vaccum circuit breaker

SCITS, KARIMNAGAR 1 DEPARTMENT OF EEE

CHAPTER I

INTRODUCTION

1.1 INTRODUCTION

Our Power system is a large interconnected system, which consisting of

Generators, transformers, transmission and distribution circuits, it is inevitable that

sooner or later some failure will occur somewhere in the system. When a failure

occurs on any part of the system, it must be quickly detected and disconnected from

the system.

There are Principal reasons for it:

Firstly, if the fault is not cleared quickly, it may cause unnecessary

interruption of service to the customers.

Secondly, rapid disconnection of faulted apparatus limits the amount of

damage to it and prevents the effects of the fault from spreading into the

system.

A Protective relay is a device that detects the fault and initiates the

operation of the circuit breaker to isolate the defective element from the

rest of the system.

Hence the Role of the circuit breaker play a major role in the power system

interconnected system

1.2 CIRCUIT BREAKER

A circuit breaker is an automatically operated electrical switch designed to

protect an electrical circuit from damage caused by overload or short circuit. Its

basic function is to detect a fault condition and interrupt current flow. Unlike a fuse,

which operates once and then must be replaced, a circuit breaker can be reset (either

manually or automatically) to resume normal operation. Circuit breakers are made in

varying sizes, from small devices that protect an individual household appliance up

to large switchgear designed to protect high voltage circuits feeding an entire city.


The modern power system deals with huge power network and huge

numbers of associated electrical equipment. During short circuit fault or any other

types of electrical fault these equipment as well as the power network suffer a high

stress of fault current in then which may damage the equipment and networks

permanently. For saving these equipment and the power networks the fault current

should be cleared from the system as quickly as possible. Again after the fault is

cleared, the system must come to its normal working condition as soon as possible

for supplying reliable quality of power to receiving ends. In addition to that for

proper controlling of power system, different switching operations are required to be

performed. So for timely disconnecting and reconnecting different parts of power

system network for protection and control, there must be some special type of

switching devices which can be operated safely under huge current carrying

condition, During interruption of huge current, there would be large arcing in

between switching contacts, so care should be taken to quench these arcs in circuit

breaker in safe manner .The circuit breaker is the special device which does all the

required switching operations during current carrying condition. This was the basic

Introduction to Circuit Breaker.

Circuit breakers are a piece of electrical device that

1. Make or break a circuit either manually or by remote control under

normal conditions.

2. Break a circuit automatically under fault conditions.

3. Make a circuit either manually or by remote control under fault

conditions.


CHAPTER II

CIRCUIT BREAKERS

2.1 INTRODUCTION

Circuit Breaker is a critical component of an electrical system they are

used to connect and disconnect transmission lines under normal operating conditions.

They are also used to a clear section of transmission grid should a short circuit occur

in the system, isolating the fault. The technology of circuit breaker evolved based on

primarily on the media in which the circuit breaker contacts are located. Every circuit

breaker used a dielectric media. When system voltages and current levels increased

oil circuit breakers are introduced later compressed air circuit breakers are developed

followed by SF6 and working breakers which is preferred technology for high voltage

circuit breaker.

The main purpose of a circuit breaker is to:

Switch load currents

Make on to a fault

Break normal and fault currents

Carry fault currents without blowing itself open.

The important characteristics from a protection point of view are:

The speed with which the main current is opened after a tripping impulse is

received.

The capacity of the circuit that the main contacts are capable of interrupting

(rupturing capacity).

The first characteristics are referred to as the ‘tripping time’ and are expressed

in cycles.

Modern high speed circuit breakers having tripping times between 3 to 8

cycles.


The Tripping or total clearing or breaking time is made up as follows:

Opening time: the time between instant of application of tripping power to the

instant of separation of the main contacts.

Arcing time: the time between the instant of separation of the main circuit breaker

contacts to the instant of arc extinction of short circuit current.

Total break or clearing time: The sum of the above (opening time + Arcing

time).

Some of the manufacturers are ABB, Alstom, General Electric, Hitachi, HYOSUNG

(HICO), Hyundai Heavy Industry (HHI), Mitsubishi Electric, Pennsylvania Breaker,

Siemens, Toshiba, Končar HVS, BHEL, CGL, and Becker/SMC (SMC Electrical

Products).

