CIRCUIT BREAKERS...Factors Affecting the Arc Resistance Degree of ionization •Less number of...

CIRCUIT BREAKERS

PRKREDDY.GNITS

PRKREDDY.GNITS

MCB

PRKREDDY.GNITS

PRKREDDY.GNITS

What is Arc ?Ionization of air molecules cause interrupting current flow from one contact to other contact becomes so heated that it glows is called arc.

PRKREDDY.GNITS

• Arc phenomenon

• when a short circuit occurs, the

contacts of circuit breaker begin to

separate

• The p.d between the contacts is

sufficient to ionize the air or oil or gas

• This ionized air acts as conductor and

ARC is struck between them even

though contacts of breaker are

opened

PRKREDDY.GNITS

PRKREDDY.GNITS

Factors Affecting the Arc Resistance

Degree of ionization

• Less number of ionized particles between the contactsincreases the arc resistance.

Length of arc

• Arc resistance is a function of arc length. More the length ofarc, more will be the arc resistance.

Cross section of arc

• Arc resistance is large when the cross section of arc is less.

PRKREDDY.GNITS

High Resistance Method

Low Resistance Method

•ARC INTERRUPTION METHODS:

PRKREDDY.GNITS

High Resistance Interruption

The resistance of arc is increased by

1.Cooling of arc

2.Increasing the length of arc

3.Reducing the cross sectional area of arc

4.Spliting of arc

PRKREDDY.GNITS

•Recovery rate theory (Slepian’s theory)

•Energy balance theory (Cassie’s theory)

PRKREDDY.GNITS

Low Resistance Interruption

Recovery Rate Theory (Slepian’s Theory)• The arc is a column of ionized gas.

• To extinguish the arc, the electrons and ions are tobe removed from the gap at current zero.

• Ions and electrons can be removed by sweepingthem away by using gas or liquid in the gap.

• The arc is interrupted if the ions are removed fromthe gap at a faster rate than the rate of ionization.

• Slepian described the process as a race between thedielectric strength and restriking voltage.

PRKREDDY.GNITS

Recovery Rate Theory (Slepian’s Theory)• In this theory, the rate at which the gap recovers its dielectric strength iscompared with the rate at which restriking voltage across the contacts rises.

• If the dielectric strength increases more rapidly than the restriking voltage, thearc will be extinguished.

• If the restriking voltage increases more rapidly than the dielectric strength, theionization continues and arc will exist for another half cycle.

PRKREDDY.GNITS

Energy Balance Theory (Cassie’s Theory)

• Immediately after current zero, the contact space contains ionized gasand the space will provide some resistance (post zero resistance).

• A current flows through this resistance and causes a power dissipation.

• This power is zero when the initial restriking voltage is zero.

• This power again becomes zero after the complete de-ionization of thecontact space.

PRKREDDY.GNITS

Energy Balance Theory (Cassie’s Theory)• Between these two limits, the power dissipated will rise to maximum andcomes back to zero.

• Power dissipation results in production of heat and dissipation of heat.

• If the heat generated exceeds the rate at which heat can be dissipated, theionization will persist and an arc is formed again.

PRKREDDY.GNITS

Terms Associated with Circuit Breaking

Arc voltage

• The voltage across the contacts of C.B. when the arc persists is calledarc voltage. This voltage becomes system voltage when the arc isextinguished.

Restriking voltage

• After the arc is extinguished, the voltage across the breaker contactsdoes not become normal instantaneously. But a transient oscillation isproduced.

• The transient voltage that appears across the breaker contacts at theinstant of arc extinction is called restriking voltage.

PRKREDDY.GNITS

Terms Associated with Circuit Breaking

Recovery voltage

• The normal power frequency voltage that appears across the breakercontacts after the final arc extinction is called the recovery voltage.

PRKREDDY.GNITS

TYPES OF CIRCUIT BREAKERS

PRKREDDY.GNITS

Classification of Circuit Breakers• Based on Voltage

• Low voltage C.B (less than 1 kV)

• Medium voltage C.B (1 kV to 52 kV)

• High voltage C.B (66 kV – 220 kV)

• Extra high voltage C.B (300 kV – 765 kV)

• Ultra high voltage C.B (above 765 kV)

• Based on Medium used for Arc Quenching• Air-Break /Blast C.B

• Oil C.B

• Vacuum C.B

• Sulphur Hexafluoride C.B

PRKREDDY.GNITS

AIR BLAST/BREAK CIRCUIT BREAKERS

• suitable for high current interruption at low voltage.

