Circuit Breakers Types & Principles

How Circuit Breakers Work

Breaker Design

The simplest circuit protection device is the fuse. A fuse is just a thin wire, enclosed in a casing, that plugs into the circuit. When a circuit is closed, all charge flows through the fuse wire -- the fuse experiences the same current as any other point along the circuit. The fuse is designed to disintegrate when it heats up above a certain level -- if the current climbs too high, it burns up the wire. Destroying the fuse opens the circuit before the excess current can damage the building wiring.

The problem with fuses is they only work once. Every time you blow a fuse, you have to replace it with a new one. A circuit breaker does the same thing as a fuse -- it opens a circuit as soon as current climbs to unsafe levels -- but you can use it over and over again.

The basic circuit breaker consists of a simple switch, connected to either a bimetallic strip or an electromagnet. The diagram below shows a typical electromagnet design. The hot wire in the circuit connects to the two ends of the switch. When the switch is flipped to the on position, electricity can flow from the bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal.

The electricity magnetizes the electromagnet . Increasing current boosts the electromagnet's magnetic force, and decreasing current lowers the magnetism. When the current jumps to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off.

A bimetallic strip design works on the same principle, except that instead of energizing an electromagnet, the high current bends a thin strip to move the linkage. Some circuit breakers use an explosive charge to throw the switch. When current rises above a certain level, it ignites explosive material, which drives a piston to open the switch.

Why Circuit Breakers Trip

The circuit breaker, the wire and even the wire insulation are all designed to work as a system—and that system has limits. Try to push more current through a circuit than it’s designed for and things start happening (Fig. B). Wires heat up under the burden of carrying the excess current. When this happens, the insulation around the wire can degrade or even melt. When insulation melts, current is no longer confined within the wire. That’s when fires start. Luckily, the circuit breaker senses the excess current and “trips” to stop the flow of power before damage occurs.

(FIG. A ) A PROPERLY FUNCTIONING 15-AMP CIRCUIT

This circuit has wires and a circuit breaker that can easily carry the amperage required by the devices on it. One Simple Equation Helps You Determine If a Circuit Is Overloaded. To start solving the problem, we need to know one simple “rule of thumb” formula. This formula will help us determine if all the electrical stuff on a particular circuit is overloading it. How do they all fit together?

(FIG. B) AN OVERLOADED CIRCUIT

This circuit has too many energy-demanding devices on it and is trying to carry more amperage than it’s designed for. Things begin to heat up. Luckily the circuit breaker senses this, trips and “breaks” the circuit. The simple formula (Fig. C) tells us how: Watts divided by voltage equals amps. The other equations shown are just other ways of saying the same thing.

FIG. C THE BASIC FORMULA

But when you plugged in the 1,200-watt space heater, the 10 amps it required, plus the draw of the other two devices, pulled 19.6 amps through a 15-amp system (Fig. B). the system just can’t handle the load. The circuit breaker tolerated this for a while. But when the excess current and resultant heat began deforming the two pieces of metal inside the breaker, they started “pulling the trigger.” And when the metal pieces bent to a certain point, the trigger snapped two contact points apart, interrupting the flow of electricity and shutting down that circuit. If there’s a huge, sudden draw on a circuit, a little electromagnet in the circuit breaker can pull the contact points apart too. If you have fuses, the excess heat melts a wire inside the fuse, which in turn stops the flow of electricity.

Appliance Power Required (watts)

Electric Range 10,000 (240 volts)

Electric Dryer 5,000 (240 volts)

Space Heater 1,000 and up

Clothes Washer 1,150

Furnace (blower) 800

Microwave 700–1,400

Refrigerator (not required) 700

Freezer (not required) 700

Dishwasher 1,400

Central Vacuum 800

Whirlpool/Jacuzzi 1,000 and up

Garbage Disposer 600–1,200

Kitchen Countertop (two circuits)

Toaster 900

Coffee maker 800

Toaster oven 1,400

Bathroom

Blow dryer 1,000–2,000

FIG. D COMMON DEDICATED CIRCUITS

FIG. E WIRE S

The process of fault clearing has the following sequence:1- Fault Occurs. As the fault occurs, the fault impedance being low, the currents increase and the relay gets actuated. The moving part of the relay move because of the increase in the operating torque. The relay takes some time to close its contacts.

2 - Relay contacts close the trip circuit of the Circuit Breaker closes and trip coil is energized.

3 - The operating mechanism starts operating for the opening operation. The Circuit Breaker contacts separate.

4 - Arc is drawn between the breaker contacts. The arc is extinguished in the Circuit Breaker by suitable techniques. The current reaches final zero As the arc is extinguished and does not restrict again.

The Trip-Circuit

STANDARD RATINGS OF CIRCUIT BREAKERS AND THEIRSELECTION

1. Rated voltage.2. Rated insulation level.3. Rated frequency.4. rated current.5. Rated short Circuit Breaking current.6. Rated transient recovery voltage for terminal faults.7. Rated short circuit making current.8. Rated operating sequence.9. Rated short time current.

Arc Formation in Circuit BreakersWhen the circuit breaker contacts are in the closed position they are pressed together by the contact force which results in elastic deformation of the material of the contact surface. Due to this fact (contact pressure) it could be said that the junction resistance is very low. As the contacts starts opening the pressure is reduced and the elastic deformation is getting reduced gradually. This reduction in pressure causes increase in the junction resistance, resulting in rapid increase in a temperature on the

contact surface. This effect is much greater if the contacts are not in a good, clean and smooth condition. As the area in contact reduces, the current density in this area increases which produces, “Hot spots”. The rise in temperature combined with the electrical stress by the voltage across the gap, at the instant of contact separation causes ionisation of the medium between the contacts. This ionisation of the medium provides a conducting path for the arc. The arc across the contacts of circuit breakers is an undesirable element. Hence, we must find some suitable means to extinguish the arc in minimum possible time. As the contact is going on separating the gap across the contact is also increasing. This results in increase in the length of the arc and the

resistance. At a pre-determined distance (gap) the resistance (length) is very high, the source supply is not able to sustain the arc, and the arc gets extinguished. Hence, it could be said that, to extinguish the arc we must increase the arc resistance.

Arc LengtheningResistance is directly proportional to length of the arc and inversely

proportional to the cross sectional area i.e. L R = ------- AAt this stage let us assume the arc to be equivalent to a conductor. Then, = Resistivity of the arcL = Length of the arcA = Area of the cross-section of the arcR = Resistance of the arc

Arc CoolingThe voltage required to maintain the ionisation increases with a

decrease of temperature so that cooling effectively increases the resistance.

Arc ConstrainingIf the arc can be constrained into a very narrow channel, the resistance

gets increased due to reduction in cross-sectional area and ionisation decreases for a given voltage.

Arc SplittingThere are two methods: -

1. The arc is forced into an arrangement of splitters by which the arc is lengthened. The Lengthening of the arc improves the cooling of the contacts with the splitters so that resistance is increased.

2. The arc is made to split into number of smaller arcs. The idea here is to ensure that the sum of the cathode – anode voltage drops of short length should be more than the supply voltage thereby the energy fed to the arc is reduced.

ARC INTERRUPTION THEORIESThere are two methods of extinguishing the arc in circuit breakers viz.

