Power System Protection

1. CIRCUUT BREAKER
Name: Muhammad Moin
MUET Jamshoro

2. CONTENT:
CIRCUIT BREAKER
TYPES OF CIRCUIT BREAERS
1-AIR-BLAST CIRCUIT BREAKER
2-SULPHUR HEXAFLUORIDE CIRCUIT BREAKERS
3-VACUUM CIRCUIT BREAKER

3. CIRCUIT BREAKER
 A circuit breaker is a piece of equipment which can make or break
a circuit either manually or by remote control under normal
conditions.
 A circuit breaker is a piece of equipment which can break a circuit
automatically under fault conditions.
 A circuit breaker is a piece of equipment which can make a circuit
either manually or by remote control under fault conditions.

4. TYPES OF CIRCUIT BREAKER
1. Air-Blast Circuit Breakers
2. Sulphur Hexafluoride Circuit Breakers
(SF6)
3. Vacuum Circuit breakers

5. 1. AIR-BLAST CIRCUIT BREAKERS
 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.

6.  This type of circuit breaker operates in air at atmospheric pressure.
After the development of the oil circuit breaker, the medium voltage
air circuit breaker has been largely replaced by oil circuit breakers
worldwide.
 Although in countries like France and Italy, ACBs are still preferable
choice up to voltage 15 kV.

7. CONSTRUCTION
 The air circuit breaker consists of three types of contacts in a
compressed air chamber. The three contacts are
 1-The main contacts which carries the load current
 2-The arching current which carries arching current
 3-The arching horns.
1-Main Contacts
The main contacts are made from a good conducting material,
usually made up of silver, copper. These offers low resistance to
the load current flowing through the circuit. This also avoids
over heating of the contacts while in service.

8. 2-Arching contacts
 The arching contacts are made of harder material with high resistance for
example tungsten. Because the aching contacts should with stand very
high arching current. Once the main contacts opens the arching current
flows through the arching current with high resistance path.
3-Arching horns and chutes
 the arching horns are also made up of hard material(tungsten or copper) to
carry the arching current. The arc is transferred from arching contacts at
last before extinction. The arc chutes made up of the fire proof insulating
material because these are the last contacts of arc. The cooling metal
plates are attached to arc chutes to convict the hot gasses way from the
gases and to cool the contacts arc the extinction of arc. The metal plates
further stretches the arc and helps in reducing the heating of contacts.

10. OPERATION
 The operating mechanisms of air circuit breaker in two modes i.e.
opening mode and closing mode are explained as follows.
1-Breaker opening mode
 Under normal operation with the breaker close the load current flows
through the main contacts because of low resistance path compared
to arching contacts.
 Under fault condition or normal opening condition, as the breaker
opens, the main contacts which are carrying the load current opens
first by transferring the current to the arching contacts. As the arching
contacts opens then arc produces due to ionization of the medium at
their faces. the arching contact faces are made up of hard materials
the damage is less for these arching current.

11.  As the arching contacts still opens the arc widens and increases due
to convection of surrounding gases. The stretched arc when enters to
arc chute area is rapidly cooled by the cooling metallic plates. The
cooled gases offers high dielectric force to the electron conduction
and hence the arc extinct.
2-Breaker Closing Mode
 Closing mode is the reverse process of breaker opening mode where
the arching contacts closes when the closing command is initiated to
the circuit breaker. The main contacts closes about a half a cycle after
the arching contacts. Then the main contacts carries the load current.

12. ADVANTAGES
 There is no chance of fire hazard caused by oil.
 The breaking speed of circuit breaker is much higher
during operation of air blast circuit breaker.
 Arc quenching is much faster during operation of air blast
circuit breaker.
 The duration of arc is same for all values of small as well as
high currents interruptions.
 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.
 Requires much less maintenance compared to oil circuit

13. DISADVANTAGES
 In order to have frequent operations, it is necessary to have
sufficiently high capacity air compressor.
 Frequent maintenance of compressor, associated air pipes and
automatic control equipment is also required.
 Due to high speed current interruption there is always a chance
of high rate of rise of re-striking voltage and current chopping.
 There also a chance of air pressure leakage from air pipes
junctions.

