Air Blast Circuit Breaker

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MSEE-01141006 CB & SUBSTATIOBS ASSIGNMENT-2 Dr. Kamran sb 1 Air Blast Circuit Breaker Working Principle. In the air blast circuit breakers the arc interruption takes place to direct a blast of air, at high pressure and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased. Consequently, the arc may be interrupted at the first natural current zero. In air blast circuit breakers, the contacts are surrounded by compressed air. When the contacts are opened the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process. A compressor plant is necessary to maintain high air pressure in the receiver. The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. Air blast circuit breakers are used for interconnected lines and important lines where rapid operation is desired. In air blast circuit breaker (also called compressed air circuit breaker) high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the circuit breaker. The circuit breaker opens and immediately after that the isolator opens, to provide addition gap.

description

Its complete description for air blast circuit breaker, worked in 2015.

Transcript of Air Blast Circuit Breaker

Page 1: Air Blast Circuit Breaker

MSEE-01141006 CB & SUBSTATIOBS ASSIGNMENT-2

Dr. Kamran sb

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Air Blast Circuit Breaker Working Principle.

In the air blast circuit breakers the arc interruption takes place to direct a blast of air, at high pressure

and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc

zone and the arc length is considerably increased. Consequently, the arc may be interrupted at the first

natural current zero. In air blast circuit breakers, the contacts are surrounded by compressed air. When

the contacts are opened the compressed air is released in forced blast through the arc to the

atmosphere extinguishing the arc in the process. A compressor plant is necessary to maintain high air

pressure in the receiver.

The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker

operates repeatedly. Air blast circuit breakers are used for interconnected lines and important lines

where rapid operation is desired. In air blast circuit breaker (also called compressed air circuit breaker)

high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized

medium between the contacts is blown away by the blast of the air. After the arc extinction the

chamber is filled with high pressure air, which prevents restrike. In some low capacity circuit breakers,

the isolator is an integral part of the circuit breaker. The circuit breaker opens and immediately after

that the isolator opens, to provide addition gap.

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The operation mechanism operates the rods when it gets a pneumatic or electrical signal. The valves

open to send the high-pressure air in the hollow of the insulator. The high-pressure air rapidly enters the

double arc extinction chamber. As the air enters into the arc extinction chamber the pressure on the

moving contacts becomes more than spring pressure and it causes the contacts to be open. The contacts

travel through a short distance against the spring pressure. At the end of contacts travel, the part for

outgoing air is closed by the moving contacts and the entire arc extinction chamber is filled with high-

pressure air, as the air is not allowed to go out. However, during the arcing period the air goes out

through the openings and takes away the ionized air of arc.

While closing, the valve is turned to close connection between the hollow of the insulator and the

reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result, the pressure of

air in the arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts

close over the fixed contacts by virtue of the spring pressure. The opening is fast because the air takes a

negligible time to travel from the reservoir to the moving contact. The arc is extinguished within a

cycle. Therefore, air blast circuit breaker is very fast in breaking the current. Closing is also fast because

the pressure in the arc extinction chamber drops immediately as the value operates and the contacts

close by virtue of the spring pressure.

The medium used to extinguish the arc created in air blast circuit breakers is a blast of very dry, clean

air, at a high pressure. The air pressure is high enough to actually open or close the contacts.

Types & Construction

Pressurized Head Air Blast Circuit Breaker

In this type, the moving contacts after opening, are kept open by maintaining the air pressure in the

main chamber (interrupting head). The contacts in this type of breaker make or break the line current

and also provide isolation, provided no loss of air pressure occurs. However, if air pressure is lost the

contacts will close and a disastrous situation may occur.

To prevent this, a motorized disconnect switch is used to ensure that isolation is safely maintained

under all circumstances. This type of breaker is used at Bruce NGS "A" & "B". Figure shows a simplified

diagram of this type of circuit breaker.

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Figure: PHABCB in closing & tripping

The circuit breaker is shown in a closed position. The closing and tripping mechanisms are identical.

Table provide the various breaker components.

Item Designation

1

1

Terminal

Terminal 2 Moving contact piston (see 3)

3 Moving contact of main chamber

4 Fixed contact of main chamber

5 Main chamber (extinguisher chamber)

6 Tripping control valve

7 Upper countershaft

8 Control valve channel (see 6)

9 Pilot valve of control valve (6)

10 Electro-valve

11 Single-pole control cubicle

12 Pilot valve of pneumatic control block

13 Intake of compressed air

14 Rod control channel (see 16)

15 Pneumatic control block

16 Rod control system

17 Horizontal metallic rod

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18 Frame-tank

19 Lower countershaft

20 Vertical insulating rod

21 Insulting supporting column

Table: Fully Pressurized Type Air Blast Circuit Breaker Components.