1. High resistance method2. Low resistance method

High Resistance Method: -In this method, arc resistance is made to increase with time so that current is

reduce to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is an enormous energy is dissipated in the arc. Therefore, it is employed only in D.C. circuit breakers and low capacity A.C. circuit breakers.

Low Resistance or Current Zero Method: -This method is employed for arc extinction in A.C. circuits only. In this

method, arc resistance is kept low until current zero when the arc extinguishes naturally and it is prevented from resisting inspire of the rising voltage across the contacts. All modern high power A.C. circuit breakers employ this method for arc extinction. In an A.C. System, the current drops to zero after every half cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact known as restriking voltage. If such a break down can occur, the arc will persist for another half cycle. After current zero, the dielectric strength of the medium between the contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted.

Terminologies of circuit breaker

Insulation LevelThis is the voltage level, which determines the principle dielectric

properties of the equipment. This is chosen on the basis of the system’s BIL (Basic Impulse Level).

Opening TimeOpening time is the time between the instant of application of tripping

power to the circuit breaker in enclosed position and the instant of separation of the contacts.

Arc DurationArc duration is the time between the instant of separation of the circuit

breaker contacts and the instant of arc extinction of the short circuit current, excluding resisting current duration if any.

Total Break TimeThe total break time is the sum of opening time and arcing time.

Breakers are now available with total break time varying from 2.5 cycles to 8 cycles.

Make TimeThe make time of the circuit breaker is the time between the initiation

of the closing operation and the instant when the contacts touch each other. It includes the operating time of any auxiliary equipment necessary to close the circuit breaker.

Rate Of Rise Of Restriking Voltage (R.R.R.V.)It is a rate, expressed in volts per micro second, representing the

increase of the restriking voltage. For a restriking voltage having a single frequency transient component, the R.R.R.V. is obtained by dividing the maximum of the oscillation by the duration of the first half wave.

Peak value of restriking voltageRRRV = ------------------------------------------

Time taken to reach to peak value

Peak Restriking VoltageIt is the maximum instantaneous voltage attained by the restriking

voltage.Restriking Voltage

The resultant transient voltage, which appears across the breaker contacts at the instant of arc extinction, is known as the restriking voltage.

Recovery VoltageThe power frequency RMS voltage that appears between the breaker

contacts after the transient oscillations die out and final extinction of arc has resulted in all the poles is called the recovery voltage.

Active Recovery VoltageIt is defined as the instantaneous recovery voltage at the instant of the

arc extinction.

Symmetrical Breaking CapacityIt is the RMS value of the AC. component of the current which the circuit

breaker is capable of breaking at a given recovery voltage and under specified conditions (viz. power factor, rate of rise of restriking voltage).

Asymmetrical Breaking CapacityIt is the RMS value of the combined AC. & DC components of the current,

which the circuit breaker is capable of breaking at a stated recovery voltage and stated reference restriking voltage under prescribed conditions.

Making CapacityThe peak value of current during the first cycle of current wave, after the

closure of circuit breaker is known as making capacity.

Short Time RatingIt is the period for which the circuit breaker is able to carry the fault current

while remaining closed.

Normal Current RatingIt is the RMS value of current, which the circuit breaker is capable of carrying

continuously at its rated frequency under specified condition.

CIRCUIT BREAKER TYPES

Circuit breaker classifications are broadly made on the location of installation Indoor circuit breaker Outdoor circuit breaker

Many insulating mediums are used for arc extinction and the medium chosen depends upon the rating and type of circuit breaker1.Air Circuit Breaker.

i. Plain break circuit breakerii. Magnetically blow out circuit breakeriii. Arc splitter type circuit breakeriv. Air blast circuit breaker.

2.Oil Circuit Breaker (tank type of bulk oil)i. Bulk oil circuit breakerii. Minimum or small oil circuit breaker

3.Gas circuit breaker-Sulphur Hexafluoride (SF6) circuit breakeri. Single pressure ii. Double pressure

4. Vacuum Circuit Breaker.

Types of circuit breaker with medium of arc quenching and rated voltage.

Operating Mechanism

Motor Operated Spring Closing Mechanism & Solenoid Operated Mechanism Electro-hydraulic operated circuit breakers Electro-Pneumatic operated circuit breakers

Air Circuit Breaker (ACB)

Of all the types mentioned in the previous chapter that the ACB is the simplest form of circuit breaker. In this circuit breaker the arc interruption process is based on the natural deionization of gases by a cooling action and lengthening of the arc. There are various types of air break circuit breakers. We shall study about ACB in brief in the following paragraphs.

Plain Break TypeIn this type of Circuit Breaker the contacts are made in the shape of two horns.

The arc initially strikes across the shortest distance between the horns, (refer figure2.1) and it is then driven steadily upwards. As the contacts goes on separating the gap between them increases and the arc also follows the contacts. When the horns are fully separated the arc extends from one tip to the other resulting in arc lengthening and cooling, thus extinguishing the arc. The relative slowness of the process and the possibilities of the arc spreading to adjacent metal work limits the application of this type to about 500 volts and in low power circuits only.

Magnetic Blow-out TypeIn a number of ACB used up to 11 kV, the extinction of the arc is

carried out by means of a magnetic blast. To achieve this the arc is subjected to the action of a magnetic field set up by coils connected in series with the circuit being interrupted. Such coils are called blow out coils because they help in the arc being magnetically blown out. The magnetic blow -out coils are shown in the figure 2.1. The arc is blown magnetically into arc chutes where the arc gets lengthened, cooled and extinguished. The arc shield prevents spreading of the arc to adjacent metal work. As the breaking action becomes more effective with heavy currents this principle has resulted in increasing the breaking capacities of these breaker to higher values. At this stage we should consider the functions of the arc chute.

This is an efficient device for quenching the arc in air, while quenching the arc it performs the following actions: It confines the arc within a restricted space. It provides mechanical protection for the personnel for any external object.

It provides rapid cooling of the gases to ensure extinction by de-ionisation.

Arc Splitter TypeIn this type of circuit breaker the blow out consists of steel inserts in the arcing

chutes. These are so arranged that the magnetic field induced by them, by the current in the arc moves it upwards still faster. The steel plates divide the arc into a number of short arcs in series (Fig.2.2).

The distribution of voltage along the length of arc across the plates are not linear but is accompanied by large voltage drops. This voltage drop automatically helps for quick extinction. When the arc comes into contact with the relatively cool surface area of steel plates, it gets rapidly and effectively cooled. The movement of arc may be natural or assisted by a magnetic blow out. Different types of air break circuit breakers; we shall study the parts of a typical unit. The breakers generally consists of three single pole units linked together by an insulated cross bar. Operating mechanisms depending upon the current ratings operates them. The following are the main parts: -

Main ContactsThese contacts carry rated continuous current and consist of moving contact

and fixed contact.

Moving Contact Assembly These are solid copper spring loaded rollers ringed with silver. The

following rollers freely roll few degrees, each time the contact is operated. This action ensures uniform wear and tear.

Fixed Contact Assembly They are two levelled copper contact bars, with silver pads at contact

faces. Rollers bridge these two fixed contacts. This arrangement provides for durability and minimum temperature rise.

Arcing Contacts These are the contacts, which undergo the effects of the arc. They close

first and open after the main contact.

Arc Chute All arc chutes are made of an insulating arc resisting material and

surrounds each pole unit. The dimensions of the chute depend on the number of arcing contacts. At the top inner surface, the arc chute is fixed with steel plates. The purposes of these steel plates are to increase the speed of upward rise of the arc into the chute by magnetic action. It also splits the arc and assists in cooling the arc.

