SF6 gas is an inert gas with dielectric strength and arc-extinguishing qualities. In SF6 breakers, the rate of rise of dielectric strength is very high and the time constant is very small. This provides another type of oil-less circuit breaker. However, the life of the contacts is short when compared to that of the vacuum circuit breaker.
The SF6 circuit breaker has other advantages that make it equally acceptable for industrial use. All circuit breaker systems up to 36 kV are three-phase systems. However, for higher voltages of up to 420 kV, three separate single-phase breakers are sometimes used to facilitate the single-phase opening and the closing for transient faults.
The advantages of SF6 breakers are as follows:
l There is no danger of explosion or fire
l Excellent arc-extinguishing capabilities with minimum time
l Contacts wear and tear is lesser
l Outdoor SF6 breakers are simple, cheap, maintenance-free, and compact
l Suitable for voltage levels ranging from 3.6 to 760 kV
l Minimum maintenance
l No contamination of moisture or dust due to sealed construction.
Virtually all current designs of circuit-breaker for use at transmission voltages now use sulphur hexafluoride (SF6) gas both as an arc-interrupting and a dielectric medium. At distribution voltages SF6 designs of circuit-breaker are also used, but here the market is still shared with vacuum and bulk-oil circuit-breaker alternatives.
Three basic types of SF6 interrupter are commonly employed. The gas-blast interrupter, the puffer interrupter and the rotating-arc interrupter.
The gas-blast interrupter tends to have a higher performance capability than the other interrupters and is more commonly applied to transmission circuit-breakers. All gas-blast interrupters cause a flow of pre-pressurised gas across or along the opening circuit-breaker contacts. These interrupters may take a number of forms. Early designs used a store of high-pressure SF6 gas kept separate from the main SF6 used for dielectric purposes. On opening of contacts a valve operates to allow the flow of the highpressure gas around the contacts to extinguish the arc. The gas is then re-compressed following an opening operation. Such a two-pressure system was used on early designs of SF6 circuit-breaker. However, it has the disadvantage that the high-pressure gas, typically stored at some 15 bar gauge, liquefies at normal ambient temperatures and it is then necessary to apply heaters to the gas in the high pressure storage cylinder to ensure that it is retained in its gaseous state. Heater failure would require the circuit-breaker to be removed from service until the heater could be reinstated. In addition, the provision of a high pressure storage chamber makes the two-pressure circuit-breaker very expensive.
The problem was resolved by the development of the ‘puffer’ circuit-breaker. Here the gas is compressed during the initial part of the opening stroke and prior to separation of the circuit-breaker arcing contacts. This requires the circuit-breaker to have a long operating stroke and a powerful operating mechanism to precompress the gas. Nevertheless, it is this arrangement that is most commonly used on transmission SF6 circuit-breakers. There are a number of variations of the puffer principle which determine the way in which the gas flows around the opening arcing contacts. These may be referred to as ‘mono-blast’, ‘partial duoblast’ or ‘duo-blast’. A typical partial duo-blast interrupter is shown in Figure 1.
Attempts have been made to overcome the necessity for the provision of a large powerful operating mechanism on puffer circuit-breakers by using the heat of the arc itself to pressurise the surrounding gas and induce arc extinction. This principle is commonly referred to as the `self-pressurisation interrupter’. Whilst clearance at high values of fault current can readily be achieved, it is usually still necessary to apply a small piston to assist in arc extinction at very low values of fault current. This design of circuit-breaker is commonly used in distribution circuits and is now being employed in circuit-breakers for use at transmission voltages.
A further alternative, widely used at distribution voltages, is the use of the rotating-arc principle. Here the arc is induced to rotate very rapidly under the influence of magnetic fields set up by a series coil inserted into the current path during the opening of the circuit-breaker contacts. Very rapid movement of the arc causes a flow of cool SF6 gas across the arc to achieve arc extinction.
The rotating-arc principle is shown in Figure below. With this design more economical operating mechanisms can be achieved but, unlike the puffer circuit-breaker, the arc duration is largely dependent on the size of current being interrupted.
At transmission voltages, SF6 circuit-breakers mainly use either pneumatic, hydraulic or spring operating mechanisms, whilst at distribution voltages spring operating mechanisms are now invariably used.
With SF6 circuit-breakers it is imperative to ensure a leak-proof assembly. The number of enclosure joints needs to be kept to an absolute minimum and the main drive for the circuit-breaker moving-contact system should preferably be taken through only one point in the enclosure. Either rotary or axial drives may be used, but special provisions must be made to ensure adequate gas sealing. Maximum gas-leakage rates are typically specified as being not greater than 1% per annum in order to ensure adequate gas retention within the anticipated maintenance periods of the circuit-breaker.
Low-gas-density alarms are usually fitted to give indication of loss of gas and, in the event of rapid loss of gas, circuit-breaker immediate tripping or trip lockout systems are usually employed.
The development of the SF6 interrupter has been such that the interrupter capability has increased rapidly and it is now common practice to use only two interrupters per phase for a typical circuit-breaker rating of 55 kA three-phase at 420 kV. A single interrupter can achieve a typical rating of 40 kA at 420 kV.
The transient recovery voltage withstand capabilities are such that parallel resistor interrupters, as used on air-blast circuit-breakers, are not required. All that is necessary is simple capacitive voltage grading across both interrupters to ensure uniform sharing. Comparison with an equivalent air-blast circuit-breaker shows that the number of interrupters at 420 kV has been reduced from 10 to 2, parallel resistor interrupters and high pressure ceramic blast tubes are no longer required because standard porcelain insulators can satisfactorily operate at the SF6 gas pressures required for interruption. This considerably reduces the number and complexity of components used and has enabled significant cost reductions to be achieved in the application of transmission switchgear.