Saturday, March 26, 2011

Air Blast Circuit breaker

5. Description of different Circuit Breaker
5. 2. Air-Blast Circuit-Breaker
5.2.1 Introduction
        Air blast circuit breakers are used today from 11 to 1100 KV, for various application. They offer several advantages such as faster operations, suitability for related operation, auto-reclosure, unit type multi-break construction, simple, assembly, modest maintenance, etc. A compressor plant is necessary to maintain high air pressure in the receiver. Air-blast circuit breakers operates repeatedly. Air-blast circuit breakers are used for interconnected lines and important lines when rapid operation is desired.
5.2.2  Construction of  Air-Blast Circuit-Breaker
       In air blast circuit breaker (also called compressed air circuit breaker) high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the circuit breaker. The circuit breaker opens and immediately after that the isolator opens, to provide addition gap. 
        In EHV circuit of today, isolators are generally independently mounted (Fig. 6).
( a ) Principle
( b ) Schematic construction  
                             Figure 6 Air blast circuit breaker isolator connection

       Figure 7 shows one pole of the EHV air blast circuit breaker. In the complete assembly there  are three identical poles.
Figure 7  One pole of an extra high voltage air blast circuit breaker

Description : High pressure air between 20 to kgf/cm2 , is is stored in the air reservoir (item 1 in Fig. 7). Air is taken from compressed air system. Three hollow insulator columns (item 2) are mounted on the resrvoir with valves (6) at their base. The double arc extinguishing chambers (3) are mounted on the top of the hollow insulator chambers. The current carrying parts (7) connect the three arc extinction chambers to each other in series and the pole to the neighboring equipment. since there exist a very high voltage between the conductor and the air reservoir, the entire arc extinction chamber assembly is mounted on insulators. 
The details of the double arc extinction chambers (3) are shown in Fig. 8. Since there three double arc extinction poles in series, there are six breakers per pole. Each arc extinction chamber (Fig. 8) consists of one twin fixed contact. There are two moving contacts  which are shown in the closing process. The moving contacts can move axially so as to open or close. Its position open or close depends on air pressure and spring pressure.

Figure 8  Extra high voltage air blast circuit breaker

       The operation mechanism (item 4 in Fig. 7) operates the rods (item 5) when it gets a pneumatic or electrical signal. The valves (6) open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the double arc extinction chamber. As the air enters into the arc extinction chamber the pressure on the moving contacts becomes more than spring pressure and contacts open.
       The contacts travel through a short distance against the spring pressure. At the end of contacts travel the part for outgoing air is closed by the moving contacts and the entire arc extinction chamber is filled with high pressure air, as the air is not allowed to go out. However, during the arcing period the air goes out through the openings and takes away the ionized air of arc.
       While closing, the valve (6) is turned so as to close connection between the hollow of the insulator and the reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result the pressure of air in the arc extinction chamber (3) is dropped down to the atmospheric pressure and the moving contacts close over the fixed contacts by virtue of the spring pressure. the opening is fast because the air takes a negligible time to travel from the reservoir to the moving contact. The arc is extinguished within a cycle. Therefore, air blast circuit breaker is very fast in breaking the current.
       Closing is also fast because the pressure in the arc extinction chamber drops immediately as the value (6)  operates and the contacts close by virtue of the spring pressure.
       The construction described below applies to air-blast circuit-breakers for EHV applications, for voltages  above 145 KV. For voltages of 420 KV and more, the construction is modified by adding required number of arc interrupting chambers in series.
       Air blast circuit breaker requires an auxiliary compressed air system.
       Air blast circuit breakers for 12 KV are generally having a different type of construction. Air blast circuit breakers are preferred for furnace duty and traction system are not satisfactory for such duties.
       Typical rating of Air Blast Circuit  Breaker are :
  • 12 KV, 40 KA
  • 22 KV, 40KA
  • 145 KV, 40 KA, 3 cycles
  • 245 KV, 40 KA, 50 KA, 2.5 cycles
  • 420 KV, 40 KA, 50 KA, 63.5 KA, 2 cycles
       The grading capacitors are connected across the interrupter unit for the equal distribution of voltage between the units. closing resistors (Fig. 9) are connected across the interrupter units for limiting the over voltages during closing operation. Opening resistors are connected across the interrupter units yo make the circuit breakers restrike free. 
Figure 9  Configuration of switching resistor

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