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Air/gas circuit breakers

Air/gas circuit breakers

Air circuit breakers (ACB):  ACBs can be used both as circuit breakers of low voltage electrical distribution systems and for protection of electrical equipment in facilities and industries.  A common breaking principle is to use the magnetic field, created by the current through the ACB, to force the arc towards insulating lamella.  As the arc goes further in between the lamella, eventually the distance to maintain the arc is exceeded and it is extinguished.

Air blast:  The air blast circuit breaker is a live tank circuit breaker design introduced in the 1930’s and common for high voltage and extra high voltage (EHV) applications.  These were the first breakers that really allowed us to reach EHV levels.  They are sparingly found in North America but more common around the rest of the world.  Air blast designs were reliable and robust though they do require regular maintenance and they are extremely noisy during operations.  Many breaks are needed for high voltages and they are often found with opening resistors.  The contacts are opened by air blast that is produced by opening a valve. Air that is compressed in a reservoir is released and directed towards the arc at high velocity. The air blast cools the arc and sweeps the arcing products away. This increases the dielectric strength of the medium between contacts and prevents the re-establishment of the arc. The arc is extinguished and the current is interrupted. The arcing time is short compared to that of an oil circuit breaker and results in less wear on the main contacts.

SF6:  SF6 insulated circuit breakers are the main type of breaker installed in transmission networks today and the only current style of breaker being manufactured for EHV applications.  Sulphur hexafluoride (SF6) is an inert, heavy gas with good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure. CAUTION must be exercised when handling the gas however because, although SF6 is in itself an inert gas, it will produce corrosive by-products under arcing conditions.  SF6 circuit breakers suffer less wear on the main contacts than do air and oil circuit breakers.  The breaking principle is to cool down the arc by blowing SF6 gas with high pressure towards the arcing contacts.

An SF6 circuit breaker’s arc extinction method may be characterized as either puffer or self-blast. The SF6 puffer circuit breaker creates the SF6 gas pressure using a piston pump whereas the SF6 self-extinguishing circuit breaker takes advantage of the SF6 gas pressure created by the heat from the arc. An SF6 Double (Dual) Pressure circuit breaker utilizes a pressurized SF6 gas chamber that is released into the arcing chamber during operation of the breaker.  The puffer type has good breaking properties for all current levels but requires more mechanical force to operate and thus a bigger operating mechanism.  The self-blast design is as effective at extinguishing the arc as the standard puffer but due to its arc assisted properties the mechanism energy can be reduced by 50%.

SF6 circuit breakers may either be of the dead-tank design (favoured in North America) or live-tank design (favoured by the rest of the world).

With dead-tank technology, the interrupter chamber is housed in a grounded metal enclosure.  The SF6 gas filling the tank insulates the high voltage, live parts of the contact assembly from the housing. SF6 gas filled bushings, which typically cannot be isolated for field testing, connect the interrupter chamber with the high-voltage terminals. Dead-tank construction carries an increased risk of internal earth fault or short-circuits within the tank. The bushings on both sides of the tank, therefore, often have CTs mounted on the upper external ground sleeve near the mounting flange that are connected to protective relays.  The dead-tank circuit breaker is better suited for earth-quake prone areas than the live-tank technology.

With live-tank technology, the interrupter chamber is housed within an insulator and is isolated from the ground by a support insulator.  The voltage level determines the length of the insulators used for both the interrupter chamber and the insulator support column.  In live-tank circuit breakers, no fault currents can occur between the interrupter unit and the housing; therefore, only one current transformer (a free-standing installation) per pole assembly is necessary. A distinguishing feature of the live-tank circuit breaker is the comparatively small gas compartment. An advantage of the low gas volume is the reduction in the amount of gas maintenance work required.

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