix) Tracking or insulation breakdown is eliminated, because there are no carbon deposits following
an arcing inside the system.
(x) Because of the excellent insulating properties of the SF6, contact gap is drastically reduced.
(xi) As these circuit breakers are totally enclosed and sealed from atmosphere, they are particularly
suitable for use in such environments where explosion hazards exist.

Disadvantages of SF6 Circuit Breakers
(i) Problems of perfect sealing. There may be leakage of SF6 gas because of imperfect joints.
(ii) SF6 gas is suffocating to some extent. In case of leakage in the breaker tank, SF6 gas may lead to
suffocation of the operating personnel.
(iii) Arced SF6 gas is poisonous and should not be inhaled or let out.
(iv) Influx of moisture in the breaker is very harmful to SF6 circuit breaker. There are several cases of
failures because of it.
(v) There is necessity of mechanism of higher energy level for puffer-types SF6 circuit breakers. Lower
speeds due to friction, misalignment can cause failure of the breaker.
(vi) Internal parts should be cleaned thoroughly during periodic maintenance under clean and dry
environment.
(vii) Special facilities are required for transporting the gas, transferring the gas and maintaining the quality
of the gas. The performance and reliability of the SF6 circuit breaker is affected due to deterioration of
quality of the gas.

OPERATING MECHANISM
To open and close the contacts of a circuit breaker, one of the following mechanisms is employed.
(1) Spring
(2) Solenoid
(3) Compressed air
• 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.
• Springs 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.
 In small circuit breakers, the spring can be charged by hand.
 For large ones, it is charged by means of a motor.

OPERATING MECHANISM Contd.
• Solenoids are very good for closing.
 The force of attraction increases when the distance between the contacts decreases.
 Compressed air is suitable for both closing as well as opening.

SELECTION OF CIRCUIT BREAKERS

SELECTION OF CIRCUIT BREAKERS Contd.

Circuit Breaker Ratings and Specifications
Stress on circuit breakers:
Usually under abnormal conditions circuit breaker undergo large thermal and mechanical stress.
It is very important to understand the stress that the C.B. usually undergo before studying about
its Ratings and Specifications.
Thermal Stress:
During the short circuit or faulty conditions the current through the circuit breakers are about 2.5-
3 times that of rated current. Which can melt the metal having diameter of 10cm, According to
Joule’s law of heating.
Mechanical Stress:
This type of stress happens at the moving parts of the circuit breakers during its operation of
preventing fault currents.

Circuit Breaker Ratings and Specifications Contd.
Duties of circuit breakers:
• It must be capable of opening on the occurrence of a fault and of clearing the fault.
• It must be capable of being closed on to a fault.
• It must be capable of carrying fault current for short time while another circuit breaker is
clearing the fault.
These duties are classified as follows:
1. Breaking capacity.
2. Making capacity.
3. Short-time capacity.
Other features of the duty include Rated voltage, Normal current rating, Operating duty and
Thermal parameters.

1. 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 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/sqrt(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
2. Making Capacity
The capacity of a circuit breaker to be closed onto a short-circuit depends upon its ability to
withstand the effects of electromagnetic forces.
The multiplication by sqrt (2) is to obtain the peak value and again by 1.8 to take the dc component
into account.

3. 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 ac and dc 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.

4. Rated Voltage, Current and Frequency
• In a power system, the voltage level at all points is not the same. It varies, depending upon the
system operating conditions. Due to this reason manufacturers have specified a rated maximum
voltage at which the operation of the circuit breaker is guaranteed.
• The rated current is the rms value of the current that a circuit breaker can carry continuously
without any temperature rise in excess of its specified limit.
• The rated frequency is also mentioned by the manufacture. It is the frequency at which the circuit
breaker has been designed to operate.

5. Rated Operating Duty
This is mechanical duty requirement of circuit breaker operating mechanism. The sequence of rated
operating duty of a circuit breaker has been specified as
• O – t – CO - t' - CO
where O indicates opening operation of CB. CO represents closing operation immediately followed
by an opening operation without any intentional time delay. t' is time between two operations which
is necessary to restore the initial conditions and / or to prevent undue heating of conducting parts of
circuit breaker. t = 0.3 sec for circuit breaker intended for first auto re closing duty, if not otherwise
specified.

