Insulating gases

Insulating gases

Insulating gases types

Electronegative Insulating gases are good insulators because ions quickly combine with ions generated in the spark. However, they tend to be caustic. However, some gases only dissociate where the discharge is (or wants to be), which makes them particularly good insulators.

Gases with electronegative species (i.e. halogens such as chlorine) are good insulators, hence the popularity of SF6, which is not only dense (the breakdown voltage is roughly proportional to density), but mainly fluorine, an element that is highly electronegative. Halogenated hydrocarbon refrigerants are also a popular insulator. CCl4, CCl2F2, CCl3F and C2Cl2F4.

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Unfortunately, the cost of insulating gases has increased significantly in recent years, largely due to the various contracts regulating halogenated hydrocarbon refrigerants. Traditional freons (R-12, R-22) are no longer manufactured and are quite expensive. As chlorine alkanes were eliminate by the regulatory push, modern refrigerants are more dependent on fluorinated or fluorinated hydrocarbons (e.g. HC-134a). Unfortunately, the capacity of the plant is limit, and the plants that used to make SF6 now produce more expensive fluorinated hydrocarbons, which results in much higher prices for SF6. In the mid-1980s, SF6 was around $ 3 to $ 4 a pound. In the mid-1990s, it was around $ 100 a pound. Now (early 2000) it has dropped to around $ 10 / pound. Since a pound is only about 10 liters, filling a large insulated tank with SF6 has become a very expensive task. More information about SF6.

SF6 Gas insulated grid substation
Gas insulated grid substation

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Breakdown voltage of most gasses can increased by increasing absolute pressure. For some gases, there is a limit set by the liquefaction point at normal operating temperatures (i.e., Freon 12 liquefies at 5 atmospheres). Gas mixtures can solve some of these problems, and a mixture of Freon 12 and Nitrogen has been popular.

A disadvantage of halogenated compounds is that cleavage products are highly corrosive. Therefore, it is important that the operating voltages remain well below the corona start voltages. Even air forms highly reactive nitrogen oxides and other caustic compounds, especially when there is water vapor. High value pressure air able to support combustion due to the oxygen contentof it. Pure nitrogen does not appear to have these disadvantages, although its degradation is only 15% better than that of air.

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The approximate decomposition of air is 30 kV / cm at 1 atm. = 30 + 1.53d where d in cm. Air breakdown is very well research to the point where the breakdown voltage of a calibrate space is used to measure high voltages. Freons: The vapor pressure of CCl2F2 (R-12) is 90 psi at 23 ° C, with the decomposition being about 17 times that of air at 1 atm. An even higher insulation resistance can be obtain by adding nitrogen to the saturated CCl2F2 to bring the total pressure to the desired value. The saturated vapor pressure of C2Cl2F4 at 23 ° C is 2 atm abs. In this case it has a relative dielectric strength of 5.6 times N2 at 1 atm. Sulfur hexafluoride (SF6): Sulfur hexafluoride is probably the most popular insulating gas, although its costs have increased dramatically recently.

SF6 Gas insulated Breaker
Gas insulated Breaker

Hydrogen: From the point of view of breakdown voltage, hydrogen gas is not a particularly good insulator (65% of the air). It’s very low viscosity and high heat capacity make it an insulating gas of choice for high-speed, high-voltage machines such as turbo generators. There is no risk of explosion if the oxygen content in the hydrogen tank is keep below the flammability limit (approx. 5%). Of course, hydrogen has many other handling problems, including the brittleness of hydrogen, it escapes through very small holes (even the pores in metal tanks) and completely colorless but very hot flames.

Relative spark breakdown strength

Relative spark breakdown strength of gases
Relative spark breakdown strength of gases

References

M.T. Rodine and R.G. Herb, Phys. Rev51, 508, 1937

E.E. Charlton and F.S. Cooper, Gen.Elect.Rev., 40, 438, 1937

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