What is SF6 Circuit Breaker? Types, Construction, Working Principle, Advantages & Disadvantages Of SF6 Circuit Breaker

SF6 Circuit Breakers (Sulphur Hexaflouride)

In such circuit breakers, sulphur hexafluoride (SF6) gas is employed because the arc quenching medium. The SF6 is an electro-negative gas and features a strong tendency to soak up free electrons. The contacts of the breaker are opened during a high flow of SF6 gas and an arc is struck between them. The conducting free electrons within the arc are rapidly captured by gas to from relatively immobile negative ions. This loss of conducting electrons within the arc quickly builds up enough insulation strength to extinguish the arc. The SF6 circuit breakers are found to be very effective for top power and high voltage service.

Types Of SF6 Circuit Breaker

There are mainly three types of SF6 circuit breakers depending upon the voltage level of the application:

1. Single interrupter SF6 CB applied for up to 245 kV (220 kV) system.

2. Two interrupter SF6 CB applied for up to 420 kV (400 kV) system.

3. Four interrupter SF6 CB applied for up to 800 kV (715 kV) system.

Construction Of SF6 Circuit Breakers

The parts of a typical SF6 breaker. It consists of fixed and moving contacts enclosed during a chamber containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contacts of breaker are opened, the valve mechanism permits a high SF6 gas from the reservoir to flow towards the arc interruption chamber. The fixed contact may be a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is additionally a hollow cylinder with rectangular holes within the sides to allow the SF6 gas to let loose through these holes after flowing along and across the arc. The ideas of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is expensive, it's reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working Principle Of SF6 Circuit Breakers

In the closed position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2.8 kg/cm2. When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the opening of a valve which allows SF6 gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. The high flow of SF6 rapidly absorbs the free electrons within the arc path to from immobile negative ions which are ineffective as charge carriers. The result's that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation (i.e., after arc extinction), the valve is closed by the action of a group of springs.

Advantages Of SF6 Circuit Breakers

Due to the superior arc quenching properties of SF6 gas, the SF6 circuit breakers have many advantages over oil or air circuit breakers. A number of them are listed below:

1. Thanks to the superior arc quenching property of SF6, such circuit breakers have very short arcing time.

2. Since the dielectric strength of SF6 gas is 2 to three times that of air, such breakers can interrupt much larger currents.

3. The SF6 breaker gives noiseless operation thanks to its closed gas circuit and no exhaust to atmosphere unlike the air blast breaker.

4. The closed gas enclosure keeps the inside dry in order that there's no moisture problem.

5. There's no risk of fireside in such breakers because SF6 gas is non-inflammable.

6. There are not any carbon deposits in order that tracking and insulation problems are eliminated.

7. The SF6 breakers have low maintenance cost, light foundation requirements and minimum off-line equipment.

8. Since SF6 breakers are totally enclosed and sealed from atmosphere, they're particularly suitable where explosion hazard exists e.g., coal mines.

Disadvantages Of SF6 Circuit Breakers

1. SF6 breakers are costly thanks to the high cost of SF6.

2. Since SF6 gas has got to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

Application Of SF6 Circuit Breakers

A typical SF6 breaker consists of interrupter units each capable of handling currents up to 60 KA and voltage within the range of 50-80 KV. Variety of units are connected serial consistent with the system voltage. SF6 circuit breakers are developed for voltages 115 KV to 230 KV, power ratings 10 MVA to 20 MVA and interrupting time but 3 cycles.