•Suppose rated duty circle of a circuit breaker is 0 – 0.3 sec – CO – 3 min – CO. This means, an
opening operation of circuit breaker is followed by a closing operation after a time interval of 0.3 sec,
then the circuit breaker again opens without any intentional time delay. After this opening operation
the CB is again closed after 3 minutes and then instantly trips without any intentional time delay.
Thermal Parameters:
It is the maximum temperature rise that is permissible for the individual parts of the equipment.
It equals to the sum of the maximum temperature rise and the fixed temperature of the ambient air.
Rated interrupting time: It is the mean time for interruption of any current not exceeding the rated
interrupting current of the breaker.

The circuit breakers are classified by various ways. The different criteria for classification of
circuit breakers are as follows
1. Interrupting medium.
2. According to service.
3. Way of operation.
4. Action.
5. Method of control.
6. Way of mounting.
7. Tank construction.
8. Contacts.

Classification of Circuit Breakers Contd.
According to Interrupting Medium: Again according to interrupting medium the circuit breakers
are classified as follows:
1. Air Circuit Breakers
2. Air Blast circuit breakers.
3. Oil Circuit breakers
4. Magnetic Blast circuit breakers.
According to service: Basically there are two types, they are:
1. Indoor Circuit Breaker.
2. Outdoor Circuit Breaker.

Depending upon Operation: According to this classification the types of circuit breakers are
1. Gravity Opened, Gravity Closed Circuit Breaker.
2. Horizontal Break Circuit Breaker.
Based on Action: This is nothing but,
1. Automatic C.B.
2. Manual C.B.
According to method of control:
According to the method of control, the circuit breaker may be controlled directly or it may be
operated remotely. The remote control may be manual, pneumatic or electrical.

The way of Mounting: According to this,
• Panel Mounted and,
• Rear of panel or remote from panel type.
Depending upon Tank construction:
• This classification is born only due to the liquid dielectric medium of separation so we can
classify it into “ A separate tank for each pole type or one tank for all poles type.
On basis of Contacts: On this type of classification the circuit breakers are classified into
1. Butt type contacts.
2. Wedge type.
3. Laminated flat contact and
4. Explosion chamber.

Auto Reclosing Scheme of Transmission System
• The extra high voltage transmission lines transmit huge amount of electric power. Hence, it is
always desirable that the continuation of power flow through the lines should not be interrupted
for a long time.
• There may be a temporary or permanent fault in the lines.
• Temporary faults get automatically cleared, and these do not require any attempt for fault
rectification.
• It is normal practice by the operators that after each initial faulty tripping of the line, they close
the line.
• If the fault is transient, the line holds after the second attempt of closing the circuit breaker, but if
the fault persists, the protection system again trips the line and then it is declared as permanent
fault.

Auto Reclosing Scheme of Transmission
System Contd.
We categorize the faults in electrical transmission system in three ways,
1. Transient Fault
2. Semi Permanent Fault
3. Permanent Fault
• The transient faults are those which automatically removed momentarily.
• Semi permanent faults are also transient in nature but there take few moments to remove.
• Semi-permanent faults may get occurred due to the falling of things on the live conductors.
• Semi-permanent faults get removed after the cause of faults is burnt away.
• During both of the above mentioned faults, line is tripped but the line can be restored if the
circuit breakers associated with the line are closed.

Numerical Example- 1
1. A circuit breaker is rated at 1500 amps, 2000 MVA, 33 kV, 3 sec, 3- phase, oil circuit breaker.
Determine the rated normal current, breaking current, making current and short time rating
(current).
Ans: The rated normal current is 1500 amps.
Making current = 2.55 x Symmetrical
breaking current
If the symmetrical braking current capacity
of the breaker is 40 KA(RMS), the making
current of the breaker must be 2.55*40=102
KA(peak).