• Compressed air 20-30 kg/cm^2 for Air blast type

• air gap between contacts=(-1.75cm)

• used up to 11 kV with air break

• Used up to 1100kV with Air blast

PRKREDDY.GNITS

• Types

• Cross blast C.B

• Axial blast C.B

• Radial blast C.B

PRKREDDY.GNITS

AIR BLAST/BREAK CIRCUIT BREAKERS

fig: cross flow ABCB

PRKREDDY.GNITS

Axial-blast typeCross-blast type

Radial-blast type

Disadvantages

1.The risk of fire is eliminated.2.Cheap compared to oil circuit breaker3.Arcing time is very less thereby less burning of contacts

Advantages

PRKREDDY.GNITS

1.Very sensitive to the variations in the rate of rise of restriking voltage.

2.Maintenance is required for compressor plant

Oil Circuit Breakers

• Mineral oil has better insulating properties than air.

• It has good cooling property.

• When arc is formed, it decomposes the oil into gases.

• Hence the arc energy is utilized in decomposing the oil.

• Less suitable for frequent operation.

• Interruption of capacitive and inductive currentscreates a problem.

• Hence resistance switching is necessary.

PRKREDDY.GNITS


• Types

• Bulk Oil C.B

• Minimum Oil C.B

PRKREDDY.GNITS

Bulk oil circuit breakers

• which use a large quantity of oil.

• The oil has to serve 2 purposes.

1. it extinguishes the arc

2. it insulated the current conducting parts

Cross sectional view of

bulk oil CB

PRKREDDY.GNITS


Minimum oil circuit breakers

• use minimum amount of oil, only for arc extinction

• conducting parts are insulated by air or porcelain or organic insulating material.

Cross sectional view of

bulck oil CBPRKREDDY.GNITS


MOCB – Operation

PRKREDDY.GNITS

Advantages:

1. It requires smaller space for installation.

2. Maintenance problems are reduced.

3. Requires less quantity of oil.

4. Risk of fire is reduced.

Disadvantages:

1. The degree of carbonization is increased due to less quantity of oil.

2. Difficulty of removing the gases from the contact space in time.

3. Possibility of explosion.

4. The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.

PRKREDDY.GNITS

Vacuum Circuit Breaker

• Vacuum of the order of 10-5 to 10-7 torr is used

• At this low pressure (high vacuum), the mean free path of electron is

large of the order of few metres.

• Hence the electrons cross the gap without any collision.

• It provides highest dielectric strength than any medium.

• space between the contacts recovers its dielectric strength 1000 times

faster than any other medium.

PRKREDDY.GNITS

Advantages

1. VCB are compact, reliable and have longer life .

2. They have low arc energy.

3. Risk of fire is eliminated.

4. Maintenance is less.

5. There is no generation of gas during and after operation.

6. They can withstand lightning surges.

Disadvantages

1. very expensive .

2. For above 36KV VCB `s are not used.

3. Require high technology for production.

4. In case of loss of vacuum, the entire breaker is useless .PRKREDDY.GNITS

Sulphur Hexafluoride (SF6) C.B

• SF6 an inert gas does not undergo any chemical

reactions and thermally stable

• good arc quenching and insulating properties, 100 times

more than air.

• Its density is 5 times that of air and heat convection is

1.6 times that of air.

• It is non–inflammable, no –poisonous and odourless.

• But by-products produced in SF6 during arcing is

dangerous for the operating personnel.

PRKREDDY.GNITS

Sulphur Hexafluoride (SF6) C.B – Operation

• In closed position, the contacts are surrounded by SF6 gas at a

pressure about 2.8 kg/cm2.

• When C.B operates, the contacts are open and arc is struck between

them.

• Valve between reservoir and arcing chamber opens up to allow SF6

gas at 14 kg/cm2 to the arc interruption chamber.

