SF6 Gas Properties

Before we begin the actual discussion on the SF6 circuit breaker, we need to recall some basic knowledge about SF6 gas. This is because SF₆ gas is the key component of the SF₆ circuit breaker. So, we call these circuit breakers SF6 circuit breakers. These belong to gas-insulated circuit breakers. Here, SF6 gas acts as both an insulating medium and the arc-quenching medium. Since we popularly use SF₆ gas for these purposes, it obviously has some definite properties.

High Dielectric Strength of SF6 Gas

The most important property of SF₆ gas is that it has a very high dielectric strength. At normal atmospheric pressure, it offers 2.5 to 3 times higher dielectric strength than air. The dielectric strength of SF6 gas is somewhat lower than that of insulating oil. When the pressure of SF₆ gas increases, its dielectric strength also increases. The dielectric strength of SF₆ gas becomes even greater than that of insulating oil when we raise the gas pressure above three times the atmospheric pressure.

At normal atmospheric pressure, the dielectric strength of air is 3 KV/ mm, of SF6 gas is 9KV / mm and for oil it is 12KV /mm. Normally, in a circuit breaker, SF₆ gas remains at a pressure of 4.5 to 7 bar. Obviously, in this pressurized state, the gas offers much better dielectric strength than insulating oil.

High Electronegativity of SF6 Gas

The next important characteristic of SF₆ gas is its high electronegativity. That means it has a very high affinity for electrons. SF6 gas absorbs the free electrons created during the arc. The lack of free electrons in the medium weakens the arc. The molecules of SF₆ are quite heavy. When SF6 molecules absorb electrons, they form SF₆^– or SF₅^– ions. These are much heavier.

As a result, the electrons lose their mobility and are no longer able to collide with other atoms and molecules in the medium. This prevents the creation of more electrons and the ionization of the medium. Therefore, the arc cannot continue with full strength.

Rapid Deionization of SF6 Gas

Another advantage of SF₆ gas is that it can recombine very quickly to its original SF₆ molecular state after the current reduces to zero during the zero crossing of the current wave. This means that SF₆^– or SF₅^– ions are rapidly deionized to form neutral SF₆ molecules. The time interval between the current wave’s zero crossing and the reestablishment of full dielectric strength between the contacts is known as the arc time constant. For SF6 circuit breakers, the arc time constant is typically a few microseconds; in some cases, it is even less than one microsecond.

Cooling Effect of SF6 gas

Another advantage of SF₆ gas is that it is a very good cooling medium. The volumetric heat absorption capability of SF6 is 3.7 times higher than that of air. As a result, it quickly absorbs heat from the arc. This reduces the ultimate temperature of the arc. Consequently, the arc diameter decreases, which facilitates better arc quenching during the zero crossing of the current wave.

Nontoxic and Nonflammable Heavy SF6 Gas

SF₆ gas is an invisible gas. That means it is a colorless gas. Also, it does not have any good or bad smell. That means it is an odourless gas. It is an ideal inert gas. It does not react with any other substances. So, it does not react with any internal or external parts of the human body. Hence, it does not have any toxic effect on the human body.

Since under normal conditions it does not react with oxygen, it is a nonflammable gas. Thus, SF6 circuit breakers are free from fire hazards. But the problem with SF₆ gas is that it is five times heavier than air, so it does not disperse easily into the atmosphere. As a result, it can create a suffocating environment in the surrounding area.

SF6 – Inert Gas

SF₆ gas is not only an inert gas, but it is also chemically very stable. Even up to 500°C, it remains unreactive with other substances. However, above 500°C, SF₆ gas is significantly affected by the presence of moisture. If moisture is present and the gas is heated above 500°C, SF₆ can decompose into several byproducts such as SF₄ (sulphur tetrafluoride), SOF₂ (thionyl fluoride), SO₂F₂ (sulphuryl fluoride), S₂F₁₀ (disulphur decafluoride or tikafluoride) and HF (hydrofluoric acid). Among these, disulphur decafluoride is highly toxic. These can affect the metal contact surfaces, gaskets and solid insulation in the circuit breaker.

During arcing, the temperature of the arc can rise above 1000°C. At such high temperatures, even pure SF₆ gas decomposes into SF₄ (sulphur tetrafluoride), SF₂ (sulphur difluoride), S₂, F₂, S, F and other sulphur–fluorine compounds. But these byproducts quickly recombine into the original SF₆ form once the temperature decreases.