14. APPLICATIONS
 The air circuit breakers are used in power plants for medium
voltage i.e. <3KV for safe operation.
 These are widely used as out door circuit breakers due to its
simplicity.
 Air break circuit breaker is suitable for the control of power
station auxiliaries and industrial plants. They do not require
any additional equipment such as compressors, etc. They are
mainly used in a place where there are possibilities of fire or
explosion hazards. Air break principle of lengthening of the
arc, arc runners magnetic blow-up is employed for DC circuit
breakers up to 15 KV.

15. 2- SULPHUR HEXAFLUORIDE (SF6) CIRCUIT
BREAKERS
 An SF6 circuit breaker is a type of circuit breaker in
which sulphur hexafluoride gas or SF6 gas is used as the arc
quenching medium, known as an SF6 circuit breaker.
 SF6 gas has excellent insulating property and it has a high
electronegativity. Due to high electronegativity, it has a high
affinity for absorbing free electrons. It absorbs electrons to
form a negative ions.
 Ions are obviously much heavier than a free electron and
therefore over all mobility of charged particles in SF6 gas is
much less as compared to the other common gases.

16.  We know that the mobility of charged particles is majorly responsible
for conducting current through a gas. Hence, for heavier and less
mobile charged particles in SF6 gas, it acquires very high dielectric
strength.
 Not only the gas has a good dielectric strength but also it has the
unique property of fast recombination after the arc is removed. The
gas also has excellent heat transfer property.
 Due to its low gaseous viscosity, SF6 gas can efficiently transfer heat
by convection. So due to high dielectric strength and high cooling
effect, SF6 gas is approximately 100 times more effective arc
quenching media than air.
 Due to these unique properties of this gas, SF6 circuit breaker is
used in a complete range of medium voltage and high voltage
electrical power system. These circuit breakers are available for the
voltage ranges from 33KV to 800 KV and even more.

17. CONSTRUCTION
 SF6 circuit breakers mainly consist of two parts, namely (1) the
interrupter unit and (2) the gas system.
1-Interrupter Unit
 This unit consists of moving and fixed contacts comprising a set of
current-carrying parts and an arcing probe. It is connected to the SF6 gas
reservoir. This unit consists slide vents in the moving contacts which
permit the high-pressure gas into the main tank.
2-Gas System
 The closed circuit gas system is employed in SF6 circuit breakers.
The SF6 gas is costly, so it is reclaimed after each operation. This unit
consists low and high-pressure chambers with a low-pressure alarm along
with warning switches. When the pressure of the gas is very low due to
which the dielectric strength of gases decrease and an arc quenching
ability of the breakers is endangered, then this system gives the warning

19. WORKING PRINCIPLE
 In the normal operating conditions, the contacts of the breaker are closed.
When the fault occurs in the system, the contacts are pulled apart, and an
arc is struck between them. The displacement of the moving contacts is
synchronized with the valve which enters the high-pressure SF6 gas in the
arc interrupting chamber at a pressure of about 16kg/cm^2.
 The SF6 gas absorbs the free electrons in the arc path and forms ions
which do not act as a charge carrier. These ions increase the dielectric
strength of the gas and hence the arc is extinguished. This process reduces
the pressure of the SF6 gas up to 3kg/cm^2 thus; it is stored in the low-
pressure reservoir. This low-pressure gas is pulled back to the high-
pressure reservoir for re-use.
 Now a day puffer piston pressure is used for generating arc quenching
pressure during an opening operation by mean of a piston attached to the
moving contacts.

20. ADVANTAGES
 SF6 gas has excellent insulating, arc extinguishing and many other
properties which are the greatest advantages of SF6 circuit breakers.
 The gas is non-inflammable and chemically stable. Their
decomposition products are non-explosive and hence there is no risk
of fire or explosion.
 Electric clearance is very much reduced because of the high dielectric
strength of SF6.
 It gives noiseless operation, and there is no over voltage problem
because the arc is extinguished at natural current zero.
 SF6 performs various duties like clearing short-line faults, switching,
opening unloaded transmission lines, and transformer reactor, etc.
without any problem.