Operating Mechanism Usually these mechanisms are designed either for manual operation or

electrical or pneumatic operations. This mechanism takes care of trip free operation, opening and closing of contacts. Also provides a lockout feature preventing closing while any work is being carried out.

Arc Runners or Arcing Horns As soon as the arc leaves the vicinity of the contacts, it commutes to a pair of

run out horns. Simultaneously the blow out coil is energised. In doing so outer blow out system is

switched on. This blow out coil provides a magnetic field, which causes the arc to travel upward and thereby length is increased. As the length of the arc is increased at a particular stage the system voltage is unable to sustain the arc and the arc gets extinguished. Figure 2.3 shows the typical diagram of truck mounted air break circuit breaker shown in closed position.

AIR BLAST CIRCUIT BREAKER

IntroductionAir blast circuit breakers are used mainly on 11 to 1100 kV

applications. They offer several advantages such as faster operations, suitability for repeated operation, auto-reclosure, unit type multi break construction, simple assembly and modest maintenance, etc. A compressor plant is necessary to maintain high air pressure in the air receiver. Air blast circuit breakers are especially suitable for railways and arc furnace, where the breaker operates repeatedly. Air blast circuit breakers are used for interconnected lines and important lines where rapid operation is desired.

Construction of Air Blast Circuit BreakerIn 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 ionised particles between the contacts are blown away by the blast of the compressed 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 there after the isolator opens immediately, to provide additional gap.

In EHV circuits of today, isolators are generally, independently mounted. Fig.3.1a&b show one pole of the EHV air blast circuit breaker. In the complete assembly there are three identical poles.

Description High-pressure air, at a pressure of 20 to 30 kg/cm2 is stored in the Air

reservoir (Item 1 in Fig.3.1a&b) Air is taken from compressed air system. Three hollow insulator columns (Item 2) are mounted on the reservoir with valve (6) at their base. The double arc extinguishing chambers (3) are mounted on the top of the hollow insulator chambers. The current carrying parts (9) connect the three arc extinction chambers to each in series and the pole to the neighbouring equipment. Since there exists a very high voltage between the conductors and the air reservoir, the entire arc extinction chamber assembly is mounted on insulators. The figure fig. 3.1(b) shows the double arc extinction chambers (3). Since there are three double arc extinction poles in series, there are six breaks per pole. Each arc extinction chamber in Fig.3.1 (b) consists of one twin fixed contact (7). There are two moving contacts (8), which are shown in opened condition. The moving contact can move axially so as to open or close. Its position open or close depends on the air pressure and spring (10) pressure. The operating mechanism (3) operates the rod (5) when it gets a pneumatic or electrical signal. The valve (6) open so as to send the high-pressure air in the hollow of the insulator. The high-pressure air rapidly enters the double arc extinction chamber [Air inlet in Fig.3.1 (b)]. As the air enters into the arc extinction chamber the pressure on the moving contact (8) becomes more than the spring pressure and the contacts open. The contacts travel through a short distance against the spring pressure. At the end of contact travel the port for outgoing air (15) is closed by the moving contact and the entire arc extinction chamber is filled with high pressure air, as the air is not allowed to goes out. However, during the arcing period the air goes out through the opening (11) and takes away the ionised air of arc.

While closing, the valve (6) is turned so as to close the 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 the air in the arc extinction chamber (3) drops down to the atmospheric pressure and the moving contacts (8) close over the fixed contacts (7) by virtue of the spring pressure.

The opening is fast because the air takes 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 valve (6) operates and the contacts close by virtue of the spring pressure. The construction described above applies to air blast circuit breakers for

EHV applications, for voltages above 145 kV. For voltages of 420 kV and more, the constructions is modified by adding required number of arc interruption chambers in series. Air blast circuit breaker requires an auxiliary compressed air system. Air blast circuit breakers for 12 kV and below have a different type of construction. In this breakers usually there will be only a single break in the interrupter chamber.

Typical ratings of Air blast circuit breakers are: -12 kV, 40 kA22 kV, 40 kA145 kV, 40 kA, 3 cycle245 kV, 40 kA, 50 kA, 2 ½ cycle420 kV, 40 kA, 50 kA, 63.5 kA, 2 cycle

The grading capacitors are connected across the interrupter unit for the equal distribution of voltage between the units. Closing resistors are connected across the interrupter units for limiting the over voltages during closing operation. Opening resistors are connected across the interrupter units to make the circuit breakers restrike free.

Principle of Arc Quenching in ABCBThe air blast circuit breaker needs an auxiliary compressed air system,

which supplies air to the air receiver of the breaker. For opening operation, the air is admitted in the arc extinction chamber. It pushes away the moving contacts against the spring pressure. In doing so, the contacts are separated and the air blast takes away the ionised particles along with it and assists in arc extinction. After a few cycles the arc is extinguished by the air blast and the arc extinction chamber is filled with high-pressure air (30 kg/cm2). The high-pressure air has higher dielectric strength than that at atmospheric pressure. Hence a small contact gap of few centimetre is enough. The nozzle shaped contacts guides the flow of air around the contacts. It may be axial, cross or a suitable combination. [Fig.3.2 (a), (b)] In the axial blast type in Fig.3.2 (a) the flow of air is longitudinal along the arc.

In axial blast type, the air flows from the high-pressure reservoir to the atmosphere through a convergent divergent nozzle. The difference in pressure and the design of nozzle is such that as the air expands into the low-pressure zone, it attains almost supersonic velocity. The mass flow of air through the nozzle is governed by the parameters like pressure ratio, area of throat, nozzle throat diameter and is influenced by the diameter of the arc itself. The air flowing at a high speed axially along the arc causes removal of heat from the periphery of the arc and the diameter of the arc reduces to a low value at current zero. At this instant the arc is interrupted and the contact space is flushed with fresh air flowing through the nozzle.

The flow of fresh air through the contact space ensures removal of hot gases and rapid building up of the dielectric strength.

The principle of cross-blast illustrated in Fig.3.2 (b) is used only in circuit breakers of relatively low rating such as 12 kV, 500 MVA and below.

Experience shows that in cross blast, the airflow pushes the arc and the length of the arc is increased. Due to the increase in the arc length the arc resistance also increases. During the period of arc extinction, the air continues to flow through the nozzle and air goes to the atmosphere. Increasing the pressure of the compressed air increases the mass flow rate. The increase in the mass flow results in increased breaking capacity. After the brief duration of airflow, the interrupter is filled with high-pressure air. The dielectric strength of air increases with pressure. Hence the fresh high-pressure air in the contact space is capable of withstanding the transient recovery voltage. After the arc extinction, the interrupter chamber is filled with high-pressure air. For closing operation, the air from this chamber is let out to the atmosphere. Thereby the pressure on the moving contacts from one side is reduced and the moving contacts close rapidly by the spring pressure (Fig.3.3a & b). The air blast circuit breakers come under the class external extinguishing energy type. The energy supplied for arc extinction is obtained from high-pressure air and is independent of the current to be interrupted.