• High pressure SF6 gas absorbs the free electrons in the arc to

formimmobile negative ions which are ineffective.

• Medium between the contacts recovers dielectric strength and arc is

extinguished.PRKREDDY.GNITS

1) The interrupter unit

This consists of fixed contacts which comprise a set of current carrying fingers and an arcing probe. As the contacts move further apart, the arc is extended and attenuated. It is finally extinguished by the gas flow the high pressure to the low pressure systems.

2) The gas system.

The low and high pressure system are fitted with low pressure alarms and a set of lock-out switches which give a warning the moment the gas pressure drops below a certain value

PRKREDDY.GNITS

Sulphur Hexafluoride (SF6) C.B – Construction

PRKREDDY.GNITS

Advantages

1. SF6 circuit breakers have very short arcing time.

2. Dielectric strength of SF6 is 2 to 3 times than that of air , these breakers can interrupt larger currents

3. Gives noiseless operation due to its closed gas circuit .

4. Risk of fire is eliminated because SF6 gas is non-flammable.

Disadvantages

1. SF6 breakers are costly due to high cost of SF6 .

2. Sealing problems arise , Imperfect joints give rise to leakage of gases.

3. After every operation SF6 gas has to be reconditioned, so additional equipment is required.

PRKREDDY.GNITS

PRKREDDY.GNITS

Fuse and Types of Fuses

A fuse is an electric / electronic or mechanical device, which is used to protect circuits from over

current, overload and make sure the protection of the circuit.

Fuse Rating = ( Power / Voltage ) x 1.25

PRKREDDY.GNITS

Characteristics of a Fuse

Current Rating & Current Carrying Capacity of Fuse

Voltage Rating of Fuse

Breaking Capacity of a Fuse

I2t Value of Fuse

Response Characteristic

Rated voltage of Fuse

Packaging Size

METAL MELTING

POINT IN

°C

SPECIFIC

RESISTANE

IN µΩ-mm

VALUE OF FUSE

CONSTANT K

FOR d in m

SILVER 980 16 -

TIN 240 112 12.8

ZINC 419 60 -

LEAD 328 210 10.8

COPPER 1090 17 80

ALUMINIUM 665 28 59

PRKREDDY.GNITS

Classification of FUSES

PRKREDDY.GNITS

The size of DC Fuse is large than AC fuse

PRKREDDY.GNITS

Low voltage fuses:

Cartridge Fuses are available up to 600A and 600V ACwidely used in industries, commercial as well as home distribution panels.

PRKREDDY.GNITS

Low voltage fuses:

Link Type Fuse

The link type fuse is also known as high rupturing capacity (HRC) or BS type fuse

PRKREDDY.GNITS

Low voltage fuses:

PRKREDDY.GNITS

Rewirable Fuses

Low voltage fuses:

PRKREDDY.GNITS

Striker type Fuse

Low voltage fuses:

PRKREDDY.GNITS

Switch type Fuse

Low voltage fuses:

PRKREDDY.GNITS

Low voltage fuses:

Drop-out fuse

PRKREDDY.GNITS

These fuses are normally charged for voltages over 1500V to 138000V.

HV (High Voltage) Fuses

The fuse part in HV fuses are fabricated with either copper, silver or in some cases Tin is used, in order to offer consistent and steady performance.

PRKREDDY.GNITS

Cartridge Type HRC Fuse


PRKREDDY.GNITS

Liquid Type HRC Fuse


PRKREDDY.GNITS

Expulsion Type HRC Fuse


PRKREDDY.GNITS

Motors & Transformers

Air-conditions

Home distribution boards

General electrical appliances and devices

Laptops

Cell phones

Game systems

Printers

Digital cameras

DVD players

Portable Electronics

LCD monitors

Scanners

Battery packs

Hard disk drives

Power convertors

Uses and Applications of Fuses

Different types of Electrical and Electronic Fuses can be used in all types of electrical and electronic systems and applications including:

PRKREDDY.GNITS

CIRCUIT BREAKERS...Factors Affecting the Arc Resistance Degree of ionization •Less number of...

Documents

Transcript of CIRCUIT BREAKERS...Factors Affecting the Arc Resistance Degree of ionization •Less number of...