Properties of SF6 Gas

5. Description of different Circuit Breaker

5.4 Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear

5.4.2 Properties of SF6 Gas

Sulphur hexafluoride (SF6) gas has good dielectric and arc extinguishing properties. The dielectric strength of the gas increase with pressure and is more than that of the dielectric oil at high pressures. SF6 is now very widely used in electrical equipments like high voltage metal enclosed cables, high voltage metal clad switchgear, capacitors, circuit breakers, current transformers, high voltage bushing, etc.

Sulphur hexafluoride gas is of low cost if manufactured on a large scale. It is transported in liquid from cylinders. Before filling the gas, the circuit breaker is evacuated to the pressure of about 4mm of mercury so as to remove the moisture and air. The gas is then filled in the C.B.

5.4.2.1 Physical properties of SF6 gas

Colorless.
Odorless.
Nontoxic. Pure SF6 gas is not harmful to health. However, impure SF6 gas contains toxic impurities.
Inflammable.
Stat: gas at normal temperature and pressure.
Density : heavy gas with density 5 times that of air at 20 ° C and atmospheric pressure.

- Liquefaction of SF6 Gas

The gas starts liquefying at certain low temperatures. The temperature of liquefaction depends on pressure. At 15 kgf/cm² the gas starts liquefying at 10 ° C . Hence this gas is not suitable for pressure above 15 kgf/cm²

The temperature at which the SF6 gas changes to liquid state depends on the pressure. With higher pressure, the temperature increases. To avoid the liquefaction of SF6 gas. the temperature of SF6 should be maintained above certain value. For atmospheric pressure, SF6 gas starts liquefying at a temperature of about 10 ° C. Hence thermostatically controlled heaters are provided, which maintain the gas temperature above about 16 ° C in case of high pressure system.

- Heat Transferability

The heat transferability of SF6 gas is 2 to 2.5 times that of air at same pressure. Hence for the equal conductor size, the current carrying capacity is relatively more.

- Enthalpy

Heat content property at temperature below 6000 ° K is much higher than nitrogen. This assists cooling of arc space after current zero, due to continuous removal of heat from the contact space by the surrounding gas.

- Low arc time constant

The time constant of the medium is defined as "the time between current zero and the instant the conductance of contact space reaches zero value". Due to the electro negativity of SF6 gas the arc time constant of SF6 gas is very low and the rate of dielectric strength is high. Hence SF6 circuit breakers are suitable for switching condition. involving high rate of rise of TRV.

5.4.2.2 Chemical properties of SF6 Gas

Stable up to 500 ° C.
Inert. The chemical inertness of this gas is advantageous in switchgear. The life of metallic part, contacts is longer in SF6 gas. The components do not get oxidized or deteriorated. Hence the maintenance requirements are reduced. Moisture is very harmful to the properties of the gas. In the presence of moisture, hydrogen fluoride is formed during arcing which can attack the metallic and insulating parts in the circuit breaker.
Electronegative gas.
Does not react with structural material up to 500 ° C.
Products of decomposition. During arc extinction process SF6 is broken down to some extent intoSF_4,SF₂. The products of decomposition recombine upon cooling to form the original gas. The remainder is removed by filters containing activated alumina ( AL₂O₃) Loss factor is small. The products of decomposition are toxic and attack certain structural materials.
The metallic fluorides are good dielectric materials hence are safe for electrical equipment.
Moisture content in the gas, due to influx from outside, present a various problems in SF6 circuit breakers. Several failures have been reported recently due to this cause.

5.4.2.3 Dielectric properties of SF6 Gas

Dielectric strength of SF6 gas atmospheric pressure is 2.35 times that of air, it is 30% less than of dielectric oil used in oil circuit breakers.
At higher pressure the dielectric strength of the gas increases. At pressure about 3 kgf/cm² the dielectric strength of SF6 gas is more than that of dielectric oil. This property permits smaller clearance and small size of equipments for the same KV.
The breakdown voltage in SF6 gas depends on several aspects such as electrode configuration, roughness of electrodes, distribution of electric field, vicinity of insulating supports, moisture, wave shape etc. Other parameters remaining constant, the breakdown voltage in SF6 increases with pressure. The gas follows paschen's law which states that "in uniformly distributed electric field, the breakdown voltage (V_b ) in a gas is directly proportional to the product of gas pressure (p) and electrod gap (d)"
V_b∝pd
With the non uniform field, the breakdown voltage versus pressure curve does not follow the paschent's law strictly. The breakdown voltage increases with pressure. However after about 2.5 kgf/cm²it starts reducing and then rises again. The pressure at which the breakdown voltage starts reducing is called 'Critical pressure'. The dielectric strength at pressure between 2-3 kgf/cm² is high. Hence this pressure range preferred in SF6 insulated metal enclosed switchgear. However, in circuit breaker compartment, the pressure of the order of is kgf/cm² preferred for arc quenching process.
Rough electrode surface reduces the breakdown voltage with rough surface the ionization starts earlier near the sharp points on conductors. Hence conductor surfaces should be smooth
Thew conductor in SF6 insulating equipment are supported on epoxy or porcelain insulators. The flashover invariably takes place along the surface of the support insulators. The breakdown can occur at extremely low values if the insulators supports are covered by moisture and conducting dust. Hence the insulators should be extremely clean and should have anti-tracking properties.
The breakdown is initiated at sharp edges of conducting parts and parts having maximum stress concentration. The limiting value of breakdown stress is of the order of 20 P KV/cm for pure SF6 and P is pressure of gas in kgf/cm² . Good stress distribution is very important in SF6 insulated equipment.
The breakdown value depends on the wave-shape characterized by peak value, wave front, wave-tall, polarity in case of impulse wave. Voltage withstand value reduces with increase in steepness and increase in duration of the wave. Negative polarity is generally more severe than positive.
SF6 gas maintain high dielectric strength even when diluted by air (Nitrogen). 30% SF6 + 70 % of air, by volume, has a dielectric strength twice that of air ( at the same pressure). Below 30% by volume, the dielectric strength reduces quickly.