What is a Sulphur hexafluoride(Sf6) circuit breaker?
A circuit breaker that uses high-pressure sulphur hexafluoride gas as arc extinction medium is called Sf6(sulphur hexafluoride circuit breaker).Sf6 gas has high dielectric strength and excellent arc quenching property. Sf6 gas proved superior over other arc quenching mediums such as oil and gas.
The sf6 gas has electro negative gas and has a strong tendency to absorb free electrons from the arc.
Construction Of Sf6 Circuit breaker
Sf6 circuit breaker consists of two parts
1. Interrupter unit
2. the Gas system unit
1. Interrupter unit
The interrupter unit consists of fixed and moving contacts with high-pressure sf6 gas and also comprises current carrying contacts and arcing probe. this chamber is called the arc interruption chamber.
The interrupter unit is connected to the sf6 gas reservoir unit. the valve mechanism is connected between the interrupter unit and the sf6 gas reservoir. when the contacts of the circuit breaker are opened, the valve mechanism permits the high-pressure sf6 gas from the sf6 gas reservoir to flow towards the arc interruption chamber of the circuit breaker.
The moving contacts and fixed contacts are hollow cylindrical current contacts. the moving contacts consist slide vent which is permitted high-pressure sf6 gas into the interrupter chamber. the tips of fixed contacts, moving contacts and arcing probe are coated with copper tungsten arc resistance material.
2. Gas System.
In the sf6 circuit breaker closed system gas system is used. the Sf6 gas is very costly so it is reclaimed after each successive operation. In a gas pressure chamber, gas pressure is very high, so special care should be taken to prevent gas leakages from joints by providing proper sealing in joints.
The gas system unit consists high-pressure chamber and a low-pressure chamber with a low-pressure alarm along with warning switches. The high-pressure chamber consists of the pressure of 16 atmospheres and the low-pressure chamber consists of the pressure of 3 atmospheric. A heater is provided into the high-pressure chamber to prevent the liquefaction of gases which maintains the temperature at 200c.
When sf6 gas pressure is low due to which dielectric strength of sf6 gas is reduced, so arc quenching ability of the circuit breaker goes into danger and the system give the warning alarm. The sf6 circuit breaker has a low alarm system that gives alarm during low gas pressure inside the arc interruption chamber.
When gas pressure becomes lower than limits controlling switches lockout the breaker. After lockout breaker can not trip manually or by the protective system. If in lockout condition breaker trips, it becomes very dangerous for human safety and damages the breaker poles.
Working of Sf6 Circuit Breaker
During normal operating conditions fixed and moving contacts remain in closed condition. When the fault occurs in the system moving contacts are separated from fixed contacts and an arc is produced between the contacts. The displacement of moving contacts are connected with the gas controlling valve which permits to enters of high-pressure gas into the arc interrupting chamber at a pressure of about 16 kg/cm^2.
The sf6 gas is electronegative gas that is absorbed electron-ion from the arc path and forms immobile negative ions which do not carry a charge carrier. These charge-free ions increase the dielectric strength of the gas and the arc is extinguished. This arc extinction process reduces the gas pressure up to 3 kg/cm^2. so, it is stored in a low-pressure reservoir. this low-pressure gas is pulled back to the high-pressure reservoir.
This type of working is used in the old type of sf6 circuit breakers. In modern type circuit breaker, it is different from old types. In modern puffer types, circuit breakers are used.
In the puffer type of circuit breaker arc energy is utilized for developing a high pressure inside the arcing chamber for arc quenching purposes. in this type of circuit, the breaker is completely filled with the sf6 gas at rated pressure.
There are two fixed contacts with some gap between them. These fixed contacts are enclosed inside the sliding cylinder. This sliding cylinder moves axially downward and upward with the fixed contacts.
There is a fixed piston inside the sliding cylinder which is fixed with some other stationary parts of the breaker, so it does not move with the sliding cylinder. The cylinder is sliding and the piston is fixed so the internal volume of the cylinder changes when the cylinder slides or moves.
During the opening of the circuit, the breaker cylinder moves downward against the position of the fixed piston. so volume inside the cylinder is decreased which cause high pressure inside the cylinder and sf6 gas inside the cylinder is compressed at high pressure. The cylinder has a number of slide gas vent which is blocked by the upper fixed contacts due to the movement of the cylinder.
When the cylinder further moves downward and fixed contacts unblock the gas vent and high-pressure high-speed sf6 gas inside the cylinder will come out towards the arc and passes through the axial hole of both the fixed contacts. During this flow of sf6 gas can quench the arc.
During the closing of the circuit, the breaker cylinder moves upwards and the piston remains at its fixed position. so volume inside the cylinder increases which cause low pressure inside the cylinder as compared to surroundings. Due to pressure difference, high-pressure sf6 gas from surrounding enters inside the cylinder through the axial hole in both fixed contacts. During this flow of sf6 gas can quench the arc.
Advantages of Sf6 circuit breaker
1. Sf6 gas has high dielectric strength, excellent arc extinguishing, and insulating properties. It is the greatest advantage of the sf6 circuit breaker.
2. The sf6 circuit breaker has high arc quenching property so such a circuit breaker has a very short arcing time.
3. The dielectric strength of Sf6 gas is 2 to 3 times of air and such a circuit breaker interrupts a larger current.
4. The sf6 circuit breaker gives noiseless operation due to its closed gas cycle as compared to the air blast circuit breaker.
5. There is no risk of fire because sf6 gas is non-flammable.
6. The closed gas enclosed keeps inside dry so that there is no moisture problem in the sf6 circuit breaker.
7. There is no carbon deposit as compared to the oil circuit breaker.
8. The sf6 circuit breaker has low maintenance, light foundations requirement, and minimum auxiliary equipment.
9. The sf6 circuit breaker is totally enclosed and sealed from the atmosphere so it is most suitable where explosion hazards are available,i.e coal mines.
10. Electrical clearance is very much reduced due to the high dielectric strength of sf6 gas.
11. there is no reduction in dielectric strength because ni carbon particles produce during arcing.
Disadvantages of Sf6 Circuit breaker
1. The sf6 circuit breaker is very costly due to the high cost of sf6 gas.
2. The sf6 gas is required to be reconditioned after every closing and opening operation so additional reconditioning equipment is required for this purpose.
3. The sf6 gas is suffocating to some extent. This gas is heavier than air and settled surroundings and causes suffocation to operating personnel.
4. Special facility requires for transportation and maintenance of sf6 circuit breaker.
Applications of Sf6 Circuit breaker
1. In the sf6 circuit breaker's single interrupter unit are capable of interrupting 60kA of fault current and voltage range of 50 to 80Kv.
2. The sf6 circuit breaker is developed for voltage from 115 Kv to 230 Kv up to 765 Kv and power ratings from 10Mva to 20 Mva and interrupting time less than 3 cycles.