SF6 CIRCUIT BREAKERS FOR MODERN POWER SYSTEM PROTECTION

Electrical faults give rise to abnormal operating conditions and can damage or disrupt the power system in many ways. It is necessary that the faulty section should be immediately disconnected so that the normal operation of the rest of the system is maintained. The protective relay should immediately detect the fault and initiate the operation of circuit breaker or breakers.

What is Circuit Breaker?

A circuit breaker is an automatic switch designed to protect an electrical circuit from damage caused by heavy currents, mainly caused due to overloading, short circuit, or earth fault. The basic function of a circuit breaker is to interrupt the current flow and isolate the faulty section from the rest of the network. Circuit breakers are made in different sizes ranging from the low voltage and low current circuit breaker used in low voltage residential, commercial and industrial premises to very high voltage and high current breakers used in electrical power stations and generating stations.

When a heavy current is interrupted, an electric arc is initiated. This arc must be quenched in a safe and reliable manner so that the breaker contacts are safe and the gap between the contacts can regain the dielectric strength in the desired time span. Different circuit breakers use different arc quenching medium and are accordingly classified as oil circuit breakers, air blast circuit breakers, vacuum circuit breakers and SF₆ circuit breakers.    

Properties of Sulphur hexafluoride (SF₆) gas:

Sulphur hexafluoride (SF₆) is a chemically stable, odourless, inert, non-inflammable and non-toxic gas. This gas has a high dielectric strength and outstanding arc quenching properties. At atmospheric pressure, the dielectric strength of SF₆ gas is about 2.5 times of air and may increase up to 5 times. 

SF₆ and its decomposition products are electro-negative. This property permits electron capture at relatively higher temperature. The ability of an atom to attract and hold electrons is called “electro-negativity”. Thus, the dielectric strength rises rapidly which enables the breaker to withstand the recovery voltage even under extreme switching conditions.

Construction of SF₆ circuit breaker:

Double pressure breaker is the early design of SF₆ circuit breaker and its operating principle is the same as that of air blast circuit breaker. Because of its complicated construction and the need for various auxiliary equipments such as compressors, control device etc., this type of SF₆ circuit breaker has become obsolete.

The puffer type or single pressure type SF₆ circuit breaker is the most popular and is available in the voltage range 3.6 kV to 765 kV. In such SF₆ breakers, the SF₆ gas is compressed by a moving cylinder and is released through a nozzle to quench the arc. Figure shows the working principle of single pressure type SF₆ circuit breaker. 

The operating mechanism, may be pneumatic or hydraulic, and is installed on the base. This operating mechanism is connected to the movable contacts located in the interrupter with the help of insulated rod of fibre glass. The interrupter and support insulator are filled with SF₆ gas at a pressure of about 5 kg/cm².

Figure shows the interrupter of a puffer type SF₆ circuit breaker in fully closed and a position in which contacts are separating. The moving cylinder or puffer cylinder and the moving contacts are coupled together. As a result, when the contacts are separated, the trapped SF₆ gas is compressed. This compressed gas is released axially through a nozzle. The gas removes the heat of the arc by axial convection and radial dissipation. The arc diameter reduces with the decrease of current and becomes very small during current zero and thus the arc is extinguished. Due to the electro-negativity and low arc time constant of SF₆ gas, it rapidly regains the dielectric strength after final current zero.     

Application of SF₆ circuit breaker

SF₆ circuit breaker has the ability to interrupt high fault currents, magnetizing and capacitive currents without too much over-voltages. Thus, it can perform duties like clearing line faults, and switching of capacitors, transformers and reactors. Because of the various advantages mentioned above, SF₆ circuit breakers are preferred for voltages above 132 kV.