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MV & HV
Circuit Breakers
Eng.: Mohamed Khaled El Sayed
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Index:(1): Introduction.
(2): Circuit breakers (General).
(3): Oil Circuit Breakers
- Bulk Oil CB.
- Minimum Oil CB.
(4): Air Blast Circuit Breakers.(5): SF6 Gas Circuit Breakers.
(6): Vacuum Circuit Breakers.
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Introduction
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The duty of the switchgears is to switch on and
offand to control, once or repeatedly severaltimes, different electrical circuits during normal
as well as abnormal operating conditions.
Short circuit current level & the behavior of circuits
during transient condition are very essential toselect proper specification of switchgears.
Good practices and knowledge of circuit
breakers and switchgears specification,operation, testing and maintenance are fatal for
engineers concerning with electrical power system.
The main subject ofthis lecture is about circuit
breaker types and application.
Introduction:
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Introduction: Circuit breaker (CB) is a device to open or close an
electric power circuit either during normal powersystem operation or during abnormal conditions.
A CB serves in the course of normal system
operation to energize or de-energize loads.
During abnormal conditions, when excessive
current develops, a CB opens to protect equipment
and surroundings from possible damage due to
excess current.
These abnormal currents are usually the result of
short circuits created by lightning, accidents,
deterioration of equipment, or sustained overloads.
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Introduction:Formerly, all CBs were electromechanical devices.
In these breakers a mechanism operates one or more pairs
of contacts to make or break the circuit.
The mechanism is powered either electromagnetically,
pneumatically, or hydraulically.
The contacts are located in a part termed the interrupter.When the contacts are parted, opening the metallic
conductive circuit, an electric arc is created between the
contacts.
This arc is a high-temperature ionized gas with an electricalconductivity comparable to graphite.
Thus the current continues to flow through the arc.
The function of the interrupter is to extinguish the arc,
completing circuit-breaking action.
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Circuit Breakers
(General)
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A CB is a complex circuit-breaking device with the
following duties:
1- Make or break both normal and abnormal currents.2- Appropriately manage the high-energy arc associated
with current interruption.
3- Rapid and successive automatic breaking and making
to aid stable system operation.4- Three pole and single pole auto-relosing arrangement.
Additionally, a CB is required to do so under the following
typical conditions:
1- Short-circuit interruption.2- Interruption of small inductive currents.
3- Capacitor switching.
4- Interruption of short-line fault.
5- Asynchronous switching.
CB Duties:
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CB Elements1. Main Contact.
2. Insulation between main contacts.
3. Interruption chamber.4. Control, supervisory and auxiliary
devices.
5. Insulating link between control unit
and main contact.
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CBs Requirements:1.Opens & closes in shortest possible time.
2.Conducts rated current.3.Withstands thermally & mechanically any
short circuit.
4.Maintains its voltage to earth across open
contacts.
5.Does not create overvoltages during
circuit opening.
6.Easy to maintain.
7.Not very expensive.
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Particulars of a CB: Medium for arc extinction.
Rated line voltage.
Rated breaking current.
Type of construction (e.g outdoor, indoor.-.. etc).
Type of operating mechanism
Total breaking time.
Other additional features such as
switching resistors, capacitor,. .
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CBs Rating:- Rated normal current:
to be carried without temperature rise.
- Rated breaking current:
= Highest current to be interrupted= rms of the peak value of the
Asymmetrical current.
- Rated MVA:
= 3 *Line voltage(kV)*breaking current(kA)
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Principles of Operation : On opening moving contact moves rapidly.
An electric Arc is formed. The intensity of the arc depends on the
current been interrupted.
As the moving contact moves further, the arcis stretched.
The gas in the chamber sweeps the arc which
is cooled. Circuit opening is achieved if the arc does not
re-ignite.
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Classification of CBs:Generally we can classify the circuit breakers
according to the constructional details as thefollowing:
Air-break circuit breaker or miniature CB.
Oil circuit breaker. Bulk Oil.
Minimum oil.
Air blast circuit breaker.
SF6 circuit breaker.
Vacuum circuit breaker.
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Oil Circuit Breakers
(OCBs)
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Oil Circuit Breakers (OCBs) The oil in OCBs serves two purposes:
It insulates between the phases and between thephases and the ground.
It provides the medium for the extinguishing of the arc.
When electric arc is drawn, the arc vaporizes the oil
and creates a large bubble that surrounds the arc. The gas inside the bubble is around 80% hydrogen,
which impairs ionization.
The decomposition of oil into gas requires energy that
comes from the heat generated by the arc.
