SF6 Circuit Breaker

SF6 Circuit Breaker

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

The attachment of electron with SF6 gas molecules may occur in tow different ways,

(1) SF6 + e = SF6 -

(2) SF6 + e = SF5 - + F

These negative ions obviously much heavier than a free electron and therefore over all mobility of the charged particle in the SF6 gas is much less as compared other common gases. We know that mobility of charged particle is majorly responsible for conducting current through a gas.

Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength. Not only the gas has a good dielectric strength but also it has the unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property. Due to its low gaseous viscosity (because of less molecular mobility) SF6 gas can efficiently transfer heat by convection. So due to its high dielectric strength and high cooling effect SF6 gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF6 Circuit Breaker is used in complete range of medium voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800KV and even more.

The SF6 insulated Switchgear contains the same components as a conventional outdoor substation. Fig (1) illustrates the construction of typical bay.

1-phase Bus enclosure.

2- Isolator.

3-Earthing Switch.

4-C.B puffer type.

5- CT's

6-Line Isolator.

7-  VT.

8-High Speed Earthing Switch.

9-Cable sealing End.

10-Operating mechanism (cabinet).

11-Conductor tube.

12-Epoxy partition

Fig (1) illustrates the construction of typical bay

Types of SF6 Circuit Breaker

There are mainly three types of SF6 CB depending upon the voltage level of application

(1) Single Interrupter SF6 CB applied for up to 245KV(220KV) system

(2)Two Interrupter SF6 CB applied for up to 420KV(400KV) system

(3)Four Interrupter SF6 CB applied for up to 800KV(715KV) system

Working of SF6 Circuit Breaker

The working of SF6 CB of first generation was quite simple it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for reutilize.

The working of SF6 circuit breaker is little bit different in moder time. Innovation of puffer type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is utilized to develop pressure in the arcing chamber for arc quenching.

Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it can not change its position during the movement of the cylinder. As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against 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 fixed contact body during closed position. As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.

During closing of the SF6 circuit breaker, the sliding cylinder moves upwards and as the position of piston remains at fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas from surrounding will try to enter in the cylinder. The higher pressure gas will come through the axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas will quench the arc.

Advantages of GIs and Application Aspects:

1-Compactness.

The space occupied by SF6 installation is only about  8 to 10 % of that a conventional outdoor

substation. High cost is partly compensated by saving in cost of space. A typical 420/525 kV SF6 GIs

requires only 920 m2 site area against 30.000 m2 for a conventional air insulated substation.

2-Choice of Mounting Site.

Modular SF6 GIS can be tailor made to Suit the particular site requirements.

This  results is saving of otherwise Expensive civil-foundation work. SF6 GIS can be suitably mounted indoor

on any floor or basement and SF6 Insulated Cables (GIC) can be taken through walls and terminated

through SF6 bushing or power cables.

3- Reduced Installation Time.

The principle of building block construction (modular construction) reduces the installation time to a few weeks. Each conventional substation requires several months for installation.

In SF6 substations, the time-consuming high cost galvanized steel structures are eliminated. Heavy foundations for galvanized steel structures,

Equipment support structures etc are eliminated. This results in economy and reduced project execution time. Modules are factory assembled, tested and dispatched with nominal SF6 gas. Site erection time is reduced to final assembly of modules.

4-Protection from pollution.

The external moisture. Atmospheric Pollution, snow dust etc. have little influence on SF6 insulated substation. However, to facilitate installation and maintenance, the substations are generally housed inside a small building.

5- Increased Safety.

As the enclosures are at earth potential there is no possibility of accidental contact

by service personnel to live parts.

6-Explosion-proof and Fire-proof installation.

Oil Circuit Breakers and oil filled equipment are prone to explosion. SF6 breakers and SF6 filled equipment are explosion proof and fire-proof.

Disadvantages of GIS:

1- High cost compared to conventional outdoor substation.

2-Excessive damage in case of internal fault. Long outage periods as

Repair of damaged part at site may be difficult.

3-Requirement of cleanliness is very stringent. Dust or moisture can cause

internal flashovers.

4-Such substations are generally in door. They need a separate building.

This is generally not required for conventional outdoor substations.

5-Procurement of gas and supply of gas to site is problematic.

Adequate stock of gas must be maintained.

6-Project needs almost total imports including SF6 Gas. Spares conventional

substation is totally indigenous up to 400 kV.