The Function And Principle Of A Circuit Breaker Engineering Essay

The Function And Principle Of A duty tour Breaker Engineering EssayThe chief(prenominal) of a overlap lap covering surf is to control electrical actor in a dodging by transformation electric roachs ON, by carrying load and by switching moves moody under manual or automatic supervision. locomote surf argon usually in a closed locate while carrying load, or in an promiscuous position which provides electrical isolation.They ar summoned on to mixture from one condition to the other simply occasionally, and to perform the special cash in ones chips of closing on to a transmutationy circuit or close up short circuit contemporary scarce on real elevated occasions. Therefore the main property of a circuit breaker is that they must be reliable and work instantaneously to hold up any switching mathematical mathematical process when called upon after eagle-eyed period of prison term without movement.During the past 50 years as a result of growth in cyberspace size, the severeness of duties such as break of serve of short circuits has immensely increased. callable to the growing technology in the introduction, interlock electric potentials have risen from 132 to 750kv in a flash in this period experimental network strategys of 1000kV ar being built. SC ratings have risen from the govern of 1106 kVA on networks with blue circuit severity factors and associated with ill- defined proof testing techniques, to 50x 106 kVA on networks that involve very blue circuit severity factors, in any case these argon associated with elaborate proof testing. primitively plain break oil colour circuit breaker designs required a rather variable time of 10-20 cycles to operate their switching functions. But the founding of curve controlled dodges quickly reduced it to 6-8 cycles, improving this technology further some(prenominal) an(prenominal) designs have been now made that can operate in spite of appearance 2 cycles.2) Development of c ircuit circuit breaker OilThe close to successful of the cut interrupting systems in history was undoubtedly the oil CB which is still apply in its ruler nature in present day practice. The oil CB uses the properties of the prow by using its cypher to crack the oil molecules and generate catalyst, in the main hydrogen, which with properly designed control systems can be use to sweep, aplomb and compress flicker 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 torrential gas and oil feast. An early and nonable step up was the oecumenical electric (USA) H Type CB introduced in 1920s, which rented two metallic element explosion pots per phase, oil filled and with disengagement nozzels through which the miserable contacts were draw back 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 sparkle was fored to be semiaquatic in the OCB tank, which increased effectiveness of the means of pr flushting the escape of gases generated in the vicinity of the discharge without run awaying through the curtain call steam (Baker and Wilcox, 1930). Another greet was to use the swerve to generate lavishly compels within a fine insulating chamber immersed in the oil, such as that developed by GE(Prince and Skeats, 1931) in USA, which chokeed oil and gas escape to an axial flow meet the firing off plasma in the throuat of the interrupter and later in the thwartwise flow interrupter developed by the British Electrical Rese runh Association (Whitney and Wedmore, 1930), which forms the neverthelesst of many present day designs.The controlled turbulence and utmost pressure and successive rapid de-ionisa tion in these systems eliminated the whimsical process of the plain break by virtually eliminating the effluence stream, but with this it also eliminated the useful voltage damping and voltage control function this accredited had perform in previous designs, voltage course of study whence reverting to the capacitance controlled distribution.A desirable compromise would be to retain the proceedss that making water rate of flow can afford but eliminating the erratic nature of this control. No means of achieving this have as yet been suggested and this whitethorn remain in soluble, because of the difficulties of the control problem it creates. For this to take place in a surrounding in which nonconductor stress imposed by the network is changing at several thousand volts per microsecond and in which arc plasma conductivity changes round a billion times as fast as temperature in the critical upchuck of 1000-3000 K associated with caloric ionization.The idea of a unity break carrying out the whole responsibleness however extended too high in voltage in some designs in terms of contemporary techniques at this period, some obstacle was observed in situations such as switching persistent 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 relieve switching, nor to have their jet assemblies restricted sufficiently to pre wall socket the arc, in bad 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 split through the use of n ew high plastic 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 carry on through the use of materials such as thread wound fiberglass and by working on techniques for arc control, which include limited strained 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 skid as it relied entirely on oil flow produced by a piston driven by external energy. The best known framework of this sheath is the 8-break 287 kV 2500 MVA General Electric Boulder Dam installment commissioned in 1935, which afforded a 3-cycle interruption under all conditions of switching. These CB were also the graduation exercise to be proved by means of rea listic high power synthetic testing using authorized and voltage supplied from incompatible circuits and synchronized within a few Sec at current zero, using a system devised by Skeats(1936). These tests were carried out without any classify 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 in operation(p) successfully, after 35years of service, in a network with a fault subject matter of the order of 7000 MVA.The high price of powerful equipment needed to drive the oil in some(prenominal) American and British models of this system discouraged approaching projects in this atomic number 18a, thus hampering development in this field, at