The Function And Principle Of A Circuit Breaker Engineering Essay

The main of a circuit breaker is to control electrical power in a system by switching circuits ON, by carrying load and by switching circuits OFF under manual or automatic supervision. Circuit breakers are usually in a closed position while carrying load, or in an open position which provides electrical isolation.

They are summoned on to change from one condition to the other only occasionally, and to perform the special function of closing on to a faulty circuit or blocking short circuit current only on very rare occasions. Therefore the main property of a circuit breaker is that they must be reliable and work instantaneously to operate any switching operation when called upon after long period of time without movement.

During the past 50 years as a result of growth in network size, the severity of duties such as interruption of short circuits has immensely increased. Due to the growing technology in the world, network voltages have risen from 132 to 750kv now in this period experimental network systems of 1000kV are being built. SC ratings have risen from the order of 1x106 kVA on networks with low circuit severity factors and associated with ill- defined proof testing techniques, to 50x 106 kVA on networks that involve very high circuit severity factors, also these are associated with elaborate proof testing.

Earlier plain break oil circuit breaker designs required a rather variable time of 10-20 cycles to operate their switching functions. But the introduction of arc controlled systems quickly reduced it to 6-8 cycles, improving this technology further many designs have been now made that can operate within 2 cycles.

2) Development of circuit breakers: Oil

The most successful of the arc interrupting systems in history was undoubtedly the oil CB which is still used in its principle nature in present day practice. The oil CB uses the properties of the arc by using its energy to crack the oil molecules and generate gas, principally hydrogen, which with properly designed control systems can be used to sweep, cool and compress arc plasma and so de-ionise itself with a self-extinguishing process. But this system was unstable and it became evident that there was a need for circuit breakers which possessed a more positive system of interruption than the forticious de-ionisation associated with uncontrolled gas and oil flow. An early and notable step up was the general electric (USA) H Type CB introduced in 1920s, which employed two metal explosion pots per phase, oil filled and with insulation nozzels through which the moving contacts were withdrawn vertically upwards, the explosion pot had been mounted on ceramic insulators within an air- insulated cubicle structure. Later, Slepian (Westinghouse) proposed a 'deion grid' in which the arc was fored to be submerged in the OCB tank, which increased 'effectiveness of the means of preventing the escape of gases generated in the vicinity of the arc without passing through the arc steam (Baker and Wilcox, 1930). Another approach was to use the arc to generate high pressures within a small insulating chamber immersed in the oil, such as that developed by GE(Prince and Skeats, 1931) in USA, which restricted oil and gas escape to an axial flow surrounding the arc plasma in the throuat of the interrupter; and later in the cross flow interrupter developed by the British Electrical Research Association (Whitney and Wedmore, 1930), which forms the basis of many present day designs.

The controlled turbulence and high pressure and resultant rapid de-ionisation in these systems eliminated the erratic operation of the plain break by virtually eliminating the leakage current, but with this it also eliminated the useful voltage damping and voltage control function this current had performed in previous designs, voltage division then reverting to the capacitance controlled distribution.

A desirable compromise would be to retain the advantages that leakage current can afford but eliminating the erratic nature of this control. No means of achieving this have as yet been suggested and this may remain in soluble, because of the difficulties of the control problem it creates. For this to take place in a surrounding in which dielectric stress imposed by the network is changing at several thousand volts per microsecond and in which arc plasma conductivity changes approximately a billion times as fast as temperature in the critical range of 1000-3000 K associated with thermal ionization.

The idea of a single break carrying out the whole duty however extended too high in voltage in some designs in terms of contemporary techniques at this period, some difficulty was observed in situations such as switching long open ended transmission lines. These limitations were associated with the electrical and mechanical strength of the insulation materials then available, which neither permitted the CN to be designed with the acceleration necessary to ensure restrike free switching, nor to have their jet assemblies restricted sufficiently to prevent the arc, in unfavorable situations, from from flashing through the jets and along the outside of the interrupter, thus by-passing the interrupting mechanism provided.

The advances in performance of present day e.h.v. dead tank oil and low oil CB construction have been brought about by using the multibreak designs, but with the added complication of positive voltage control; by reducing the inertia of the moving parts through the use of new high tensile materials or eliminating mechanical linkages by the use of high preassure oil drives; by improved containment of the arc with the interrupter as the result of the grater pressures that can be sustained through the use of materials such as thread wound fiberglass; and by working on techniques for arc control, which include limited forced oil flow pressurizing of the interrupter. The overall complication of low oil circuit isolation switches, made possible by the improved internal dielectric parameters following shorter arc time.