The oil surrounding the bubble conducts the heat
away from the arc and thus also contributes to
deionization of the arc.
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Disadvantages of Oil CBs: It is inflammable and there is a risk of fire.
It may form an explosive mixture with air.
It require continuous maintenance .
Absorb moisture, so dielectric strengthreduces.
Oil leakage problem.
Oil has to be replace after some operations
because of the carbonization of oil
(i.e. changing and purifying the oil).
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Bulk Oil
Circuit Breakers
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Bulk Oil CBs: Bulk oil CBs are enclosed in metal-grounded
weatherproof tanks that are referred to as dead tanks. The original design of bulk OCBs was very simple and
inexpensive. The arc was drawn directly inside of the container tank
without any additional arc extinguishing.
Plain break breakers were superseded by arccontrolled oil breakers.
The arc controlled oil breakers have an arc control
device surrounding the breaker contacts. The purpose of the arc control devices is: To improve operating capacity. To speed up the extinction of arc.
To decrease pressure on the tank.
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Plain Bulk Oil CB
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Arc ControlBulk Oil CB
1- bushing 6- plunger guide
2- oil level indicator 7- arc control device
3- vent 8- resistor
4- current transformer 9- plunger bar
5- dashpot
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Separate PhasesBulk Oil Dead Tank
CBs
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Minimum Oil
Circuit Breakers
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Minimum Oil Breakers: The minimum oil breakers were developed to
reduce the oil volume only to amount needed forextinguishing of the arc (about 10% of the bulk- oilamount).
The arc control for the minimum oil breakers is
based on the same principle as the arc controldevices of the bulk oil breakers.
To improve breaker performance, oil is injectedinto the arc.
The interrupter containers of the minimum oilbreakers are made ofinsulating material and areinsulated from the ground.
This is usually referred to as live tank construction.
Mi i Oil B k
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Minimum Oil Breaker:
1- vent valve
2- terminal pad3- oil level indicator
4- moving contact
5- lower fixedcontact
6- separating piston
7- terminal pad8- upper drain valve
9- lower drain valve
Minimum Oil CB
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Minimum Oil CB:
Electric arc in Oil CBs
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Electric arc in Oil CBs: In oil CBs, the arc is drawn in oil inside a special
compartment of the interrupting chamber called the
explosion pot. The intense heat of the arc decomposes the oil and
produces gases, mainly composed ofhydrogen,generating high pressure that produces a fluid flow
through the arc and out of the explosion pot throughvents situatedon its walls.
Thus extending the arcs column and carrying itsenergy away until its total extension.
At transmission voltages below 345 kV, oil breakersused to be popular.
They are increasingly losing ground to gas-blast circuitbreakers such as air-blast breakers and SF6 circuit
breakers.
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31/99Representation of electric arc in Oil CB
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Contact opening
sequence of
minimum oil CB at
short circuit current
Lif T k 2 S k Mi i Oil CB
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Life Tank 2 Stroke Minimum Oil CB:
HV Mi i Oil B k
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HV Minimum Oil Breakers: For HV (above 132 kV), the interrupters are
arranged in series. It is essential to ensure that each interrupter carries
its share of the duty. Care must be taken that all breaks occur
simultaneously, and that the restriking voltage isdivided equally across the breaks during theinterrupting process.
The natural voltage division depends on stray
capacitances between the contacts and to theground, and therefore is in very uneven.
This is corrected by connecting capacitances orresistors in parallel with the interrupting heads.
V lt th B k
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Voltage across the Breaker:
Breaker Capacitances:
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Breaker Capacitances:
Breaker Capacitances:
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Breaker Capacitances:
B k C it
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Breaker Capacitors:
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Air Blast Circuit Breaker
(ABCB)
Air blast circuit breaker:
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Air blast circuit breaker:Most of the main features of air blast circuit
breakers are:
Fast operations.
Suitability for repeated operation. Auto reclosure.
Unit type multi break constructions. Simple assembly.
Modest maintenance.
Air Blast CB:
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Air Blast CB: A compressors plant necessary to maintain high
air pressure in the air receiver. The air blast CBs are especially suitable for
railways and arc furnaces, where the breaker
operates repeatedly.
Air blast CBs is used for interconnected lines and
important lines where rapid operation is
desired.
High pressure air at a pressure between 20 to 30
kg/ cm2 stored in the air reservoir.
Air is taken from the compressed air system.
Air Blast CB Construction:
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Air Blast CB Construction: Three hollow insulator columns are mounted on
the reservoir with valves at their basis.
The double arc extinguished chambers are
mounted on the top of the hollow insulator
chambers.
