Vacuum Circuit Breaker

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1 “Study of 6.6kV Vacuum Circuit Breaker” Project Report submitted in the partial fulfilment of the requirement for the degree of Bachelor of Engineering in Power Engineering Under the guidance of Shri. P.K. Yadav Director NPTI, Nagpur (WR) Submitted by: Ashwina Gharde Nishtha Sharma Sinni Pawar Vaishali Wakde Yogita Rachchawar National Power Training Institute, Nagpur (WR) RTM Nagpur University

description

Contains study of Vacuum Circuit Breaker with description, maintenance, trouble shooting and merits-demerits

Transcript of Vacuum Circuit Breaker

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“Study of 6.6kV

Vacuum Circuit Breaker”

Project Report submitted in the partial fulfilment of the requirement for the

degree of

Bachelor of Engineering in Power Engineering

Under the guidance of

Shri. P.K. Yadav

Director

NPTI, Nagpur (WR)

Submitted by:

Ashwina Gharde

Nishtha Sharma

Sinni Pawar

Vaishali Wakde

Yogita Rachchawar

National Power Training Institute, Nagpur (WR)

RTM Nagpur University

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(April 2016)

CERTIFICATE

This is to certify that the project entitled

“STUDY OF 6.6KV VACUUM CIRCUIT BREAKER”

is being submitted by Ashwina Gharde, Nishtha Sharma, Sinni Pawar,

Vaishali Wakde, Yogita Rachchawar, in partial fulfilment of B.E. Power

Engineering from National Power Training Institute, Nagpur (WR) to

RTM Nagpur University and is a record of their work carried out

under the guidance of Shri P.K. Yadav.

Shri. P.K. Yadav Shri. S.I. Mahant

Project Guide Course Co-ordinator

Director, NPTI(WR) Dy. Director, NPTI(WR)

Shri. N.C. Moharil

Director, NPTI(WR)

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ACKNOWLEDGEMENT

It is a great pleasure and moment of immense satisfaction for us to

express our profound gratitude and indebtedness toward our project guide

Shri P. K. Yadav(Director) whose enthusiasm was a source of great inspiration

to us. We are thankful for the able guidance and untiring attention which he

conferred on us from beginning to completion of the project. We are extremely

grateful to Shri A. G. Vinchurkar, (Principal Director), Shri N. C. Moharil

(Director) for providing an excellent academic climate in institution which has

made this endeavor possible. We also take this opportunity to express our

gratitude to Shri. S. I. Mahant (Course Co-ordinator) for his valuable

suggestion and support.

We also thank officials of Khaparkheda Power Plant, Nagpur for giving

us an opportunity to undergo project training and special thanks to

Shri. Pantavne (Dy. Executive Engineer) for his Guidance throughout the

training.

PROJECT ASSOCIATES:

Ashwina Gharde

Nishtha Sharma

Sinni Pawar

Vaishali Wakde

Yogita Rachchawar

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** CONTENTS **

Sr.

No.

Topics Page

No.

1 History of Circuit Breaker 5

2 Introduction 6

3 General construction 15

4 Description of VM-3 Vacuum Circuit Breaker 20

5 Interlocks 26

6 Ratings and Specifications 29

7 Maintenance 33

8 Troubleshooting 35

9 Power Circuitry 40

10 Merits and Demerits 41

11 Index of Diagrams 43

12 References 44

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1. History of a Circuit Breaker:

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

ciruits 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) 458392. 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.

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2. INTRODUCTION:

2.1 Description:

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.

2.2 Operation and Arc Formation:

All circuit breaker systems have common features in their operation. Although

details vary substantially depending on the voltage class, current rating and type

of the circuit breaker.

The circuit breaker must detect a fault condition; in low voltage circuit breakers

this is usually done within the breaker enclosure. Circuit breakers for large

currents or high voltages are usually arranged with protective relay pilot

devices to sense a fault condition and to operate the trip opening mechanism.

The trip solenoid that releases the latch is usually energized by a separate

battery, although some high-voltage circuit breakers are self-contained with

current transformers, protective relays and an internal control power source.

Once a fault is detected, the circuit breaker contacts must open to interrupt the

circuit; some mechanically-stored energy (using something such as springs or

compressed air) contained within the breaker is used to separate the contacts,

although some of the energy required may be obtained from the fault current

itself. Small circuit breakers may be manually operated, larger units have soleno

ids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive

heating, and must also withstand the heat of the arc produced when interrupting

(opening) the circuit. Contacts are made of copper or copper alloys, silver alloys

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and other highly conductive materials. Service life of the contacts is limited by

the erosion of contact material due to arcing while interrupting the current.

Miniature and molded-case circuit breakers are usually discarded when the

contacts have worn, but power circuit breakers and high-voltage circuit breakers

have replaceable contacts.

2.2.1 Arc Phenomenon:

When a short circuit occurs, a heavy current flows through the circuit breaker

before they are opened by the protective system. At the instant when the

contacts begin to separate, the contact area decreases rapidly and large fault

current causes increased and hence rise in temperature. The heat produced in the

medium between the contacts is sufficient to ionize the air or vaporize and

ionize the oil. This acts as a conductor and arc is formed between the contacts.

The potential difference between the contacts is small and just sufficient to

maintain the arc. The arc provides a low resistance path and consequently the

current in the circuit remains uninterrupted till the arc persists.