The multibreak (Prince, 1935) impulse CB already referred to was a special case as it relied entirely on oil flow produced by a piston driven by external energy. The best known example of this type is the 8-break 287 kV 2500 MVA General Electric Boulder Dam installation commissioned in 1935, which afforded a 3-cycle interruption under all conditions of switching. These CB were also the first to be proved by means of realistic high power synthetic testing using current and voltage supplied from different circuits and synchronized within a few µSec at current zero, using a system devised by Skeat's(1936). These tests were carried out without any sort of failure to an equivalent SC level in excess of 4000 MVA, and it is of historic interest to the world of synthetic testing, on which modern high power breakers rely largely for proof of rating, to note that these CB were still operating successfully, after 35years of service, in a network with a fault capacity of the order of 7000 MVA.

The high price of powerful equipment needed to drive the oil in both American and British models of this system discouraged future projects in this area, thus hampering development in this field, at a time when the modular construction of the air blast CB made possible began to be apparent. This together with a change away from oil and it should be considered that even engineering is not free from the influence of fashion encouraged a swing to airblast construction. Nevertheless the difficulties ingerent in deciding on such long term development policies in switchgear are exemplified by the decade or more which passed before the HV irblast CB matched the best oil CB practice in both their performance and reliability.

Miniature Circuit Breakers:

Miniature CB are only used at LV, mainly in domestic or light industrial or commercial operations. In general they are used in the same applications as semi-enclosed or cartridge fuses and offers an alternative for protecting radial or ring circuits. They are usually only single phase devices and have a typical rated load current range of up to 100A with a maximum SC rating of 16kA at 240V. Manually operated over center spring operating mechanisms are used. MCB's usually employ a series overload coil for rapid SC tripping and bimettalic element for tripping on overloads. All miniature CB operate on the air- break principle where an arc formed between the main contacts is forced, by means of an arc runner, and the magnetic effects of the SC currents, into metallic arc splitter plates. These cause a no. of series acrc to be formed and at the same time extract energy from the arc and cool it to achieve a state called arc extinction.

With some design modifications of the MCB this arc interruption process can be so rapid that current cut-off can be achieved in much the same way as described for a current-limiting fuse.

MCB's do not provide rapid operation for very low values of earth leakage current. In today's world wiring regulations require that a very rapid operation is achieved in the occasion of an earth fault to subsidize the harms of electrocution. This requires operation for earth fault currents as low as 30mA in a time of 2-3ms.

C:UsersMohmed TalhaDesktopdoosanproject report (mid sem)MCB working.jpg

Fig 3.1 - Working principle of a MCB

To achieve this requirement on MCB a variation on the basic construction is done. Such a modified device is known as 'earth leakage CB'. Tripping at such low values of earth leakage current is done by using an internal current transformer to pass feed and return conductors. Resultant flux of the CT core is zero. Under EF condition the feeding and return currents will be of different values, this current difference cause flux to generate with the CT core which produces an output voltage at its secondary terminals. The tripping circuit of the residual current device is energized from the secondary winding terminals.

The contacts of the MCB and residual current devices are not maintainable and have to be replaced after a limited number of operations is necessary. This problem is seldom and eroded contacts can be usually detected by overheating which causes unnecessary tripping of the device.

Air Circuit Breakers:

Atmospheric air is used as an interrupting medium in an ACB. The arc is drawn between its contacts and extended via arc runners on to an arc chute where it is presented with a large cooling surface of arc splitter plates. These break the arc into a number of series arcs. The running principle of an ACB is the same as that of an MCB. Free air circuit breakers are often used in LV and MV applications up to approximately 20KV. A rated current of typically 4000A and also work perfectly in case of a SC current of up to 90kA at 12kV. Fault level, number of operations and types of load are applications of LV switchgears where tireless operation is required. Also due to economic considerations molded case CB have replaced many LV applications where previously ACB's were used. But, ACB still dominate in areas where high performance, long term reliability and maintainability are basic requirements. A very typical application to support this statement is in generating station's LV auxiliary supply.

The main application of HV ACB's has been in applications where the exclusion of flammable materials is a fundamental requirement. Again a typical application being in a generating stations HV auxiliary supplies, mainly 11kV.

