In an SF6 Circuit breaker, sulphur hexafluoride gas is applied as the arc quenching medium.
The sulphur hexafluoride gas (SF6) is an electronegative gas and has a strong aptitude to absorb free electrons. The contacts of the breaker are opened in a high-pressure flow sulphur hexafluoride (SF6) gas and an arc is propelled between them. The gas captures the conducting free electrons in the arc to form relatively obsolete negative ions. This loss of conducting electrons in the arc quickly builds up sufficient insulation strength to extinguish the arc.
SF6 Circuit Breaker are used in substations for all voltages ranging from 144 to 765 kV or even above. Continuous currents up to 8000 A, and symmetrical interrupting ratings up to 63 kA at 765 kV and 80 kA at 230 kV.
Sulphur Hexafluoride (SF6) Circuit Breaker
Construction of SF6 Circuit Breaker
The SF6 circuit breaker consists of two (2) main parts:
- The Interrupter Unit
- The Gas System
The interrupter unit of sulphur hexafluoride (SF6) circuit breaker structure of fixed and moving contacts enclosed in a chamber.
The chamber is named the arc interruption chamber that contains the SF6 gas. This chamber is connected to the sulphur hexafluoride (SF6) gas reservoir. A valve mechanism is there to consent the gas to the arc interruption chamber.
When the contacts of the breaker are unlocked, the valve mechanism consent a high-pressure sulphur hexafluoride gas from the receptacle to flow towards the arc interruption chamber.
The fixed contact is a swelled cylindrical current-carrying contact fitted with an arcing horn. The moving contact is also a swelled cylinder with rectangular holes in the sides.
The holes consent the sulphur hexafluoride gas (SF6) gas to let out through them after flowing along and across the arc.
The tips of fixed contact, moving contact, and arcing horn are involved with a copper-tungsten arc-resistant material.
Since the gas pressure is very high, a lot of care is to be taken to prevent gas leakage at joints by providing perfect scaling.
The low and high-pressure systems are fitted with low-pressure alarms and set of lockout switches which give a warning the moment the gas pressure drops below a certain value because otherwise there will be a reduction in the dielectric strength and arc quenching ability of the breaker is imperilled.
If the danger limit is reached the security devices immobilize the breaker. The overpass safety devices see to it that a fault in the control circuit does not consent the compressor to build up excessive pressure in the high-pressure receptacle or continue to pump gas into the atmosphere in the happening of a major leak.
The gas holds on within the hard-hitting chamber at 16 atmospheres whereas the pressure level on the unaggressive aspect is three atmospheres.
The degree is 20◦C in order to prevent liquefaction of gas in the high-pressure chamber at low temperature; a heater is fitted in the high-pressure chamber. A thermostat is set to the switch on when the ambient temperature falls under 16◦C.
Working of SF6 Circuit Breaker
The working of SF6 CB of the first generation was quite simple and it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high-pressure receptacle. By operation of SF6 circuit breaker, this highly compressed gas is released through the arc in the breaker and collected to a relatively low-pressure receptacle and then it is pumped back to the high-pressure receptacle for reutilizing.
The working of SF6 circuit breaker is a small bit different in modern time. The newness of puffer type design makes the operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to reveal pressure in the arcing chamber for arc quenching. Here the breaker is loaded with SF6 gas at rated pressure. There are two permanent contacts fitted with a specific contact gap. A sliding cylinder bridges these to permanent contacts. The cylinder can axially slide upward and downward by the contacts. There is one stationary piston inside the cylinder which is permanent with other stationary parts of the sulphur hexafluoride circuit breaker, in such a way that it cannot alternative its position during the movement of the cylinder. As the piston is fixed and the cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.
During the opening of the breaker, the cylinder moves downwards against the position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper permanent contact body during closed position.
As the cylinder a lot of more downward, these vent openings pass the higher permanent contact, and become unblocked and then shrunk SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of both permanent contacts. The arc is quenched by this flow of SF6 gas.
Properties of Sulphur hexafluoride Circuit Breaker
Sulphur hexafluoride possesses very good insulating and arc quenching feature. These features are
- It is colourless, odourless, non-toxic, and non-inflammable gas.
- SF6 gas is extremely stable and inert, and its density is five times that of air.
- It has high thermal conductivity better than that of air and help in better cooling current-carrying parts.
- SF6 gas is strongly electronegative, which means the free electrons are easily removed from discharge by the structure of negative ions.
- It has a unique feature of fast recombination after the source energising spark is removed. It is 100 times more effective as compared to arc quenching medium.
- Its insulator strength is a pair of.5 times than that of air and half-hour but that of the insulator oil. At high the insulator strength of the gas will increase.
- Moisture is very harmful to SF6 circuit breaker. Due to a combination of chill and SF6 gas, hydrogen fluoride is formed which can attack the parts of the circuit breakers.
Advantages of SF6 Circuit Breaker
The important advantages of sulphur hexafluoride circuit breaker are :
- The superior arc quenching feature of sulphur hexafluoride gas (SF6). Due to this sulphur hexafluoride circuit breakers have very short arcing time. Hence sulphur hexafluoride gas is used in Gas Insulated Substations.
- Dielectric strength of sulphur hexafluoride gas is 2 to 3 times compared to air. Thus sulphur hexafluoride circuit breakers can interrupt much larger currents.
- The sulphur hexafluoride gas (SF6) circuit breaker gives silent operation due it’s closed gas circuit and no exhaust to atmosphere, improper the air blast circuit breaker.
- The dense design of SF6 gas CBs substantially reduces place requirements and building installation costs.
- The SF6 gas circuit breakers can manage all known switching phenomena.
- SF6 gas circuit breakers perfectly can acclimatize to environmental requirements. They have completely enclosed gas system that eliminates any exhaust by switching operations.
- Contact hiatus is minimum in SF6 gas circuit breakers due to dielectric strength provided by the high-pressure SF6.
Disadvantages of SF6 Circuit Breaker
In general, the important disadvantage of the sulphur hexafluoride circuit breaker compared to other circuit breakers is their relatively high costs. The cost has been somewhat coming down in recent years.
Some other disadvantages of the sulphur hexafluoride circuit breaker are:
- Heavy-duty switching operations can produce harmful by-products from decomposition. By typical sealed operation, these by-products will recombine into sulphur hexafluoride. However, during arcs or in the event of switchgear failure, sulphur hexafluoride and its by-products can be released into the environment.
- Imperfect joints in the Sf6 Circuit breaker lead to leakage of the sulphur hexafluoride gas. Thus continuous monitoring devices are required.
- Arced sulphur hexafluoride gas is poisonous and should not be inhaled.
- While sulphur hexafluoride in its pure form is non-toxic, it does not backing life and can, therefore, become a breathing hazard if it has collected. As it is grave than air, it can typically collect close to the land, in cable trenches or drainage systems, for example. Therefore, site personnel is advised to take care of such environments.
- Exposure to solid by-products, if inhaled, include irritation to manifested skin and eyes, the nose, throat, and lungs; further syndrome can occur if sufficient volume reaches the gastrointestinal tract.
- Particular facilities are required for transportation of gas, passes of gas and for maintaining the quality of the gas. The corrosion of quality of gas affects the performance and hence reliability of the SF6 circuit breaker.
Types of SF6 Circuit Breaker
The sulphur hexafluoride circuit breaker has been developed by several manufacturers and several designs have emerged. The types of SF6 circuit breakers can be extensively identified as
- Double pressure Type
- Single Pressure Puffer Type