Circuit Breakers with External Extinguishing EnergyIf the pressure generated in the arc extinction chamber is derived from arc

current e.g. by decomposition of oil in the oil circuit breaker, the circuit breaker is said to be of internal energy source. If the pressure is independent of arc current the circuit breaker is said to be of external energy source. The behaviour of these two types is inherently different. In the air blast circuit breakers the air pressure used for the arc interruption is constant and does not depend on the arc current. The compressed air pressure is of such magnitude that it can break the rated breaking current (say 20 kA) satisfactorily at natural zero.

The arcing time does not change appreciably for lower magnitudes of currents, as the air pressure is independent of arc current. Now, consider that the breaker has to interrupt small currents. For this current if the air pressure used for the arc interruption is too high, the current gets chopped out before reaching natural zero. This current chopping gives rise to high restriking voltage. The resistance of contact space being high, the contact space is not likely to break down. Hence resistance switching should be employed to take care of restriking voltages.

The arcing time of ABCB is almost independent of arc current (Fig.3.4). Whereas in oil circuit breaker the arcing time is more for lower currents [Fig.3.3 (a)] and the restriking voltages are damped out even with low contact space due to higher

dielectric strength. In the circuit breakers with external energy source the pressure of extinguishing medium determines the breaking capacity of the unit. In circuit breakers with internal energy source the design features determine the capacity limit.

Resistance Switching in ABCBWe have noted earlier that the post zero resistance of contact space is high in

air blast circuit breakers. This is because, the contact clearance space is filled with high-pressure air after final current zero and also the high-pressure air has high dielectric strength. The high restriking voltage appearing across the contacts does not damp out through the contact gap because of the high post zero resistance. Further, voltages of the order of several times the normal voltage appear across the contacts because of current chopping. If these voltages are not allowed to discharge, they may cause break down of insulation of the circuit breaker or the neighbouring equipment. To overcome this difficulty, “Resistance switching” is adopted. The usual procedure is to connect a resistance across the arc.

Fig.3.5 shows another popular arrangement used for a double arc-extinguishing chamber. During the opening operation air is admitted in the arc-extinguishing chamber. It separates main contacts and pushes the auxiliary contacts. The auxiliary contacts close, thereby the resistors are connected a cross the arc for a short time. The auxiliary contacts are located in the inclined V shaped insulators while the resistors are located in the vertical insulators. Immediately after arc extinction, the pressure on either side of the piston of auxiliary contacts gets so adjusted that the auxiliary contacts open and resistor circuit is interrupted. Ceramic resistors of non-

linear characteristics similar to those used in the lightning arrestors are used for resistance switching.

These resistors are made of silicone carbide, bound by inorganic binders subjected to heat treatment. During high current non-linear resistor offers low resistance. Thus the main arc currents is partially diverted through resistor unit. When the current reduces, the resistance offered by non-linear resistor increases, causing a greater drop across the resistor units. Thereby the voltage available for arc between auxiliary contacts is no more sufficient and arc between auxiliary contacts is automatically extinguished.

Merits Of Air Blast Circuit Breaker and Air Circuit Breaker1. Can be used at high pressure.2. Reliable operation due to external source of extinguishing energy.3. Free from decomposition.

4. Clean, non-inflammable.5. Freely available everywhere.6. Fresh medium is used every time. Hence the breaker can be repeatedly operated. 7. At high pressure, a small contact travel is enough.8. The same air serves the purpose of moving the contact and arc extinction.

The demerits of ABCB, Complex design of arc extinction chambers Complex operating mechanism, Problems due to switching over voltages. (Switching over voltage is reduced by

pre-closing resistors) Auxiliary high-pressure air system is necessary. The cost can be justified if there

are several breakers in the switching yard. Problem arising out of compressed air system

Oil Circuit breaker

Constructional Details of an OCBThe tank type or bulk oil circuit breaker has 3 separate tanks for 72.5 kV and

above. For 36 kV and below single tank construction is popular. In single tank construction phase barriers are provided between each phase. These types of circuit breaker are used for indoor metal-clad draw out type switchgear up to 12 kV. (referfig.4.1) Above 12 kV, it is usually of out door type. Dielectric oil (Transformer oil) is used in circuit breakers as an arc extinction medium as well as insulating medium between the live contacts, oil tank and earth. The contact separation takes place in steel tanks filled with oil. The gases formed due to the heat of the arc expand and set turbulent flow in the oil. This turbulence of the oil and the gases formed in the oil cause the cooling effect. Thus the arc is extinguished.

Mainly there are two types of oil break circuit breakers: - Plain break oil circuit breaker Arc control circuit breaker

Plain Break Circuit BreakerIn this type of circuit breaker when the contacts are separated, the arc is burning

freely in the oil. This can be seen in the fig.4.2. As shown in the figure, the steel tank is filled with transformer oil and the tank is sealed air tight leaving the required space for the fitting of the terminal bushings. The bushing carries the fixed contacts. A tension rod is (which is subjected to the up and down motions) attached to the moving contact bridge as shown in the figure. The contact tips are made of copper and the main contacts are made up of hard drawn copper coated with silver. The tension rod is operated like any other operating mechanism as in the air circuit breaker.

OperationWhen the arc is struck in the oil at the instant of contact separation, the

oil in the proximity of the contacts gets decomposed. The arc thus becomes surrounded by the gas bubbles, and oil vapours. The gas produced due to the decomposition of oil comprises of 60 to 80% hydrogen and smaller proportions of acetylene and other gases. Because of good thermal conductivity of hydrogen, de-

ionisation of arc takes place. Hence hydrogen is one of the most efficient extinguishing media. The gas produced creates a turbulent action, which causes particles of oil to penetrate into the arc core where they draw off the heat by evaporation. Because of the absence of effective control over the arc, the arcing time and the amount of energy released before interruption often vary over a wide range. This factor calls for a larger safety in the oil tank and airtight sealing of the tank. In case there is an air cushion the hydrogen may pass through the oil and mix up with air thus forming an explosive mixture. The arcing time can be effectively reduced (for faster quenching of the arc) by introducing the arc control device in the plain break circuit breaker.

Arc Control Circuit BreakerThe bulk oil circuit breakers generally employed in our power systems have an

arc control device. In this type of breaker the gases produced during arcing are confined to small volumes by the use of an insulating rigid arc chamber surrounding the contacts. The working principle and the basic constructions are detailed here. The contacts of the OCB are immersed in oil in a cylindrical container made of insulating material. The cylindrical container is called as Explosion Port or Explosion chamber. There is a hole in the bottom of the container through which a rod passes. This rod is attached to the switching bridge of the device. The other general construction is the same as for a CB with arc burning freely in oil. At the instant of contact separation, an arc is struck between the contact, which forms gas bubbles on account of heat generated (as seen in the given figure 4.3). The gas so produced thus builds up high pressure since the gas is confined in a small volume in the explosion chamber and forces the gas through or around the arc, to extinguish it. Thus arc quenching is done effectively.

Advantages of Explosion Chamber

By incorporating the explosion chamber the following advantages are achieved The initial pressure impulse acts upon the explosion chamber and does not affect

the container of the circuit breaker. The explosion chamber is designed to withstand fairly high pressure, high temperature and high dielectric strength.

Due to better cooling de-ionisation is considerably accelerated. The duration of arc is many times shorter than the breakers without the explosion

chamber. Hence the quantity of energy generated in the arc is proportionately smaller. As a result the arc is extinguished faster.

In order to meet the increased system operating voltage the capacity of the breakers were enhanced by incorporating various modifications to the basic design of the explosion chamber.