Fig 2.1 Fault Clearing Time


2.1.1 ORIGIN

An early form of circuit breaker was described by Thomas Edison in an

1879 patent application, although his commercial power distribution system used

fuses. Its purpose was to protect lighting circuit wiring from accidental short-circuits

and overloads. A modern miniature circuit breaker similar to the ones now in use was

patented by Brown, Boveri & Cie in 1924. Hugo Stotz, an engineer who had sold his

company, to BBC, was credited as the inventor on DRP (Deutsches Reichspatent).

Stotz's invention was the forerunner of the modern thermal-magnetic breaker

commonly used in household load centers to this day.

Interconnection of multiple generator sources into an electrical grid

required development of circuit breakers with increasing voltage ratings and

increased ability to safely interrupt the increasing short circuit currents produced by

networks. Simple air-break manual switches produced hazardous arcs when

interrupting high currents; these gave way to oil-enclosed contacts, and various forms

using directed flow of pressurized air, or of pressurized oil, to cool and interrupt the

arc. By 1935, the specially constructed circuit breakers used at the Boulder Dam

project use eight series breaks and pressurized oil flow to interrupt faults of up to

2,500 MVA, in three cycles of the AC power frequency.

2.1.2 SHORT CIRUIT CURRENT

Circuit breakers are rated both by the normal current that they are

expected to carry, and the maximum short-circuit current that they can safely interrupt.

Under short-circuit conditions; the calculated maximum prospective short

circuit current may be many times the normal, rated current of the circuit. When

electrical contacts open to interrupt a large current, there is a tendency for an arc to form

between the opened contacts, which would allow the current to continue. This condition

can create conductive ionized gases and molten or vaporized metal, which can cause

further continuation of the arc, or creation of additional short circuits, potentially

resulting in the explosion of the circuit breaker and the equipment that it is installed in.


Therefore, circuit breakers must incorporate various features to divide and extinguish the

arc.

In air-insulated and miniature breakers an arc chutes structure consisting (often) of

metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc

into the arc chute. Larger circuit breakers such as those used in electrical power

distribution may use vacuum, an inert gas such as sulfur hexafluoride or have contacts

immersed in oil to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by

testing. Application of a breaker in a circuit with a prospective short-circuit current

higher than the breaker's interrupting capacity rating may result in failure of the

breaker to safely interrupt a fault. In a worst-case scenario the breaker may

successfully interrupt the fault, only to explode when reset.

MCB used to protect control circuits or small appliances may not have sufficient

interrupting capacity to use at a panel board; these circuit breakers are called

"supplemental circuit protectors" to distinguish them from distribution-type circuit

breakers.

2.1.3 ARC INTERRUPTION

Low-voltage MCB uses air alone to extinguish the arc. Larger ratings have metal

plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils or

permanent magnets deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to

blast a jet of oil through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using

a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride

(SF6) to quench the stretched arc.


Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other

than the contact material), so the arc quenches when it is stretched a very small

amount (less than 2–3 mm (0.079–0.118 in)). Vacuum circuit breakers are frequently

used in modern medium-voltage switchgear to 38,000 volts.

Air circuit breakers may use compressed air to blow out the arc, or alternatively,

the contacts are rapidly swung into a small sealed chamber, the escaping of the

displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically

the arc is extinguished between 30 ms and 150 ms after the mechanism has been

tripped, depending upon age and construction of the device.

2.1.4 STANDARD CURRENT RATINGS

Circuit breakers are manufactured in standard sizes, using a system of preferred

numbers to cover a range of ratings. Miniature circuit breakers have a fixed trip

setting; changing the operating current value requires changing the whole circuit

breaker. Larger circuit breakers can have adjustable trip settings, allowing

standardized elements to be applied but with a setting intended to improve protection.

For example, a circuit breaker with a 400 ampere "frame size" might have its over

current detection set to operate at only 300 amperes, to protect a feeder cable.

