Air Blast Circuit Breaker (ABCB)

We have so far discussed three main types of circuit breakers. These are Vacuum Circuit Breaker, Gas Circuit Breaker or SF6  Circuit Breaker (GCB) and Oil Circuit Breaker. The other type of circuit breaker that we discuss here is Air Blast Circuit Breaker(ABCB). This type of breakers are also becoming obsolete. Once Air Blast type of breakers were preferred in Extra High Voltage substations. Now it is difficult to find new HV/EHV substations equipped with Air Blast Circuit Breakers.

One should not be confused between Air Circuit Breaker and Air Blast Circuit Breaker. Air Circuit Breakers are usually used in low voltage applications below 450 volts. You can today find these in Distribution Panels (below 450 volts). Air Blast Circuit Breakers are high capacity breakers and can be seen in old substations mainly above 132 kV. The working principle of these two circuit breakers are quite different. Here we will only discuss the working of ABCB.

In Air Blast Circuit Breaker, air at high pressure is blast upon the arc formed between the contacts. The air blast blows away the ionized air between the contacts.

See the Sketches (Figs-A and B) illustrating the arc extinction process of the axial blast type breaker . The contacts are in closed position by spring pressure. For opening the contacts. Air at high pressure from the air receiver (Fig-C) is blasted to the interruption chamber. This pressure exceeds the spring pressure and pushes the moving contact away from the fixed contact. This opens the contacts and air at high pressure passes through the nozzle and port to the atmosphere. This axial flow of air at high speed extinguishes the arc within 2 or 3 cycles of current wave and ionized gas is blown away.  Then the port is closed by the moving contact arm(Fig-B)  and the space between the contacts is filled with fresh air at high pressure. This enables the breaker to withstand high Transient recovery Voltage (TRV). Compare Fig-A with Fig-B. In Fig-B the arc is extinguished and spring is in compressed state.

To close the contacts, a valve arrangement lets the air from the chamber to pass to the outside atmosphere. This makes the spring pressure to close the fixed and moving contacts.

Some main advantages of the Air Blast Circuit Breaker(ABCB) are:
  • Arc extinction is very fast. Hence it is suitable for frequent opening and closing operation.
  • Due to refilling of separated contacts space by fresh air at high pressure,  the separation requirement between the contacts is quite less in comparison to OCB. This makes the size of the breaker smaller.
  • The ionized gas flushed out to the atmosphere. Hence unlike OCB here the arc quenching medium does not deteriorate with time. This eliminates some maintenance burden.
  • It is non-inflammable.
  • Finally one important advantage is that in ABCB the arc quenching depends on the high pressure air which is obtained from a compressor, an external source. So in case of ABCB the arc extinction or arcing time does not depends upon the arc current. (In case of OCB the arcing time depends on the current to be interrupted).
  • The breaker breaking capacity depends upon the external source, the high pressure air.
The Air Blast Circuit Breakers has some disadvantages. The important one is that  Air Blast Circuit Breakers  require a compressor plant (not shown in Fig-C) which requires regular maintenance. Hence ABCB is not economical for low voltage applications. There are other issues like current chopping and restriking voltage which requires to be handled by proper design and damping mechanism.

In last few articles we have discussed the working principles of all the major types of breakers used in High Voltage and Extra High Voltage Substations. Perhaps this is enough in developing some basic concepts on an important substation equipment like Circuit Breaker. In subsequent articles we will discuss some other equipment used in HV/EHV substations.