DIFFERENT TYPES OF ARC CONTROL DEVICESide Vented explosion pot

It consists of laminations (vulcanised fibre glass materials) punched with centre hole for free movement of moving contacts and with laminations punched with centre hole and side vent for release of gas. These laminations are arranged as shown in the figure4.4.The moving contact is withdrawn vertically downwards through a tight fit throat (formed by the centre hole). When the moving contact separates, due to high temperature the oil molecules get ionised and thus an arc is struck. Due to arc the oil gets decomposed and hydrogen and other gases are produced. These gases are entrapped until the moving contact moves slightly below the vent; till then high gas pressure is developed. The pressurised gas escapes through the vent at a high velocity pulling the arc towards the vent thus increasing the arc length. Due to these process of cooling and lengthening the arc is quenched quickly.

In the above design the quantity of oil in the explosion chamber is less leading to starvation of oil for gassification for higher capacity breakers. This deficiency is overcome by providing for additional oil supply in the explosion chamber as discussed below.

Cross jet explosion potThe construction of cross jet is similar to that of side vented explosion

pot except for the extra provision for supply of the oil to avoid starvation. As the clearance around the moving contact is small, the gasses evolved during arcing operation remain in arc path. The arc splitters (vents) help in increasing the arc length

and help in quenching the arc. Refer fig.4.5. The arc is struck and gas is formed around the arc. As the moving contact passes the first arc splitter, the gases are expelled through it, simultaneously as the gases get released the space is replenish by fresh oil through the back passage. The arc will be quenched if arc current goes to zero; the arc can restrike if the de-ionisation of the arc is not complete. When the arc is restruck, large amount of gas is produced; thus, gas pressure builds up again and is released when the arc current again reaches zero. The gas pressure is not released (the arc products are not pushed across the arc path) because of the backpressure built by the fresh oil until the current goes to zero.

Oil blast explosion pot

This type of explosion pot consists mainly of three components, viz. upper fixed contact, intermediate (floating) contact and a hollow moving contact in the bottom. When the circuit breaker is the closed condition the fixed and floating contacts are under compressive force due to buffer spring and throw-off spring and the moving contact is subjected to tensile force due to trip spring. Hence all the three contacts are under pressure. The oil blast explosion pot has two chambers, the upper and the lower, and they are connected together through holes. When a fault occurs

the two lower (intermediate and moving) contacts move downwards together and an arc is established between the top fixed and intermediate moving contact. Due to this arc, a high pressure is developed in the upper tank and there is no relief for this pressure until the intermediate contact comes to rest after its maximum travel. Now, the moving contact detaches from the intermediate contact and another arc is established between the intermediate and the moving contact. The pressure created by the arc is subsided by the movement of the oil through the hollow moving contact. When the arc current goes to zero, the oil is forced through the arc and it is quenched. The only draw back is that arcing time is long. (Refer fig.4.6.)

Turbulator type explosion potIt is an arc control device, which is fitted to the top fixed contact. It

consists of oil impregnated vulcanised fibre plates which are held under compression. These plates are arranged so as to form a series of vents on one side of the arc and a series of oil pockets on the other side. The tip of the fixed contact is so arranged that when the circuit breaker opens the arc is so focussed that its extinction is facilitated. i.e. the arc is drawn in front of the vents. This arrangement of oil pockets generate turbulence to the gases during arcing which ensures the faster cooling and evacuation of the gases through the vents. The spring-loaded valves are provided at the top casing.

The function of spring loaded valve is as follows: -When arc is struck, the gas pressure closes the valve and when arc is

extinguished the pressure dies out and the valve is opened by the spring. The opening of the valve permits the oil to get replaced.

Minimum Oil circuit breaker

In the bulk oil circuit breaker large quantity of oil is required though only a small quantity is necessary for arc extinction. The large quantity is necessary to provide insulation between the live parts and the earthed steel tank. The entire oil in the tank is likely to get deteriorated due to sludge formation in the proximity of arc. Then the entire oil needs replacement. The tanks are too big in 72.5 kV and above, so the tank type oil circuit breaker looses its simplicity. The above mentioned reason led to the development of Minimum Oil Circuit Breaker. As the name itself signifies, this type of circuit breaker requires less oil. There is no steel tank but the arc extinction takes place in porcelain containers as shown in the figure 4.7. In the minimum oil circuit breaker the current interruption takes place inside the Interrupter chamber. The interrupter chamber is made of insulating material like porcelain for outdoor or fibre glass for indoor circuit breakers. The interrupter chamber encloses a fibre glass enclosure which accommodates fixed contact, arc control device and the moving contact. The clearance between the live parts and the enclosure is reduced and hence less quantity of oil is required. There are two chambers and they are separated from each other and filled with oil. The upper chamber is the interrupter chamber and the lowerchamber is the support chamber. The oil from the upper chamber does not mix up with bottom chamber, the lower chamber also acts as a dielectric support to the interrupter chamber. The arc extinction device is fitted to the upper fixed contact, which is ring shaped with many contact fingers. The moving contact moves upward for closing operation, a resin bonded glass fibre cylinder enclose the contact

assembly, which is also filled with oil. The figure 4.7 shows one pole as such there are three poles, one for each phase and they operate simultaneously

Construction Minimum oil circuit breaker mainly consists of:

Breaker pole Base frame Operating mechanism Support structureBreaker Pole

The primary functions of a circuit breaker is to interrupt short circuit current, to carry normal current, to switch in and out normal loads and to provide necessary insulation between live and earthed parts.

Breaker pole mainly consists of: Interrupter unit Support insulator Operating insulator (Not applicable to HLC)

Typical rating of the BHEL MOCB’s:S

.No.Type of

MOCBRated

service voltage in KV

Rated normal current in

Amps.

Rated Breaking

Capacity (KV)

1.

HLC 1/52/630

25 630 20

2.

HLC 72.5/1600

72.5 1600 20

3.

HLD 145/1250 B

145 1250 20

4.

HLR 84/2501 B

72.5 2500 40

5.

HLR 145/2502 B

145 2500 40

6.

HLR 245/2503 B

245 2500 40

7.

HLR 420/2505 B

420 2500 40

8.

HLR 145/2501 E

145 2500 31.5/40

9.

HLR 245/2502 E

245 2500 40

10.

HLR 420/2501 E

420 2500 40

Interrupter UnitThis is the top half of the pole filled completely with oil and is

supported by the support insulators. Interrupter unit mainly consists of two terminals, contact system with fixed and moving contacts, extinguishing chamber, etc. The fixed contact is a socket type, mounted at the top of the interrupting chamber and is electrically connected to the top terminal. The fixed contact consists of a set of spring loaded contact fingers providing necessary contact pressure and an arcing contact of high arc resistance material. The moving contact is a plug type which initiates making and breaking of the currents and moves vertically up wards and downwards. While opening, the moving contact separates from the fixed contact and an electric arc is drawn between the arcing contact on the fixed contact and the tip of the moving contact. The extinguishing chamber is the vital part of the breaker in which the arc gets extinguished smoothly and effectively. This is built up of insulating materials capable of withstanding high mechanical and electrical stresses. The extinguishing chamber is a contraction type with axial blast in case of HLC and HLD MOCB. In case of HLR MOCB the extinguishing chamber is a cross blast type. The interrupter chamber is filled with nitrogen to the required pressure, in avoid ingress of moisture in the oil and to achieve re-strike free operation while interrupting capacitive currents.

