Sulfur Hexafluoride Gas Characteristics:
Physical Properties:
As an insulating gas, SF6 has many advantages. It is a colorless, odorless, non-toxic, non-flammable inert gas, and has excellent cooling arc characteristics, especially when the switchgear has high arc temperature. cooling effect, avoiding the possibility of localized high temperature.
The insulation performance of SF6 far exceeds the traditional oil and air insulation medium. When used in electrical equipment, the size of the equipment can be reduced and the reliability of equipment insulation can be improved. The disadvantage is that during arc discharge, it decomposes to form low-fluorine compounds of sulfur, which are not only toxic, but also have a corrosive effect on some insulating materials and metals.
SF6 is a heavier gas. Under the same conditions, its density is 5 times that of air. The relationship between its pressure and temperature follows the ideal gas law. The critical temperature is the highest temperature at which SF6 gas liquefies, and the critical pressure represents the gas pressure required for liquefaction at this temperature. SF6 can remain gaseous only when the temperature is higher than 45°C. Under normal use conditions, it has the possibility of liquefaction, so SF6 cannot be used at too low temperature and too low pressure.
The excellent thermal conductivity of SF6 is one of the reasons for the arc extinguishing performance of SF6. The good thermal conductivity of SF6 can be attributed to two reasons. One is that the molecular weight of SF6 is large, the specific heat is large, and its convective heat transfer ability is better than that of air, and the other is the cooling effect produced at high temperatures.
Chemical Properties
The chemical effect of SF6 gas itself is very stable and has very high dielectric strength.
Under atmospheric pressure and at a temperature of at least 500°C, SF6 has high chemical stability, and within the normal temperature range, it has no reaction with metals commonly used in electrical equipment.
The dangerous temperature of SF6 decomposition is about 600℃. At this time, SF6 decomposes to form a low-fluorine compound of sulfur. Therefore, SF6 is quite stable at least at the A-class insulation temperature of electrical equipment, that is, within 105℃.
Electrical Characteristics
SF6 is a medium with high dielectric strength.
Under a uniform electric field, the dielectric strength of SF6 is about 2, 5~3 times that of air under the same pressure, and its dielectric strength is equivalent to that of transformer oil under 3 atmospheres.
Practice has proved that: adding a small amount of sulfur hexafluoride gas in the air, the dielectric strength of the air is significantly improved; on the contrary, adding a small amount of air to the sulfur hexafluoride gas will also significantly reduce the dielectric strength of the sulfur hexafluoride gas.
Due to the high dielectric strength of SF6, for circuit breakers of the same voltage level and similar breaking current, the number of series breaks of SF6 is less.
For example, the Ln-type SF6 circuit breaker developed by our company has a single breaking voltage of 220KV; another example is a 500KV oil-less circuit breaker with 6~8 breaks, while the SF6 circuit breaker has only 3~4 breaks. The corona onset voltage of SF6 is much higher than that of air, and the dielectric strength is independent of the frequency of the applied voltage, but attention should be paid to the uniformity of the electric field, impurities, electrode shape and irregularity, etc. Strength has a certain strength.
Moisture in SF6 gas will affect insulation. When the water content in SF6 exceeds a certain concentration, SF6 may decompose at a temperature above 200 °C. The decomposed product contains hydrofluoric acid, which is a highly corrosive and highly toxic acid. In addition, moisture condensation is also detrimental to edgewise insulation. Therefore, in the electrical equipment of SF6, the moisture content should be strictly controlled.
Precautions
SF6 gas is very stable at room temperature, but when there is a lot of water, it may decompose above 200°C, and the decomposed product contains hydrofluoric acid, which is a highly corrosive and highly toxic acid.
The hazard of moisture is more important in the chemical reaction caused by the decomposition of SF6 gas under the high temperature of the arc, and the final products in the chemical reaction also increase with the increase of moisture content. In order to limit the generation of toxic substances, the moisture content in SF6 gas must be limited.
The influence of moisture on insulation, the condensation on the surface of the insulation will have an adverse effect on the insulation performance. Usually the mixed moisture in the gas is affected in the form of water vapor. When the temperature drops suddenly, it may condense into dew and attach to the surface of the insulating part, causing a surface discharge accident.
