The chief of a circuit ledgeman is to command electrical power in a system by exchanging circuits ON, by transporting burden and by exchanging circuits OFF under manual or automatic supervising. Circuit surfs are normally in a closed place while transporting burden, or in an unfastened place which provides electrical isolation.
They are summoned on to alter from one status to the other merely on occasion, and to execute the particular map of shuting on to a faulty circuit or barricading short circuit current merely on really rare occasions. Therefore the chief belongings of a circuit ledgeman is that they must be dependable and work outright to run any shift operation when called upon after long period of clip without motion.
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During the past 50 old ages as a consequence of growing in web size, the badness of responsibilities such as break of short circuits has vastly increased. Due to the turning engineering in the universe, web electromotive forces have risen from 132 to 750kv now in this period experimental web systems of 1000kV are being built. SC evaluations have risen from the order of 1×106 kVA on webs with low circuit badness factors and associated with ill- defined cogent evidence proving techniques, to 50x 106 kVA on webs that involve really high circuit badness factors, besides these are associated with luxuriant cogent evidence testing.
Earlier apparent break oil circuit ledgeman designs required a instead variable clip of 10-20 rhythms to run their shift maps. But the debut of discharge controlled systems rapidly reduced it to 6-8 rhythms, bettering this engineering farther many designs have been now made that can run within 2 rhythms.
2 ) Development of circuit surfs: Oil
The most successful of the arc disrupting systems in history was doubtless the oil CB which is still used in its rule nature in present twenty-four hours pattern. The oil CB uses the belongingss of the discharge by utilizing its energy to check the oil molecules and generate gas, chiefly H, which with decently designed control systems can be used to brush, cool and compress discharge plasma and so de-ionise itself with a self-extinguishing procedure. But this system was unstable and it became apparent that there was a demand for circuit surfs which possessed a more positive system of break than the forticious de-ionisation associated with uncontrolled gas and oil flow. An early and noteworthy measure up was the general electric ( USA ) H Type CB introduced in 1920s, which employed two metal detonation pots per stage, oil filled and with insularity nozzels through which the moving contacts were withdrawn vertically upwards, the detonation pot had been mounted on ceramic dielectrics within an air- insulated cell construction. Later, Slepian ( Westinghouse ) proposed a ‘deion grid ‘ in which the discharge was fored to be submerged in the OCB armored combat vehicle, which increased ‘effectiveness of the agencies of forestalling the flight of gases generated in the locality of the discharge without go throughing through the arc steam ( Baker and Wilcox, 1930 ) . Another attack was to utilize the discharge to bring forth high force per unit areas within a little insulating chamber immersed in the oil, such as that developed by GE ( Prince and Skeats, 1931 ) in USA, which restricted oil and gas flight to an axial flow environing the discharge 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 footing of many present twenty-four hours designs.
The controlled turbulency and high force per unit area and attendant rapid de-ionisation in these systems eliminated the fickle operation of the field interruption by virtually extinguishing the escape current, but with this it besides eliminated the utile electromotive force damping and electromotive force control map this current had performed in old designs, electromotive force division so returning to the electrical capacity controlled distribution.
A desirable via media would be to retain the advantages that leakage current can afford but extinguishing the fickle nature of this control. No agency of accomplishing this have as yet been suggested and this may stay in soluble, because of the troubles of the control job it creates. For this to take topographic point in a surrounding in which dielectric emphasis imposed by the web is altering at several thousand Vs per microsecond and in which arc plasma conduction alterations about a billion times every bit fast as temperature in the critical scope of 1000-3000 K associated with thermic ionisation.
The thought of a individual interruption transporting out the whole responsibility nevertheless extended excessively high in electromotive force in some designs in footings of modern-day techniques at this period, some trouble was observed in state of affairss such as exchanging long unfastened ended transmittal lines. These restrictions were associated with the electrical and mechanical strength of the insularity stuffs so available, which neither permitted the CN to be designed with the acceleration necessary to guarantee restrike free shift, nor to hold their jet assemblies restricted sufficiently to forestall the discharge, in unfavourable state of affairss, from from blinking through the jets and along the exterior of the interrupter, therefore by-passing the interrupting mechanism provided.
