Circuit breaker

Circuit Breaker

as a

protection device

Main point will covered in this presentation

1- General2- circuit breaker

2.1- General2.2- Rate terms of circuit breaker2.3- operating principle2.4- the insulation fluids commonly used as insulation

medium during the separation of contact3- Arc phenomena

3.1- General3.2- initiation of an arc3.3- Maintenance of arc3.4- Arc voltage

4- Arc Extinction 4.1- General4.2- Method of Arc phenomena

4.2.1- High Resistance Method 4.2.2- LOW Resistance Method

5- Classification of circuit breaker

6- Main type of circuit breaker

7- scoop on some types with some details

8-Tesying of circuit breaker

1. General

In a power system switchgear serves two basic purposes:

1. Switching during normal operating condition for purpose of operation and maintenance.

2. Switching during abnormal condition such as short-circuits and interrupting the fault current.

A switch is used for opening and closing of an electric circuit. Every electric circuit needs a switching device and protective device. SwitchgearSwitchgear is a general term covering a wide rang of equipment concerned with switching and protection.

The first of above could be served by relatively simple switches because it is relatively simple as it involves

normal current which are easy to interrupt. The second function is however complex.

With the advancement of electrical power system the lines and other equipment operate at very high voltage and carry large current. Whenever a short-circuit occurs, a

heavy current flows through the equipment causing considerable damage to the equipment and interruption

the service.

In order to avoid such damage every part of the power system is provided with a protective relaying system and

an associated switching device. The relaying equipment is aided in this task by circuit breaker.

2. Circuit breaker2.1 General

The automatic circuit breakers perform the following duties:1. It carries the full-load current continuously without

overheating or damage.2. It open and close the circuit on no load.

3. It makes and breaks the normal operating current.4. It makes and breaks the short-circuit current of magnitude

up to which it designed for.

Circuit breaker are a mechanical device designed to close or open the contact members, thus closing or opining of an electrical circuit under normal or abnormal condition.

2.2 Rate terms of circuit breaker

• Max voltage• NO, of poles• Frequency• Maximum continuous current

carrying capacity• Max interrupting capacity• Max momentary

2.3 Operating principle

Whenever a fault occurs on any part of the power system, the trip coils of the

breaker get energized and the moving contacts are pulled apart by some

mechanism, thus opening the circuit. The separation of current carrying production

an arc. The current is thus able to continue until the discharge ceases.

A circuit breaker as a switching and current interrupting consist of fixed and moving contacts, which are touching each other and carry the current under normal condition. When the circuit breaker is closed, the current carrying contacts, called the electrodes, engage each other under the pressure of a spring.

Therefore the main problem in circuit breaker is to extinguish the arc within the shortest possible time so

that heats generated by it not reach a dangerous value.

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 breaker itself.

The basic construction of circuit breaker requires the separation of the contacts in an insulating fluid which serve two functions:

1. Extinguishes the arc drawn between the contacts when the circuit breaker opens.

2. Provides insulation between the contact and from each contact to earth.

The insulating fluids commonly used for this purpose are as follows:

1. Air and atmospheric pressure.2. Compressed air.3. Oil producing hydrogen for arc extinction.4. Ultra high vacuum5. Sulphur hexa-fluoride (SF6)

3. Arc phenomena

The heats generated from the arc ionize the air molecules. So

the +ve ions attracted to the –ve contact and the –ve ions

attracted to the +ve contact. Thus current flow is caused due

to movement of electrons.

3.1 Initiation of an arcInitiating electrons are thought of

produced by the following two processes:

1. By high voltage gradient at the cathode resulting into field emission

2. By increase of temperature resulting into thermal emission

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3.2 maintenance of arc

The ionization is further by:1. High temperature of the medium around the contacts

caused by high current densities.2. The field strength or voltage gradient which increases

the kinetic energy of moving electrons and increases the chances of detaching from neutral molecules.

3. An increase of mean free path-the distance through which the electron moves freely.

All above three processes (thermal emission – ionization – and field emission) may start either one after the other or almost simultaneously and enable the arc to be initiated and maintained.