But such high rated ACB's are very expensive and are not recommended, thus this is diminishing and the scales are tipping over to the more favorable SF6 circuit breakers. A further application of the ACB is for use with DC supplies, this method of interruption still being the most suitable for d.c. circuits. DC circuit breakers are widely used where ratings of up to 3 kV exist.

AIR BLAST CIRCUIT BREAKERS:

These use a blast of compressed air at a pressure of 25-75 bar which is derected across the arc patch to cool and remove ionized gas. Only when arc lengths are short and at first or zero current the air blast circuit breakers perform fast in interruption. Also in the receiver of the CB compressed air has to be stored locally. This local reserve has to be replenished from a local air compressor. Usually a suitable ring main network is used as a central system to feed the circuit breakers.

2 types of Air Blast Circuit Breaker exist:

Sequentially isolated circuit breaker - recloses after air blast

Pressurised head circuit breaker- remains open after air blast

SF6 circuit breakers

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.

SF6 has an excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

C:UsersMohmed TalhaDesktopdoosanproject report (mid sem)SF6 CB.jpg

Fig 3. - Working of an SF6 CB

Disadvantages of SF6 breakers

SF6 is considered as a greenhouse gas and though it is very efficient in some circuit breakers, laws are being passed which restrict the emission of this gas into the atmosphere in some countries.

Also the energy requirement of an SF6 breaker is 5 times that of an oil circuit breaker which is not very economical

Fig 3. - One type of SF6 rotating arc principle

Types of SF6 circuit breakers:

Single interrupter- 220kV system

Double interrupter- 400kV system

Four interrupter- 715kV system

Working of the SF6 CB (ref. http://www.electrical4u.com/electrical-switchgear/sf6-circuit-breaker.php)

The working of SF6 CB of first generation was quite simple, it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for reutilize, Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching. Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it cannot change its position during the movement of the cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.

During closing of the SF6 circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.

Vacuum Circuit Breakers:

Vacuum CB's do not require an interrupting medium or an insulation medium. The interrupters do not contain ionizable material

During the separation of current-carrying contacts, contact pressure reduces real contact surface reduces and the temperature of contacts increases to melting temperature, this produces metal vapors which initiates the vacuum arc, maintaining until the next current zero. Due to the special geometry of spiral contacts, the arc column is kept rotating by the radial magnetic field produced in order to involve a wider surface than that of a fixed contracted arc. Thus, overheating and erosion of the contacts are prevented. So the lifespan of the CB is increased.

Since there is no interruption or insulation material in the medium there is definitely no decomposition of gases or particles.

Advantages of vacuum circuit breakers:

Very long lifetime of the contacts

Less maintenance required

Less moving parts in mechanism

Less force needed to separate the contacts

Environment friendly. Since interruption takes place in a vacuum medium, VCB's do not require gas or liquid addition. This reduces the possibility of leakage of gas that can be harmful to the environment.

Requirements of a circuit breaker:

The power dealt by the circuit breakers is quite large and serves as an important link between the consumers and suppliers. The following are the necessary requirements for a circuit breaker or switchgear

It must safely interrupt the normal working current as well as the short circuit current

After occurrence of fault the switchgear must isolate the faulty circuit as quickly as possible i.e. keeping the delay to minimum

It must have a high sense of discrimination i.e. in systems where an alternate arrangements have been made for continuity of supply it should isolate the only faulty circuit without affecting any of the healthy ones.

It should not operate when an over current flows under healthy conditions

Circuit breaker Tripping schemes

Relay with make contact type

Relay with break contact type

The make type contact necessities auxiliary DC supply for operation, while the break type contact relays derive their tripping energy from main supply source, they are discussed as follows;

Relay with make contact type: The relays are connected in star, while their three contacts are connected in parallel and this parallel unit of contacts is connected in series with breaker auxiliary switch and trip coil to battery supply.

When a fault occurs on any of the phase the relay will close the contact this energizes the respective trip coil which opens the CB and along with it auxiliary switch is opened and the trip coil De energized, the supply of current to fault path is stopped and the relay contact comes to normal position. The advantage of the auxiliary switch is that breaking of the tripping circuit takes place only across this switch and arcing, etc. which is harmful to contacts over the relay contacts is avoided.

Relay with break type contact: The tripping circuit derives its energy from the main supply source through CT's or PT. The relay elements and the trip coil of each phase are connected in series and are so connected as to form a star connection. Under the normal working conditions the relay contacts are closed and at the same time the trip coils energized. When a fault occurs, the relay contacts open and CB trip coils are energized to open the CB.