During the operation of a power system, it is often desirable and necessary to switch on or off the various circuits (e.g., transmission lines, distributors, generating plants etc.) under both normal and abnormal conditions. In earlier days, this function used to be performed by a switch and a fuse placed in series with the circuit. However, such a means of control presents two disadvantages. Firstly, when a fuse blows out, it takes quite some time to replace it and restore supply to the customers. Secondly, a fuse cannot successfully interrupt heavy fault currents that result from faults on modern high-voltage and large capacity circuits. Due to these disadvantages, the use of switches and fuses is limited to low-voltage and small capacity circuits where frequent operations are not expected e.g.. for switching and protection of distribution transformers, lighting circuits, branch circuits of distribution lines etc.
With the advancement of the power system, the lines and other equipment operate at very high voltages and carry large currents. The arrangement of switches along with fuses cannot serve the desired function of Switchgear in such high capacity circuits. This necessitates to employ a more dependable means of control such as is obtained by the use of circuit breakers.
A circuit breaker can make or break a circuit either manually or automatically under all conditions, viz, no-load, full-load and short circuit conditions. This characteristic of the circuit breaker has made it a very useful equipment for switching and protection of various parts of the power system.
CIRCUIT BREAKERS
A circuit breaker is a piece of equipment which can
(i)make or break a circuit either manually or by remote control under normal conditions
(ii) break a circuit automatically under fault conditions
(iii)make a circuit either manually or by remote control under fault conditions
Thus a circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions. The latter control employs relays and operates only under fault conditions.
Operating Principle : A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energised and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.
ARC PHENOMENON
When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour acts as conductor and an arc is struck between the contacts. The p.d. between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists.
During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors:
(i) Degree of ionisation – the arc resistance increases with the decrease in the number of ionised particles between the contacts.
(ii) Length of the arc – the arc resistance increases with the length of the arc i.e., separation of contacts.
(iii) Cross Section of arc – the arc resistance increases with the decrease in area of X-section of the arc.
CLASSIFICATION OF CIRCUIT BREAKERS
There are several ways of classifying the circuit breakers. However, the most general way of classification is on the basis of the medium used for arc extinction. The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6) or vacuum. Accordingly, circuit breakers may be classified into:
(i) Oil circuit breakers – which employ some insulating oil (e.g., transformer oil) for arc extinction.
(ii) Air-blast circuit breakers – in which high pressure air-blast is used for extinguishing the arc.
(iii) Sulphur hexafluroide circuit breakers – in which sulphur hexafluoride (SF6) gas is used for arc extinction.
(iv) Vacuum circuit breakers – in which vacuum is used for arc extinction. Each type of circuit breaker has its own advantages and disadvantages. In the following sections, we shall discuss the construction and working of these circuit breakers with special emphasis on the way the arc extinction is facilitated.
Oil Circuit Breakers
In such circuit breakers, some insulating oil (e.g., transformer oil) is used as an arc quenching medium. The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen at high pressure. The hydrogen gas occupies a volume about one thousand times that of the oil decomposed. The oil is, therefore, pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc region and adjacent portions of the contacts. The arc extinction is facilitated mainly by two processes. Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionisation of the medium between the contacts. Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. The result is that the arc is extinguished and the circuit current is interrupted.
Advantages : The advantages of oil as an arc quenching medium are:
(i) It absorbs the arc energy to decompose the oil into gases which have excellent cooling properties.
(ii) It acts as an insulator and permits smaller clearance between live conductors and earthed components.
(iii) The surrounding oil presents a cooling surface in close proximity to the arc.
Disadvantages : The disadvantages of oil as an arc quenching medium are:
(i) It is inflammable and there is a risk of a fire.
(ii) It may form an explosive mixture with air
(iii) The arcing products (e.g., carbon) remains in the oil and its quality deteriorates with successive operations. This necessitates periodic checking and replacement of oil.
Types of Oil Circuit Breakers
The oil circuit breakers find extensive use in the power system. These can be classified into the following types:
(i) Bulk oil circuit breakers – which use a large quantity of oil. The oil has to serve two purposes. Firstly, it extinguishes the arc during opening of contacts and secondly, it insulates the current conducting parts from one another and from the earthed tank. Such circuit breakers may be classified into :
(a) Plain break oil circuit breakers.
(b) Arc control oil circuit breakers.
In the former type, no special means is available for controlling the arc and the contacts are directly exposed to the whole of the oil in the tank. However, in the latter type, special arc control devices are employed to get the beneficial action of the arc as efficiently as possible.