International Standard--- IEC 60898-1 and European Standard EN 60898-1

define the rated current I of a circuit breaker for low voltage distribution applications

as the maximum current that the breaker is designed to carry continuously (at an

ambient air temperature of 30 °C). The commonly-available preferred values for the

rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A,

100 A and 125 A.


2.2 TYPES OF CIRCUIT BREAKERS

Many different classifications of circuit breakers can be made, based on

their features such as voltage class, construction type, interrupting type, and structural

features, the types of breakers basically refer to the medium in which the breaker

opens and closes. The medium could be

OIL

AIR BLAST

SF6

VACUUM

Fig 2.2 Types of circuit Brekers


1. Arc control Device

Fig 2.2.1 Arc chamber

A breaker consists of moving and fixed contact, and during the breaker

operation, the contacts are broken and the arc created during such separation needs to

be controlled. The arc control devices, otherwise known as turbulator or explosion

plot achieves this:

1. Turbulence caused by Arc bubble.

2. Magnetic forces tend to force main contacts a art and movements causes

oil to be sucked in through ports and squirted past gap.

3. When arc extinguished (at current zero), ionized gases get swept away and

prevents re-striking of the arc.

2.2.1 OIL CIRCUIT BREAKERS

Oil Circuit breakers are high voltage circuit breakers that are

usually operated mechanically, using a powerful spring press but use oil to insulate

and therefore, minimize the foot print occupied by circuit breaker. As the medium for

extinguish dark created when the circuit breaker trips under fault condition.

Oil Circuit breakers are used in voltage range in between 11Kv to 132Kv.


1. In modern installations, oil circuit breakers, which are becoming obsolete, are

being replaced by vaccum and SF6 breakers.

2. The main contacts are immersed in oil and the oil acts as the ionizing medium

between the contacts. The oil is mineral type, with high dielectric strength to

withstand the voltage across the contacts under normal conditions.

3. Arc energy decomposes oil into 70% hydrogen, 22% acetylene, 5% methane and

3% ethylene. Arc is in a bubble of gas surrounded by oil.

Fig 2.3 Oil Circuit breaker


ADVANTAGES:

1. Ability of cool oil to flow into the space after current zero and arc goes out

2. Cooling surface presented by oil

3. Absorption of energy by decomposition of oil.

4. Action of oil as an insulator lending to more compact design of switchgear.

DISADVANTAGES:

1. Inflammability (Especially if there is any air near hydrogen)

2. Maintenance (changing and purifying).

3. Not suitable for high current interruption at low voltages due to carbonization of

oil.

4. The whole breaker unit is immersed in the oil.

2.2.2 TYPES OF OIL CIRCUIT BREAKERS

1) Bulk oil circuit breakers

a) Plain break oil circuit breaker

b) Arc control oil circuit breakers

i) Self-blast oil circuit breaker

(1) Plain explosion pot.

(2) Cross jet explosion pot

(3) Self-compensated explosion pot

ii) Forced-blast oil circuit breaker

2) Low oil circuit breakers


2.2.3 PLAIN BREAK OIL CIRCUIT BREAKER

The plain-break oil circuit breaker is the earliest type from which all other circuit

breakers have developed. It has a very simple construction. It consist of fixed and

moving contacts enclosed in a strong weather tight earthed tank containing oil up to

certain level and air cushion above the oil level . The air cushion provides sufficient

room to allow for the circuit breaker.

WORKING PRINCIPLE

1. The hydrogen gas bubble generated around the arc cools the arc column and aids

the deionization of the medium between the contacts.

2. The gas sets up turbulence in the coil and helps in eliminating the arcing products

from the arc path.

3. As the arc lengthens due to the separating contacts, the dielectric strength of the

medium is increased.

Fig 2.4 Plain Oil C.B

ADVANTAGES

1. For successful interruption long arc length is necessary and it is formed here.


DISADVANTAGES

1. There is no special control over the arc other than the increase in length by

separating the moving contacts.

2. These breakers do not permit high speed interruption.

2.2.4 SELF-BLAST OIL CIRCUIT BREAKER

In this type of circuit breaker, the gases produced during arcing are confined to a

small volume by the use of an insulating rigid pressure chamber or pot surrounding the

contacts.

a) Plain explosion pot:

It is a rigid cylinder of insulating material and encloses the fixed and moving

contacts. The moving contact is a cylindrical rod passing through a restricted opening (Called

throat) at the bottom. When a fault occurs, the contacts get separated and an arc is struck

between them. The principal limitation of this type of pot is that it cannot be used for very lower

for very high fault currents.

b) Cross jet explosion pot:

This type of pot is just a modification of plain explosion pot. It is made of

insulating material and has channels on one side which act as arc splitters. The arc

splitters help in increasing the arc length, thus facilitating are extinction.