Refer to Figure, The main chambers (5) are permanently pressurized because they are connected

directly to the air tank (18) via the insulating support column. Piston (2) drives the moving, contacts (3).

Normally, the pressure on the two sides of the piston (2) is the same. Piston (2) is actuated by creating a

difference of pressure, between its two faces.

Sequence of Operation

(a) Opening Cycle

An opening signal to the coil of electro-valve (10) causes the opening of pilot valve {12). Pressure is then

established in the channel (14) to actuate the piston controlling the rod assembly (16). The movement

of the piston causes the horizontal metal rods (17) to be pulled, as well as the vertical insulating rods

(20), through the counter shafts (19). On being pulled, the rods (20) open the pilot valve (9). Pressure

is then established in the channels (8) and on the piston of each of the opening control valves (6) causing

them to open. There is one opening control valve in each main chamber. As the valves (6).open, the

inside of the tubular moving contacts (3) is open to the atmosphere, as well as the annular space on the

rear face of the pistons ( 2). The pressure on the front face of the piston (2} is greater than the rear face.

This causes the contacts to open.

(b) Closing Cycle

For the closing of this type of air blast circuit breaker, there is another set of valves called closing valves

(not shown). The principal of operation is similar to the opening cycle, except the closing value will exit

the other side of the piston (2) to atmosphere. Air blast circuit breakers are used today from 11 to 1100

KV, for various applications. They offer several advantages such as faster operations, suitability for

related operation, auto-reclosure, unit type multi-break construction, simple, assembly, modest

maintenance, etc. A compressor plant is necessary to maintain high air pressure in the receiver. Air-blast

circuit breakers operate repeatedly. Air-blast circuit breakers are used for interconnected lines and

important lines when rapid operation is desired.

Atmospheric Head Air Blast Circuit Breaker

In this type of air blast circuit breaker, the fault current is interrupted by the contacts in the interrupting

heads and the isolating heads, but isolation is maintained only by the isolating contacts. The isolation

contacts are an integral part of the circuit breaker. Figure shows such a circuit breaker.

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Figure: Atmospheric Head Air Blast Circuit Breaker (AHABCB)

Sequence of Operation

Figure shows the operation of this type of circuit breaker. Each interrupting head contains a fixed and a

moving contact.

(a) Opening Cycle

An electrical signal to the blast valve opens the valve. Air pressure acts on the lower part of the moving

contact and depresses the spring causing the moving contact to move in the cavity. Figure 2(B) shows

the interrupting head of an AHABCB in closed position. As the moving contact moves away from the

fixed contact and arc is developed, Figure 2(C). A blast of air from the air reservoir rushes past the

moving contact and provides cooling. This extinguishes the arc, Figure 2 (D). After the contacts in all of

the interrupting heads are open, the isolating head contact opens. After the isolating contact has

opened, the interrupting contacts· reclose. Circuit isolation is maintained by the isolation contacts

remaining open.

(b) Closing Cycle

To close a non-pressurized type of air blast circuit breaker, only the isolation contacts need to be closed

(with an air blast). Isolation contacts, as well as the interrupting contacts are designed to make or break

large currents.

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In air blast circuit breaker (also called compressed air circuit breaker) high pressure air is forced on the

arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is

blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air,

which prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the

circuit breaker. The circuit breaker opens and immediately after that the isolator opens, to provide

addition gap. In EHV circuit of today, isolators are generally independently mounted.

PHABCB with Sequential Isolator

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Typical ratings of Air Blast Circuit Breaker are:

12 KV, 40 KA

22 KV, 40KA

145 KV, 40 KA, 3 cycles

245 KV, 40 KA, 50 KA, 2.5 cycles

420 KV, 40 KA, 50 KA, 63.5 KA, 2 cycles

Advantages and Disadvantages of Air Blast Circuit Breakers

Advantages

(a) By connecting several breaker heads in series the voltage rating of the breaker can be increased.

(b) By careful design, the interrupting capacity rating can be increased to over 50,000 MVA.

(c) Fast clearance of fault currents. Breakers used at Bruce NGS can clear a fault within 2 cycles.

Disadvantages

(a) Expensive.

