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Protection Question No.36 - SF6 CB
i 'd like to know the mechanizm of SF6 CB working.
Author : Mohammed - From: Libya
Mon, June 1, 2009 - 19:22
SF6 is the latest technology in the field of arc extinction.
It was introduced in the 1960s and attempts to achieve a high dielectric strength between the contacts. At room temperature SF6 is a chemically inert, non-toxic and noninflammable, colorless, odorless gas, having a molecular weight of 146 and provides excellent arc quenching as a result of electronegative behavior.
At atmospheric pressure, its dielectric strength is two to three times that of air, and its arc-quenching ability many times more than air.
This gas undergoes no chemical change at high temperatures, except small decomposition into SF2 and SF, gases and some metallic fluorine in the form of an insulating powder while interrupting and quenching an arc. These gases and powder, however, are readily absorbed by activated alumina placed in the filters in the closed-loop circuit of the gas, as discussed later. The gas cycle is such that after every interruption the consumed gas is replenished through a reservoir filled with SF6 gas at a high pressure, say, sixteen times that of the atmosphere and connected to the main interrupting chamber through pressure valves and filters. As soon as the pressure in the interrupting chamber falls below a pre-set value, the valve in the reservoir opens and builds up the lost pressure.
Due to the very high pressure in the reservoir, compared to only almost three times the atmosphere in the chamber, it is possible that the pressure inside the interrupting chamber may sometimes exceed the required value. In the interrupting chamber, therefore, are also provided high-pressure release valves to pump the excess gas back to the reservoir through a compressor and a filter. The total gas circuit is a closed cycle without any venting to the atmosphere. This gas is electronegative and its molecules quickly absorb the free electrons in the arc path between the contacts to form negatively charged ions. This apparent trapping of the electrons results in a rapid build-up of dielectric strength after a current zero. The detailed sequence of arc extinction may be summarized as follows.
The contacts begin to compress a quantity of SF6 gas as soon as they start opening. This opening also causes arc plasma between the contacts. The temperature of the arc plasma ionizes the gas into sulphur and fluorine atoms and quickly becomes quenched through the turbulence of the compressed gas through a very strange process of negative ion formation. At higher temperatures, the S atoms become ionized into S+ protons and SN neutrons.
The S" electrons of the neutrons are immediately absorbed by the fluorine atoms to form fluorine ions (F) which are heavy and are sluggish. They contribute little to maintaining the conductivity of the arc plasma. This quickly immunizes the free electrons, restores its dielectric strength, quickly quenches the arc plasma, extinguishes the arc and builds up the dielectric strength after a current zero. After a current zero, the process quickly, quenches the arc in the beginning itself by sweeping away the arc plasma, thus improving the dielectric strength between the parting contacts and achieving successful extinction of the arc. The arc extinction process may be slightly delayed when the contacts open very close to the next current zero, and the quenching medium blows it out with force, before the current zero, leading to a case of current chopping. But with continuous improvement in the techniques of arc extinction, it has been possible to achieve an interruption devoid of a current chopping or a strike of the arc plasma.

Is it enough?
Author : Hamid - From: IRAN
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