When a fault occurs, the moving contact of the circuit breaker begins to

separate .as the moving contact is withdrawn, the arc is initially struck h in the stop of

the pot. The gas generated by the arc exerts pressure on the oil in the back passage.

When the moving contact uncovers the arc splitter ducts, fresh oil so forced across the

arc path. The arc is therefore driven sideways into the arc splitters which increase the

arc length, causing arc extinction. The cross jet explosion pot is quite efficient for

interrupting heavy fault currents. However, for low fault currents, the gas pressure is

small and consequently the pot does not give a satisfactory operation.


c) Self-compensated explosion pot:

This type of pot is essentially a combination of plain explosion pot and

cross jet explosion pot.therefore; it can interrupt low as well as heavy short circuit

currents with reasonable accuracy.

Fig 2.5 Self compensated

2.2.5 FORCED –BLAST OIL CIRCUIT BREAKER

In a forced –blast oil circuit breaker, oil pressure is created by the piston-

cylinder arrangement. The movement of the piston is mechanically coupled to the

moving contact .when a fault occurs, the contacts get separated by the protective

system and an arc is struck between the contacts .the piston forces a jet of oil towards

the contact gap to extinguish the arc. It may be noted that necessary oil pressure

produced does not in any way depend upon the fault current to be broken.


ADVANTAGES

1. The quantity of oil required is reduced considerably.

2. The performance at low currents is more consistent than with self-blast oil circuit

breakers.

APPLICATIONS OF BULK OIL CIRCUIT BREAKER

1. Used up to to12Kv,500MVA

MERITS

1. Simplicity in construction.

2. High rupturing capacity

3. Suitability for automatic as well as manual operation.

4. Possibility of locating CT’s in bushings.

DEMERITS

1. Large size and greater weight.

2. Unsuitable for indoor installation.

3. Greater wear and tear of the contacts resulting in their frequent replacement.

4. Fire hazard.

2.2.6 LOW OIL CIRCUIT BREAKERS

1. It is found only a small percentage of oil is actually used for arc extinction while

major part is utilized for insulation purposes.

2. For this reason, the quantity of oil in bulk oil reaches a very high figure as the

system voltage increases.

3. This not only increase the expenses, tank size and weight of the breaker but it also

increases the fire risk and maintenance problems.


Fig 2.6 Minimum Oil Circuit breaker


WORKING

In a minimum oil circuit breaker, the arc drawn across the current carrying

conductor is contained inside the arcing chamber; hence the hydrogen bubble

formed by the vaporized oil is trapped inside the chamber. As the contacts

continue to move, after its certain travel an exit vent becomes available for

exhausting the trapped hydrogen gas. There are two different types of arcing

chamber is available in terms of venting are provided in the arcing chambers. One

is axial venting and other is radial venting. In axial venting gases (mostly

hydrogen), produced due to vaporization of oil and decomposition during arc, will

sweep the arc in axial or longitudinal direction.

Fig 2.7 MOCB working

Fig 2.8 MOCB Arcing chamber


Fig 2.9 MOCB Arc Formation

Fig 2.10 MOCB Arc Extinction

MERITS

1. It requires lesser quantity of oil & it requires smaller space.

2. There is reduced risk of fire

3. Maintenance problems are reduced.

DEMERITS

1. Due to smaller quantity of oil, the degree of carbonization is increased.

2. There is a difficulty of removing the gases from the contact space in time.

3. The dielectric strength of the oil deteriotes rapidly due to high degree of

carbonization


2.3 AIR BLAST CIRCUIT BREAKER

These breakers employ a high pressure air blast as an arc quenching medium

Arc is chopped into a number of small arcs by the Arc-Shute as it rises due to heat

and magnetic forces.

The contacts are opened in a flow of air blast

The air circuit breakers are normally employed for 380-480V distribution.

Suitable for high current interruption at low voltages.

2.3.1 PRINCIPAL OF OPERATION

Fig 2.11 Air blast C.B Principle of operation


2.3.2 TYPES OF AIR BLAST CIRCUIT BREAKER

1. Axial-blast CB

2. Cross-blast CB

3. Radial-blast CB

Fig 2.12 Types of Air Blast C.B

ADVANTAGES

The risk of fire is eliminated.