Now, the current flowing between the contacts depends on the resistance in the

path. This resistance depends on,

a. Degree of Ionization: The arc resistance increases with the increase in the

degree of ionized particles.

b. Length of Arc: The arc resistance increases with the increase in the length

of the arc (also the separation of the contacts).

c. Cross-section of the arc: The arc resistance increases with the decrease in

the X-section of the arc.

2.2.2 Important Characteristics:

a. Arc Voltage: It is the voltage that appears across the contacts of the circuit

breaker during the arcing period. As soon as the contacts of the circuit

breaker separate, the arc is formed. The voltage that appears across the

contacts during the arcing period is called the arc voltage. Its value is low

except for the period the fault current is at or near zero current point. At

current zero the arc voltage rises rapidly to the peak value and this value tries

to maintain the current flow in the arc.

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b. Restriking Voltage: It is the voltage which appears across the terminals of

each pole of a circuit breaker immediately after the breaking of the circuit

i.e, at current zero.

c. Recovery Voltage: It is the normal frequency r.m.s voltage that reappears at

the poles of a circuit breaker after final arc extinction.

d. Rate of Rise of Restriking Voltage (R.R.R.V): It is the rate expressed in

volts per micro second, representative of the increase of the restriking

voltage.

e. Peak Restriking Voltage: It is the maximum instantaneous voltage attained

by the restriking voltage.

2.3 Arc Extinction:

The various methods of arc extinction are,

2.3.1 High Resistance Method:

In this method the arc resistance is mad to increase with time so that the

current reduces to a value where it's insufficient to maintain the arc. The

disadvantage of this method is that enormous amount of heat is

dissipated. Therefore, it is applied only in d.c. or low capacity a.c. circuit

breaker.

The resistance of the arc may be increased by,

i. Lengthening of the arc: Since resistance is directly proportional to its

length thus the gap between contacts is increased to increase its

length.

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ii. Cooling of the arc: Cooling helps in the deionization of the medium

between the contacts. This increases the arc resistance. this can be

obtained by a gas blast directed along the arc.

iii. Reducing the X-section area: If the X-section is reduced then the

voltage required to maintain the arc increases. Thus resistance

increases. This can be achieved by allowing the arc to pass through a

narrow opening or by having smaller area of contacts.

iv. Splitting the arc: The resistance ca be increased by splitting the arc

into a number of smaller arcs in series. This can be done by

introducing some conducting plates in between the arcs.

2.3.2 Low Resistance Method:

This method is used for arc extinction in a.c. circuits only. In this method

the resistance is kept low until current turns zero and then arc extinguishes

naturally and is prevented from restriking in spite of the rising voltage across

the contacts.

In an a.c. system current drops to zero after every half cycle. At every

current zero the arc extinguishes for a brief moment. Now the medium between

contacts contain ions and electrons so that it has small di-electric strength and

can be easily broken down by the rising voltage called the restriking voltage. If

such a breakdown occurs the arc will persist for another half cycle. But, if

immediately after current zero, the dielectric strength of the medium is built up

more rapidly than the restriking voltage, the arc fails to restrike and the current

will be interrupted. This can be achieved by,

i. causing the ionized particles in the space between to recombine into

neutral molecules

ii. sweeping the ionized particles away and replacing them with unionized

particles.

Thus rapidly deionizing can be done by,

a. Lengthening of the gap: The dielectric depends on the length of the

gap, thus if the contacts are opened rapidly, higher dielectric strength of the

medium can be achieved.

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b. High pressure: If pressure in the vicinity is increased the, the density

of the particles constituting the discharge also increases. This increased density

of particles because higher rate of de-ionization and consequently the dielectric

strength of the medium will increase.

c. Cooling: Natural combination of ionized particle take place more

rapidly if they are allowed to cool. Therefore, dielectric strength of the medium

can be increased by cooling.

d. Blast Effect: If the ionized particles between the contacts are swept

away and replaced by unionized particles, the dielectric strength of the medium

can be increased considerably.

2.4 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.

2.4.1 Low-voltage circuit breakers:

Low-voltage (less than 1,000 VAC) types are common in domestic, commercial

and industrial application, and include:

a. MCB (Miniature Circuit Breaker): Rated current not more than

100 A. Trip characteristics normally not adjustable. Thermal or thermal-

magnetic operation. Breakers illustrated above are in this category.

There are three main types of MCBs:

1. Type B - trips between 3 and 5 times full load current;

2. Type C - trips between 5 and 10 times full load current;

3. Type D - trips between 10 and 20 times full load current.

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b. MCCB (Molded Case Circuit Breaker): Rated current up to 2,500 A.

Thermal or thermal-magnetic operation. Trip current may be adjustable in larger

ratings.

Low-voltage circuit breakers are also made for DC applications. Direct

current requires special breakers because the arc is continuous—unlike an AC

arc, which tends to go out on each half cycle. A direct current circuit breaker

has blow-out coils that generate a magnetic field that rapidly stretches the arc.

Small circuit breakers are either installed directly in equipment, or are arranged

in a breaker panel.

