In this article we will discuss about operation and mounting of circuit breaker.
Operation of Circuit Breaker:
Each circuit breaker is provided with an operating mechanism.
The functions of this mechanism are:
(i) To close the breaker contacts rapidly without hesitation at all currents from zero to rated making current capacity
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(ii) To hold the breaker contacts together against the action of powerful opening springs and
(iii) To open the breaker contacts without delay immediately on receiving tripping signal.
The small low voltage circuit breakers are manually operated while medium and large circuit breakers are power operated either electrically or pneumatically.
The force for opening a circuit breaker must be instantaneously available on receipt of the tripping impulse from the protective gear, and the universal practice is to employ a previously-charged spring for oil circuit breakers and compressed air for air-blast circuit breakers. The operating time between instant of receiving trip signal and final contact separation is of the order of 1.5 cycles or 0.03 s in the modern EHV circuit breakers and in slow circuit-breakers employed in distribution systems it may be about 3 cycles or 0.06 second.
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With spring-opening, the circuit-breaker closing mechanism compresses the spring and latches the moving contacts in the closed position. The tripping impulse then energises a small solenoid that releases the latch and allows the spring to exert its full force to open the contacts; suitable buffers must be provided to absorb the kinetic energy of the moving parts at the end of the stroke.
With an air-blast circuit breaker the contacts are normally held closed by a spring and forced open by the admission of compressed air, the compressed air thus performing the double function of opening the contacts and extinguishing the arc. The air supply must therefore be maintained until after the auxiliary isolating switch (also operated by compressed air) has opened, after which it is cut off and the main contacts return to the closed position under the action of their spring.
The force for closing a circuit breaker may be applied by hand, by a solenoid, by a spring or by compressed air. With spring-opened circuit breakers, the closing force has to compress the spring and must therefore be considerable. For a large circuit breaker the power required for closing may be of the order of 25 kW, but the time occupied is only a fraction of a second so that the energy is relatively small e.g., 6.25 kW-s or 1.75 Wh. Manual operation is thus impracticable for circuit breakers having short-circuit ratings above about 150 MVA.
Solenoid closing is the most widely used method, the solenoid being fed from a battery, usually of 110 or 220 V. For large circuit breakers, the current may be as high as 600 A at 110 V dc.
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For medium-sized circuit breakers, spring-closing may be used, the spring being pre-charged by hand or by a small motor. In such cases the provision of a battery may not prove economical.
The use of compressed air in connection with the air-blast circuit breaker has been referred to above, closing of circuit breakers being affected by the isolating switch. With oil circuit breakers in extensive substations, where the heavy cabling and voltage drop resulting from the solenoid currents may cause difficulties, compressed air closing is sometimes employed.
In all cases the closing mechanism must be trip-free i.e., if a tripping impulse comes during the closing operation the circuit breaker must immediately trip even though the closing force be still maintained; with spring-opened circuit breakers this is affected by a mechanical toggle mechanism, while with air-blast circuit breakers it is inherent in their operation.
Mounting of Circuit Breaker:
Types of Circuit Breaker Mountings and Enclosures:
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Circuit breakers are usually connected to a common bus-bar. On both sides of circuit breakers isolators are provided to make dead or disconnect the circuit breakers from live parts whenever required for the purpose of maintenance and repairs. Some electrical or mechanical interlocks must be provided so that circuit breaker is open before opening of its isolators and isolators are open before the opening or removal of any cover or barrier. Current transformers (CTs) and potential transformers (PTs) are usually connected.
There are four types of circuit breaker mountings, which are described below:
1. Cubicle Type or Cellular Type:
In this type of mountings the circuit breakers and their ancillary equipment are mounted in individual chambers or cells made of slate marble or concrete with doors of sheet-steel or in completely sheet-steel cubicles. In this arrangement complete isolation of the cell or cubicle from bus-bars may be obtained. Every part in the cell is accessible and any visible fault can be traced out easily but is not very safe.
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2. Truck Type:
In this type of mounting the circuit breaker and its auxiliaries are mounted on the movable truck, which is maintained in a masonry cell and can be withdrawn from the cubicle for maintenance and repairs purposes. On the two walls of the cells two sockets are fitted. Shutters must be arranged automatically to cover the live sockets whenever the truck is taken out, and the truck must be locked in position whenever the circuit breaker is closed. This type is suitable up to voltage of 11 kV and a short-circuit rating of 250 MVA.
3. Metal-Clad Type:
In this case each and every live part of the switchgear is completely enclosed in metallic enclosures which are earthed. The enclosures are air, oil or compound filled. It provides proper safety, protects the switchgear from the ingress of dirt and dust from outside and reduces the space occupied to minimum. The circuit breaker can be moved away from the bus-bars either horizontally or between the guides provided by this purpose after disconnecting from live parts. This type is suitable up to voltage of 66 kV and a short-circuit rating of 1,000 MVA.
4. Open Type:
For the largest ratings, it is economical to install the circuit breakers in open to avoid building costs. Bulk oil, low oil-content and air-blast circuit breakers are mainly used for outdoor installations. In some cases it is found economical to install open type switchgear in a building, the switchgear in this case being of low oil-content type.