The arcing products are completely removed by the blast.

The arcing time is very small due to the rapid buildup of dielectric

strength between contacts.

DISADVANTAGES

Air has relatively inferior arc extinguishing properties.

Air blast circuit breakers are very sensitive to the variations in the rate of

restriking voltage.

Considerable maintenance is required for the compressor plant which

supplies the air blast.

APPLICATIONS

The air blast circuit breakers are finding wide applications in high voltage

installations. Majority of the circuit breakers for voltages beyond 110 kV are of this type.


2.4 SULPHUR HEXAFLOURIDE GAS (SF6)

A circuit breaker in which the current carrying contacts operate in sulphur

hexafluoride or SF6 gas is known as an SF6 circuit breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That means

it has high affinity of absorbing free electron. Whenever a free electron collides with

the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

The attachment of electron with SF6 gas molecules may occur in two different

ways,

These negative ions obviously much heavier than a free electron and therefore

over all mobility of the charged particle in the SF6 gas is much less as compared other

common gases. We know that mobility of charged particle is majorly responsible for

conducting current through a gas.

Fig 2.13 SF6 C.B


2.4.1 WORKING OF SF6

The working of SF6 CB of first generation was quite simple it is some extent

similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high

pressure reservoir. During operation of SF6 circuit breaker this highly compressed

gas is released through the arc in breaker and collected to relatively low pressure

reservoir and then it pumped back to the high pressure reservoir for re utilize.

The working of SF6 circuit breaker is little bit different in modern time.

Innovation of puffer type design makes operation of SF6 CB much easier. In buffer

type design, the arc energy is utilized to develop pressure in the arcing chamber for

arc quenching.

Fig 2.14 SF6 CB Working


Fig 2.15 SF6 CB Operation

Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very

high dielectric strength. Not only the gas has a good dielectric strength but also it has the

unique property of fast recombination after the source energizing the spark is removed.

The gas has also very good heat transfer property. Due to its low gaseous viscosity

(because of less molecular mobility) SF6 gas can efficiently transfer heat by convection.

So due to its high dielectric strength and high cooling effect SF6 gas is approximately 100

times more effective arc quenching media than air. Due to these unique properties of this

gas SF6 circuit breaker is used in complete range of medium voltage and high voltage

electrical power system. These circuit breakers are available for the voltage ranges from

33KV to 800KV and even more.


Types of SF6 Circuit Breaker

There are mainly three types of SF6 CB depending upon the voltage level of

application-

1. Single interrupter SF6 CB applied for up to 245 KV (220 KV) system.

2. Two interrupter SF6 CB applied for up to 420 KV (400 KV) system.

3. Four interrupter SF6 CB applied for up to 800 KV (715 KV) system.

ADVANTAGES

Excellent insulating, arc extinguishing, physical and chemical properties

of SF6 gas is greater advantage of SF6 circuit breakers

Electrical clearances are very much reduced because of high dielectric

strength of SF6

Its performance is not affected due to variation in atmospheric conditions

It gives noiseless operation it does not make sound like air-blast circuit

breaker during operation

Same gas is re-circulated into the circuit thereby reducing the requirement

of SF6 gas.

No over voltage problem. The arc is extinguished at natural current zero

without the current chopping and associated over-voltages originating

across the circuit breaker terminals

DISADVANTAGES

Imperfect joints leading to leakage of the SF6 gas. Continuous monitoring

devices are required

Arced SF6 gas is poisonous and should not be inhaled

The internal parts need thorough cleaning during periodic maintenance

under clean and dry environment. Dust of Teflon and sulphides should be

removed

APPLICATIONS

SF6 Circuit breakers are mostly employed for High voltage applications


2.5 TESTING OF CIRCUIT BREAKERS

Why test circuit breakers?

Some of the most important of the many reasons for testing circuit breakers

are to ensure they are:

1. Provide protection for expensive equipment.

2. Prevent outages that lead to loss of income.

3. Ensure reliability of the electricity supply.

4. Prevent downtime and darkness.

5. Verify breaker performance.

Substation breaker testing is an important task for any power utility .The

breakers are there to facilitate the flow of current during normal operation and to

interrupt current flow in the event of a fault.however,all electricity operated

devices are, sooner or later, likely to experience some kind of failure. That failure

can be caused by many facors, including ageing and external faults. The power

utility operator has to be prepared and have a plan in place to handle every

situation.