2.4.2 Medium-voltage Circuit Breakers:

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled

into metal-enclosed switchgear line ups for indoor use, or may be individual

components installed outdoors in a substation. Air-break circuit breakers

replaced oil-filled units for indoor applications, but are now themselves being

replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high

voltage circuit breakers described below, these are also operated by current

sensing protective relays operated through current transformers. The

characteristics of Medium-voltage breakers are given by international standards

such as IEC 62271. Medium-voltage circuit breakers nearly always use separate

current sensors and protective relays, instead of relying on built-in thermal or

magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to

extinguish the arc:

a. Vacuum circuit breakers: With rated current up to 6,300 A, and

higher for generator circuit breakers. These breakers interrupt the current by

creating and extinguishing the arc in a vacuum container - aka "bottle". Long

life bellows are designed to travel the 6-10 mm the contacts must part. These

are generally applied for voltages up to about 40,500 V, which corresponds

roughly to the medium-voltage range of power systems. Vacuum circuit

breakers tend to have longer life expectancies between overhaul than do air

circuit breakers.

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In such breakers, vacuum (degree of vacuum being in the range of 10-7 to 10-5

torr) is used as the arc quenching medium. Since vacuum offers the highest

insulating strength, it has far superior arc quenching properties than any other

mediums. For eg, when contacts of a breaker are open in vacuum, the

interruption occurs at first current zero with dielectric strength between the

contacts building up at a rate of 1000 times higher than that obtained with other

circuit breaker.

When contacts of a breaker are open in vacuum, an arc is produced between the

contacts by the ionization of metal vapour of contacts. However, the arc is

quickly extinguished because the metallic vapours, electrons and ions produced

during arc rapidly condense on the surface of the circuit breaker contacts

resulting in a quick recovery of dielectric strength as soon as the arc produces in

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vacuum it is quickly extinguished due to the fast weight of recovery of

dielectric strength.

b. Air circuit breakers: Rated current up to 6,300 A and higher for

generator circuit breakers. Trip characteristics are often fully adjustable

including configurable trip thresholds and delays. Usually electronically

controlled, though some models are microprocessor controlled via an integral

electronic trip unit. Often used for main power distribution in large industrial

plant, where the breakers are arranged in draw-out enclosures for ease of

maintenance.

BULK OIL CIRCUIT BREAKER

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur

hexafluoride gas. Moreover, Bulk Oil and Medium Oil are now phasing out.

Medium-voltage circuit breakers may be connected into the circuit by bolted

connections to bus bars or wires, especially in outdoor switchyards. Medium-

voltage circuit breakers in switchgear line-ups are often built with draw-out

construction, allowing breaker removal without disturbing power circuit

connections, using a motor-operated or hand-cranked mechanism to separate the

breaker from its enclosure. Some important manufacturer of VCB from 3.3 kV

to 38 kV are ABB, Eaton, Siemens, HHI (Hyundai Heavy Industry), S&C

Electric Company, Jyoti and BHEL.

2.4.3 High-voltage circuit breakers:

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Electrical power transmission networks are protected and controlled by high-

voltage breakers. The definition of high voltage varies but in power

transmission work is usually thought to be 72.5 kV or higher, according to a

recent definition by the International Electrotechnical Commission (IEC). High-

voltage breakers are nearly always solenoid-operated, with current sensing

protective relays operated through current transformers. In substations the

protective relay scheme can be complex, protecting equipment and buses from

various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish

the arc.

• Bulk oil(phasing out)

• Minimum oil(phasing out)

• SF6

Due to environmental and cost concerns over insulating oil spills, most new

breakers use SF6 gas to quench the arc. High-voltage AC circuit breakers are

routinely available with ratings up to 765 kV. 1,200 kV breakers were launched

by Siemens in November 2011, followed by ABB in April the following year.

High-voltage direct current circuit breakers are still a field of research as of

2015. Such breakers would be useful to interconnect HVDC transmission

systems.

a. Sulfur hexafluoride (SF6) circuit breakers:

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur

hexafluoride gas to quench the arc. They are most often used for transmission-

level voltages and may be incorporated into compact gas-insulated switchgear.

In cold climates, supplemental heating or de-rating of the circuit breakers may

be required due to liquefaction of the SF6 gas.

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3. GENERAL CONSTRUCTION

The 6.6kV Vacuum Circuit Breaker-type VM metal clad switchgear is of

horizontal draw out pattern suitable for easy extension of switchboard on both

directions for systems up to 12kV. The design incorporates single bus bar

system and a set of interlocks for safety of operations and is fully

compartmentalized.

A panel consists of fixed portion and moving portion of modular construction

having four high voltage chambers namely breaker chamber, bus bar chamber,

CT chamber and pressure relief chamber. Instrument panel is a separate low

voltage chamber. Moving portion comprises of wheel mounted truck fitted with

an operating mechanism, vacuum interrupters and isolating contacts. Motor

operated spring closing mechanism keeps the springs charged after every

closing operation making it ready for the next operation. Springs can also be

charged manually in case of failure of auxiliary power to the spring charging

motor.

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3.1 Main Breaker Chamber:

The main breaker chamber is a welded steel structure which accepts the moving

portion at the floor level. The isolating contacts are the multi finger type with

the copper fingers silver plated. The isolating contacts assemblies are mounted

on epoxy support insulators and are a self aligning type. This chamber also

includes features like secondary isolating contacts socket, guides for the moving

portion, earthing contact which mates with an earthing strip on the moving

portion and safety shutters.

The moving portion of the VCB can be kept either in the SERVICE or TEST

position inside the breaker chamber. The front door can be kept closed giving a

neat and flush appearance and making the switchgear dust and vermin proof.