21. DISADVANTAGES
 SF6 gas is suffocating to some extent. In the case of leakage in
the breaker tank, the SF6 gas being heavier than air and hence
SF6 are settled in the surroundings and lead to the suffocation
of the operating personnel.
 The entrance of moisture in the SF6 breaker tank is very
harmful to the breaker, and it causes several failures.
 The internal parts need cleaning during periodic maintenance
under clean and dry environment.
 The special facility requires for transportation and
maintenance of quality of gas.

22. APPLICATIONS
 A typical SF6 circuit breaker consists of interrupter units each
capable of dealing with currents up to 60kA and voltages in
the range of 50-80kV.
 A number of units are connected in series according to the
system voltage.
 SF6 circuit breakers have been developed for voltages 115 kV
to 230 kV, power ratings 10 MVA to 20 MVA and interrupting
time less than 3 cycles.

23. 3- VACUUM CIRCUIT BREAKERS
 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.
 The vacuum interrupter consists of a steel arc chamber in the
Centre symmetrically arranged ceramic insulators. The
vacuum pressure inside a vacuum interrupter is normally
maintained at 10– 6 bar.

24.  The material used for current carrying contacts plays an important
role in the performance of the vacuum circuit breaker. Cu/Cr is the
ideal material to make VCB contacts. Vacuum interrupter technology
was first introduced in the year 1960. But still, it is a developing
technology.
 As time goes on, the size of the vacuum interrupter is being reduced
from its early 1960’s size due to different technical developments in
this field of engineering. The contact geometry is also improving
with time, from butt contact of early days it gradually changes to
spiral shape, cup shape and axial magnetic field contact. The vacuum
circuit breaker is today recognized as most reliable current
interruption technology for medium voltage switchgear. It requires
minimum maintenance compared to other circuit breaker
technologies.

25. CONSTRUCTION
 The figure below shows the construction of vacuum circuit
breaker. It consists of fixed contact, moving contact and arc
shield mounted inside a vacuum chamber. The movable
member is connected to the control mechanism by stainless
steel bellows. This enables the permanent sealing of the
vacuum chamber so as to eliminate the possibility of leak. A
glass vessel or ceramic vessel is used as the outer insulating
body. The arc shield prevents the deterioration of the
internal dielectric strength by preventing metallic vapors
falling on the inside surface of the outer insulating cover.

27. WORKING PRINCIPLE
 This is working of vacuum circuit breakers. When the breaker
operates, the moving contact separates from the fixed contact
and an arc is struck between the contacts. The production of
arc is due to the ionization of metal ions and depends very
much upon the material of contacts. The arc is quickly
extinguished because the metallic vapors, electrons and ions
produced during arc are diffused in a short time and seized by
the surfaces of moving and fixed members and shields. Since
vacuum has very fast rate of recovery of dielectric strength,
the arc extinction in a vacuum circuit breaker occurs with a
short contact separation (say 0·625 cm).

28. ADVANTAGES
 They are compact, reliable and have longer life.
 There are no fire hazards.
 There is no generation of gas during and after operation.
 They can interrupt any fault current. The outstanding feature of a
VCB is that it can break any heavy fault current perfectly just before
the contacts reach the definite open position.
 They require little maintenance and are quiet in operation.
 They can successfully withstand lightning surges.
 They have low arc energy.
 They have low inertia and hence require smaller power for control
mechanism.

29. DISADVANTAGES
 Requirements of high technology for production of vacuum
interrupters.
 It needs additional surge suppressors for the interruption of
low magnetizing currents in a certain range.
 Loss of vacuum due to transit damage or failure makes the
entire interrupter useless, and it cannot be repaired on site.
 When the inductive load or the capacitive load is switched off,
it is easy to cause overvoltage due to interception, oscillation
and resignation.


30. APPLICATIONS
 For a country like Pakistan, where distances are quite large
and accessibility to remote areas difficult, the installation of
such outdoor, maintenance free circuit breakers should prove a
definite advantage. Vacuum circuit breakers are being
employed for outdoor applications ranging from 22 kV to 66
kV. Even with the limited rating of say 60 to 100 MVA, they
are suitable for a majority of applications in rural areas.