Testing of circuit breakers is more difficult than other electrical equipment

like transformer or machine because the short circuit current are very large .Also

there is no satisfactory method of testing circuit breaker at reduced power.

The testing plan of circuit breaker consists of essentially of a specially

designed alternator capable of giving about 2000MVA under short circuit

condition the prime mover of the alternator is 750KV motor end is disconnected

just before the short circuiting is done. The kinetic energy of rotor is sufficient to

give the desired energy for testing purpose.

Testing of circuit breaker can be classified in to two main groups


TYPE TEST

ROUTINE TEST

TYPE TEST

These test are conducted on first few proto type circuit breakers of each

type for the purpose of providing the capabilities and confirming the rated

characteristics of the circuit breaker of that design. Such test is conducted in

specially built testing laboratories.

Type test are broadly classified as

Mechanical Performance test.

Thermal test.

Insulation test

Short circuit test

MECHANICAL TEST

These are mechanical endurance type tests involving repeated opening and

closing of the circuit breaker. A circuit breaker must open and close at the correct

speed and perform its designated duty and operation without mechanical failure.

THERMAL TEST

Thermal test are carried out to check the thermal behavior of the circuit

breakers. The breaker under test is subjected to study state temperature rise due to

flow of its rated current through its poles in closed condition. The temperature rise

for rated current should not exceed 40oC for current less than 800A normal

current and 50oC for normal value of current 800A and above.

In such sets the contact drops or contact resistances are also measured as

these contacts surfaces are responsible for generation of heat and subsequent

temperature rise.


DIELECTRIC TESTS

These tests are performed to check power frequency and impulse voltage

withstand capacity.

Power frequency tests are conducted on clean new circuit breaker, the test

voltage varies with circuit breaker rated voltage. The test voltage with a frequency

between 15 to 100HZ is applied as follows.

Between poles with circuit breaker closed

Between poles and earth with circuit breaker open.

Across terminals with circuit breaker open

The voltage is gradually increased and maintained at test value for one

minute. In impulse voltage of specified shape and magnitude applied to the

breaker. For outdoor circuit dry and wet test are conducted.

SHORT CIRCUIT TESTS

Circuit breakers are subjected to sudden short circuit in the short circuit

test laboratories and oscillograms are taken to know the behavior of circuit

breaker at the time of switching in, during contact breaking and after arc

extinction. The oscillograms are studied with particular reference to the making

and braking currents, both symmetrical and asymmetrical.

ROUTINE TEST

Once type tests are conducted and a particular design is found to be

satisfactory the product becomes prototype and a large number of circuit breaker

of similar design manufactured. However, each and every circuit breaker still

subjected to a few more tests before commissioning. These tests are called routine

test.


2.6 APPLICATIONS OF CIRCUIT BREAKER

Depending on its application in the network the CB’s Service life differs.

For instance, line circuit breakers operate seldom and have a longer service life

than.

Generator CB

High Voltage CB

Capacitor CB

Reactor CB

High voltage DC CB

Distribution CB

Traction CB

Industrial CB


CHAPTER III

VACCUM CCIRCUIT BREAKER

3.1 INTRODUCTION

A vacuum circuit breaker is such kind of circuit breaker where the arc

quenching takes place in vacuum. The technology is suitable for mainly medium

voltage application. For higher voltage vacuum technology has been developed

but not commercially viable. The operation of opening and closing of current

carrying contacts and associated arc interruption take place in a vacuum chamber

in the breaker which is called vacuum interrupter. The vacuum interrupter consists

of a steel arc chamber in the centre symmetrically arranged ceramic insulators.