Also the position of moving portion can be seen through a glass window on the

door. The control cables enter at the front compartment of this chamber or

elsewhere as per specific requirement.

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3.2 Instrument and Relay Chamber:

The instrument and relay chamber is of folded sheet steel construction with a

hinged instrument panel suitable for flush mountings at instruments in the front

to provide better access for maintenance etc. A removable cover is provided on

the top of the instrument and relay chamber.

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3.3 Busbar Chamber:

The bus bar chamber is of welded steel construction and houses the bus bars

which consist of multiple parallel aluminum bars or copper bars supported on

epoxy support insulators.

The bus bars are of air insulated bar type and are easy to erect needing no

special techniques of insulation for jointing. The bus bar chamber is provided

with bolted covers at the top and back. The insulated barriers provide partition

between two adjacent bus bar chambers. In the rear side a compartment is

provided for mounting additional relays and fitments.

3.4 C T Chamber:

The Cable and termination chamber is also of welded angle iron construction

with ample space for cable termination and current transformers of various

types.

Access to various CT’s can be made by opening the bolted back cover which

can be removed without disturbing the HT cables.

3.4 Moving Portion:

The moving portion consists of a truck frame with four wheels on which three

vacuum interrupters and the operating mechanism are mounted. The interrupters

are mounted on epoxy support insulators and are shielded from each other by

means of insulating barriers.

3.5 Secondary Plug and Socket Arrangement:

A standard 32 pin plug and socket arrangement is provided on every panel. This

meets all the requirements of low voltage connections between the cubicle and

the truck. The plug is assembled at the end of a flexible conductor hose

provided in the instrument and relay chamber. An interlocking link is assembled

in the plug body. The socket is always mounted on the truck top. Two spring

loaded interlocking pins are assembled in the socket mounting bracket. The pins

when kept in free position allow the interlocking link of plug to push the

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blocking levers and the plug can be inserted in to the socket. The plug is locked

over the socket by pressing down the flap over the pins on the socket body. The

interlocking pins shall then be brought to the locked positions so that the

interlocking link is on plug body is kept in its place. The pin not brought into

locked position will not allow the movement of truck further inside the cubicle

as in the free position the projection of pins will foul with the position plate in

the breaker chamber. Also when the truck is at the SERVICE or TEST position

it is not possible to disengage the plug from the socket.

The interlocking pins can be brought to free position only before the TEST

position when the truck is being pushed in. the blocking levers do not allow the

interlocking pins to be brought to locked position unless the plug with

interlocking link is engaged with socket.

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4. Description of VM-3 Vacuum Circuit Breaker

The VM3AF vacuum circuit breakers are designed to handle all recognised

switching duties. The Breakers are extremely reliable in service, required only a

minimum of maintenance and have long life expectancy. Moreover, their Small

size and weight, quiet and low vibrations operation and the fact that they are not

affected by temperature or represent a fire risk enable the Breakers to be used in

locations subject to adverse conditions.

The three breaker poles, each with its vacuum interrupters, are mounted on a

common mechanism housing. The energy storing spring mechanism is motor

operated and can be actuated by hand also.

4.1 Construction:

The construction of vacuum breaker is shown in figure 3. The breaker polls are

fixed to the rear of the mechanism housing by 2 cast resin insulators, each end

fitted with phase barriers.

The insulated support are either aluminium castings or are made of sheet Steel,

depending upon the rated normal current and rated short circuit breaking

current.

The pole terminals and are designed for direct bus connections.

The energy storing mechanism and all the control and actuating devices are

installed in the mechanism housing. The Breakers can be operated by hand and

also electrically by means of solenoids.

The ON/OFF indicator, the spring charging indicator and the operation counter

are fitted on the front of the mechanism housing.

4.2 Breaker Pole:

The vacuum interrupter is rigidly fixed by the by the upper terminal to pole

support. The lower ceremic part of the interrupter is established against lateral

forces by a centring ring on pole support. The external forces due to switching

operations and the contact pressure are absorbed by the insulating Struts.

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4.3 Current Part Assembly:

The current part assembly consists of upper terminal and pole support the fixed

contacts and the moving contact stem is connected by the terminal clamp to

Copper flexible.

4.4 Vacuum Interrupter:

The basic construction can be seen in figure 4. The moving contact moves in

guide. The metal bellows follows the travel of the moving contact and seals the

interrupter against the surrounding atmosphere.

4.5 Arc Quenching Principle:

When the contact separate, the current to be interrupted initiates the metal

vapour arc discharge and flows through the plasma until next current zero. The

arc is then extinguished and the conductive metal vapour condenses on the

metal surfaces within a matter of microseconds. As a result, the dielectric

strength in the break builds up very rapidly.

The contacts are so designed that the self-generated field causes the arc to

travel. This prevents the local overheating of the contacts when interrupting

large currents.

The metal vapour arc discharge can only be maintained if a certain minimum

current flows. A current that does not attain this level is chopped prior to current

zero. This chopping current is kept to a minimum in order to prevent unduly

high over voltage build up with inductive circuits are switched. The use of

special contact material ensures that the current chopping is limited to low

value.

The Rapid build of the dielectric strength in the break enables sucks to be safely

extinguished even if the contact separation occurs immediately prior to the

current zero.