Support InsulatorSupport Insulator column is the bottom half of the breaker pole. This is

of solid core porcelain in case of HLD and HLR MOCB and hollow porcelain in case of HLC MOCB. The support insulator provides the necessary insulation between the lower terminal and the earthed base frame and provides support to the interrupter chamber.

Operating insulatorThe operating force to the moving contact is transmitted by means of a

solid core porcelain-operating insulator. This rotary or operating insulator is coupled to the moving contact in the interrupting chambers by means of mechanical linkages housed in the external mechanism housing. (In case of HLC breakers, the operating force to the moving contact is transmitted by fibreglass pull rod housed in the hollow support insulator).

Base FrameIn case of HLD and HLR MOCB, the support and operating insulators

are mounted on each base frame. The number of base frames per phase in the case of HLR MOCB may be one or more depending upon the number of breaks per phase, whereas the HLD MOCB in variably consists of only one base frame for each pole. In case of HLC MOCB, there is a common base frame on which all the three phases are erected. The base frame essentially consists of an operating arm to connect the pull rods transmitting motion and houses the opening spring either inside or outside, depending upon the type of breaker, which gives the required opening speed at the time of contact separation.

Operating MechanismThis is a motor operated spring closing type either BLF or BLG. BLF

mechanism is used for 36 kV HLC MOCB where as BLG mechanism is used for above 36 kV. The operating mechanism mainly consists of a set of closing springs to close the breaker at the required speed, spring charging motor for charging of the closing springs. Limit switch is mainly to break and make the power supply to the motor depending upon the position of the closing spring.

Support structureThe base frame, on which the breaker poles rest will be supported at an

elevation from the ground on the support structure. In case of HLC MOCB, the support structure will be invariably supplied along with the breaker. For HLD and HLR MOCB the structure will be supplied, if specially ordered. The support structure shall be mounted on concrete foundations.

Mode of OperationThe circuit breakers are designed to perform the operating duty of O-

0.3Sec-CO-3Min-CO. The closing and opening operations are described below.

Closing operationOn the initiation of closing command to the closing coil, the catch

system gets released and closing force on account of discharging of closing springs is

transmitted to the operating rod by link system. The operating rod connected to the operating mechanism moves in a horizontal direction, in case of HLD and HLR MOCB and this motion is converted into rotary motion by operating axle in the base frame. This is further transmitted to the operating lever in the external mechanism of the interrupter unit through rotary insulator. This movement is converted into rectilinear motion by a set of levers and the moving contacts are moved up to close the breaker.

In case of HLC breakers there will not be any rotary insulators. The operating rod connecting the mechanism and breaker moves in a vertical plane. This motion is transmitted to the operating axle in the base frame and from there to the operating lever to which the fibre glass rod along with the moving contact is connected.Opening operation

The opening spring in the base frame at the end of the pull rod system which will be charged during the closing operation always exerts force on the operating rod connected to the operating mechanism and in turn on the moving contact system to pull them to “OFF” position. This is prevented by the tripping catch system. When trip coil is supplied with operating voltage, the catch system gets released and the moving contact is brought back to “OFF” position.

Principle of Arc extinctionIn case of HLC & HLD MOCB the extinguishing chamber is of

contraction type employing the principle of axial blast. The pressure of the gases generated at the initiation of arc is used on a differential piston to force high pressure oil on the gas envelope of the arc to contact it and maintain a high pressure and high speed gas flow along with the arc path. Thus the insulation is again reinstated in the arc path and the current is broken around its natural current zero.

With regard to HLR MOCB, the extinguishing chamber is based on the cross-blast type design with no moving parts. The gases (mainly hydrogen) evolving from the decomposition of the oil by the arc, flow out through a number of slots, which are opened one by one as the moving contact moves downwards. The gases are thus forced to pass through the arc, which is cooled. When the current passes through zero, no further energy is generated for a brief instant.

A very rapid temperature drop occurs in the arc column, which de-ionizes the gases. A high dielectric strength is therefore built up very rapidly between the two contacts and re-ignition of the arc is prevented. The interrupters of HLR MOCB are pressurised with nitrogen to achieve restrike free operation while interrupting capacitive currents.

Arc Control Device1. These should be inspected and cleaned and if there is a change in shape and size

of the contact moving path, vents size. If badly burnt or cracked (laminations or discs), they should be renewed.

2. Care should be taken that vent holes and contact entry orifices are cleaned and the arc control device should be flushed with clean oil before refitting.

3. Resistors and connections if fitted, should be checked for continuity and resistance value.

4. It is important to ensure when refitting arc control devices that vent holes and contact orifice are in their correct positions relative to contact system.

5. Inspect the venting system to ensure that a free passage for oil and gases exists. Where there is a joint between fixed and movable portions of the gear ensure that it is in sound condition.

6. In no circumstances should the vents be made larger than the design values.

Advantage of Oil Circuit Breaker Reduced arc length due to better cooling resulting in minimum clearance between

the contacts. Very good insulation between live and earth is achieved due to oil. Where there is danger of ignition and explosion the oil separates the arc from the

dangerous atmosphere. This feature makes the OCB a unique device.

Disadvantage of Oil Circuit Breaker The gases produced during the operation are inflammable and it is essential they

are not permitted to leak or else a highly explosive mixture will be produced. Larger contacts suffer more wear from current breaking operations than they do in

Air Break Circuit Breaker. Due to frequent operation of the contacts in oil, carbonisation takes place and

insulating properties of oil rapidly deteriorates requiring special equipment for oil purification.

SULPHUR HEXA FLUORIDE (SF6) CIRCUIT BREAKER

IntroductionThe last few years have seen notable progress being made in the field of

circuit breakers. At extra high voltage Sulphur Hexafluoride (SF6) breakers employing the single pressure puffer type have more or less replaced the minimum oil and air blast technologies.SF6 Circuit BreakersWhy SF6?

Because of its electronegative nature, the gas has a very high dielectric strength, which is a function of its density and its high relative heat transfer properties. Fig.5.1 compares the temperature rise in air and SF6. This feature offers the designer the opportunity of either reducing the current carrying materials within the interrupter or obtaining a higher current rating than would otherwise possible.

Fig.5.2 compares the breakdown voltage with power frequency voltage being applied across two 50-mm spheres with a gap spacing of 10 mm.

Fig.5.3 shows the pressure Vs Temperature isochores in SF6 for constant density. Because of its de-ionisation characteristics, it is particularly suited to withstand a high rate of rise of dielectric voltage stress appearing across the breaker contacts during interruption. Due to its electronegative property it can absorb free ions rapidly to become a heavy negative ion. The gas is non-toxic, non-corrosive,

colourless, odourless, non-inflammable and physiologically inert. Because of these physical and chemical properties SF6 is ideal for switchgear application. Lastly the gas is easy to carry and transport and leakage can be easily detected by means of halogen detecting instruments.