The progresss in public presentation of present twenty-four hours e.h.v. dead armored combat vehicle oil and low oil CB building have been brought about by utilizing the multibreak designs, but with the added complication of positive electromotive force control ; by cut downing the inactiveness of the moving parts through the usage of new high tensile stuffs or extinguishing mechanical linkages by the usage of high preassure oil thrusts ; by improved containment of the discharge with the interrupter as the consequence of the grater pressures that can be sustained through the usage of stuffs such as yarn wound fibreglass ; and by working on techniques for discharge 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 parametric quantities following shorter arc clip.
The multibreak ( Prince, 1935 ) impulse CB already referred to was a particular instance as it relied wholly on oil flow produced by a Piston driven by external energy. The best known illustration of this type is the 8-break 287 kilovolt 2500 MVA General Electric Boulder Dam installing commissioned in 1935, which afforded a 3-cycle break under all conditions of exchanging. These CB were besides the first to be proved by agencies of realistic high power man-made proving utilizing current and electromotive force supplied from different circuits and synchronized within a few µSec at current nothing, utilizing a system devised by Skeat ‘s ( 1936 ) . These trials were carried out without any kind of failure to an tantamount SC degree in surplus of 4000 MVA, and it is of historic involvement to the universe of man-made testing, on which modern high power surfs rely mostly for cogent evidence of evaluation, to observe that these CB were still runing successfully, after 35years of service, in a web with a mistake capacity of the order of 7000 MVA.
The high monetary value of powerful equipment needed to drive the oil in both American and British theoretical accounts of this system discouraged future undertakings in this country, therefore haltering development in this field, at a clip when the modular building of the air blast CB made possible began to be evident. This together with a alteration off from oil and it should be considered that even technology is non free from the influence of manner encouraged a swing to airblast building. Nevertheless the troubles ingerent in make up one’s minding on such long term development policies in switchgear are exemplified by the decennary or more which passed before the HV irblast CB matched the best oil CB pattern in both their public presentation and dependability.
Miniature Circuit Surfs:
Miniature CB are merely used at LV, chiefly 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 option for protecting radial or pealing circuits. They are normally merely individual stage devices and have a typical rated load current scope of up to 100A with a maximal SC evaluation of 16kA at 240V. Manually operated over centre spring runing mechanisms are used. MCB ‘s normally employ a series overload spiral for rapid SC tripping and bimettalic component for stumbling on overloads. All illumination CB operate on the air- interruption rule where an discharge formed between the chief contacts is forced, by agencies of an discharge smuggler, and the magnetic effects of the SC currents, into metallic arc splitter home bases. These cause a no. of series acrc to be formed and at the same clip pull out energy from the discharge and cool it to accomplish a province called arc extinction.
With some design alterations of the MCB this arc break procedure can be so rapid that current cut-off can be achieved in much the same manner as described for a current-limiting fuse.
MCB ‘s do non supply rapid operation for really low values of earth leakage current. In today ‘s universe wiring ordinances require that a really rapid operation is achieved in the juncture of an Earth mistake to subsidise the injuries of burning. This requires operation for Earth mistake currents every bit low as 30mA in a clip of 2-3ms.
Degree centigrades: UsersMohmed TalhaDesktopdoosanproject study ( mid sem ) MCB working.jpg
Fig 3.1 – Working rule of a MCB
To accomplish this demand on MCB a fluctuation on the basic building is done. Such a modified device is known as ‘earth escape CB ‘ . Stumbling at such low values of earth leakage current is done by utilizing an internal current transformer to go through provender and return music directors. Resultant flux of the CT nucleus is zero. Under EF status the eating and return currents will be of different values, this current difference cause flux to bring forth with the CT nucleus which produces an end product electromotive force at its secondary terminuss. The stumbling circuit of the residuary current device is energized from the secondary twist terminuss.