The material used for current carrying contacts plays an important role in

the performance of the vacuum circuit breaker. CuCr is the most ideal material to

make VCB contacts. Vacuum interrupter technology was first introduced in the

year of 1960. But still it is a developing technology. As time goes on, the size of

the vacuum interrupter is being reducing from its early 1960’s size due to

different technical developments in this field of engineering. The contact

geometry is also improving with time, from butt contact of early days it gradually

changes to spiral shape, cup shape and axial magnetic field contact. The vacuum

circuit breaker is today recognized as most reliable current interruption

technology for medium voltage switchgear. It requires minimum maintenance

compared to other circuit breaker technologies

3.2 PRINCIPLE

Two contacts called electrode remains closed under normal operating

conditions. When fault occurs on any part of the system, the trip coil of the

Vaccum circuit breaker gets energized and contacts are separated by the vaccum

pressure which is having around (10-7 to 10-5 torr). The arc is quickly


extinguished because the metallic vapors, electrons, and ions produced during arc

condense quickly on the surfaces of the circuit breaker

3.3 OPERATION

The main aim of any circuit breaker is to quench arc during current zero

crossing, by establishing high dielectric strength in between the contacts so that

reestablishment of arc after current zero becomes impossible. The dielectric

strength of vacuum is eight times greater than that of air and four times greater

than that of SF6 gas. This high dielectric strength makes it possible to quench a

vacuum arc within very small contact gap. For short contact gap, low contact

mass and no compression of medium the drive energy required in vacuum circuit

breaker is minimum. When two face to face contact areas are just being separated

to each other, they do not be separated instantly, contact area on the contact face

is being reduced and ultimately comes to a point and then they are finally de-

touched. Although this happens in a fraction of micro second but it is the fact. At

this instant of de-touching of contacts in a vacuum, the current through the

contacts concentrated on that last contact point on the contact surface and makes a

hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due

to that hot spot and create a conducting media for arc path. Then the arc will be

initiated and continued until the next current zero.

3.4 CONSTRUCTION

A schematic diagram of the vaccum C.B is shown in below fig .it is a very

simple device as compared to an air or oil C.B. The outer envelope is normally

made of glass due to the ease of joining it to the metallic end caps and also

because the glass envelope makes it easy to examine from outside state of the

contacts after the breaker has interrupted the current. This is important since a

change from a silvery mirror like finish to a milky white color shows that the

baffle is losing its vacuum. A sputter shield is provided in between the contacts

and the envelope in order to prevent the metal vapour reaching the envelope as it


reduces the breakdown strength between the contacts. This is generally made of

stainless steel. Inside the sputter shield the breaker has two contacts, one fixed

and the other moving contact which moves through a short distance of 5 to 10 mm

depending upon the operating voltage. The metallic bellows made of stainless

steel is used to move the lower contact. The design of the bellows is very

important as the life of vacuum breaker depends upon the ability of this part to

perform repeated operations satisfactorily.

Fig 3.1 Vaccum Circuit Breaker


The periphery of the end cap is sealed to the envelope and the fixed contact

stem is an integral part of one end cap .one end of the fixed as well as moving

contact is brought out of the chamber for external connections. The lower end of

the circuit breaker is fixed to a spring operated or solenoid operated mechanism

so that the metallic bellows inside the chamber are moved downward and upward

during opening and closing operation respectively. It is to be noted that the

operating mechanism should provide sufficient pressure for a good concentration

between the contacts and should avoid any bouncing action

Fig 3.2 Front view

Fig 3.3 Rear view


Fig 3.4 Cross section view of VCB

3.5 THE VACUUM ARC

The vacuum arc results from the neutral atoms, ions and electrons emitted

from the electrodes themselves. As the current carrying contacts are separated,

cathode spots are formed depending upon the current to be interrupted. For low

currents a highly mobile cathode spot is formed and for large currents a multiple

number of cathode spots are formed. These spots constitute the main source of

vapour in the arc. The processes involved in drawing the arc will be due to high

electric field between the contacts or resistive heating produced at the point of

operation or a combination of the two. The cathode surfaces normally are not

perfectly smooth but have many micro projections. When the contacts are

separating, the current flowing in the circuit will be concentrated in these

projections as they form the last point of contact. Due to their small area of cross

section, the projections will suffer explosive evaporation by resistive hating and

supply sufficient quantity of vapour for the arc formation. Since in case of

vaccum breakers the emission occurs only at the cathode spots and not from the

entire surface of the cathode, the vaccum arc is also known as cold cathode arc. In

cold cathode the emission of electrons could be due to any of the combinations of

the following mechanisms:

Field emission

Thermionic emission


Field and thermionic emission

Secondary emission by positive ion bombardment

Secondary emission by photons

Pinch effect.

3.6 VACUUM ARC STABILITY

In a.c. circuit the current passes through zero value 100 times in a second.