Numerical Example- 2
2. A generator connected through a 3-cycle C.B. to a transformer is rated 10 MVA, 13.8 kV with
reactances of Xd″ = 10%, Xd′ = 15% and Xd = 100%. It is operating at no load and rated voltage when a
3-phase short circuit occurs between the breaker and the transformer. Determine (i) the sustained
short circuit current in the breaker; (ii) the initial symmetrical r.m.s. current in the breaker; (iii) the
maximum possible d.c. component of the short circuit current in the breaker; (iv) the momentary
current rating of the breaker; (v) the current to be interrupted by the breaker; and (vi) the
interrupting kVA.
Ans: (i) Since the steady reactance is 100%
The steady state short circuit MVA = 10 MVA
(ii) The initial symmetrical r.m.s. current is the current corresponding to sub-transient state
where the % reactance is 10.

Numerical Example- 2 Contd.
(iii) The maximum possible d.c. component = peak value of the subtransient current = sqrt (2) × 4180
= 5910 amps.
(iv) Momentary current rating = 1.6 × 4180 = 6688 amps
(v) Since it is a 3-cycle breaker the current to be interrupted by the breaker = 1.2 × symmetrical
breaking current = 1.2 × 4180 = 5019 amps.
(vi) The interrupting kVA = sqrt (3) × 13.8 × 5016 = 119897 kVA or 119.897 MVA

Assignment 1.1
(Submission date: 14/09/2020)
1. Explain how arc is initiated and sustained in a circuit breaker when the circuit breaker contacts
separate.
2. Discuss the principle of arc interruption in (i) an oil C.B.; and (ii) air blast circuit breaker.
3. Compare the performance and characteristics of (i) minimum oil breakers and air blast C.B.; (ii) air
blast C.B. and bulk oil C.B.
4. Explain the terms (i) Symmetrical breaking current; (ii) Asymmetrical breaking current; and (iii)
making current. Explain clearly how these currents can be determined from oscillograms taken
during short circuit tests on a 3-phase C.B.
5. Explain the terms (i) restriking voltage; (ii) recovery voltage; and (iii) RRRV. Derive an expression
for the restriking voltage in terms of system voltage, inductance and capacitance, across a C.B.
contact when a 3-phase fault takes place. Assume the neutral of the system to be solidly grounded.

Assignment 1.2
1. Differentiate between type tests and routine tests. What different tests are carried out to prove the
ability of a C.B.?
2. What are the requirements of the contact material for a vacuum circuit breaker? Why is current
chopping not a serious problem with such circuit breakers?
3. Describe the construction, principle of operation and application of a vacuum breaker.
4. Compare the performance of SF6 gas with air when used for circuit breaking.
5. Explain the process of ‘current chopping’ in SF6 breakers.
6. Describe the construction, principle of operation and application of SF6 circuit breaker. How does
this breaker essentially differ from an air blast breaker?

Assignment 1.3
1. In a short circuit test on a C.B. the following readings were obtained on a single frequency transient: (i)
Time to reach the peak restriking voltage 40 μsec; (ii) the peak restriking voltage 100 kV. Determine the
average RRRV and the frequency of oscillation.
2. An 11 kV, 50 Hz alternator is connected to a system which has inductance and capacitance per phase of 10
mH and 0.01 μF respectively. Determine (i) the maximum voltage across the breaker contacts; (ii) Frequency
of transient oscillation; (iii) the average RRRV; and (iv) the maximum RRRV.
3. A 66 kV, 50 Hz, 3-phase alternator has an earthed neutral. The inductance and capacitance per phase of the
system are 7 mH and 0.01 μF respectively. The short circuit test gave the following results: Power factor of
fault 0.25, fault current symmetrical recovery voltage is 90% of full line voltage. Assuming that the fault is
isolated from the ground, calculate the RRRV.
4. A circuit breaker is rated as 2500 A, 1500 MVA, 33 kV, 3 secs, 3-phase oil C.B. Determine the rated
symmetrical breaking current, rated making current, short time rating and rated service voltage.

Circuit Breaker: Part 2

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Circuit Breaker: Part 2