The arc drawn in the vacuum breaker is not cooled. The metal vapour plasma is

highly conductive and the resulting arc voltage only attains values between 20

to 200 volts. For this reason and because of the short arcing times the arc energy

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developed in the break is very small. This also account for the long electrical

life expectancy of the vacuum interrupter. Owing to the high vacuum (<109 bar)

in the interrupter contact clearances of only about 6 to 20 mm depending upon

the rated voltage are needed in order to attain a high dielectric strength.

4.6 Switching Operation:

When closing command is initiated the charged closing spring actuates the

moving contact through breaker shaft, lever, insulated coupler and drive link.

The forces that occur when the movement of the insulated coupler is converted

into the vertical motion of the moving contacts are absorbed by the drive link,

which pivots on the pole Support and adaptor. During closing, the tripping

Springs and the contact pressure springs are charged and latched by Pawl.

The recharging of the closing spring takes place automatically immediately after

closing if the supply of motor is on, otherwise hand charging can be done with

the help of manual charging handle. In the closed condition the contact pressure

springs and atmospheric pressure, maintained the necessary contact pressure.

The contact pressure springs automatically compensates for arc erosion which is

very small

When at ripping command is given, the energy stored in the tripping and contact

pressure springs is released by the Pawl. The opening sequence is similar to

closing. The Residue forces of the tripping springs arrests the moving contacts

in the open position .

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4.7 Operating Mechanism:

The motor operated spring closing mechanism of vacuum circuit breaker is of

modular design. It contain separate models for all of the functions required for

switching operation. As well as for indication and controls. Each module can be

dismantled and install easily without it being necessary to carry out any

adjustments

4.8 Mode of Operation:

The operating mechanism is of stored energy types that is the charging of

spring is not followed automatically by the closing opening or opening breaker

contacts. When the Stored energy mechanism is charge, the instant of operation

can be chosen as desired. The mechanical energy for carrying out and open-

close - open sequence for auto reclosing duty is stored in the closing and

tripping springs.

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a. Charging of the Closing Spring with Circuit Breaker in Open

Condition:

It shows the operating mechanism linkage position prior to closing spring

Charging when the circuit breaker is an open conditions. The charging shaft is

supported in the charging mechanism but is not couple mechanically with the

charging mechanism. Fitted to it are the crank at 1 end the camp together with

liver at the other

When the charging mechanism is actuated by hand or by Motor, the flange

turns until the driver locate in the cut away part of CAM disc thus causing the

spring charging shaft to follow. The crank charges the closing springs. when

this has been full tensioned, the crank actuates the linkage for the “closing

spring charged” indicator via and also the limit switches for interrupting the

motor supply. When the closing spring is charged, cam Disk follows idly that is

it is bought in 2 position for closing. figure 6 shows the final position after the

closing spring is fully charged and the breaker is an open condition.

b. Closing Operation:

After the completion of charging operation of the closing springs as explained

in (A) the breaker is ready for closing. If the breaker is to be closed locally the

spring is released by pressing “ON” button fig 6. In case of remote operation the

closing Solenoid unlatched the closing spring.

As the closing spring is under discharge process, the charging shaft is turned by

crank. The cam disc at the other end of the charging shaft actuates the driver

lever with the results that break shaft is turned by lever via coupling rod. At the

same time, lever, fixed on the breaker Shaft operate the three insulated coupler

for the breaker poles. Lever charges the tripping springs during closing and the

breaker is latch in the closed position by Lever with Pawl roller and bipawl

The crank on the charging shaft moves linkage and thus the closing “spring

charged” indication is cancelled by the limit switches in the motor supply

circuit. Manual recharging of the closing spring is also possible immediately

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c. Opening Operations:

If the breaker is to be trip locally, the spring is released by pressing the “OFF”

button. In the case of an electrical command being given, the tripping solenoid

unlatches the tripping spring . The tripping spring turn the breaker shaft via

lever the sequence being similar to that for closing

d. Manual Operation for Spring Charging:

Normally the closing spring is charged by an electrical Motor. A built in feature

also facilitates manual charging of spring if the power supply to Motors fails.

This is achieved by inserting the hand crank in the opening and turning in clock

wise until the indicators show the “closing sprint charged”.

The hand crank couples with charging mechanism via a trip -free facility the

operator is thus not expose to any risk if the control supply recover during the

manual spring charging operation.

MANUAL OPERATING MECHANISM

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5. Interlocks

The various interlocks provided on the breaker ensure that:

a. The VCB truck cannot be racked in or out unless circuit breaker is in

‘open’ condition.

b. The VCB truck cannot be read racked in unless secondary plug and

socket are engaged.

c. The circuit breaker closing operation is not possible unless secondary

plug and socket are engaged.

d. The secondary plug and socket cannot be disengaged when the VCB

truck is not in ‘service’ test, or any intermediate position between these

two positions.

e. The circuit breaker closing operation is not possible unless the truck is in

‘Service’ or ‘Test’ position.

f. The interlock mechanism cannot be operated unless the circuit breaker is

an “Open” condition.

g. Inter changeability of trucks of different current ratings is not possible.

