In this article we will discuss about:- 1. Construction of SF6 Circuit Breakers 2. Properties of Sulphur Hexafluoride Gas 3. Arc Extinction 4. Advantages 5. Problems Associated.
Contents:
- Construction of SF6 Circuit Breakers
- Properties of Sulphur Hexafluoride Gas
- Arc Extinction in SF6 Circuit Breakers
- Advantages of SF6 Circuit Breakers
- Problems Associated With SF6 Circuit Breakers
1. Construction of SF6 Circuit Breakers:
Such a circuit breaker essentially consists of two parts, namely:
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(i) The interrupter unit and
(ii) The gas system.
(i) The Interrupter Unit:
This unit consists of moving contact and fixed contacts comprising a set of current carrying fingers and an arcing probe. In the close position of the breaker, the fingers make contact round the circumference of the moving contact having the arcing probe enclosed within its hollow end. The contacts are surrounded by interrupting nozzles and blast shield to control the arc displacement and the movement of the hot gas. The moving contact is in the form of hollow nozzle sliding in a second set of spring loaded fingers.
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Side vents in the moving contact permits the high pressure gas into the main tank. When the moving contact is withdrawn from the fixed finger contacts an arc is struck between the moving nozzle and the arcing probe. With the increasing separation between the contacts the arc is extended and attenuated and it is finally extinguished by the flow of gas from the high pressure to low pressure systems.
(ii) The Gas System:
The closed circuit gas system is employed in SF6 circuit breakers. Since the gas is costly, it is reconditioned and reclaimed after each operation of the breaker. Necessary auxiliary system is provided for such purpose. The low and high pressure systems are provided with low pressure alarms and a set of lock-out switches which give a warning the moment the gas pressure drops below a certain value, failing which dielectric strength will be reduced and arc quenching ability of the circuit breaker will be endangered.
On reaching danger limit the safety devices immobilize the breaker. The gas is stored in the high pressure chamber at 16 atmospheres while the gas pressure on the low pressure side is 3 atmospheres. Lot of care is required to prevent gas leakages at joints by providing perfect sealing. The temperature is kept 20°C. A heater backed with a thermostat at 16°C is provided in the high pressure chamber to prevent liquefaction of the gas in the high pressure chamber at low temperature.
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SF6 Circuit Breaker with Puffer Piston Principle:
With the puffer-piston principle the SF6 arc quenching pressure is produced during an opening operation by means of a piston attached to the moving contacts. The arc extinction takes place in the insulating nozzle. A generously dimensioned current carrying path concentrically arranged around the arcing contacts permits high load currents to be accommodated.
SF6 breakers are made from 72-550 kV with rated interrupting current of 20-63 kA and rated current of 1,200-12,000 A.
2. Properties of Sulphur Hexafluoride Gas:
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Sulphur hexafluoride (SFe) gas is prepared by burning coarsely crushed sulphur in fluorine gas, in a gas-tight steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The gas thus obtained contains other fluorides such as S2 F10 SF4 and must be purified further. SF6 gas is generally supplied by chemical firms. The gas costs less 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 in the circuit breaker. The gas can be reclaimed by the gas-handling unit.
The sulphur hexafluoride gas possesses very good insulating properties and outstanding arc-quenching characteristics which make it an ideal medium for circuit interruption.
The physical, chemical and dielectric properties and arc-quenching characteristics of SF6 gas are given below:
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1. It is colourless, odourless, non-toxic and non-inflammable gas.
2. This gas is extremely stable and inert, and its density is five times that of air.
3. It has a thermal conductivity higher (about 2-2.5 times) than that of air and assists in better cooling of current carrying parts.
4. The gas starts liquefying at certain low temperatures. The temperature at which the SF6 gas changes to liquid state depends on pressure. With higher pressure, this temperature increases. To avoid the liquefaction of SF6 gas the temperature of SF6 should be maintained above certain level. For 15 atm pressure SF6 gas starts liquefying at a temperature of about 10°C. Thermostatically controlled heaters are, therefore, provided so that the gas temperature above about 16°C could be maintained in case of high pressure system.
