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| Fig 1 : A 2-pole miniature circuit breaker |
(i) Switching on and off during normal operation for maintenance etc.
(ii) Switching during abnormal conditions- short circuits, earthing etc. to protect the associated equipment.
What is a Circuit Breaker ?
A circuit breaker is an apparatus in electrical systems that has the capability to, in the shortest possible time, switch from being an ideal conductor to an ideal insulator and vice-versa.
Furthermore, the circuit breaker should be able to fulfill the following requirements:
1. In the stationary closed position, conduct its rated current without producing impermissible heat rise in any of its components.
2. In its stationary positions, open as well as closed, the circuit breaker must be able to withstand any type of overvoltages within its rating.
3. The circuit breaker shall, at its rated voltage, be able to make and break any possible current within its rating, without becoming unsuitable for further operation.
In earlier times, oil and compressed air were typical insulating and extinguishing medium. Nowadays they are almost entirely replaced by SF6 gas for economical and practical reasons, and also due to increased demands for higher ratings.
The circuit breaker is a crucial component in the substation, where it is used for coupling of busbars, transformers, transmission lines, etc. The most important task of a circuit breaker is to interrupt fault currents and thus protect electric and electronic equipment. The interruption and the subsequent reconnection should be carried out in such a way that normal operation of the network is quickly restored, in order to maintain system stability. In addition to the protective function, the circuit breakers are also applied for intentional switching such as energizing and de-energizing of shunt reactors and capacitor banks. For maintenance or repair of electrical equipment and transmission lines, the circuit breakers, together with the disconnectors, earthing switches or disconnecting circuit breakers with built-in disconnecting function, will ensure personnel safety. So a circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
In short, a circuit breaker is a sort of automatic switch which can interrupt the fault currents. Two important parts of a circuit breaker that need consideration are:
(i) Arc extinction system and (ii) Relay for operation
Principles of arc extinction :
The current interruption process in a high-voltage circuit breaker is a complex matter due to simultaneous interaction of several phenomena. When the circuit breaker contacts separate, an electric arc will be established, and current will continue to flow through the arc. Interruption will take place at an instant when the alternating current reaches zero. When a circuit breaker is tripped in order to interrupt a short-circuit current, the contact parting can start anywhere in the current loop. After the contacts have parted mechanically, the current will flow between the contacts through an electric arc, which consists of a core of extremely hot gas with a temperature of 5,000 to 20,000 K. This column of gas is fully ionized (plasma) and has an electrical conductivity comparable to that of carbon. When the current approaches zero, the arc diameter will decrease, with the cross-section approximately proportional to the current. In the vicinity of zero passage of current, the gas has been cooled down to around 2,000 K and will no longer be ionized plasma, nor will it be electrically conducting.
Two physical requirements (regimes) are involved:
Thermal regime : The hot arc channel has to be cooled down to a temperature low enough that it ceases to be electrically conducting.
Dielectric regime : After the arc extinction, the insulating medium between the contacts must withstand the rapidly-increasing recovery voltage. This recovery voltage has a transient component (transient recovery voltage, TRV) caused by the system when current is interrupted. If either of these two requirements is not met, the current will continue to flow for another half cycle, until the next current zero is reached. It is quite normal for a circuit breaker to interrupt the short-circuit current at the second or even third current zero after contact separation.
Arc Extinction Process :
Whenever a circuit carrying current is interrupted by a circuit breaker an arc is inevitably formed between the contacts which prolongs the current interrupting process for a duration ranging from 10 to 100 or more milliseconds. Since arc is produced in every circuit breakers, therefore suitable energy dissipating device must be incorporated in the design of circuit breaker. Unless carefully controlled, arc can lead to danger of fire or explosion. The arc consists of a column of ionized gas i.e. gas in which the molecules have lost one or more of their negative electrons, leaving positive ions. The negative electrons are attracted towards the positive contact and being light, more towards it very rapidly. The positive ions attracted towards the negative contact. Due to electron movement the current flows. The ionization process is accompanied by the emission of light and heat. Also some portion of power is dissipated as heat. The temperature of arc may be as high as 60000 C. Two methods commonly used are:
(i) High resistance interruption :
In this the arc is controlled in such a way that its resistance is caused to increase rapidly, thereby reducing the current until it falls to a value that is insufficient to maintain the ionization process. The arc resistance may be increased by
(a) Arc lengthening
(b) Arc cooling
(c) Arc splitting
(ii) Low resistance interruption :
In this the arc resistance is kept low, in order to keep the arc energy to a minimum and use is made of a natural or artificial current zero when the arc extinguishes itself and is then prevented from re striking.
Protection of contacts :
During arcing mechanical as well as electrical erosion of contacts occurs. Therefore the resistance to erosion by arching is the important property of contact materials. In case of dc circuits the process of erosion is represented by loss of material from one contact and the deposition of part of this material on to the other contact. However, in case of ac circuits there is no marked direction of transfer, as either contact becomes successively positive and negative.
There are two distinct forms of protections which may be employed with the object of reducing the rate of erosion of contacts by arcing thereby prolonging their useful life.
(a) Arc dispersion :
In this the destructive effects of the arc are minimized, using one of the following methods:
1. Oil immersion of contacts
2. Multiple break contacts
3. De ionization of arc path
4. Magnetic blow out of arc
5. Blast principle using air, oil, gas or water.
(b) Arc prevention :
In this the occurrence or arc is prevented by reducing the current and voltage below the minimum arcing values or reducing its destructive effects as far as possible. The principle devices used to quench circuits of this kind are :
(i) Discharge resistance (ii) Rectifiers (iii) Condensers
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