5.1 Interlock features:

As explain in general description, of system of full proof interlocks has been

provided to ensure safety of operation. The interlocking features are described

below:

a. Main Interlock:

This consists of interlock shaft assembly and Cam. The cams mounted on either

side of the shaft prevents the movement of the breaker truck unless the

operating sequence is in order. Also the cam moves the breaker trucks by

approximately 35 mm in the last stage of plugging in or in the beginning of

withdrawal. The cams in the logged vertical position at as locks for the breaker

trolley in the service position and prevent throwing out of truck during short

circuit. Rotation of the interlock shaft also actuates the interlocking linkage

which allow the circuit breaker closing operation only at specific locations of

the truck in the fixed portion. During the withdrawal of truck from fixing

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housing, interlocking feature takes care that this is possible only when the

circuit breaker is an open condition.

b. Truck Stops:

To ensure the right matching of moving and fix positions, that is the rating

wise/function wise, truck stops are provided on the moving and fixed portions.

The projected portion of the stop on the moving potion fouls with the stop on

fixed portion on attempt of inserting a wrong moving portion.

5.2 Operation of interlocks:

a. Isolation of breaker from service position:

For isolating circuit breaker from the service position it must first be tripped.

Inside the handle in to the hole of interlock shaft assembly. Move the handle

upward it comes to ‘Free’ position. During the process the truck with VCB will

get pushed out disengaging Isolating contacts. The truck can now be further

pulled out manually. The truck will get stopped at ‘Test’ position. In this

position the shutters with close there by physically separating the breaker

chamber from the other high voltage compartments. Remove the handle and

close the door if the truck is to be left in this position.

At this position no load operation of the breaker can be carried out. For this

rotate the inter lock shaft to ‘Locked’ condition

For withdrawal of truck out of the cubicle from ‘Test’ position check –

Wheel covers are slided out.

Interlock is in ‘Locked’ position.

Closing the tripping springs are free.

Secondary plug and socket is disconnected. Breaker will be

in-operative without secondary connection and interlock

shaft indicating ‘locked’ position. For operation of breaker

outside the cubicle use jumper connections.

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b. To insert the truck:

The truck can be pushed in only with the breaker in tripped position. Make sure

that the interlock is in ‘lock’ position. Open the front door and slide out the

wheel covers. Bring the truck in front of the breaker chamber. The wheels

initially get aligned by welded guide strips at the floor level. Push the truck till

the interlock cam is stopped at the stop pin of the breaker chamber. This is the

‘test’ position of the truck inside the housing. Now rotate the interlock shafts to

‘free’ position. Engage the secondary plug and socket.

Push the truck further inside till the interlock cam is stopped by interlock block.

During this movement the truck is guided by the guide rollers and the shutters

get lifted up to open the entry of isolating contacts of HV chamber.

Now on rotating the interlock handle to the ‘locked’ position, the truck will get

moved further into ‘service’ position now the breakers can be closed.

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6. RATINGS AND SPECIFICATIONS

1 Time required to charge the closing spring 15 second

2 Does the provision exist for immediate

recharging of closing spring after closing of

breaker

Yes

3 Whether o-co operating is possible with one

charging of spring

Yes

4 Mechanical indicator

Breaker service/ test

Breaker open/close

Closing spring charged/discharged

No

Yes

Yes

5 Whether operating counter is provided Yes

6 Auxiliary contacts On moving position

7 Rated voltage 220 V

8 Continuous current 15 A

9 Making current 15

10 Breaking current 10 resistance

1 inductance

11 Maximum number of (NO+NC) contacts

mounted on the truck

6NO+6NC

12 Maximum number of NO+NC mounted on

panel (Actuated in service position only)

VAJC relay to provided

having 6NO+6NC

contacts

13 Whether the contacts are changeover type No

14 Whether (NO+NC) contacts actuated with

spring charging mechanism provided

Yes

(4 changeover contacts)

1 Whether contacts are silver plated Vacuum interrupter

design is proprietary of

BHEL Bangalore

2 Thickness of silver plating and contact load

on each finger

Not applicable for V

interrupter

3 Type of operating mechanism

Closing

Motor charged spring

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Tripping close

Spring operated

4 Whether fixed trip or trip free Trip free

5 Antipumping feature trip free Yes antipumping plug in

relay provided

6 Does provision exist for operating breaker

During control

Supply failure

Closing

Tripping

Manual closing

Push

Button

Manual emergency trip

push

Button accessible with

door

7 Whether slow operating of breaker is

possible for maintenance

Yes

8 Rated voltage of closing/trip coil variation in

voltage permissible

220 V DC

9 Variation in voltage permissive

Closing

Tripping

85-100%

No entry 70-110%

10 Power required at rated supply voltage

Closing coil

Trip coil

200 W

200 W

11 Whether manual trip/close is provided for

emergency operation

Yes

12 Spring charging motor

Voltage

Watts

RPM

Class of insulation

220 V DC

600 W appox.

8000

E

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1. Breaking capacity

2. Making capacity

3. Short- time capacity

6.1 Breaking Current Capacity:

The breaking current capacity of a circuit-breaker is a term used to express the

highest current that the circuit breaker is capable of breaking at a stated

recovery voltage and a stated reference restriking voltage under prescribed

conditions of use and behavior. If the current is symmetrical it is referred to as

symmetrical breaking capacity, whereas if the current is asymmetrical the

breaking capacity is referred to as asymmetrical breaking capacity.

The breaking capacity of a circuit breaker is of two two types

Symmetrical breaking capacity

Asymmetrical breaking capacity

Symmetrical breaking current capacity: It is the rms value of the ac

component of the fault current that the circuit breaker is capable of breaking

under specified condition of recovery voltage.

