SF6 HT Voltage Brakers

SF6 breakers are used in high voltage power distribution applications. The gas inside the arc chamber (sodium hexaflouride) stops the arc of the separating contacts during a breaker trip. Other technologies doing the same thing are vacuum enclosed contact bottles. The SF6 technology has advantages in the areas of overall apparatus size, weight, maintenance, and cycle interrupt, ampacity versus its older oil-filled or air-gap technologies. The dis-advantage is the enviromental hazards associated with SF6 gas. Vacuum-breakers are a good option up to about 72 kilovolts, after that, SF6 dominates the market.

Westinghouse, Siemens, and ABB are good sites to get maintenance information.

I believe that it is sulphur hexafluoride. Sodium would form the compound NaF while Sulphur forms SF₆.

SF6 & VCB Medium Voltage Circuit breakers were developed after experiencing various short comings of OCBs, which were bulky cumbersome and accident prone. Many accidents were reported due to oil getting ignited during operation resulting in major electrical accidents ,deaths and fire in substations.In SF6 breakers instead of oil ,SF6 gas is used as medium in the circuit breaking chamber.The current continue to flow after contact separation through the arc whose plasma consists of ionised SF6 gas as long as it is burning the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at the current zero when the heat is extracted by the falling current . The continued flow of gas finally de ionises the contact gap and establishes the Di electric strength required to prevent a re strike.

The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. The gas pressure can be very simply and permanently supervised. This function is not needed where the interrupters are sealed for life.

Some features of SF6 breakers are:-

The arc energy is low because of the high conductivity of the arc in the SF6 gas, Due to the low energy the contact erosion is small.

SF6 breakers are useful for operation in Medium voltage substations for switching transformers distribution feeder cables, special application for switching off short circuit , current with high DC components ,reactors ,capacitors etc.

We have been using SF6 breakers and VCBs in our new substations and for replacement of oil circuit breakers for modernisation of old substations.

The maintenance cost got reduced drastically. The mechanical fitments and replacement of bottles are rather simple and less time consuming.

In SF6 beakers gas leakage is a defect which can happened must be inspected at regular intervals.Some manufacturers incorporated gas pressure indicators in their bottles for certain models. Once or twice we experienced gas leakage and bursting of bottles. The breakers are fitted with operation counters so that preferably after 5000 normal operations /10 short circuit tripping preventive maintenance /replacement of bottles can be carried out. Records of number of operations/ short circuit or earth fault tripping should be maintained for preventive maintenance trouble free operation.

VCBs ( Vacuum Circuit breakers) are also a comparable product for switching operation in substations and in my experience they are superior.

In a Vacuum circuit breaker, vacuum interrupters are used for breaking and making load and fault currents. When the contacts in vacuum interrupter separate, the current to be interrupted initiates a metal vapour arc discharge and flows through the plasma until the next current zero. The arc is then extinguished and the conductive metal vapour condenses on the metal surfaces within a matter of micro seconds. As a result the dielectric strength in the breaker builds up very rapidly.

thanks to given information khan