3 Circuit Breakers

7/28/2019 3 Circuit Breakers

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MV & HV

Circuit Breakers

Eng.: Mohamed Khaled El Sayed

[email protected]


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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


22/99Indoor Withdrawable Bulk Oil CB


23/99Three Phase Bulk Oil CBs


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Separate PhasesBulk Oil Dead Tank

CBs


25/99Dead Tank & Live 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.


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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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



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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.



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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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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

3 Circuit Breakers

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