INTERRUPTER DESIGNSDual Pressure

The early designs were dual pressure designs where the gas stored at around 14 kg/cm2 pressure, maintained in a gaseous state by special heaters, was exhausted on parting of contacts into the lower pressure region through the nozzles of the movable contacts of the circuit breaker. When an arc is drawn in SF6 the temperature within the arc column will dissociate the gas into its various by products and these will tend to recombine very rapidly in the cooler zones away from the immediate vicinity of the arc core. The heat involved in dissociation is thus extracted from the arc to be released subsequently in the cooler regions where the original properties of the gas are restored. This can be achieved either by arranging the gas to flow over an arc occupying a relatively fixed position (puffer) or alternatively causing the arc to move rapidly through gas which is relatively stationary (rotating arc) or by a combination of these two principles (puffer + rotating arc). The puffer principle is the basic principle of all EHV breakers presently in the market. These breakers require a longer stroke and the pressure increases on the upstream end due to heating effects of the arc that could tend to reduce the opening forces. The opening springs and the mechanism driving them should be sufficiently strong to overcome this effect.

Puffer single pressure breakersThe puffer type breakers represents the second generation of SF6

breakers employing a single pressure. The mono-blast, partial dual-blast and full dual-blast concepts are illustrated in Figs.5.4, 5.5 & 5.15.

The single pressure puffer type breakers use a gas compression cylinder attached to the moving contact. The cylinder on opening causes the gas in the annular region between the cylinder and piston to be compressed and when the contact separates, this compressed gas is caused to flow through the annular space thereby ensuring a high dielectric recovery rate once the arc extinguishes. The mono-blast is directed in one direction alone, the partial dual-blast has a second nozzle smaller than the main nozzle that causes the blast in two directions and the full dual-blast uses two nozzles of the same size to give an equal blast in both directions.

The forces that oppose contact movement in the event of an interruption during faults differ considerably than those occurring during no load operation. Fig.5.6 gives an idea of the variation in these values. The energy for gas compression arises principally from the opening springs and it does not depend upon the magnitude of the fault current. By suitably employing the mono, partial dual and full dual blast concepts the designer aims at arriving at an economical solution. The relation of the circuit breaker weight in Kg. to the breaking capacity in MVA has progressively reduced from about 1.5 kg/MVA in the 60’s to around 0.3 Kg/MVA presently.

A significant area of complexity within the puffer type can be found in the operating mechanism. To a large extent this comes from the interaction between the operating mechanism and the interrupter. The heating of the gas, due to the arc interruption of the fault current, enhances the gas pressure within the blast cylinder to very high values due to the low boiling point, high thermal coefficient and higher density of the gas. This high pressure has to be overcome by the driving mechanism, which is simultaneously driving the mass of the interrupter towards the open position. It

follows that the total power required to trip any given type of puffer interrupter will be a function of fault current.

The driving mechanism must therefore be capable of delivering the power necessary for the maximum fault rating of the circuit breaker and also be provided with a means of absorbing the excess energy which will be present when breaking normal load. The difference between that level necessary for high interruption is largely governed by the efficiency of the interrupter design.Fig.5.7 shows the equal section double nozzle configuration interrupting a fault current.

In dual-blast configuration the arc emanating between the nozzles burns on either side, within a constructed zone, being subjected to a considerable pressure differential. Within this zone, subject to the axial pressure gradient, the extremely light arc plasma is accelerated to very high axial velocities of the order of 10,000 m/sec. The expelled plasma is replaced by arc heating.

Typical Construction of SF6 Circuit BreakerConstruction

The SF6 circuit breaker type 3AR1/EG is of outdoor type and it is suitable for mounting on a support structure. It mainly comprises of the following as shown in Figure5.15. Breaker poles Base tube Electro - Hydraulic operating mechanism

Control unit

Breaker PoleThe primary functions of a circuit breaker is such as interrupting short circuit

currents, carrying normal currents, switching ON and OFF normal loads and providing necessary insulation between live and earthed parts.

The breaker pole consists of: Interrupter Unit Support insulator

Interrupter UnitThe interrupter unit consists of fixed contact tube, guide tube, moving

contact tube, blast cylinder and blast piston. The fixed contact tube is connected to the top terminal via contact support. The guide tube is fastened to the lower terminal. The other end of the fixed contact tube and the guide tube, which are subjected to arcing during arc interruption, are provided with arc quenching nozzles. The nozzles are made up of graphite material, which keeps the contact wear to a minimum. The moving contact tube consists of spring loaded finger contacts arranged in the form of a ring. The front end of the moving contact tube is provided with an arc resistant insulating ring and an arcing ring of high arc resistant material.

The blast cylinder is made up of high arc resistant insulating material and the moving contact tube, are rigidly coupled to each other and connected to the operating rod in the support insulator through fork. The blast piston, which is made up of aluminium, is fastened to the lower terminal pad by stay bolts. The fixed contact tube, guide tube, moving contact tube, blast cylinder and blast piston are all housed inside a porcelain insulator. When the circuit breaker is in closed position, current flows from top terminal to bottom terminal through contact support, fixed contact tube, moving contact tube and guide tube.

Arc InterruptionWhen the circuit breaker is in closed position, the moving contact

assembly bridges the fixed contact tube and guide tube. When an opening operation is initiated, the blast cylinder moves towards the stationary blast piston, so that the SF6

gas in the blast cylinder is compressed to the required pressure to quench the arc. The SF6 gas compressed during the above process is released only when the contacts are separated, with the moving contact assembly acting as a slide valve.

At the instant of contact separation, arc strikes between the front end of the arc quenching nozzle of the fixed contact tube and the arcing ring of the moving contact tube. The compressed gas in the blast cylinder is released into the break readily as the contacts are separated. As the moving contact assembly moves further, the arc between the front end of the fixed contact nozzle and the arcing ring of the moving contact is transferred from the arcing ring of the moving contact to the nozzle of the guide tube by gas flow. The arc is further elongated by the gas flow axially into the nozzles and gets extinguished. While the arc is being interrupted, the blast cylinder which is made up of arc resistant insulating material

encompasses the arc quenching assembly, thereby protecting the porcelain insulator from arcing effects. The moving contact assembly and blast cylinder move further to reach fully open position. (Refer 5.16)

Support InsulatorSupport insulator apart from supporting the interrupted unit provides

insulation between live parts and the earthed base tube. It houses the guide and operating rod, which is connected to the interrupter unit. The operating rod is made of insulating material like fibreglass. The other end of the operating rod is connected, through a coupling rod to the lever in the base tube.

Base TubeThe gas tight base tube, which supports all the three breaker poles,

encloses the complete layer system to transmit the operating force from the hydraulic mechanism to the breaker poles. A diaphragm and a filter provided on the left hand

side of the base tube are for releasing excessive pressure developed and absorbing the products of the decomposed SF6 gas as well as to keep the gas dry respectively. The lever system, to the differential piston of the hydraulic mechanism is fastened to the right hand side of the base tube. The horizontal motion of the differential piston is converted to the vertical motion of the operating rod by the lever system. The base tube, support insulator, interrupter unit are all filled with SF6 gas for insulating and arc quenching purposes.

Merits Of SF6 BreakerThe SF6 breakers are most efficient and reliable one than all other

types of breakers due too their following merits: -1. High dielectric and insulating properties of the gas.2. Excellent extinguishing ability.3. Non-flammable.4. Non-corrosive.5. Odourless and colourless.6. Toxicity is almost zero for all practical purpose.7. Heavier than air. 8. Longer life of contacts due to absence of oxygen and carbon molecules in the gas.9. Breaker size and dimensions are smaller as compared to ABCB/OCB of the same

kV rating. 10. Maintenance involves only on the operating system. This is an advantage since

maintenance aspects of the interrupter parts are very minimum (Practically nil).