The current carrying parts connect the three arcextinction chambers to each other in series and
the pole to the neighboring equipment.
Since there exists a very high voltage between theconductor and the air reservoir, the entire arc
extinction chambers assembly is mounted on
insulators.
HV Air Blast CB
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HV Air Blast CB
HV Air Blast CB:
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HV Air Blast CB:
Double Arc Extinction Chamber
Air Blast Breaker Modules
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Air Blast Breaker Modules
Principle Of Arc Quenching In ABCB:
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Principle Of Arc Quenching In ABCB :It needs an auxiliary compressed air system which
supplies air to the air receiver of the breaker.
For opening operation, the air is admitted in the arcextinction chamber. It pushes away the movingcontacts.
In doing so, the contacts are separated and the air blast
takes away the ionized gases along with it and assists inextinction.
After a few cycles the arc is extinguished by the air blastand the arc extinction chamber is filled with highpressure air (20 kg/cm2).
The high pressure air has higher dielectric strength thanthat of atmospheric pressure. Hence a small contactgap of few centimeters is enough.
Principle Of Arc Quenching
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Principle Of Arc Quenching
(Axial Flow)
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Representation of electric arc in ABCB
Principle Of Arc Quenching In ABCB:
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Principle Of Arc Quenching In ABCB :In axial blast type air flow, the air flows from high
pressure reservoir to the atmospheric through a
convergent divergent nozzle.
The difference is pressure and the design of the
nozzle is such that the air expands into the low
pressure zone, it attains almost supersonic velocity.The mass flow of air through the nozzle is governed
by the parameters like:
Pressure ratio,Area of throat,
Nozzle throat diameter.
and is influenced by the diameter of the arc itself.
Principle Of Arc Quenching In ABCB:
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Principle Of Arc Quenching In ABCB :The air flowing at a high speed axially along the
arc causes the removal of heat from theperiphery of the arc and the diameter of the arc
reduces to a low value at current zero.
At this instant the arc is interrupted and thecontact space is flushed with fresh air flowing
through the nozzle.
The flow of fresh air through the contact spaceensures removal of hot gases and rapid building
up of dielectric strength.
Sequence Of Operation In ABCB:
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Sequence Of Operation In ABCB : The principle of operation ofcross blast flow is, the air
flows around the arc and the diameter of arc is likely to
remain stable for higher values of current.
During the period of arc extinction, the air continues to
flow through the nozzle to the atmosphere.
The mass flow rate can be increased by increasing thepressure of high pressure system. The increase in the
mass flow results in the increased breaking capacity.
The air blast circuit breakers come under the class
external extinguishing energy type.
The energy supplied for arc extinction is obtained from
high pressure air and is independent of current to be
interrupted.
Sequence Of Operation
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Sequence Of Operation
T li C i Ai Bl CB
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Tulip Contacts in Air Blast CB
Contact shown in closed-arc starting
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Contact shown in closed arc startingstep-opened position (ABCB)
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MV Single phase of ABCS Resistance Switching:
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Resistance Switching:The post zero resistance of contact space is high in
air blast circuit breakers.This is because the contact clearance space is filled
with high pressure air after final current zero and
high pressure air has high dielectric strength.The high restriking voltage appears across the
contacts does not damp out through the gap
because of the high post zero resistance.Further, voltages of the order of several times the
normal voltage appear across the contacts because
of current chopping.
Resistance Switching:
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Resistance Switching: Further, voltages of the order of several times the
normal voltage appear across the contacts because of
current chopping.
If these voltages are not allowed to discharge, they may
cause break down of insulation of the circuit breaker or
the neighboring equipment.
To overcome this difficulty, resistance switching is
adopted.
The usual procedure is to connect a resistance in shuntwith the arc.
During the opening operation, air is admitted in the arc
extinguishing chamber.
Resistance Switching:
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Resistance Switching:It separates the main contacts and pushes the auxiliarycontacts.
The auxiliary contacts close, thereby the resistors areconnected across the arc for a short time of arcing.The auxiliary contacts are located in the inclined Vshaped insulators while the resistors are located in the
vertical insulators.Immediately after the arc extinction the pressure oneither side of the piston of auxiliary contacts gets soadjusted that the auxiliary contacts open and resistor
circuit is interrupted.Ceramic resistances of non linear characteristics, similarto those used in the lightning arresters are used forresistance switching.
Non liner Resistance function:
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Non liner Resistance function:These consist of silicone carbide, bound byinorganic binders subjected to heat treatment.
During high current, non liner resistor offers lowresistance.