5. Heat content property at temperatures below 6,000°K is much higher than that of nitrogen. This assists in cooling of arc space after current zero, due to continuous removal of heat from the contact space by the surrounding gas.
6. The thermal time constant of SF6 is low and as a result the pressure at which it should be stored and used are relatively smaller as compared to air.
7. Due to the electronegativity of SF6 gas the arc time constant (the time between current zero and the instant the conductance of contact space reaches zero value) of SF6 gas is very low (< 1 µs) and rate of rise of dielectric strength is high. Hence SF6 circuit breakers can withstand severe RRRV and thus are more suitable for short line faults without switching resistors and can interrupt capacitive currents without re-striking.
8. It is chemically stable up to 500°C and does not react with structural material up to 500°C.
9. The gas is strongly electronegative, which means that free electrons are readily removed from a discharge by the formation of negative ions through processes by which a free electron is attached to a neutral gas molecule.
The attachment of the electron with the neutral gas molecule may occur in two ways:
(i) As direct attachment SF6 + e– → SF6–
(ii) As dissociative attachment SF6 + e– → SF5 + F–
The negative ions formed are relatively heavier and immobile as compared to free electrons and are, therefore, ineffective as current carriers so that ionized SF6 has as high an electric strength as unionized gases such as nitrogen at equal density.
10. The SF6 gas not only possesses a good dielectric strength but it has the unique property of fast recombination after the source energizing the spark is removed. This property of SF6 makes it very effective in quenching arc. SF6 is approximately 100 times as effective as air quenching arc.
11. It has exceptionally low reactivity and does not attack metals, plastics etc. The chemical inertness of SF6 gas is advantageous in switchgear. The life of metallic part, contacts is longer in SF6 gas. The components do not get oxidised or deteriorated. Hence the maintenance requirements are reduced—the breaker can be operated for several years without opening for the purpose of overhauling.
12. Moisture is very harmful to the properties of SF6 gas. In the presence of moisture, hydrogen fluoride is formed during arcing which can attack the metallic and insulating parts of the circuit breaker.
13. During arc extinction process SF6 is decomposed, into atoms, electrons and ions. These atomic components do not recombine completely to form the original SF6 gas on cooling. They form low molecular gaseous sulphur fluorides and compounds with the contact metals, e.g., copper fluorides. The percentage of gaseous decomposition products have been found extremely small. These products and any other secondary gaseous reaction products are removed from the gas circuit by filters containing activated alumina (Al2O3) when the gas is pumped back into the high pressure tank.
The metallic fluorides are good dielectric materials and are, therefore, safe for electric equipment. However, they must be removed during periodic maintenance as they absorb moisture and lose their dielectric property. This happens if the breaker is dismantled during rainy season.
14. Dielectric strength of SF6 gas at atmospheric pressure is 2.35 times that of air and 30% less than that of dielectric oil. At higher pressure the dielectric strength of the gas increases. At pressure about three times the atmospheric pressure, it is more than that of oil. This property permits smaller clearances and small size of equipments for the same voltage level.
Actually speaking the dielectric strength of the SF6 gas depends upon the nature of field existing between the electrodes, which in turn depends on the shape and configuration of electrodes, and the gap between the electrodes. The dielectric strength may actually increase to about 5 times depending upon the non-homogeneity of the field.
15. SF6 gas maintains high dielectric strength even when diluted by air (nitrogen). 30% SF6 and 70% air by volume has a dielectric strength twice that of air at a given pressure. Below 30% by volume the dielectric strength falls sharply.
3. Arc Extinction in SF6 Circuit Breakers:
The arc extinction process in SF6 circuit breakers is different from that in air-blast circuit breakers.
During the arcing period, SF6 gas is blown axially along the arc. The heat is removed from the arc by axial convection and radial dissipation. Consequently, the arc diameter reduces during the decreasing node of the current wave. The diameter becomes small during current zero. Turbulent flow is introduced around current zero for extinguishing the arc.
