circuit-breaker-lt-col-mawla-200328210340 (1).ppt

circuit-breaker-lt-col-mawla-200328210340

2. Contents
Introduction
Operating mechanism
Types of circuit breakers
Air Blast circuit breaker
Vacuum circuit breaker
Oil circuit breaker
SF6circuit breaker
Conclusion

3. Introduction
Acircuitbreakerisamechanicalswitching
device,capableofmaking,carryingand
breakingcurrentsundernormalcircuit
conditions.Itisalsocapableofmakingand
carryingcurrentsforaspecifiedtimeand
breakingcurrentsunderspecified
abnormalcircuitconditions,suchasthose
ofashortcircuit.

5. Circuit Breakers
A circuit breaker is a piece of equipment which can
(i)make or break a circuit either manually or by
remote control under normal conditions
(ii)break a circuit automaticallyunder fault
conditions
(iii)make a circuit either manually or by remote
control under fault conditions

6. Circuit Breakers
IEEE definition:
“A device designed to open and close a
circuit by nonautomatic means, and to open
the circuit automatically on a
predetermined overcurrent without damage
to itself when properly applied within its
rating.”

7. OperatingPrinciple
Circuit Breaker consists of two contacts:
Fixed contact.
Moving contact.
Under normal operating conditions, these contacts
remain closed and will not open automatically until
and unless the system becomes faulty.
When a fault occurs on any part of the system, the trip
coils of the circuit breaker get energized and the
moving contacts are pulled apart by some mechanism,
thus opening the circuit.

8. OperatingPrinciple
When the contacts of a circuit breaker are separated
under fault conditions, an arc is struck between
them.
The production of arc not only delays the current
interruption process but it also generates enormous
heat which may cause damage to the system or to the
circuit breaker itself.
Therefore, the main problem in a circuit breaker is to
extinguish the arc within the shortest possible time .

9. Fixed
contact
Moving
contact
ARC
Fixed
contact
Moving
contact
ARC
ARC IS
QUENCHED BY
MEDIUM
IN A CIRCUIT BREAKER
OPERATING
PRINCIPLE
OF BREAKER

10. Arc Phenomenon
Duringtheseparationofcontacts,duetolarge
faultcurrentandhighcurrentdensityatthe
contactregionthesurroundingmediumionizes
andthusaconductingmediumisformed.Thisis
calledtheARC.
During the arcing period, the current flowing
between the contacts depends upon the arc
resistance.
The greater the arc resistance, the smaller the
current that flows between the contacts.

11. Arc Phenomenon
Thearcresistancedependsuponthefollowing
factors:
Degreeofionization
Lengthofthearc
Cross-sectionofarc
Factors responsible for the maintenance of arc
between the contacts. These are :
p.d. between the contacts
ionized particles between contacts

12. Arc Extinction

13. Arc Extinction
Temperature zones in
arc

14. Methods of Arc Extinction
There are two methods of extinguishing the arc in circuit
breakers viz.
High resistance method.
Low resistance or current zero method
High resistance method. In this method, arc resistance is
made to increase with time sothat current is reduced to a
value insufficient to maintain the arc.
The resistance of the arc may be increased by :
(i)Lengthening the arc.
(ii)Cooling the arc.
(iii)Reducing X-section of the arc.
(iv)Splitting the arc.

15. Methods of Arc Extinction
Low resistance or Current zero method. This
method is employed for arc extinction ina.c. circuits
only.
In this method, arc resistance is kept low until current
is zero where the arc extinguishes naturally and is
prevented from restrikinginspiteof the rising voltage
across the contacts.
All modern high power a.c. circuit breakers employ this
method for arc extinction.
There are two theories to explain zero current method
Recovery Rate Theory
Energy Balance Theory

16. Methods of Arc Extinction
In an a.c. system, current drops to zero after every half-cycle.
At every current zero, the arc extinguishes for a brief moment.
Now the medium between the contacts contains ions and
electrons so that it has small dielectric strength and can be
easily broken down by the rising contact voltage known as
restrikingvoltage.
If such a breakdown does occur, the arc will persist for another
half-cycle.
If immediately after current zero, the dielectric strength of the
medium between contacts is built up more rapidly than the
voltage across the contacts, the arc fails to restrikeand the
current will be interrupted.