The contacts of the MCB and residuary current devices are non maintainable and have to be replaced after a limited figure of operations is necessary. This job is rarely and scoured contacts can be normally detected by overheating which causes unneeded tripping of the device.
Air Circuit Surfs:
Atmospheric air is used as an disrupting medium in an ACB. The discharge is drawn between its contacts and extended via discharge smugglers on to an discharge chute where it is presented with a big chilling surface of arc splitter home bases. These break the discharge into a figure of series discharge. The running rule of an ACB is the same as that of an MCB. Free air circuit surfs are frequently used in LV and MV applications up to about 20KV. A rated current of typically 4000A and besides work absolutely in instance of a SC current of up to 90kA at 12kV. Fault degree, figure of operations and types of burden are applications of LV switchgears where tireless operation is required. Besides due to economic considerations molded instance CB have replaced many LV applications where antecedently ACB ‘s were used. But, ACB still dominate in countries where high public presentation, long term dependability and maintainability are basic demands. A really typical application to back up this statement is in bring forthing station ‘s LV subsidiary supply.
The chief application of HV ACB ‘s has been in applications where the exclusion of flammable stuffs is a cardinal demand. Again a typical application being in a generating Stationss HV subsidiary supplies, chiefly 11kV.
But such high rated ACB ‘s are really expensive and are non recommended, therefore this is decreasing and the graduated tables are tipping over to the more favourable SF6 circuit surfs. A farther application of the ACB is for usage with DC supplies, this method of break still being the most suited for d.c. circuits. DC circuit surfs are widely used where evaluations of up to 3 kilovolts exist.
AIR BLAST CIRCUIT Surfs:
These use a blast of tight air at a force per unit area of 25-75 saloon which is derected across the discharge spot to chill and take ionized gas. Merely when arc lengths are short and at first or zero current the air blast circuit surfs perform fast in break. Besides in the receiving system of the CB compressed air has to be stored locally. This local modesty has to be replenished from a local air compressor. Normally a suited ring chief web is used as a cardinal system to feed the circuit surfs.
2 types of Air Blast Circuit Breaker exist:
Consecutive stray circuit ledgeman – recloses after air blast
Pressurised caput circuit breaker- remains unfastened after air blast
SF6 circuit surfs
A circuit ledgeman in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.
SF6 has an first-class insulating belongings. SF6 has high electro-negativity. That means it has high affinity of absorbing free negatron. Whenever a free negatron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.
Degree centigrades: UsersMohmed TalhaDesktopdoosanproject study ( mid sem ) SF6 CB.jpg
Fig 3. – Working of an SF6 CB
Disadvantages of SF6 surfs
SF6 is considered as a nursery gas and though it is really efficient in some circuit surfs, Torahs are being passed which restrict the emanation of this gas into the ambiance in some states.
Besides the energy demand of an SF6 ledgeman is 5 times that of an oil circuit ledgeman which is non really economical
Fig 3. – One type of SF6 revolving arc rule
Types of SF6 circuit surfs:
Single interrupter- 220kV system
Double interrupter- 400kV system
Four interrupter- 715kV system
Working of the SF6 CB ( ref. hypertext transfer protocol: //www.electrical4u.com/electrical-switchgear/sf6-circuit-breaker.php )
The working of SF6 CB of first coevals was rather simple, it is some extent similar to air blare circuit ledgeman. Here SF6 gas was compressed and stored in a high force per unit area reservoir. During operation of SF6 circuit ledgeman this extremely compressed gas is released through the discharge and collected to comparatively low force per unit area reservoir and so it pumped back to the high force per unit area reservoir for reutilize, Innovation of blowfish type design makes operation of SF6 CB much easier. In buffer type design, the discharge energy is utilised to develop force per unit area in the arcing chamber for discharge extinction. Here the ledgeman is filled with SF6 gas at rated force per unit area. There are two fixed contact fitted with a specific contact spread. A skiding cylinder Bridgess these to fixed contacts. The cylinder can axially skid 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 ledgeman, in such a manner that it can non alter its place during the motion of the cylinder. As the Piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder alterations when the cylinder slides.