(ii) Low oil circuit breakers – which use a minimum amount of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts are insulated by air or porcelain or organic insulating material.
Air-Blast Circuit Breakers
These breakers employ a high pressure air-blast as an arc quenching medium. The contacts are opened in a flow of air-blast established by the opening of the blast valve. The air-blast cools the arc and weeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents re-establishing the arc. Consequently, the arc is extinguished and flow of current is interrupted.
Advantages : An air-blast circuit breaker has the following advantages over an oil circuit breaker:
(i)The risk of fire is eliminated.
(ii)The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil replacement is avoided.
(iii) The growth of dielectric strength is so rapid that the final contact gap needed for arc extinction is very small. This reduces the size of the device.
(iv) The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction of that in oil circuit breakers, thus resulting in less burning of contacts.
(v) Due to lesser energy, air-blast circuit breakers are very suitable for conditions where frequent operation is required.
(vi) The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted.
Disadvantages : The use of air as the quenching medium offers the following disadvantages:
(i) The air has relatively inferior extinguishing properties.
(ii) The air-blast circuit breakers are very sensitive to the variations in the rate of rise of restriking voltage.
(iii) Considerable maintenance is required for the compressor plant which supplies the air-blast. The air blast circuit breakers are finding wide applications in high voltage installations. Majority of the circuit breakers for voltages beyond 110 kV are of this type.
Types of Air-Blast Circuit Breakers
Depending upon the direction of air-blast in relation to the arc, air-blast circuit breakers are classified into:
(i) Axial-blast type – in which the air-blast is directed along the arc path.
(ii) Cross-blast type – in which the air-blast is directed at right angles to the arc path.
(iii) Radial-blast type – in which the air-blast is directly radially.
Sulphur Hexaflouride (SF6) Circuit Breakers
In such circuit breakers, sulphur hexaflouride (SF6) gas is used as the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The SF6 circuit breakers have been found to be very effective for high power and high voltage service.
Construction : It consists of fixed and moving contacts enclosed in a chamber (called arc interruption chamber) containing SF6 gas. The chamber is connected to the SF6 gas reservoir. When the contacts of the breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber. The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is costly, it is reconditioned and reclaimed by a suitable auxiliary system after each operation of the breaker.
Working : In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2-8 kg/cm². When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronised with the opening of a valve which permits SF6 gas at 14 kg/cm pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation (i.e., after arc extinction), the valve is closed by the action of a set of springs.
Advantages : Due to the superior arc quenching properties of SF6 gas, the SF circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below:
(i) Due to the superior arc quenching property of SF6 such circuit breakers have very short arcing time.
(ii) Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger current.
(iii) The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.
(iv) The closed gas enclosure keeps the interior dry so that there is no moisture problem.
(v) There is no risk of fire in such breakers because SF6 gas is non-inflammable.
(vi) There are no carbon deposits so tracking and insulation problems are eliminated.
(vii) The SF6 breakers have low maintenance cost, light foundation requirements and minimum auxiliary equipment.
(viii) Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazards exist e.g., coal mines.
Disadvantages :
(i) SF6 breakers are costly due to the high cost of SF6.
(ii) Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.
Applications : A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents upto 60 kA and voltages in the range of 50-80 kV. A number of units are connected in series according to the system voltage. SF6 circuit breakers have been developed for voltages 115 kV to 230 kV. power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.
Vacuum Circuit Breakers (VCB)
In such breakers, vacuum (degree of vacuum being in the range from 10-7 to 10-5 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers.
Principle : The production of arc in a vacuum circuit breaker and its extinction can be explained as follows: When the contacts of the breaker are opened in vacuum (10-7 to 10-5 torr), an arc is produced between the contacts by the ionisation of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. The reader may note the salient feature of vacuum as an arc quenching medium. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.
Construction : It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.
Working : When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionisation of metal ions and depends very much upon the material of contacts. The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the faces of moving and fixed members and shields, Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0-625 cm).
Advantages : Vacuum circuit breakers have the following advantages:
(i) They are compact, reliable and have longer life.
(ii) There are no fire hazards.
(iii) There is no generation of gas during and after operation.
(iv) They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position.
(v) They require little maintenance and are quiet in operation.
(vi) They can successfully withstand lightning surges.
(vii) They have low arc energy.
(viii) They have low inertia and hence require smaller power for control mechanisms.
Applications : For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance free circuit breakers should prove a definite advantage. Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited ratings of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas.
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