EEE 4508 Switchgear and Protection
Lecture Notes 6
6.0 Switching and Protection Circuit Breakers Cont.:
The circuit breakers that shall be considered in this lecture are the Air blast, Oil, Vacuum,
Metal clad and Sulphur hexa-fluoride types of circuit breakers. The circuit breakers shall be
discussed in their mod of arc extinction and general operation including application.
6.1 Air Blast Circuit breakers
Air blast circuit breakers employ a high-pressure gas blast sweeping across the contacts in the
space so as to create a low resistance arc. The circuit breaker employs gases that are compressed
like; air, nitrogen, carbon dioxide, hydrogen and Freon. Nitrogen has equivalent circuit
breaking properties to compressed air and therefore there is no advantage in using nitrogen.
Carbon dioxide has the drawback of its being difficult to control owing to freezing at valves
and other restricted passages. Tests have shown an increased breaking capacity by the use of
hydrogen, but its cost and that of the ancillary apparatus make its use expensive compared to
air. On the other-hand, Freon has high dielectric strength and good extinguishing properties,
but it is expensive and it is decomposed by the arc into acid forming elements. Therefore, it
follows that compressed air is the accepted circuit-breaking medium for gas-blast circuit
breakers.
6.1.1 Contact Separation
All Air Blast Circuit Breaker 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. Arrangements vary but can be grouped into three types as shown
in Figure 6.1:
(i) Axial blast,
(ii) Radial blast
(iii) Cross blast.
Axial and radial blast are preferred for the higher voltages although the cross blast breakers
particularly for voltages of about 15 KV and heavy current (up to 100 KA) have proved to
perform better and require less air than would an axial-blast breaker at these high currents.
1
, Figure 6.1 Air blast circuit breaker
The main structural advantage of the axial-blast circuit breaker over the cross blast is its easier
adaptability to high voltage insulation particularly for outdoor applications. This is because the
interrupting chambers can be fully enclosed in porcelain tubes. The axial blast type of circuit
breaker thus is ideal for high and super voltage application and outdoors where dust and
corrosive fumes are prevalent. In indoor high power medium voltage use, this class of circuit
breaker with currents of 2000-4000A are common, requiring special multi-finger contacts in
order to keep the temperature low enough to prevent damaging through oxidation. A multiple
interruption by air blast can be arranged for very high voltages and combining the joint radial
and axial cooling by direct air convection. In order to secure high air velocities in Air Blast
Circuit Breakers, a relatively short wide passages for the air flow between pressure reservoir
and arc is provided. If the necessary volume of air is available near the arc, velocities exceeding
that of sound may be attained at the critical instant of extinction.
6.1.2 Principle of Operation
Gas blast interruption is dependent on turbulent cooling, and is therefore influenced by
aerodynamic configuration, including nozzles, gas flow passages and mass flow. Compressed
air as an excellent insulator is forced on the arc at the instant of contact separation. The
compressed air acts on the arc through the nozzle which helps exhaust the hot gas and the
arcing products to the atmosphere. In this way the interrupter of an Air Blast Circuit Breaker
performs its operating cycle. Extinction occurs at the first current zero when the flow of
compressed air increases rapidly to establish the dielectric strength between the electrodes to
withstand re-striking voltage. The growth of dielectric strength is rapid and pressure of air is
so high that the final gap caused by interposition of insulating layer of air between the contacts
need only be small, thus reducing the size of the device. The energy supplied for arc extinction
is obtained from high pressure air and is independent of the fault current to be interrupted.
2
Lecture Notes 6
6.0 Switching and Protection Circuit Breakers Cont.:
The circuit breakers that shall be considered in this lecture are the Air blast, Oil, Vacuum,
Metal clad and Sulphur hexa-fluoride types of circuit breakers. The circuit breakers shall be
discussed in their mod of arc extinction and general operation including application.
6.1 Air Blast Circuit breakers
Air blast circuit breakers employ a high-pressure gas blast sweeping across the contacts in the
space so as to create a low resistance arc. The circuit breaker employs gases that are compressed
like; air, nitrogen, carbon dioxide, hydrogen and Freon. Nitrogen has equivalent circuit
breaking properties to compressed air and therefore there is no advantage in using nitrogen.
Carbon dioxide has the drawback of its being difficult to control owing to freezing at valves
and other restricted passages. Tests have shown an increased breaking capacity by the use of
hydrogen, but its cost and that of the ancillary apparatus make its use expensive compared to
air. On the other-hand, Freon has high dielectric strength and good extinguishing properties,
but it is expensive and it is decomposed by the arc into acid forming elements. Therefore, it
follows that compressed air is the accepted circuit-breaking medium for gas-blast circuit
breakers.
6.1.1 Contact Separation
All Air Blast Circuit Breaker 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. Arrangements vary but can be grouped into three types as shown
in Figure 6.1:
(i) Axial blast,
(ii) Radial blast
(iii) Cross blast.
Axial and radial blast are preferred for the higher voltages although the cross blast breakers
particularly for voltages of about 15 KV and heavy current (up to 100 KA) have proved to
perform better and require less air than would an axial-blast breaker at these high currents.
1
, Figure 6.1 Air blast circuit breaker
The main structural advantage of the axial-blast circuit breaker over the cross blast is its easier
adaptability to high voltage insulation particularly for outdoor applications. This is because the
interrupting chambers can be fully enclosed in porcelain tubes. The axial blast type of circuit
breaker thus is ideal for high and super voltage application and outdoors where dust and
corrosive fumes are prevalent. In indoor high power medium voltage use, this class of circuit
breaker with currents of 2000-4000A are common, requiring special multi-finger contacts in
order to keep the temperature low enough to prevent damaging through oxidation. A multiple
interruption by air blast can be arranged for very high voltages and combining the joint radial
and axial cooling by direct air convection. In order to secure high air velocities in Air Blast
Circuit Breakers, a relatively short wide passages for the air flow between pressure reservoir
and arc is provided. If the necessary volume of air is available near the arc, velocities exceeding
that of sound may be attained at the critical instant of extinction.
6.1.2 Principle of Operation
Gas blast interruption is dependent on turbulent cooling, and is therefore influenced by
aerodynamic configuration, including nozzles, gas flow passages and mass flow. Compressed
air as an excellent insulator is forced on the arc at the instant of contact separation. The
compressed air acts on the arc through the nozzle which helps exhaust the hot gas and the
arcing products to the atmosphere. In this way the interrupter of an Air Blast Circuit Breaker
performs its operating cycle. Extinction occurs at the first current zero when the flow of
compressed air increases rapidly to establish the dielectric strength between the electrodes to
withstand re-striking voltage. The growth of dielectric strength is rapid and pressure of air is
so high that the final gap caused by interposition of insulating layer of air between the contacts
need only be small, thus reducing the size of the device. The energy supplied for arc extinction
is obtained from high pressure air and is independent of the fault current to be interrupted.
2