During gap of the ledgeman the cylinder moves downwards against place of the fixed Piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has Numberss of side blowholes which were blocked by upper fixed contact organic structure during closed place. As the cylinder move further downwards, these vent gaps cross the upper fixed contact, and go unblocked and so compressed SF6 gas inside the cylinder will come out through this blowholes in high velocity towards the discharge and base on ballss through the axial hole of the both fixed contacts. The discharge is quenched during this flow of SF6 gas.
During shutting of the SF6 circuit ledgeman, the skiding cylinder moves upwards and as the place of Piston remains at fixed tallness, the volume of the cylinder additions which introduces low force per unit area inside the cylinder compared to the surrounding. Due to this force per unit area difference SF6 gas from environing will seek to come in in the cylinder. The higher force per unit area gas will come through the axial hole of both fixed contact and enters into cylinder via blowhole and during this flow ; the gas will slake the discharge.
Vacuum Circuit Surfs:
Vacuum CB ‘s do non necessitate an interrupting medium or an insularity medium. The interrupters do non incorporate ionizable stuff
During the separation of current-carrying contacts, contact force per unit area reduces existent contact surface reduces and the temperature of contacts additions to runing temperature, this produces metal bluess which initiates the vacuity discharge, keeping until the following current nothing. Due to the particular geometry of coiling contacts, the arc column is kept revolving by the radial magnetic field produced in order to affect a wider surface than that of a fixed contracted discharge. Therefore, overheating and eroding of the contacts are prevented. So the lifetime of the CB is increased.
Since there is no break or insularity stuff in the medium there is decidedly no decomposition of gases or atoms.
Advantages of vacuity circuit surfs:
Very long life-time of the contacts
Less care required
Less traveling parts in mechanism
Less force needed to divide the contacts
Environment friendly. Since break takes topographic point in a vacuity medium, VCB ‘s do non necessitate gas or liquid add-on. This reduces the possibility of escape of gas that can be harmful to the environment.
Requirements of a circuit ledgeman:
The power dealt by the circuit surfs is rather big and serves as an of import nexus between the consumers and providers. The following are the necessary demands for a circuit ledgeman or switchgear
It must safely disrupt the normal on the job current every bit good as the short circuit current
After happening of mistake the switchgear must insulate the faulty circuit every bit rapidly as possible i.e. maintaining the hold to minimum
It must hold a high sense of favoritism i.e. in systems where an surrogate agreements have been made for continuity of supply it should insulate the lone faulty circuit without impacting any of the healthy 1s.
It should non run when an over current flows under healthy conditions
Circuit ledgeman Tripping strategies
Relay with make contact type
Relay with break contact type
The brand type contact necessities auxiliary DC supply for operation, while the interruption type contact relays derive their tripping energy from chief supply beginning, they are discussed as follows ;
Relay with make contact type: The relays are connected in star, while their three contacts are connected in analogue and this parallel unit of contacts is connected in series with ledgeman subsidiary switch and trip spiral to battery supply.
When a mistake occurs on any of the stage the relay will shut the contact this energizes the several trip spiral which opens the CB and along with it subsidiary switch is opened and the trip spiral De energized, the supply of current to blame way is stopped and the relay contact comes to normal place. The advantage of the subsidiary switch is that interrupting of the stumbling circuit takes topographic point merely across this switch and arcing, etc. which is harmful to contacts over the relay contacts is avoided.
Relay with interruption type contact: The stumbling circuit derives its energy from the chief supply beginning through CT ‘s or PT. The relay elements and the trip spiral of each stage are connected in series and are so connected as to organize a star connexion. Under the normal on the job conditions the relay contacts are closed and at the same clip the trip coils energized. When a mistake occurs, the relay contacts unfastened and CB trip spirals are energized to open the CB.