Asymmetrical breaking current capacity: It is the rms value of the

total current comprising of both ac and dc components of the fault current that

the circuit breaker can break under specified conditions of recovery voltage.

6.2 Making Current Capacity:

The possibility of a circuit-breaker breaker being closed on a dead short must be

taken account of. The capacity of a circuit breaker to make current depends

upon its ability to withstand the effect of. The capacity of a circuit breaker to

make current depends upon its ability to withstand the effect of electromagnetic

forces. The maximum force in any phase is a function of the square of the peak

making capacity.

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6.3 Short Time Capacity:

The short-time current rating is based on thermal and mechanical limitations.

The circuit breaker must be capable of carrying short-circuit current for a short

period while another circuit breaker is cleaning the fault. The rated short-time

current is the rms value of the current that the circuit breaker can carry safety

for a specified short period.

6.4 Rating of Circuit Breaker:

Rated voltage 17.8kV

Rated frequency 50Hz

Rated symmetrical breaking current 20.4kA

Rated asymmetrical breaking current 25.5kA

Rated making current 52kA

Rated short time current (1sec) 20.4sec

Rated operating duty 0-3mm-co-3mm-co.

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6. Maintenance of Vacuum Circuit Breaker

A vacuum circuit breaker needs less maintenance as compare to other

breakers and no inflammable substance that supports fire compared with that of

oil in the oil circuit breaker.

The Table shows the types of Maintenance needed for vacuum circuit breaker.

Sr.No Classification Check point Method of Inspection

1.

2.

3.

General

Vacuum

Opening

mechanism

Cleaning for every

point or every

component

Contact wear

Judgement of

vacuum

Lubrication of each

component

Coupling section

Springs

Clamping section

Clean the circuit breaker to remove the dust

and dirt. The envelope of the vacuum

interrupter and insulating materials should

wiped with a clean cloth.

Check the contact for wear replace if the

contact piece has worn out.

Withstand test at 22kv ac for one minute.

Apply small amount of oil to rotary

sections, sliding surfaces,pins,coupling

section etc. use grease for the operating

shaft and bearings and machine oil for the

parts operating shaft and bearing and

machine oil for the parts.

Check for rust and wear

Visual check for rust and deformation

Check for loosened bolts and nuts broken

springs washers and snap pins, their

incoming off, etc.

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Control circuit

Control Switches

Check for loosened connections, rust

damaged lead wires.

Check whether the contacts are damaged

by arcing.

6.1 Checks for Maintenance:

Visual inspection of the breaker shall be connected systematically and

checks shall be made for any loose or damaged parts.

Perform slow closing operation as suitable for better operation of the

breaker spring.

Contact erosion on the interrupter shall be checked by locating the point

below the Circuit Breaker in Closed Position.

Check the vacuum of the interrupter.

Mechanism shall be lubricated with grease/oil as per the instruction

manual.

Electrical operation of the breaker shall be checked either by using test

cabinet or by putting the truck in test position of the metal clad enclosure.

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7. Trouble Shooting

Sr.no Trouble Reasons Remedial action

1.

2.

Motor not

running

Motor keeps on

running after

fully charging the

springs .

a)Improper wiring

b) Supply not available

c) Failure of motor

cutoff switch

d) Failure of Rectifier

e) Failure of motor

f) Gearbox jam

Motor cutoff not taking

place

Trip latch setting

disturbed

a) Check motor circuit wiring as per

schematics drawing and tighten. The

loose connection found.

b) Check incoming supply to the

motor at rectifier terminal. If not

available then restore incoming

supply.

c) Check limit switch operation if

found defective then replace limit

switch.

d) Check supply at incoming terminal

if available then check at outgoing

terminal. If supply is not available

replace rectifier.

e) Check supply at motor terminal if

available and motor is not running,

replace the motor.

f) Even after replacing motor, motor is

not running it indicates gearbox is

jammed / drawing more current

replace gear box.

Check the opening arrangement of

motor cut off switch . if not

functioning properly replace the lever

fitted on the LHS top of the gear box.

Adjust the setting of stud provided on

tripping mechanism to restore the

latching by rotating 1 & 2 turn

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3.

Trip latch does

not hold the

vacuum circuit

breaker in closed

position

maximum check the tripping operation

at minimum operating voltage 5 times

,if unable to do setting contact BHEL.

7.1 Inspections, Checks and Test Without Control Power

Vacuum circuit breaker are normally shipped with their primary contact

open and their springs discharged however, it is critical to first verify the

discharged condition of the springs loaded mechanism after the de- energizing

contact power.

a. Spring – Discharged Check:

Perform the spring discharge check before removing the circuit breaker

from the pallet or removing from the switchgear.

The spring discharged check consist of simply performing the following

tasks in the order given. This check assures that both the opening and closing

springs are fully discharged.

1. Press red open push button

2. Press black close push button

3. Again press red open push button

4. Verify springs condition indicator show discharged

5. Verify main contact status indicator shows open

b. Manual Spring Charging Check

1. Insert the manual spring charging crank into the manual change handle

socket.

2. Turn the crank clockwise until the spring conductor indicator shows the

closing spring charged.

3. Repeat the spring discharged check.

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4. Verify that the spring are discharged and the circuit breaker primary

contact open by indicator positions.

c. Automatic Spring Charging Test

The automatic spring charging features of the circuit breaker must be

checked. Control power is required for automatic spring charging to take place

1. Open the control power circuit by opening the controls power

disconnect device.