(b) Complicated construction, requiring air receivers and high-pressure pipework.

(c) Maintenance is time consuming, as access is difficult.

(d) A supply of very dry, compressed air is required to ensure no condensation or ice formation on

the insulators or contacts.

(e) The breaker, when opened, unless fitted with silencers, is very noisy and consequently cannot

be used in built-up residential areas.

Bulk Oil Circuit Breaker

In Bulk Oil Circuit Breaker oil serves a two-fold purpose, i.e., as means of extinguishing the arc and also

for providing insulation between the live parts and the metallic tank.

This is the oldest amongst the three types having been developed towards close

of the nineteenth century. In its simplest form the process of separating the current carrying

contacts was carried out under oil with no special control over the resulting arc other than the increase

in length caused by the moving contacts.

As the power systems began to develop resulting in higher voltages and higher fault levels, plain break

type breaker could no longer keep pace with the requirements. Various methods of controlling the

breaking process were investigated and developed.

This led to the development of controlled break oil Circuit Breaker. This employed pressure chamber, is

still widely used because it is relatively cheap to make, and gives greatly improved performance in

terms of final extinction, gap length and arcing time, as against the plain break oil Circuit Breaker.

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Various designs exist according to the preferences and requirements of individual manufacturers and

designations such as ‘Cross Jet Type’, ‘Explosion Pot’ and ‘Baffle pot’, etc.

Many oil Circuit Breakers feature special arc control devices most of which are based on the simple

pressure chamber principle but incorporate certain modifications aimed at improving the breaking

capacity. Depending on the working principle of these special pressure chambers the breakers are

designated as: impulse oil Circuit Breakers design grid breakers, breakers with double arc pressure

chambers and axial jet pressure chamber oil Circuit Breakers.

For general illustration, a view of the contact actuating mechanism of 33 kV,

type OKM, bulk oil breaker manufactured by M/s English Electric Co. is shown in Figure.

The contacts are actuated by a lever assembly L housed within the top-plate and connected to the lifting

bridge N by links M.

The beam lever assembly is pivoted on a shaft H fixed in bearings in the top-plate and is operated by a

tie rod G connected by an adjustable coupling J to the vertical pull-rod K from the Circuit Breaker

operating mechanism. An oil seal F is fitted to prevent leakage from top-plate and a pin E on the driven

end of the beam lever operates an indicator arm. The lifting bridge N which carries the lift rods Q and

moving contacts R moves vertically on guide I, rods D fixed in the top-plate, At the top end of each guide

rod and fastened to the top plate by clips A is an accelerating spring C. These springs are compressed by

the lifting bridge during the closing stroke and provide a throw off force when the breaker is tripped

open. The mechanism is prevented from over traveling the closed position by adjustable stops B in the

top-plate. At the lower end of each guide rod is an oil dashpot assembly P.

These oil buffers arrest the downward or contact opening movement. The working part of the breaker is

cylindrical chamber known as an interrupter pot. The view of the interrupter is shown in Figure the

interrupter pot is screwed and locked on to an interrupter top block. The interior of the chamber is

fitted with insulating dividing plates, which form labyrinths and oil flow passages. Assembled in the top

of the chamber is the fixed spring loaded cluster type contact, the fingers of which are arranged in a

circular formation to engage with the moving contact, which is of the solid rod of candle type.

Alternate cluster fingers are extended to form arcing contacts. These parts carry the arc current and

protect the normal current carrying parts from burning. The moving contacts are clamped by pinch bolts

at each end of a cross bar which is bolted to the lift rod. The separation of the contacts and drawing out

of the arc take place in the interrupter pot which almost completely restricts the movement of the oil

within it.

The internal space available for gas is thus little more than that swept out by the moving contact, and a

pressure is set up which depends upon the rate of gas production and its rate of flow through the vents.

The pressure rise and the condition resulting there from are believed to play a large part in giving this

type of oil Circuit Breaker a very much higher breaking capacity than the plain break type.

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Figure

Minimum Oil CBs

As the system voltages and fault levels increased the Bulk Oil Breakers required huge quantities of

insulating oil and became unwieldy in size and weight. This added enormously to the cost of a power

system. Simultaneously improvements were made in the technique of ceramics. The function of oil as

insulating medium in the Bulk Oil Breakers was transferred to the porcelain containers.

Only a small quantity of oil was used to perform its functions as arc quenching medium. This led to the

development of small oil volume or low oil content breakers in the continent of Europe.