Thus the main arc current is partly diverted through
resistor unit.As current reduces, the resistance offered by nonlinear resistors increases causing a greater dropacross the resistor units.
Thereby the voltage available for arc betweenauxiliary contacts is no more sufficient and arcbetween auxiliary contacts is automaticallyextinguished.
Configuration of switching resistor
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Configuration of switching resistor
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HV Three phase ABCS ABCB Advantages:
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ABCB Advantages:An air blast CB has the following advantages:
1- The risk of fire is eliminated.
2- The arcing products are completely removed by the blast.
3- The growth of dielectric strength is so rapid that final
contact gap needed for arc extinction is very small (this
reduces the size of device).4- The arcing time is very small due to the rapid build up of
dielectric strength between contacts. Therefore, the arc
energy is less, thus resulting in less burning of contacts.
5- Due to lesser arc energy, air blast CBs are very suitable forconditions where frequent operation is required.
6- The energy supplied for arc extinction is obtained from
high pressure air and is independent of the current to be
interrupted.
ABCB Disadvantages:
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ABCB Disadvantages:1- Air has relatively inferior arc extinguishing
properties.2- Air blast circuit breakers are very sensitive to
the variations in the rate of restriking voltage.
3- Considerable maintenance is required for the
compressor plant which supplies the air blast.
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SF6 Gas Circuit Breaker
(SF6 CB)
Introduction:
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Introduction:Sulfur hexafluoride (SF6) gas is an alternative to air as aninterrupting medium.
SF6 is a colorless nontoxic gas, with good thermalconductivity and density approximately five times that ofair.
SF6 is chemically inert up to temperature of 150o
C andwill not react with metals, plastics, and other materialscommonly used in the construction of high voltage CBs.
The principle of operation is similar to the air blast
breakers, except that the SF6 gas is not discharged intothe atmosphere.
A closed compartment completely sealed and self-contained construction is used.
Why SF6?
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Why SF6?Sulfur Hexafluoride (SF6) is an excellent gaseousdielectric for high voltage power applications.
It has been used extensively in HV CBs and otherswitchgears employed by the power industry.Applications for SF6 include gas insulated transmissionlines and gas insulated power distributions.
SF6 combined electrical, physical, chemical and thermalproperties offer many advantages.Some of the properties of SF6 making it desirable to usein power applications are:
High dielectric strength.
Unique arc-quenching ability.
Excellent thermal stability.
Good thermal conductivity.
SF6 Gas Technical Data:
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SF6 Gas Technical Data: In short, SF6 at atmospheric pressure is a
heavier gas than air, it becomes liquid at (-63.2C)and in which noise propagates badly.
Appearance: Colorless, Odorless gas
Formula: SF6
Molecular Weight: 146.06 g/mol
Specific Volume: 2.5 ft/Lb.
(SF6 gas density is 5.11 times as heavy as air).
Boiling Point: -82 F Vapor Pressure: 325 psi
Melting point: -64C, Decomposes at 500C
Solubility in water: low.
SF6 Standard Specifications:
http://en.wikipedia.org/wiki/Image:Sulfur-hexafluoride-3D-balls.pnghttp://en.wikipedia.org/wiki/Image:Sulfur-hexafluoride-3D-balls.png -
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SF6 Standard Specifications:
Component ASTM 2472 D IEC 376
Sulfur Hexafluoride (by wt.) 99.8% 99.8%
Water (vol. %) 8 ppmv 15 ppmvDew Point -62C -40C
Hydrolysable Fluorides (HF) 0.3 ppmw 1.0 ppmw
Air (wt. %) 500 ppmw 500ppmw
Carbon Tetrafluoride (CF4) 500 ppmw 500
ppmw
SF6 CB Types:
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SF6 CB Types:
There are two major types for arcextinguish principals of SF6
Circuit Breakers:
1- Rotating Arc Principal
2- Puffer Principal.
Rotating arc SF6 CBs:
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Rotating Arc Principal M. Khaled
Rotating arc SF6 CBs:
SF6 CB Pole
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(Arc Revolving Principal)
M. Khaled
1 Main Connections.
2 Cylindrical Coil.
3 Load Current Contact.
4 Fixed arcing contact.
5 Moving contact.6 Breaking Chamber.
7 Pressure Equalizing Chamber.
8 Operating Lever.
9 Operating Shaft with rotary Seal.
10 Auxiliary Compression Piston.
11 Transmission Casing.
Circuit-breaker works
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The circuit-breaker is closedOpening of the main contactsSeparation of the arcing contacts
The Circuit-breaker Is Open
(a): On opening of the
main contacts.
