What are the Important Characteristics of SF6 Circuit Breakers?

The SF6 Gas

The Sulphur Hexafluoride (SF6) is an electron negative, inert, heavy gas having good dielectric and arc quenching properties. The strength of the dielectric of this gas increases with pressure and more than the strength of the dielectric of oil at a pressure of 3 bars. This gas is widely used in electrical industry and is used as an arc extinguishing medium for high voltage applications like  high voltage metal-clad switchgear, capacitors, circuit breakers, current transformers, bushings and others. 

The chemical properties of SF6 gas includes: 

1. This gas is stable up to 500 degrees C.
2. SF6 gas is inert which make it advantageous for switchgear applications. Thus, the life of metallic part, contacts is longer in SF6 gas because the components do not get oxidized or deteriorated.  
3. SF6 is an electron negative gas which can easily arrest electrons from the arc.
4. Does not react with structural material - up to 500 degrees C.
5. The products of decomposition after arc suppression can be broken down into SF4 and SF2 and he products of decomposition recombine upon cooling to form the original gas. The remainder is removed by filters containing activated alumina (Al2O3), therefore the loss factor is small compare to other gasses. 
6. The metallic fluorides component of SF6 are good dielectric materials hence are safe for electrical equipment.

SF6 Circuit Breakers

Several circuit breaker manufacturers around the world has developed SF6 circuit breaker for the last 2 decades with voltage range from 4.16 kV to 700 kV. SF6 gas insulated-metal-clad switchgear comprises factory assembled metal clad, sub-station equipment like circuit breakers, isolators, grounding switches, bus-bars, etc. These are filled with SF6 gas. Such sub-stations are compact and are being favored in densely populated urban areas.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The steel box is made gas-tight. The gas thus obtained contains other fluorides such as S2F10, SF4 and must be purified further SF6, gas is generally supplied by chemical firms. The cost of gas is low if manufactured on a large scale. The gas is transported in liquid form in cylinders. Before filling the gas, the circuit breaker is evacuated to the pressure of about 4 mm of mercury so as to remove the moisture and air. The gas is then filled into the circuit breaker. The gas can be reclaimed by the gas-handling unit.

Essential Properties of SF6 Circuit Breakers

1. Odorless
2. Non- flammable
3. State-Gas at normal pressure and temperature
4. Colorless
5. Non-toxic (however impure SF6 gas contains toxic elements)
6. High density gas (with a density of 5 times compare to air at 20 degrees C. and atmospheric pressure)

SF6 Circuit Breakers Design

Different manufacturers developed the design of Sf6 over the years. Nowadays, there are unique design features that can be identified as follows: 

Double Pressure Type

In this design, the gas from high pressure system is released into the low pressure system through a nozzle, during the arc extinction processes. Most of models adapting this design are phased out. 

Single Pressure Puffer Type 

In this design, the gas is compressed by means of moving cylinder system and is released through a nozzle while extinguishing the arc. This is widely used for voltages up to 760 kV. 

Live Tank Design 

The interrupters under this design category are supported by porcelain insulators.

Dead Tank Design 

The interrupters of this category are installed within SF6 gas-filled tank at ground potential. Each of the design has certain advantage and disadvantages depending on the applications. Single pressure puffer type live tank breakers are being preferred for conventional outdoor switchyards.

Puffer Principle Method of Arc Suppression

The puffer cylinder moves downwards for the opening stroke, the pressure radio P1/P2 rises. The pressure rise depends upon the throat-diameter of nozzle and speed of puffer cylinder. The pressure ratio P1/P2 increases to about five during opening condition. The compressed gas is released through the convergent-divergent nozzle. The arc is quenched at a current zero. For higher interrupting ability, the flow pattern is optimized. 

Single pressure puffer type SF6 circuit breakers are sealed units filled with SF6 gas at a pressure 5 kg/cm2. Both dead-tank and live tank designs have been developed for voltages from 3.3 to 760 kV and breaking currents from 20 to 80 kA. The designs are being continuously optimized for higher capacity per interrupter. There are two types of designs in single pressure puffer type SF6 circuit breakers. 

• Puffer type SF6 breaker with insulating nozzle 
• Puffer type SF6 circuit breaker with conducting nozzle

Advantages of SF6 Circuit Breakers 

1. For outdoor Extra High Voltage application, the SF6 has less number of interrupters per pole than Air Blast CB and Minimum Oil CB. Outdoor SF6 CB is simple, less costly, maintenance free and compact. 
2.  The decomposition products are not explosive and therefore there is no danger of fire or explosion. Accordingly, the gas is non-flammable and chemically stable.
3. The gas requirement is less since the same gas is recirculated in the circuit. 
4. The current carrying capacity of SF6 is 150% more than the current carrying capacity of air blast circuit breakers. This is due to its superior heat transferability of SF6 gas. 
5. Compare to air blast circuit breakers, SF6 is silent. 
6. The sealed construction avoids the contamination by moisture, dust, sand etc. No costly compressed air system like Air Blast CB. 
7. SF6 breakers requires less maintenance. 
8. SF6 has the ability to interrupt low and high fault currents, magnetizing currents, capacitive current, without excessive over-voltages. 
9. The SF6 gas circuit breaker can perform the various duties like clearing short-line faults, opening unloaded transmission lines, capacitor switching, transformer, reactor switching, etc. much smoothly. 
10. Excellent insulating, arc extinguishing, physical and chemical properties of SF6 gas is the greatest advantage of SF6 breakers. No frequent contact replacement. Contact corrosion is very small due to inertness of SF6 gas. Hence contacts do not suffer oxidation. 
11. Due to unique characteristics of SF6 gas, the arc is extinguished at natural current zero without current chopping and associated over-voltage originating in circuit breakers. 

Disadvantages of SF6 Circuit Breaker

1. Sealing problems arise due to the type of construction used. Imperfect joints lead to leakage of gas. The arced SF6 gas is poisonous and strong sealing must be available to prevent it from being inhaled.  
2. The influx of moisture in the system is very dangerous to SF6 gas circuit breakers since this gas can be poisonous if mixed with impurities.  
3. The double pressure SF6 circuit breakers are relatively costly due to the type of construction and the complex gas system. 
4. The internal parts should be cleaned thoroughly during periodic maintenance under clean, dry environment. Dust of Teflon and sulfides should be removed. 
5. Special facilities are needed for transporting the gas, transferring the gas and maintaining the quality of gas. The deterioration of quality of the gas affects the reliability of the SF6 circuit breaker.