Like the Bulk Oil Breakers these have also since then passed through many stages of development with

varying designs of the arcing chambers. Today the small oil volume breakers are available for voltages

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up to 36 kV and the fault levels associated therewith.

Contrary to the operation of the impulse type Circuit Breaker, such as air blast Circuit Breaker, in which

arc extinction and dielectric recovery are affected by means of an external quenching medium, the

process of arc extinction in the small oil volume Circuit Breaker is of internal

thermo- dynamic origin.

During the tripping operation an arc strikes in oil between the moving contact and the fixed contacts.

This arc is elongated vertically in the explosion pot until the distance traveled is sufficient to withstand

the voltage between contacts. The increase in internal pressure due to the Splitting up and vaporization

of oil by the arc creates a rapid movement of the extinguishing

Medium round the arc this self-quenching effect causes a rapid cooling of the ionized column along its

whole length due to partition of the explosion pot and the dielectric recovery is sufficiently rapid.

To prevent the arc restricting after a natural Passage Through zero. The electric arc itself has, therefore,

supplied the necessary energy for its own extinction. There are now numerous manufacturers of small

oil volume breakers. However, to illustrate the principles of working, the sectional view of working

portion of 170 kV 3500 MVA. Breakers of

M/s Delle France has been shown in Figure the most important part of the breaker is its extinguishing

chamber. This takes the form of an insulating cylinder containing oil, in the axis of which moves the

contact rod and within which breaking occurs.

The arcing chamber is supported at its base by a casing enclosing a mechanism whose function is to

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move the contact rod. According to the impulses given by the control mechanism. In the on position, the

current flows from the Upper current terminal (1) to the contact fingers, (2) Follows the movable

contact rod (7) and reaches the current terminal (10) across the lower contact fingers (8). At the

beginning of the stroke and before breaking, the contact rod strongly pulled down. Wards by the

tripping springs, starts a high-speed opening motion. Then, an arc strikes between the contact rod tips

(6) and the stationary. Arcing ring (3) protecting the upper contact fingers.

At this moment gases escape without hindrance towards top of the apparatus.

The contact rod rapidly reaches a very high linear speed; it moves the arc downwards and forces it to

enter the explosion pot (5) where it is maintained rectilinear and is elongated in a direction opposite to

the release of gases towards fresh oil. Since the arc is as short as possible, the arc voltage is minimized

and the energy dissipated is reduced.

Still, since the gases can no longer develop freely, they generate a considerable pressure in the

explosion pot (5), thus producing a violent upward axial blast of oil vapor, exhausting the highly ionized

gaseous mass.

The optimum distance is thus obtained, the jet of oil causes the dielectric strength to be rapidly

increased, and at the following current zero, the arc is impeded from restricting and the breaking is thus

achieved. The explosion pot (5) is intended to withstand high pressures.

It is partitioned into several components by means of discs whose function is to retain a certain quantity

of fresh oil while the first break is proceeding; this allows a second break to occur with complete safety

at the full short circuit current. The low oil content Circuit Breakers require separate current

Transformers of wound type. Still at all voltages from 33 kV and above the costs of these breakers

inclusive of current Transformers compete favorably with that of the Bulk Oil Breakers.

In addition there are certain other advantages which may be summed up as under:

(I) Light and reduced size rendering transport

(ii) Simple construction making erection easy.

(iii) Quick and simple maintenance.

One of the limitations put forward against this class of breakers is frequent maintenance, owing to

reduced quantity of oil and consequent liability to quick carbonization, on circuits susceptible to

frequent trappings because of too many faults.

Interruptions on lines carried on pin insulators are rather too many on account of poor workmanship,

inadequate and improper maintenance.

However, for this reason alone, it may not be worthwhile to reject these breakers unless the difference

in cost with Bulk Oil Breakers is meager. For this very reason doubt was expressed about the ability of

these breakers for rapid reclosing duty. However, low oil content breakers have been designed and

constructed for rapid reclosing duties by established makers of this class of breakers. Rated breaking

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capacities in general are covered securely by a circuit breaking of any design but, depending on the arc

extinguishing principles employed, difficulties are sometimes encountered in performing certain specific

duties.

The situations where the small oil volume breakers are, presently considered at disadvantage are:

(I) Switch unloaded lines.

(II) Evolving faults.

(III) Out of phase disconnection.

The small oil volume breakers have distinct advantage over the air blast breakers under the following

conditions:

1. Kilometric faults.

This is because the oil Circuit Breakers are much less sensitive to the natural frequency of the restricting

voltage.