17. Methods of Arc Extinction
The rapid increase of dielectric strength of the
medium near current zero can be achieved by :
(a)causing the ionized particles in the space between
contacts to recombine into neutral molecules.
(b)sweeping the ionized particles away and replacing
them by unionized particles.
The de-ionization of the medium can be achieved by:
(i)lengthening of the gap.
(ii)high pressure.
(iii)cooling.
(iv)blast effect.

18. Restriking voltage. It is the transient voltage that appears
across the contacts at or nearcurrent zero during arcing
period.
At current zero, a high-frequency transient voltage appears
across the contacts and is caused by the rapid distribution of
energy between the magnetic and electric fields associated
with the plant and transmission lines of the system.
This transient voltage is known as restriking voltage .
The current interruption in the circuit depends upon this
voltage. If the restriking voltage rises more rapidly than the
dielectric strength of the medium between the contacts, the
arc will persist for another half-cycle. On the other hand, if
the dielectric strength of the medium builds up more
rapidly than the restriking voltage, the arc fails to restrike
and the current will be interrupted
Restrikingvoltage

19. Restrikingvoltage

20. Recovery voltage.
It is the normal frequency (50 Hz) r.m.s. voltage that appears
across thecontacts of the circuit breaker after final arc
extinction. It is approximately equal to the system voltage.
When contacts of circuit breaker are opened, current drops
to zero after every half cycle.
At some current zero, the contacts are separated sufficiently
apart and dielectric strength of the medium between the
contacts attains a high value due to the removal of ionized
particles.
At such an instant, the medium between the contacts is
strong enough to prevent the breakdown by the restriking
voltage.
Recovery voltage

21. Recovery voltage.
Consequently, the final arc extinction takes place
and circuit current is interrupted.
Immediately after final current interruption, the
voltage that appears across the contacts has a
transient part (See Fig. 19.1).
However, these transient oscillations subside
rapidly due to the damping effect of system
resistance and normal circuit voltage appears
across the contacts. The voltage across the contacts
is of normal frequency and is known as recovery
voltage.
Recovery voltage

22. Recovery Rate Theory-By slepain
Slepian’s Theory of Arc
Extinction
•Arc extinction process is a race
between dielectric strength and
restriking voltage
•Residual column of ionized gas
exists after current zero
•If Dielectric strength builds up
faster than Restriking voltage-arc
extinguishes.
•This theory incomplete
–compares restrike voltage and
dielectric strength
–does not cover arcing phase
–does not consider energy

23. Energy Balance Theory
Cassie’s Theory of Arc Extinction
•Arc consists of column at uniform temp, well defined
boundary
•Uniform distribution of energy in column, temp remains
constant
•Arc cross section adjusts itself to accommodate current
•Power dissipation proportional to column cross-section
•Energy equation
•Breakdown occurs if power fed in arc > power loss

24. Arc Extinction in Oil
Arc extinction in OIL
•Arc decomposes dielectric oil
•Gasses produced increase chamber pressure
•Flow of gasses channelized through vents
•Arc gets extended into vents cooled by flowing gases
•Gas contains 70% hydrogen -good dielectric strength
•Contact area filled with fresh dielectric for arc
extinction
•Chamber may be pressurized with inert gas

25. Arc Extinction in Vacuum
Arc extinction in VACUUM
•Current leaves contact from small intensely hot
spots
•Metal vaporizes from spots
•Vapor constitutes the plasma in vacuum arc
•Rate of vapor emission -current in arc
•At current zero plasma may vanish
•Vacuum has very high dielectric strength-though
arc may not restrike

26. Arc Extinction in Air Blast
Arc extinction in AIR-BLAST
•Air flows from high pressure reservoir during
arc extinction process
•Flow rate governed by throttle diameter of
nozzle, pressure difference, nozzle profile
•Almost supersonic speed of air flow-rapid
reduction of arc diameter
•Arc does not reappear after final current zero

27. Arc Extinction in SF6 Gas
Arc extinction in SF6 gas
•SF6 atoms and molecules attracts electrons,
forms ‘–ve’ ions
•‘–ve’ ions heavier than electrons-resistance of
plasma increases rapidly
•Gas flows through nozzle over arc -takes the
heat away
• Medium regain dielectric strength rapidly