SF6 gas, due to its electronegativity and low arc time constant, regains its dielectric strength rapidly after the final current zero, the rate of rise of dielectric strength is very high and the time constant is very small. SF6 is a remarkable medium for arc extinction. The arc extinguishing properties are improved by moderate rates of forced gas flow through the arc space. The SF6 gas at atmospheric pressure can interrupt currents of the order of 100 times the value of those can be interrupted in air with a plain breaker interrupter.
The basic requirement in arc extinction is not primarily the dielectric strength, but high rate of recovery of dielectric strength. In SF6 gas, the dielectric strength is quickly regained, as already explained. Problems connected with current chopping are, therefore, minimum.
In SF6 circuit breakers, the gas is made to flow from a high pressure zone to a low pressure zone through a convergent-divergent nozzle. The nozzle is located such that the gas flows axially over the arc length. The flow of gas attains almost supersonic speed in the divergent portion of the nozzle, thereby the gas takes away the heat from the arc periphery and arc diameter is reduced. Finally the arc diameter becomes almost zero at current zero and the arc is extinguished. The contact space is filled with fresh SF6 gas and its dielectric strength is rapidly regained because of electronegativity of the gas and turbulent flow of fresh gas in the contact space.
4. Advantages of SF6 Circuit Breakers:
SF6 circuit breakers have the following advantages over conventional breakers:
1. Excellent insulating, arc-extinguishing, physical and chemical properties of SF6 gas is the greater advantage of SF6 circuit breakers.
2. The gas is non-inflammable and chemically stable. The decomposition products are non-explosive i.e., there is no risk of fire or explosion.
3. Electrical clearances are very much reduced because of high dielectric strength of SF6 .
4. Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to air- blast circuit breaker and minimum oil circuit breaker. Outdoor SF6 circuit breaker is simple, comparatively cheaper in cost, maintenance free and compact.
5. Its performance is not affected due to variation in atmospheric conditions.
6. It gives noiseless operation-it does not make sound like air-blast circuit breaker during operation.
7. No frequent contact replacement-arcing time is small owing to outstanding arc quenching properties of SF6 and, therefore contact erosion is less. Hence contacts do not suffer oxidation.
8. There is no reduction in dielectric strength of SF6 since no carbon particle is formed during arcing.
9. Minimum maintenance. The breaker may require maintenance once in four to ten years.
10. The sealed construction avoids the contamination by moisture, dust, sand etc. No costly compressed air system is required as in case of air-blast circuit breaker.
11. Same gas is re-circulated in the circuit thereby reducing the requirement of SF6 gas. One triple pole, 145 kV SF6 circuit breaker needs only about 30 kg of SF6 gas for first filling and no replacement is required for about 5 years.
12. No overvoltage problem. The arc is extinguished at natural current zero without current chopping and associated overvoltage originating in circuit breakers.
13. The SF6 gas circuit breaker can perform various duties like clearing short line faults, opening unloaded transmission lines, capacitor switching, and transformer reactor switching etc., without any problem.
14. Ample overload margin. For the same size of conductors, the current carrying capability of SF6 circuit breakers is about 1.5 times that of air-blast circuit breakers because of superior heat transferability of SF6 gas.
5. Problems Associated With SF6 Circuit Breakers:
Problems associated with SF6 circuit breakers are given below:
1. Sealing problems. Imperfect joints lead to leakage of SF6 gas.
2. SF6 gas is suffocating to some extent. In case of leakage in the breaker tank, SF6 gas being heavier than air settles in the surroundings and may lead to suffocation of the operating personnel. However, it is non-poisonous.
3. Arced SF6 gas is poisonous and should not be inhaled or let-out.
4. Influx of moisture in the breaker is very harmful to SF6 gas circuit breakers. Several failures are reported because of it.
5. Mechanism of higher energy level is required for puffer type SF6 circuit breakers. Lower speeds due to friction, misalignment can cause failure of the breaker.
6. The internal parts need thorough cleaning during periodic maintenance under clean and dry environment. Dust of Teflon and sulphides should be removed.
7. Special facilities are required for transportation of gas, transfer of gas and maintenance of quality of gas. The deterioration of quality of gas affects the performance and hence reliability of the SF6 circuit breaker.