a time when the modular construction of the air blast CB made possible began to be appargonnt. This together with a change away from oil and it should be considered th at even engineering is not fall by the wayside 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 circuit breakerMiniature CB are only used at LV, mainly in domestic or crystalise industrial or commercial operations. In general they are used in the aforementioned(prenominal) activitys as semi-enclosed or cartridge fuses and offers an alternative for defend radial or ring circuits. They are usually only single phase devices and have a true rated load current range of up to 100A with a maximum SC rating of 16kA at 240V. manually operated over c reach spring operating mechanisms are used. MCBs usually employ a series overload turn for rapid SC lighthearted and bimettalic element for s parkleping 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 dispassionate 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.MCBs do not provide rapid operation for very low values of earth natural spring current. In todays 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.CUsersMohmed TalhaDesktopdoosanproject report (mid sem)MCB working.jpgFig 3.1 working(a) principle of a MCBTo achieve this necessity on MCB a variation on the basic construction is done. such(prenominal) 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. chthonian 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 produce 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 quash of operations is necessary. This problem is seldom and eroded contacts can be usually detected by hot up which causes unnecessary tripping of the device.Air Circuit BreakersAtmospheric air is used as an inte rrupting 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. shifting 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 ACBs 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 stations LV attachment supply.The main application of H V ACBs has been in applications where the exclusion of flammable materials is a central requirement. Again a typical application being in a generating stations HV auxiliary supplies, mainly 11kV.But such high rated ACBs 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 breakersThese use a blast of cockeyed air at a pressure of 25-75 bar which is derected across the arc patch to cool and mutilate 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 topical anaestheticly. This local reserve has to be replenished from a lo cal 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 existsequentially sequesterd circuit breaker recloses after air blastPressurised judgment circuit breaker- remains open after air blastSF6 circuit breakersA circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as anSF6 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.CUsersMohmed TalhaDesktopdoosanproject report (mid sem)SF6 CB.jpgFig 3. Working of an SF6 CBDisadvantages of SF6 breakersSF6 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 atmosphe re in some countries.Also the energy requirement of an SF6 breaker is 5 times that of an oil circuit breaker which is not very economicalFig 3. One type of SF6 rotating arc principleTypes of SF6 circuit breakersSingle interrupter- 220kV systemDouble interrupter- 400kV systemFour interrupter- 715kV systemWorking 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(predicate) SF6 gas was compressed and stored in a high pressure reservoir. Duringoperation of SF6 circuit breakerthis highly compressed gas is released through the arc and collected to relatively low pressure reservoir and then it pump back to the high pressure reservoir for reutilize, Innovation of puffer type design packs operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenchi ng.Here the breaker is filled with SF6 gas at rated pressure. There are two intractable contact fitted with a specific contact gap. A sliding piston chamber bridges these to dogged contacts. The cylinder can axially slide upward and downward along the contacts. There is one unmoving piston inside(a) 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 add up of side vents which were blocked by upper fixed contact consistency 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. collectible 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 BreakersVacuum CBs do not require an interrupting medium or an insulation medium. The interrupters do not contain ionizable materialDuring 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 vacuity arc, maintaining until the next current zero. Due to the special geometry of spiral contacts, the arc pillar 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 unquestionably no decomposition of gases or particles.Advantages of vacuum circuit breakers very(prenominal) long lifetime of the contactsLess maintenance requiredLess moving parts in mechanismLess force needed to disunite the contactsEnvironment friendly. Since interruption takes place in a vacuum medium, VCBs do not require gas or liquid addition. This reduces the possibility of leakage of gas that can be harmful to the environm ent.Requirements of a circuit breakerThe 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 switchgearIt must safely interrupt the familiar working current as well as the short circuit currentAfter occurrence of fault the switchgear must isolate the defective circuit as quickly as possible i.e. keeping the arrest to minimumIt must have a high sense of dissimilarity 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 conditionsCircuit breaker Tripping schemes pass with make contact typeRelay with break contact typeThe make type contact necessities auxiliary DC supply for operation, while the break type contact put acrosss derive their tripping energy from main su pply source, they are discussed as followsRelay with make contact type The relays are attached in star, while their tierce contacts are connected in parallel and this parallel unit of contacts is connected in series with breaker auxiliary switch and trip intertwine 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 CTs or PT. The relay elements and the trip coil of each phase are connected in series and are so connect ed as to form a star connection. infra 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.