In this article we will discuss about:- 1. Principle of Arc Extinction in Air-Blast Circuit Breakers 2. Types of Air-Blast Circuit Breakers 3. Features 4. Drawbacks.
Principle of Arc Extinction in Air-Blast Circuit Breakers:
The air-blast circuit breaker requires an auxiliary compressed air system which supplies air to the breaker air receiver. When opening is required, compressed air is admitted to the arc extinction chamber. It pushes away the moving contacts. In doing so the contacts are separated and the air blast takes away the ionized gases along with it and assists arc extinction.
Air blast extinguishes the arc within one or two cycles and the arc 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 isolator opens to provide additional gap. In EHV switchyards, isolators are generally independently mounted.
The air-blast circuit breakers fall under the category of external extinguishing energy type. The energy supplied for arc extinction is obtained from high pressure air and is independent of current to be interrupted.
Types of Air-Blast Circuit Breakers:
All air-blast circuit breakers follow the principle of separating their contacts in a flow of air established by the opening of a blast valve. The arc which is drawn is usually rapidly positioned centrally through a nozzle where it is kept to a fixed length and is subject to maximum scavenging by the air flow.
The air-blast circuit breakers, according to the type of flow of blast of compressed air around the contacts are of three types namely:
(i) Axial Blast
(ii) Radial Blast
(iii) Cross Blast
The different ways of flow of the blast of compressed air around the contacts are shown in Fig. 10.14.
Axial or radial blast seems to be favoured for the higher voltages although cross-blast breakers particularly for voltages of about 15 kV and heavy current (up to 100,000 A) have proved satisfactory and need less air than would an axial-blast circuit breaker at such high currents.
1. Axial-Blast Air Circuit Breaker:
In axial-blast type circuit breaker, the flow of air is longitudinal along the arc [Fig. 10.14 (a)]. Axial-blast circuit breakers may be single blast or double blast [Fig. 10.14 (b)]. Breakers employing double blast arrangement are sometimes called radial blast circuit breakers as the air blast flows radially into the nozzle or space between the contacts.
The essential components of a typical axial-blast circuit breaker are shown in Fig. 10.15. The fixed and moving contacts are held in closed position by spring pressure under normal operating conditions. The air reservoir tank is connected to the arc chamber through an air valve, which is opened by a tripping impulse.
On occurrence of a fault, the tripping impulse causes opening of the air valve connecting the reservoir to the arcing chamber. The air entering the arc chamber exerts pressure on the moving contacts which move when the air pressure exceeds the spring force. The moving contact is separated and an arc is struck. The air flowing at a 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 following through the nozzle.
The flow of fresh air through the contact space ensures removal of hot gases and rapid building up of dielectric strength. After the brief duration of air flow, the interrupter is filled with high pressure air. The dielectric strength of air increases with pressure. Thus the fresh high pressure air in the contact space is capable of withstanding the transient recovery voltage.
It is noteworthy here that the air pressure form the reservoir is maximum initially and falls thereafter. It is known that for a particular reservoir pressure there is a certain optimum contact at which rupturing capacity is maximum. This gap is usually small (ten of mm) and may reach very quickly if the moving parts are of small inertia. The shorter the gap, relatively smaller amounts of energy is released in the arcing chamber. The arc is kept in the high velocity blast of air converging into the nozzle throat.
The falling reservoir pressure and short optimum gap result in three important features of the axial blast principle:
1. The interruption must take place at the first current zero after the optimum gap has reached otherwise restrikes may take place at subsequent zeros of current due to decreasing pressure. In oil circuit breakers it is otherwise, i.e., chances of interruption increase if arcing exists beyond the first current zero.
2. The axial blast circuit breaker gives high speed clearance because of the small gap required for interruption. This is desirable for improving transient stability on hv transmission and interconnection networks.
3. The small contact gap after interruption may constitute inadequate clearance for the normal system voltage. Therefore, an isolating switch is incorporated as a part of this circuit breaker. This switch opens immediately after fault interruption to give the necessary clearance for insulation.
For low voltages the isolating switch is not needed and adequate travel is provided instead for the moving contact.
The arcing time of arc controlled circuit breaker varies considerably depending upon the breaking current. The higher the breaking current (within the rating of the breaker), the smaller the arcing time. The arcing time in case of air-blast circuit breaker is independent of the breaking current due to fixed air pressure and the optimum small contact gap. The short gap along with an isolating switch provides a total break time of 2 to 5 cycles.
The operation of the air-blast circuit breaker is very much affected by the circuit natural frequency. When the current is passing through zero value the residual column has relatively high resistance which reduces the chances of the re-striking voltage transient being damped.
Now the effect of RRRV during this zero current condition is more serious especially where the chance of extinction decreases after the optimum gap has reached. The effect of natural frequency on the performance of the air-blast circuit breaker is overcome by shunting the arc with resistors of suitable values.