28. Classification of Circuit Breaker

29. According to their location
1) Outdoor Circuit Breaker
2) Indoor Breaker
Based on External Design
1) Dead Tank Type
2) Live Tank Type

30. According to the voltage level of
installationtypes of circuit breakerare
referred as
1) Ultra High Voltage( Above 765 KV)
2) Extra High Voltage (300KV to 765 KV)
3) High Voltage Circuit Breaker(66KV to 220 KV)
4) Medium Voltage Circuit Breaker (1KV to 52 KV)
5) Low Voltage Circuit Breaker(Less than 1KV)

31. Based on Medium used for
ARC Quenching
OilCircuitBreakers
VacuumCircuitBreakers
AirBlastCircuitBreakers
SF6CircuitBreakers

32. OIL CIRCUIT BREAKER
It is designed for 11kv-765kv.
These are of two types
•BOCB (Bulk oil Circuit Breaker)
•MOCB (Minimum oil Circuit Breaker)
The contacts are immersed in
oil bath.
Oil provides cooling by
hydrogen created by arc.
It acts as a good dielectric
medium and quenches the arc.

33. Advantages:
Oil has good dielectric strength.
Low cost.
Oil is easily available.
It has wide range of breaking capability.
Disadvantages:
Slower operation , takes about 20 cycles for arc
quenching.
It is highly inflammable , so high risk of fire.
High maintenance cost.

34. VACCUM CIRCUIT BREAKER
It is designed for medium voltage
range (3.3-33kv).
This consists of vacuum of pressure
(1*10
-6
) inside arc extinction chamber.
The arc burns in metal vapor when the
contacts are disconnected.
At high voltage, it’s rate of dielectric
strength recovery is very high.
Due to vacuum arc extinction is very
fast.
The contacts loose metals gradually
due to formation of metal vapors.

35. Advantages:
Free from arc and fire hazards.
Low cost for maintenance & simpler mechanism.
Low arcing time & high contact life.
Silent and less vibrational operation.
Due to vacuum contacts remain free from corrosion.
No byproducts formed.
Disadvantages:
High initial cost due to creation of vacuum.
Surface of contacts are depleted due to metal vapours.
High cost & size required for high voltage breakers.

36. AIR BLAST CIRCUIT BREAKERS
This operates using high velocity blast of air which
quenches the arc.
It consists of blast valve , blast tube & contacts.
Blast valve contains air at high pressure.
Blast tube carries the air at high pressure & opens the
moving contact attached to spring.
There is no carbonization of surface as in VCB.
Air should be kept clean & dry to operate it properly.

38. Advantages:
High speed operation as compared to OCB.
Ability to withstand frequent switching.
Facility for high speed reclosure.
Less maintenance as compared to OCB.
Disadvantages:
Little moisture content prolongs arcing time.
Pressure should be checked frequently for frequent
operation.
Risk of fire hazards due to over voltages.
It can’t be used for high voltage operation due to
prolonged arc quenching.

39. SF6 CIRCUIT BREAKERS
It contains anarc interruption chamber containing SF
6gas.
In closed position the contacts remain surrounded
bySF
6gas at a pressure of 2.8 kg/cm
2
.
During opening high pressure SF6 gas at 14 kg/cm
2
from its
reservoir flows towards the chamber by valve mechanism.
SF
6rapidly absorbs the free electrons in the arc path to
form immobile negative ions to build up high dielectric
strength.
It also cools the arc and extinguishes it.
After operation the valve is closed by the action of a set of
springs.
Absorbent materials are used to absorb the byproducts and
moisture.

41. Advantages:
Very short arcing period due to superior arc quenching
property ofSF
6.
Can interrupt much larger currents as compared to other
breakers.
No risk of fire.
Low maintenance, light foundation.
No over voltage problem.
There are no carbon deposits.
SF
6breakers are costly dueto high cost ofSF
6.
SF
6gas has to be reconditioned after every operation of the
breaker, additional equipment is required for this purpose.
Disadvantages:

42. CONCLUSION:
Therefore, we conclude that circuit breaker is
the most essential part of the electrical
networks as it protects every device from
damage. It helps us to detect the fault and area
affected by it. Nowadays vacuum and SF6
circuit breakers are widely used due to their
reliable and fast operations.