(b): The current is shunted
into the breaking circuit.(c): The expansion volume.
(d): Arc rotates under the
effect of the magnetic field
created by the coil.(e): The tubular arcing
contact.
Circuit breaker works
under
Revolving Arc principle
SF6 CBs:
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SF6 CBs:The rotating arc interrupter is as inexpensive as oil or air
with reliability and performance of a vacuum interrupter.
SF6 gas decomposes during arcing process but very quicklyrecombines at current zero regaining its dielectric
properties.
Fault currents up to 20kA can be cleared at 12kV with a
50mm gap (approx) at a gas pressure of a 2.0 bar.
The arc current is transferred during contact separation
through a solenoid of a few turns of copper strip generating
a magnetic field causing the arc to rotate around the innerring of the solenoid.
The rapid movement of the arc through the SF6 gas causes
cooling and enables extinction at the first current zero at
high fault currents.
SF6 CB Pole
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(Puffer Principal)
M. Khaled
1 Terminal2 Insulating case
3 Blasting nozzle
4 Moving arcing contact5 Main moving contact
6 Fixed arcing contact
7 Main fixed contact
8 Insulating tie-rod
9 Anti-explosion valve
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Puffer Type SF6
CB Interrupter
Principle
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Step by Step(Off) Switching
Operation according
to(Puffer Principal)
Operating Mechanism:
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Operating Mechanism:
Operating Mechanism:
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Operating Mechanism:
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Step by Step(Off) Switching
Operation according
to(Puffer Principal)
For HV SF6 CB
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UHV Puffer-type SF6 CB Advantages of SF6 circuit breakers:
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Simple construction.
Less cost. SF6 gas is non flammable, non toxic and
chemical inert gas.
Same gas is re-circulated in the circuit. Maintenance free CB.
Ability to interrupt low and high fault
current.
Excellent arc extinction.
g
Advantages of SF6 over Oil CBs:
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Shorter arcing time.
Can interrupt much longer current. Gives noiseless operation due to its close
gas circuit.
No moisture problem. No risk of fire.
No carbon deposits, so no tracking and
insulation problems.
Low maintenance cost.
g
HV SF6 Circuit Breaker
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HV SF6 Puffer Circuit Breaker
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Vacuum Circuit Breaker
(VCB)
Vacuum CBs:
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Vacuum CB are used for MV and HV circuits.
A vacuum CB is a breaker in which a pair of contacts is
tightly sealed in a vacuum envelope.
The contacts are separated by using a bellows to move
one of them.
An arc is produced by metallic vapor boiled from theelectrodes, and is extinguished when the vapor particles
condense on solid surfaces.
Because the contacts are in a vacuum, arcing betweenthe contacts is suppressed, ensuring that the circuit
remains open.
As long as the circuit is open, it will not be energized.
Vacuum CBs:
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A vacuum CB comprises a vacuum vessel made up of an
insulating casing and end plates closing the opposite
ends and a pair of separable electrodes in the vacuumvessel.
Vacuum CBs include separable main contacts which are
disposed within an insulating housing. One of the contacts is fixed relative to the housing and
the other separable main contact is movable.
The moveable contact assembly usually comprises a
stem of circular cross-section having the contact at oneend enclosed within a vacuum chamber and a driving
mechanism at the other end which is external to the
vacuum chamber.
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Vacuum Vessel
(Interrupter)
Vacuum CBs:
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In general, an arc electrode is required to
satisfy basic characteristics such as: A large breaking capacity.
High withstand voltage.
Small contact resistance (high electricconductivity).
High fusion resistance.
Small wastage of a contact.
Small chopped current.
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One Pole of Vacuum CB
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VCB Operating Mechanism (Charged Springs)
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Operating Mechanism Unit with Stored-energy
Mechanism and Secondary Equipment
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Vacuum CB Module
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VCB Operating Mechanism (Actuator)
Magnetic actuator
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Actuator is a bi-stable Permanent Magnet. Switchover of the
armature is effected by magnetic field created by the coils.
The armature is held magnetically in limit position by the
field of permanent magnets.
Switching is released by excitation of a coil until the retaining
force of the permanent magnet is exceeded.
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Advantages of Vacuum circuit breakers:
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Compact, reliable and have longer life.
No fire hazards. No generation of gas during and after
operation.
Can interrupt any fault current.
No noise is produced while operating.
Require less power for control operation (lesspower for operating mechanism).
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Comments
andQuestions
Th k
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Thank you
for
Attention
Mohamed Khaled El Sayed