3.3 Arc voltage

The voltage that appears across the contact of circuit breaker is called the arc voltage.For moderated values of current and voltage the arc characteristic can be expressed by Ayrton's equation

Ea = A + B/ia

Where:The constant A and B vary linearly With the arc length l

A=a+g l B= b+δ lAverage values of a,g,b,δ for arcs In air between copper electrodes are as following: a=30v ,g=10v/cm ,b=10VA ,δ=30VA/cm

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1.1.Arc can be used to generate musicArc can be used to generate music2.2.It can be used in drawingIt can be used in drawing

4. Arc extinction

The conductance of the arc is proportional to 1. The number of electrons per cubic centimeter

produced by ionization 2. The square of the diameter of the arc3. Reciprocal of the length

The arc extinction can, therefore, be facilitated by deionizing the arc path. This may be achieved by cooling the arc or bodily removing the ionized particles from the

space between the circuit breaker contacts.

4.1 Method of arc extinction

4.1.1 High Resistance Method

In this case the arc is controlled in such a way that its effective resistance increases with the time so that the current is reduced to such a value that heat produced by it is not

sufficient to maintain the arc and the current is interrupted or the arc is extinguished.

The resistance of the arc can be increased by:1. Cooling of arc2. Increasing the length of arc3. Reducing the cross-section of arc4. Splitting of arc

4.1.2 Low resistance OR Current zero interruption

This method is applicable only in ac circuit interruption because there is natural zero current 100 times in a second for 50Hz supply system.

This method of arc extinction is employed in all modern high power ac circuit.

In this method the arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from restriking after it has gone out at current zero.

5. Duties of circuit breakers

1. Interruption of small inductive current

2. Switching of unloaded transmission lines and unloaded

cables

3. Switching of capacitor banks

4. Interruption of terminal faults

5. Interruption of short-line faults (kilometric fault)

6. Asynchronous switching

6. Resistance switching

It is a deliberate connection of a resistance in parallel with the

contact space (or arc).

With the shunted by resistance R a part of arc current

is diverted through this resistance. This result in the

decrease of arc current and increase in the rate of

deionization of the arc path.

On occurrence of fault, the contacts

of the circuit breaker open and an

arc is struck between the contacts.

7. Classification of circuit breaker

1. Oil circuit breakers

2. Water type circuit breaker

3. Air-Break circuit breaker

4. Air blast circuit breaker

5. Sulphur Hexafluoride (SF6 ) circuit breaker

6. Vacuum circuit breaker

7.1 Oil circuit breakers

Bulk oil circuit breaker :

using a large quantity of oil, also

called the dead tank type

because the tank is held at

ground potential

Low oil circuit breaker: which

operate with a min amount of oil,

also sometimes called the live tank circuit breaker because

the oil tank is insulated from the ground.

Advantage:

1. Arc energy is absorbed in decomposing of oil.

2. The gas formed which mainly hydrogen is provided good

cooling properties.

3. The oil has a high dielectric insulating between the

contacts after the arc extinguished.

4. The very good insulation medium allows smaller clearance

between line conductors and earth components

Disadvantage

1. Oil is inflammable and may cause fire hazards.

2. There is a risk of formation of explosive mixture with air.

3. Due to decomposition of oil in the arc, the oil becomes

pouted by carbon particles, which reduce its dielectric

strength.

Maintenance of oil circuit breaker

1. All current carrying parts be checked and arcing

contacts be attended if necessary.

2. Dielectric strength condition and level of oil should be

checked.

3. Inspect the insulation for possible damage. Clean the

surface and remove deposits of carbon

4. Check closing, tripping and interlock mechanism

5. Check indicating devices and lamps

Automatic resisting soled state type (PTC)

A polymer PTC (for positive temperature coefficient) is a special type of circuit breaker called thermistor ( thermal resistor). PTC resistor increase resistance as its temperature in increase, PTC’s are made of conductive polymer , are solid state device , which means they have no moving parts.

Polymer PTC construction and operation

In its normal state the material in the polymer PTC is in the form of a dense crystal with many carbon particles packet together. The carbon particles provide conductive pathways for current flow. This resistance is low. When the material is heated from excessive current the polymer expands, pulling the carbon chains apart, in this expanded ” tripped ” state there are few pathways for current. When current floe exceeds the trip threshold, the device remain in the “open circuit” state as long as voltage is remain applied to the circuit. It reset only when the voltage is removed and the polymer is cools.