VACUUM CIRCUIT BREAKERS (VCB)Introduction

High voltage vacuum contacts is a completely new type of switching device for frequent switching of alternating current circuits of medium voltage using vacuum as a dielectric and interrupting medium. The advantage of vacuum as an interrupting medium is primarily due to its extremely high dielectric strength and outstanding arc recovery characteristic. This is because, at high vacuum very few molecules are available for ionisation process. In vacuum, arc is mainly supported by metal vapour from the contacts. Nowadays these breakers are also used in higher voltage ranges 110 &220KV etc by cascading.

Advantage Of Vacuum ContactorsBecause of the various advantages vacuum contactors are becoming

increasingly popular and are utilised in various applications. The salient features of vacuum contactors are listed below: -1. High Interrupting speed2. Extremely low contact erosion3. Noiseless operation4. Stable contact resistance5. Low operating power6. Freedom from catastrophic failures7. Long life8. Less maintenance9. Reduction in size and weight compared to other breakers of similar rating.10. Economical in long run

11. Non-inflammable and non-toxic12. Environmentally safe.

For 3.3 kV & 6.6 kV motors, minimum oil / air break circuit breakers were being used in Power/Steel/Cement plants. Because of inherent advantage of minimum maintenance and suitability for inching duty, vacuum contactors are ideal for these applications and replacing the older MOCB &ACB. Vacuum contactors are also now being extensively used in LT & HT. The compactness of vacuum contactors helps in reducing the size of flameproof enclosure for colliery applications and thereby reducing the costs.

ConstructionThe vacuum contactor basically consists of three numbers of vacuum

switches and a solenoid operating mechanism. A sectional diagram of a vacuum switch is shown in Fig.6.1.

The vacuum switch consists of a glass or ceramic envelope containing a pair of contacts made out of special alloy. The fixed contact stem is brazed to the top flange and the moving contact stem is connected to the

bottom flange with stainless steel bellows, which provide necessary seal against atmosphere in addition to the movement of the moving contact. The entire switch is sealed at a pressure of 10-7 mm Hg or less after special processing to de-gas copper contacts and other parts. In order to prevent the metal vapour formed during the arcing from reaching the envelope resulting in the reduction of the breakdown voltage level between contacts, a sputter shield is provided to collect these particles. Gettering is done inside the switch to absorb gas molecules which come out of the contacts or which may remain inside the switch after evacuation. A properly processed switch will have a shelf life of more than 20 years. All live parts are insulated generally by epoxy resin insulating materials. Vacuum switch has a natural tendency to close its contacts due to differential atmospheric pressure, acting on metal bellows. This tendency is overcome by means of two springs, which act against the atmospheric pressure. The armature is closed Electro-magnetically. This compresses the two reaction springs and allows contacts to close with an adequate pressure and be held in this position. This is called electrically held type contactor. In case, where operating interval is not so frequent, mechanical latching device is provided. In this design, the armature is held in close position by means of a mechanical device during working and de-latched by means of a shunt trip coil. This design economises the operating power of the coil, in addition to the contactor remaining closed during momentary supply failure. The above operating mechanism is mounted in a rigid steel frame assembly out side the switch.Arc Interruption in Vacuum

Vacuum power interrupters differ from other types in many respects, but the major difference lies in the nature of the arc. While in the other interrupting media the arc exists through a gaseous ionisation process, whereas in vacuum a metal vapour arc carries the current between the two contacts within the interrupter. The main requirement for interruption of arc in vacuum is the proper contact separation (10-20 mm depending on rating) and a current zero. Current is interrupted at a current zero because of lack of the conducting metal vapour and electrons between the open contacts. Since there are no gaseous contaminants or any product of an ionisation process present, arc reignition is a rare phenomenon. It occurs only when contact parting is initiated at about the same time as a current zero. Then the arc will be carried for the next half cycle only. (Refer 6.2) This perfection in vacuum technology has been achieved after many years of research in overcoming the problems. Some of the problems overcome were: - Current chopping tendencies and the resulting high over voltages. Gross melting of contacts and the liability to weld when making or carrying high

currents. Contamination of the vacuum due to gas produced by the action of the arc on

contact metals with high gas content. Deterioration of the insulation due to condensing metal vapour on the inner

surfaces of the insulating container.

The Solutions to these problems have required many years of work on such matters as: The geometry of the contact to ensure that the arc rotates over a large area and

thus prevents grosses melting. The processing techniques including degassing of contact materials and the

surface cleanliness of components within the envelope.

Most fundamentally, on contact metallurgy to produce new materials of low gas content, good anti weld properties and low current chopping probabilities.

Selection of envelope materials capable of withstanding high processing temperatures and the protection of the inner insulation by shielding.

Long life vacuum seals.Vacuum bottles with ratings up to 40 kA, 31.5 kA and 12 kA are available at

present. It is recognised that for same MVA rating vacuum interrupter has more number of breaks per pole than SF6 or ABCB. In view of this the use of vacuum switchgear is restricted at present to medium voltage. Vacuum bottles used, as contactors are similar to those used for circuit breakers except that the latter has higher short time ratings. Vacuum contactors with ratings of 600 Amps current at 6.6 kV and maximum occasional chopping current as low as 0.75A are now available.

Advantage Of Vacuum InterruptersThis simple, and most efficient power interrupting device having a multitude of

advantages, which are brought out below.

Higher Reliability Vacuum circuit breakers provide the most reliable protection for

medium-voltage power distribution system and the equipment being protected. The reliability of the vacuum interrupting technology has proven to be excellent over more than 15 years of commercial services. Over 60,000 vacuum interrupters put in the field have accumulated more than 2,50,000 interrupter years of field experience with less than 50 failures. It is estimated that the failures involving the vacuum interrupter for all reasons are in the range of 0.1% and for vacuum breaker is approx. 0.3%. This is comparable to industry statistics of 0.36% for metal clad drawn out breakers in

industrial plants. The basic feature behind this reliability concept is, the main contacts of the power circuit breaker are in a sealed vacuum environment which: - Ensure that external contamination will not affect the interrupting process. There will be no ionised gas produced during an interruption. If vacuum interrupter fails on loss of vacuum the back up breaker will clear the

circuit before any damage is done to the vacuum breaker or cubicle. With other breakers, if the breaker fails to clear, the breaker and the cubicle will most likely be destroyed.

Minimum MaintenanceThe use of vacuum circuit breaker permits reduction of maintenance work

because Only periodic cleaning and lubrication of mechanism and an occasional wipe

spring adjustment is required; no maintenance of interrupter is required as they are sealed.

Low contact arc erosion offers exceptionally long switching and interrupting life without maintenance (at least 30 operations at full breaking capacity, against less than 10 for air magnetic circuit breakers).

High level of vacuum (better than 10-5 mm of Hg) permits greater number of operations without maintenance (10,000 electrical operations and 40,000 mechanical operations without parts replacement).

No need to inspect or change the contact during the life of the interrupter, nor there is any insulating medium to filter or replace. A simple contact wear indicator inspection is only required, together with an occasional high potential test of the interrupter, if a vacuum check is absolutely needed.

Test experience shows electrical life of vacuum interrupters is more than that required by the IEC standards.

In a properly processed vacuum interrupter loss of vacuum situation is not likely to arise. It has become customary to guarantee 20 years of life for vacuum interrupters by leading manufacturers.