2. Cross-Blast Air Circuit Breaker:
In such a breaker, an air-blast is directed at right angles to the arc.
The principle used in the cross-blast type air circuit breakers is fundamentally different from the axial-blast one.
A moving contact arm operates in close proximity to an arc chute to draw an arc which is forced by a transverse blast of air into the splitter plates within the arc chute, thereby lengthening it to the point when it cannot restrike after current zero.
The consistent high speed operation of the axial-blast type is not reproduced in this type, but as the air blast is constant regardless of current magnitude, it is quite efficient in switching small currents. Because the moving arm is not restricted (relatively) in its travel, full isolation is obtained without the need for a series isolator as in other types.
Resistance switching is not normally required as the lengthening of arc automatically introduces some resistance to control the re-striking voltage transient but if extra resistance is thought desirable, it is possible to introduce it by connecting it in sections across the arc splitters.
Features of Air-Blast Circuit Breakers:
The desirable features to be found in air-blast circuit breakers are:
1. High Speed Operation:
It is very necessary on large interconnected networks in order that system stability can be maintained and in the air-blast circuit breaker this is achieved because the time interval between the receipt of a tripping impulse (derived from the protective gear applied) and contact separation is very short.
Once the contacts part and an arc is drawn it should, ideally, be interrupted in the shortest possible time and this time duration should be reasonably consistent at all values of current which the circuit breaker may be called upon to break i.e. from small line charging or transformer magnetising current up to the highest value of fault current. This the air-blast circuit breaker does and arc durations throughout the current range are of the order of one-half to one cycle.
In the oil circuit breakers this consistency rarely exists and because the deionisation processes are largely dependent on the current value, it is usual to find longer arcing times at low current values than at the higher values, and that the whole range of arcing times is higher in the air-blast-design.
2. Suitability for Frequent Operation:
Repeated switching by an air-blast circuit breaker is possible simply because of absence of oil, which rapidly carbonises with frequent operation, and because there is an insignificant amount of wear and tear at the current-carrying contact surfaces. But it must be remembered that if frequent switching is anticipated, the maintenance of an adequate air supply is essential.
3. Facility of High-Speed Reclosure:
High-speed reclosure by automatic means is an advantage on hv interconnected networks to assist and maintain system stability during the clearance of transient faults, a type of fault which is perhaps in majority on overhead lines. Provided that the time interval between fault interruption and reclosure is chosen to permit insulation recovery, then a system can often be restored to normal by breaker reclosure, the cause of the interruption (insulator or line flash-over) having disappeared. The low inertia of the moving contacts in air-blast circuit breakers and the relative ease with which compressed air mechanisms can be reversed, all help in very short restoration times being achieved.
4. Negligible Maintenance:
The ability of the air-blast circuit breaker to cope with repeated switching also means that negligible maintenance is required. For example, the relatively large quantities of dielectric oil essential in the oil circuit breaker need an installation of oil-filtering plant and very regular treatment of oil. No such requirement arises with an air-blast circuit breaker.
5. Elimination of Fire Hazard:
Because of absence of oil the risk of fire is eliminated.
6. Reduced Size:
The growth of dielectric strength is so rapid in air-blast circuit breakers that final gap required for arc extinction is very small. This reduces the size of the device.
Drawbacks of Air-Blast Circuit Breakers:
It is noteworthy that compressed air at the correct pressure, clean and dry, must be available at all times, involving in the largest installation of a plant with two or more compressors and an excessive air supply network in the form of a ring main or duplicate bus system. The maintenance and upkeep of this plant and the problem of air leakages at the pipe fittings are factors which operate against air-blast circuit breakers in many (smaller) installations and it is a costly adjunct for lower voltage systems as compared with the use of oil or air-break circuit breakers.
Because of fixed air pressure, it is obviously available regardless of the magnitude of current to be interrupted by the breaker. It must naturally, be sufficient to deal with the highest value of anticipated fault current, but this means that it can be very drastic in its effect on small currents and the problem of current chopping arises leading to serious over-voltages.
Sensitivity of the air-blast circuit breaker to circuit severity, i.e., the rate of rise of re-striking voltage. Many of the air-blast circuit breakers (and some very high voltage circuit breakers) overcome this problem by resort to resistance switching where in external resistors are automatically connected in shunt with the contact gap and thereby damp out very high re-striking voltage transients caused by current chopping.
Not all air-blast circuit breakers incorporate resistance switching. It is usual in the design known as axial blast and is less frequently employed in the design known as cross-blast.
During the period 1950-1970, air-blast circuit breakers were preferred for 220 kV and above. However today SF6 circuit breakers, which are maintenance free and of superior switching performance, are preferred for this range. For 11 kV and 33 kV applications vacuum circuit breakers are preferred. Thus air-blast circuit breakers have become almost obsolete.