Sulphur Hexa Fluoride (SF6) is the recent development in the field of high voltage switchgear circuit breaker. In this a gas called sulphur hexa flouride is used as the medium of insulation and arc interruption. SF6 is about 5 times heaver than air. It is chemically very stable, odourless, inert, non-flammable and non-toxic, The gas has a high dielectric strength and outstanding arc quenching characteristics.
In SF6, the arc voltage remains low until immediately before current zero so that arc energy does not attain high value. Moreover, the arc time constant for SF6 is also very low. Furthermore, SF6 and decomposition products are electronegative permitting electron capture at relatively high temperature.
Thus, the dielectric strength rises rapidly and enables the breaker to withstand the recovery voltage, even under extreme switching conditions. In air blast circuit breakers, air is allowed to escape following the quenching operation.
This obviously would be uneconomical in the case of SF6 breakers. Actually, sealed circuit breaker chambers are therefore developed in which even the gas pressure remains practically constant over long periods. Owing to low contact erosion in SF6 and almost negligible decomposition of gas in arc, the breaker can be operated for several years without having to be opened for the purpose of overhauling.
Dielectric strength vs pressure for air, oil and SF6…
an insulator with an overall height 160 mm the impulse and power frequency withstand voltages are shown…
Dielectric properties of SF6
At atmospheric pressure the dielectric strength of SF6 is about 2.5 times that of air. Actually speaking, this value will depend on the nature of the field existing between the electrodes, which in turn will depend 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. The graph shows the relation of dielectric strength vs pressure.
It may be seen from curves that dielectric strength which is 30 % less than that of oil at atmospheric pressure increases rapidly with increase of pressure. It attains a value equal to that of oil at pressure of 650 gm/cm2 – and at a pressure of 1.25 kg/cm2, it is about 15% higher.
For an insulator with an overall height 160 mm the impulse and power frequency withstand voltages are shown in figures as a function of SF6 pressure. At pressure of 3.5 atmosphere the withstand voltages are almost equal to those for outdoor post insulators measuring 2100 mm.
Fig-A: Puffer Type SF6 Circuit Breaker…
This gas is strongly electronegative, which means that free electrons are rapidly removed from discharge by the formation of negative ions through process by which a free electron is attached to neutral gas molecule. The attachment may occur in two ways:
(A) As direct attachment – SF6 + e-
(B) As dissociative attachment
SF6 + e- = SF5 +F-
The resulting ions, which are heavy and relatively immobile are thus ineffective as current carriers – so that ionized SF6 has as high an electric strength as unionised gasses such as N2 at equal density.
Quenching Properties of SF6
The extinction of AC arc at the instant of current zero is primarily influenced by the speed with which the dielectric strength in the contact gap regenerates immediately before and after the passage of current zero. Its efficiency as an arc quenching medium can be explained by the low dynamic time constant (about 1 micro sec compared with about 100 microsecond in N2) of arcs drawn in it. In the case of cylindrical arc, the time constant (H) is a function of temperature, the thermal conductivity is low.
The low time constant of SF6 is due to its ability for free electrons to be captured by molecules of SF6 gas, These SF6 ions surround the arc and form an insulating barrier. This reduces the diameter of arc column, and hence results in reduction of time constant, which aids arc quenching. Figure shows time constants of SF6 and air as functions of pressure.
Condition is much less favourable where the arc burn in N2. No thin core forms in the critical temperature range between 3000 and 7000K because of good thermal conductivity of N2.
The diameter of the arc approaching extinction remains considerably larger and its time constant which varies as the square of the radius, is therefore very much greater. The bounty regions below the ionization temperature does not have the same dielectric strength as SF6, because nitrogen is not electronegative. SF6 and almost all its decomposition products are electronegative and have an affinity for electrons. During cooling, the dielectric strength of the breaker, therefore, rises more rapidly than, for example, with air. The influence of low arc time constants on circuit breakers can be seen as follows.
Meyer’s equation for the limiting value of recovery voltage after current has passed through zero above which arc restrikes is given by
Where Ea = arc voltage.
Wo = 2 πf0 where f0 is the natural frequency of the mains.
H= arc time constant.
Since H is 100 times smaller for SF6 than air, for the value of limiting voltage the natural frequency of mains may be 100 times greater. In other words SF6 breakers can withstand severe RRRV, and thus are most suitable for shot line faults without switching resistors and can interrupt capacitive currents without restriking.
Essential parts of SF6 gas in arc
The essential parts of a SF6 breaker are :
- The tank
- The interrupter unit
- The operating mechanism
- The bushing
- The gas system
Tank: The distance between line and earth parts inside the tank is very much reduced due to better insulating properties of SF6 . As already illustrated in figure, the dielectric withstands 510 kV at 50 hz and a kV BIL Test. Even at atmospheric pressure the insulation distances are sufficient to withstand nearly twice the rated voltage to earth. No large pressure rises are caused due to the operation of SF6, the tanks being designed for pressure of nearly four times and tested at six times the pressure
Interrupter Units: Organic insulation like fibre or micarta should not be located in the arc path since they will be decomposed thus diluting the gas:
The arc is extinguished by SF6 gas under a pressure of 14 kg/cm2 which reduces the mechanical energy for operation of the breaker. The important parts of the interrupter are:
- Main reservoir containing gas at 14 kg/cm2
- Blast valve and control mechanism
- Piping for the gas under pressure
- Axial flow Interrupter
- Tripping spring.
Capacitor units are placed across each break to ensure equal voltage distribution. Metallic parts are surrounded by electrostatic shields, which provide correct distribution of electric field between the intrupter and tank. The various parts are supported by two insulating bars running the whole length of the interrupter.
Operating mechanism: In operation the tripping spring drives the moving contacts and simultaneously opens the valve of the pressure reservoir. The gas under pressure flows into the breaking chambers and extinguishes the arc. At the end of the operation, the mechanism releases the valve of pressure reservoir, which is closed by the action of a set of springs.
Bushings: These contain SF6 at a pressure of 2 kg/cm2 and are much simpler than the condenser bushings. They contain a hollow conductor, a fixing flange. the upper and lower porcelain insulators and the springs, which hold the assembly together. The SF6 gas in bushings communicate with that in the tank through small holes in the upper part of the hollow conductor. The gas in the bushings is thus unaffected by any disturbances in the tank at the instant the current is broken. A filter containing activated alumina is placed at the bottom of the hollow conductor delaminating all chance of contamination of SF6 inside the bushings .
Gas system: A compressor sends the gas back after each break to the high pressure reservoir. Being a closed circuit, no gas escapes to the atmosphere.
An auxiliary reservoir of SF6 at 14 kg / cm2 is located below each tank, containing enough gas for 4 consecutive breaks without the need for starting up the compressor.
Recent results and prospects for future development
Whether these techniques based on new principles will succeed is more difficult to estimate. It is not probable that a better multiautomic gas than SF6 gas will be found , but perhaps a better liquid than oil is feasible. There are attractive possibilities of the combination of SF6 insulation with vacuum interrupters. Another recent suggestion is to use liquid SF6 in a container much like oil in minimum oil breakers. This may provide a solution to the problem of higher speed of operation, on the basis of the results obtained so far, and in view of the fact that future development looks promising.
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