Benefits of SF6 Circuit Breakers

Controlled Gas Pressure

High SF6 gas pressure can cause current chopping in the breaker. This means that if we increase the pressure of the SF6 circuit breaker, there may be a very high rate of rise in dielectric strength between the contacts during arcing. This can lead to current chopping, which is not desirable for a healthy power system. So, by controlling and optimizing the pressure in the SF6 circuit breaker, one can avoid current chopping in the breaker.

Additionally, if the pressure in the arcing chamber is much reduced, the restriking voltage level becomes low, which may cause the arc to restrike again and again. Therefore, to avoid restriking, the pressure needs to be increased to an optimized level of SF6 gas to be maintained in CBs.

Compact Design

Due to the high dielectric strength of SF₆ gas, the contact gap between the moving and fixed contacts can be maintained much smaller compared to that of air and oil circuit breakers. Also due to higher insulating strength of pressurized SF6 gas. As a result, the overall size of the SF₆ circuit breaker can be made significantly more compact than air and oil circuit breakers.

Longer Life Span

Another advantage of SF₆ gas is that it is an inert gas, meaning it does not react with the surrounding oxygen. As a result, it is nonflammable. This nonflammable nature ensures that SF6 circuit breakers are free from fire hazards. The chemically inert nature of SF₆ gas prevents contact erosion. Hence, it increases the longevity of the contacts and the internal components of the circuit breaker.

Hermetically Sealed from Atmosphere

Also, the SF₆ gas used in the circuit breaker is hermetically sealed from the surrounding atmosphere. This sealing ensures that moisture, dust, and other atmospheric conditions do not affect the performance of the SF6 circuit breaker.

Silent Performance of SF₆ Circuit Breakers

The SF6 circuit breaker operates much more quietly compared to air blast or oil circuit breakers. In air blast circuit breakers, there is a huge and irritating sound during operation due to the blast of air. In oil circuit breakers also, the noise level is also very high due to the movement of larger contacts in the bigger arc quenching chamber. But due to its compact design and reduced movement of the contacts, the SF6 circuit breaker offers a much quieter operation compared to an equivalently rated air or oil circuit breaker.

No Carbonization in SF₆ Circuit Breakers

There is another advantage. When an arc forms in SF₆ gas, it does not create any carbon or carbon-based components. Because SF₆ gas does not contain any carbon. In contrast, oil is a hydrocarbon product, and hence, during arcing, it may produce carbon. This carbon deposition degrades the overall insulation system of the breaker.

Additionally, due to this decomposed carbon, the dielectric strength of the insulating medium (oil) deteriorates. If we do not maintain oil properly, over time, the breaker may fail to operate or even explode due to the deterioration of insulation levels caused by carbon deposition. For this reason, after a specific number of faulty operations, we must replace the oil in the circuit breaker with new oil. However, in the case of SF6, this medium-changing process can be avoided. Since there is no carbonization in SF6 gas, ideally, there is no degradation of the insulation level inside the circuit breaker.

Disadvantages of SF₆ Circuit Breakers

Gas Leakage

But SF₆ gas does have some disadvantages as well. The first major drawback is the possibility of SF₆ gas leakage. Due to leakage at the joints of the circuit breaker, the pressurized SF₆ gas escapes into the atmosphere. This causes two main problems.

Firstly, SF₆ gas is highly suffocating and heavier than air, so it does not disperse easily. As a result, it can accumulate around the circuit breaker, creating a hazardous environment. Secondly, the leakage leads to a reduction in the internal pressure of the circuit breaker. When the pressure drops below the required level, the arc during switching operations may not be quenched effectively. This can result in arc re-striking even after the circuit breaker has fully opened, causing incomplete current interruption. Continuous arcing under such conditions can potentially lead to a catastrophic failure or explosion of the circuit breaker.

Environmental Impact

Moreover, SF₆ is a very strong greenhouse gas. It is around 23,500 times more effective than carbon dioxide in trapping heat. If released into the atmosphere, it can have a serious environmental impact.

High Cost

The SF6 circuit breaker is more expensive than the vacuum circuit breaker. So, for 33 kV and 11 kV systems, we generally use vacuum circuit breakers. Earlier, we used to use SF6 breakers in 33 kV systems. But now, due to high cost, we prefer vacuum breakers. However, for 132 kV and above, we mostly use SF6 breakers. This is because vacuum breakers are not yet available for such high voltages.