2.2. Air-Blast Circuit-Breaker 2.2.

2.2. Air-Blast Circuit-Breaker 2.2.1 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 operate repeatedly. Air-blast circuit breakers are used for interconnected lines and important lines when rapid operation is desired. 2.2.2 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. 2.2). ( a ) Schematic construction
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 30
( b ) Principle
Figure 2.2 Air blast circuit breaker isolator connection Figure 2.3 shows one pole of the EHV air blast circuit breaker. In the complete assembly there are three identical poles. Figure 2.3 One pole of an extra high voltage air blast circuit breaker
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 31
Description: High pressure air between 20 to kgf/cm2, is is stored in the air reservoir (item 1 in Fig. 2.3). 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. 2.4. Since there three double arc extinction poles in series, there are six breakers per pole. Each arc extinction chamber (Fig. 2.4) 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 2.4 Extra high voltage air blast circuit breaker
The operation mechanism (item 4 in Fig. 2.3) 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
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 32
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 systems are not satisfactory for such duties. Typical ratings 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. 2.5) are connected across the interrupter units for limiting the over voltages during closing operation. Opening resistors are connected across the interrupter units to make the circuit breakers restrike free.
Figure 2.5 Configuration of switching resistor
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 33
2.2.3 Principle of Arc Quenching in Air Blast Circuit Breaker
The air blast circuit breaker needs an auxiliary compressed air system which supplies air to the air receiver of the breaker. For opening operation, the air is admitted in the arc extinction chamber. It pushes away the moving contacts against spring pressure. In doing so, the contacts are separated and the air blast takes away the ionized gases along with it and assists s arc extinction. After few cycles the arc is extinguished by the air blast and the arc extinction chamber is filled with high pressure air (30kgf/cm2 ). The high pressure air has higher dielectric strength than that of atmospheric pressure. Hence a small contact gap of a few centimeters is enough. The flow of air around contacts is guided by the nozzle shaped contacts. It may be axial, across or a suitable combination {Fig. 2.6(a), (b)}. Figure 2.6 Flow of air around contacts in air blast circuit breaker
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 34
In the axial blast type air flow Fig. 2.6 (a) the flow air is longitudinal, along the arc. In axial blast type air flow, the air flows from high pressure reservoir to the atmospheric pressure through a convergent divergent nozzle. The difference in pressure and the design of nozzle is such that as the air expands into the low pressure zone, it attains almost supersonic velocity. The mass flow of air through the nozzle is governed by the parameters like pressure ratio, area of throat, nozzle throat diameter and is influenced by the diameter of the arc itself. The air flowing at high speed axially along the arc causes removal of heat from the periphery of the arc and the diameter of the arc reduces to a low value at current zero. At this instant the arc is interrupted and the contact space is flushed with fresh air flowing through the nozzle. The flow of fresh air through the contact space ensures removal of hot gases and rapid building up of the dielectric strength. The principle of cross blast illustrated in Fig. 2.6 (b) is used only in the circuit breaker of relatively low rating such as 12 KV, 500 MVA. The experience has shown that in the cross blast flow, the air flows around the arc and the diameter of arc is likely to remain stable for higher values of current. During the period of arc extinction, the air continues to flow through the nozzle to the atmosphere. The mass flow rate can be increased by increasing the pressure system. The increase in the mass flow results in increased breaking capacity. After the brief duration of air flow, the interrupter is filled with high pressure air. The dielectric strength of air increases with pressure. Hence the fresh high pressure air in the contact space is capable of withstanding the transient recovery voltage. After the arc extinction the interrupter chamber is filled with high pressure air. For closing operation, the air from this chamber is let out to the atmosphere. Thereby the pressure on the moving contacts from one side is reduced and the moving contacts close rapidly by the spring pressure (Fig. 2.7). The air blast circuit breakers come under the class external extinguishing energy type. The energy supplied for arc extinction is obtained from high pressure air and is independent of current to be interrupted.
Circuit Breakers & Substations 4th Y ear Electrical Power Eng. Dep.
Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 35
Figure 2.7 Principle of Operation in ABCB 2.2.4 Merits and Demerits of Air Blast Circuit Breaker Merits of Air Blast Circuit Breaker:
1. Can be used at high pressure.
2. Reliable operation due to external source of extinguishing energy.
3. Free from decomposition.
4. Clean, non-inflammable.
5. Air is freely available everywhere.
6. Fresh medium is used every time. Hence the breaker can be repeatedly operated, if designed for such duty.
7. At high pressure the small contact travel is enough.
8. The same are serves purpose of moving the contact and arc extinction.
9. High speed of operation. The compressed air moves very fast and brings ab