2. Disconnection of Transformers on load.

The current chopping phenomenon which causes over voltages, before natural zero, is not serious in this

class of breakers as the arc extinguishing. Energy is always proportional to the broken current.

1 – Circuit Breaker pole

2 – Mechanism housing

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2a – cover of mechanism housing

3 – Pole head

4 – Pole cylinder

5 – Crank housing

6 – Upper main terminal

13 – Bottom main terminal

22 – Vent housing

23 – 0il level indicator

39a – square on charging shaft

47 – Spring condition indicator

82 – off push - button

88 – on push – button

98 – Circuit Breaker indicator

99 – Operation counter

119 – Lifting hole for transport

Summary of Oil Circuit breaker operation & construction is given as below.

1) Bulk oil circuit breakers

Which use a large quantity of oil. In this circuit breaker, the oil serves two purposes. Firstly, it

extinguishes the arc during opening of contacts and secondly it insulates the current conducting

parts from one another and from the earthed tank. Such circuit breakers are classified into:

a) Plain oil circuit breakers

b) Arc control circuit breakers

In the former type no means is available for controlling the arc and the contacts are exposed to the

whole of the oil in the tank. In the latter special arc control devices are employed to get the beneficial

action of the arc as efficiently as possible

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2) Minimum oil circuit breakers ,

Which use minimum amount of oil. In such circuit breakers oil is used only for arc extinction, the current

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

Construction

There are two chambers in a low oil circuit breaker, the oil in each chamber is separated from each

other. The main advantage of this is that low oil is required and oil in second chamber won’t get

polluted. Upper chamber is called the circuit breaker chamber and lower one is called the supporting

chamber. Circuit breaking chamber consists of moving contact and fixed contact. Moving contact is

connected with a piston it’s just for the movement of the contact and no pressure build due to its

motion. There are two vents on fixed contact they are axial vent for small current produced in oil due to

heating of arc and radial vents for large currents. The whole device is covered using Bakelite paper and

porcelain for protection. Vents are placed in a tabulator.

Operation

Under normal operating conditions, the moving contacts remain engaged with the upper fixed contact.

When a fault occurs, the moving contact is pulled down by the tripping springs and an arc is struck. The

arc vaporizes oil and produces gases under high pressure. This action constrains the oil to pass through a

central hole in the moving contact and results in forcing series of oil through the respective passages of

the turbulator. The process of turbulation is orderly one, in which the sections of arc are successively

quenched by the effect of separate streams of oil, moving across each section in turn and bearing away

its gases.

Advantages

A low oil circuit breaker has following advantages compared to bulk oil circuit breaker

1. It requires lesser quantity of oil

2. It requires smaller space

3. There is reduced risk of fire

A low oil circuit breaker has following disadvantages compared to bulk oil circuit breaker

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

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

3. The dielectric strength of oil deteriorates rapidly due to high degree of carbonisation.

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Advantages & Disadvantages of Oil Circuit Breakers

Advantages

(a) By connecting several interrupting mechanisms in series, the voltage rating of the breaker can

be increased.

(b) By careful design the interrupting capacity rating can be increased up to 26,000 MVA.

(c) Quiet operation.

Disadvantages

(a) The breaker contains flammable oil, consequently should be located outdoors.

(b) Oil breakdown at high temperatures forms carbon, which is dissolved in the oil. This increases

the oil conductivity. To keep the oil insulating properties at an acceptable level, it must be

purified after a predetermined number of breaker operations. This requires oil treatment

equipment on site.*

(c) may become an environmental hazard if spillage occurs.**

*Oil purification standards for the breakers are the same as for the transformer oil.

**Askerol is both an environmental and a health hazard.

Comparison Air blast Circuit breakers with Oil Circuit Breaker

Advantages:

How air blast circuit breaker is better than oil circuit breaker:

1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small.

This reduces the size of device.

2. The risk of fire is eliminated.

3. Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent

operation is required.

4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive

operations; the expense of regular oil is replacement is avoided.

5. The energy supplied for arc extinction is obtained from high pressure air and is independent of the

current to be interrupted.

6. The arcing time is very small due to the rapid buildup of dielectric strength between contacts. Therefore,

the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts.

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Disadvantages:

1. Considerable maintenance is required for the compressor plant, which supplies the air blast.

2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage.

3. Air blast circuit breakers are finding wide applications in high voltage installations. Majority of circuit

breakers for voltages beyond 110 kV are of this type.