2. Energize (close) the control power circuit disconnect.

3. Use the close & open controls to first close & then open the circuit

breaker contacts. Verify the contacts position visually by observing

the open/closed indicator on the circuit breaker.

4. De-energize control power by repeating step-1.

Disconnect the plug jumper from the switchgear first and next from

the circuit breaker.

5. Perform the spring discharged check open. Verify that the closing

spring is discharged and the primary contacts of the circuit breaker are

open.

d. Final Mechanical Inspection without Control Power –

1. Make a final mechanical inspection of the circuit breaker verify

that the contacts are in the open position and the closing spring is

discharged.

2. Verify mechanical condition of springs.

3. Check for loose hardware.

7.2 Tests Performed:

a. Checking the Contact Erosion:

Checking the contact erosion in the vacuum interrupter can be done in simple

manner. After taking out the VCB mounted truck from the switchboard, check

the position of the white mark ‘A’ on the interrupter moving contact. Wear is

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still within the permissible limits as long as this mark can be seen when the

contacts are in closed position. In the event, the mark is not visible the

interrupter needs a replacement.

b. Checking the Vacuum:

Before putting the breaker in service, or an interrupter is suspected of leaking

because of mechanical damage, the vacuum may be checked as follows:

Open and isolate the breaker and detach the insulated coupler from lever.

The atmospheric pressure will force the moving contact of a hermetically

sealed interrupter into ‘close’ position. A vacuum interrupter may be

assumed to be intact if it shows the following characteristics:

An appreciable closing force has to be overcome when lever is moved to

‘open’ position by hand. When the lever is released, it must automatically

return to the ‘close’ position and the contacts must be heard to close.

After checking the vacuum, the lever should be refitted to the insulated

coupler.

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c. Lubrication of Operating Mechanism:

The main points to be lubricated with centoplex 24 DL or Molykote.

All the points not marked (bearings, articulated joints and auxiliary switch)

should be treated with Molykote spray. To lubricate the mechanism points

detach its cover. Lubricate all the appropriate points, starting at the top left and

working through systematically. Parts that are not rigidly fixed should be moved

slightly to and fro to let the oil penetrate.

After lubrication, the breaker shall be operated several times to test it.

Articulated joints and bearings that cannot be dismantled should not be cleaned

with a cleaning agent prior to being oiled.

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9. Power Circuitry

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10. Merits of Vacuum Circuit Breakers

The vacuum switchgear has been successfully developed and is gaining rapid

popularity. The vacuum switches are likely to be popular for wide range of

applications. These switches devices have several merits such as :

1. VCB is self-contained and does not need filling of gas or oil. They do not need

auxiliary air system, oil handling system, etc. No need for periodic refilling.

2. No emission of gases, pollution free.

3. Modest maintenance of the breaker, no maintenance of interrupters. Hence

economical over long period.

4. Breakers forms a unit which can be installed at any required orientation.

Breaker unit is compact and self contained.

5. Non-explosive

6. Silent operation.

7. Large number of operation on load, or short circuit. Suitable for repeated duty.

8. Long life of the order of several hundred operations on rated normal current.

9. Constant dielectric. There are no gas decomposition products in vacuum and the

hermetically sealed vacuum interrupter keeps out all environmental effect.

10. Constant contact resistance. In vacuum, the contacts cannot be oxidized, a fact

which ensures that their very small resistance is maintained through their life.

11. High total current switched. Since contact piece erosion is small, rated normal

interrupted current is up to 30.000 times; and rated short circuit breaking current

is on the average of a hundred times.

The above reasons, together with the economic advantages offered, have

boosted acceptance of the vacuum arc quenching principle.

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Demerits of Vacuum Circuit Breakers

1. The vacuum interrupter is more expensive than the interrupter devices in other

types of interrupters and its cost is affected by production volume. It is

uneconomical to manufacturer vacuum interrupters in small quantities.

2. Rated voltage of single interrupter is limited until very recently to about

36/√3 = 20 KV above 36 KV, two interrupters are required to be connected in

series. This makes the breaker uneconomical for voltage rated above 36 KV.

3. Vacuum interrupters required high technology for production.

4. In the event of loss of vacuum, due to transient damage or failure, the entire

interrupter is rendered useless. It cannot be required at site.

5. For interrupter low magnetizing currents, in certain range, additional surge

suppressors are required in parallel with phase of a VCB.

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11. Index of Diagrams

Page No. Name of Diagram

5 Circuit Breaker

8 Graph of Restriking Voltage

12 Vacuum Interrupter

13 Bulk Oil Circuit Breaker

14 SF6 Circuit Breaker

15 Side View of Vacuum Circuit Breaker

16 Sectional View of Vacuum Interrupter

17 Operating Mechanism - - Closing spring charged and

Breaker in Open Position

23 Operating Mechanism - Closing spring charged and

Breaker Closed

25 Manual Operating Mechanism

38 Contact Erosion Test

40 Power Circuitry

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12. References:

1. Operation and Maintenance Manual – Indoor Metalclad Vacuum Circuit

Breaker

2. Power System Engineering – Soni, Gupta, Chakrobarty & Bhatnagar

3. Switchgear protection and Power Systems - S. Rao

4. BHEL Manuals

5. NPTI Manuals

6. Wikipedia.org