It is desirable to interrupt the current chopping .Therefore, it is necessary for

successful arc interruption that it be stable for half cycle duration and particularly

it should continue to exist when the current approaches zero. The stability of arc

in vacuum depends upon

The contact material and its vapour pressure.

Circuit parameters such as voltage,current,inductance and capacitance

3.7 VACUUM ARC-RECOVERY PHENOMENON

When the arc interruption is over, the space between the surrounding the

electrodes is filled with vapour and plasma. The presence of this residue affects

very much the ability of an interrupter to withstand high voltages. the process by

which this residue decays and by which the vaccum gap regains its dielectric

strength is known as arc recovery phenomenon at current zero the cathode spot

extinguishes within in 10-8 second and after this the original dielectric strength is

established very soon this quick build up of dielectric strength is due to the

condensing, quick diffusion and of metal vapour to the glass walls in absence of

gas molecules. After the arc is interrupted, the recovery strength during the first

few micro seconds is 1kV/µsec for an arc current of 100A, as compared with

50V/µsec in case of air gap.


3.8 CURRENT CHOPPING PHENOMENON

Post-arc current phenomena that occur when interrupting high currents

with vacuum circuit breakers have been investigated. High resolution measuring

equipment has been used to measure both the post-arc current and the arc voltage

in the current-zero regions. Three examples of frequently observed phenomena

are described. The first describes the phenomenon that in the event of a current-

chopping, the current is zero for a short period of time just before the natural

alternating current zero, but continues to flow afterwards, in the form of a post-arc

current. The second and third example deal with the post-arc phenomena after

currents those are much higher than the test breakers rated short-circuit current.

These examples show a low-voltage period after current-zero. Apparently, during

this post-arc period, the residual plasma between the breaker's contacts conduct

well. In addition to the voltage-zero periods, the voltage trace in the third example

also shows evidence of current-chopping. This means that the plasma conducts

poorly just before current-zero, but conducts well immediately afterwards.

Fig 3.5 Current chopping phenomenon


3.9 SPECIFICATIONS

It is designed for medium voltage range (3.3-33kv).

This consists of vacuum of pressure (10-7 to 10-5) inside arc extinction

chamber.

Vacuum is used as an arc quenching medium

At high voltage, its rate of dielectric strength recovery is very high.

Have greatest insulating strength

ADVANTAGES

Free from arc and fire hazards.

Low cost for maintenance & simpler mechanism.

Low arcing time & arc extinction is very fast.

Silent and less vibrational operation.

Can interrupt any fault current.

It has higher dielectric strength.

Requires small amount of power for operation

APPLICATIONS

For outdoor applications ranging from 11 KV to 33 KV.

Suitable for majority of applications in rural area.

They can be used where the switching frequency is high.

They can be used along with static over current relays.

This gives a fast RRRV and vacuum circuit breakers are the best solutions.


CHAPTER IV

CONCLUSION

A vacuum circuit breaker (VCB) has demonstrated its ability to interrupt short

circuits with faster than normal rates of rise of transient recovery voltage (TRV) at levels

greater than those produced by most transformer secondary faults. Two recent

exploratory test programs evaluated the interrupting ability of a 15 kV VCB containing

interrupters of the rotating arc type with contacts made from a chromium-copper powder

metal mixture. The interrupting conditions covered a wide range of currents from 10% to

130% of the 28 kA rated short circuit current of the tested circuit breaker and a wide

range of TRV rates of rise. These tests showed that the interrupting performance of the

tested VCB was unaffected by the TRV rate of rise to the fastest rates available in the test

laboratory. Such a VCB can therefore be used without TRV modifying capacitors to slow

down the rate of rise provided by the power system. This ability is particularly important

if analysis shows that the expected TRV from a transformer secondary fault has a fast

rate of rise beyond the recognized ability of an older circuit breaker to acceptably

interrupt.


REFERENCES

1.Electrical power systems by C.L.Wadhwa, New Age international Publishers.

2.Switch Gear Protection and Power Systems by Sunil S.Rao, Khanna Publishers.

3.Modern Power System Analysis by D. P Kothari & I.J Nagrath. Eastern

Economy Edition.

4.Power Systems by J.B GUPTA, Katson Books.

5.Power system Protection and Switchgear, by B.Ravindranath and Michener,

Wiley Eastern

vaccum circuit breaker

Engineering