SF6 Gas Properties IntroductionSF6 is a combination of sulfur and fluorine its first synthesis wasrealized in 1900 by French researchers of the PharmaceuticalFaculty of Paris.It was used for the first time as insulating material,In the United States about 1935.In 1953, the Americans discovered its properties for extinguishingtheelectric arc. This aptitude is quite remarkable. Physical propertiesIt is about five times heavier than air, and has a density of 6.1 4kg / m3.It is colorless, odorless and non-toxic.Tests have been carried out replacing the nitrogen content of air by SF6 (the gaseous mixtureconsisted of 79 % SF6 and 24 % oxygen): five mice were then immersed in this atmosphere for24 hours, without feeling any ill effects.It is a gas which the speed of sound propagation is about three times less than in air, atatmospheric pressure. The interruption of the arc will therefore be less loud in SF6 than in air.The dielectric strength of SF6 in on average 2.5 times that of air, and, by increasing pressure, itcan be seen that the dielectric strength also increases and than around 3.5 bar of relativepressure, SF6 has the same strength as fresh oil. The principal characteristics of the gas are as follows:Molar mass 146.078Critical temperature 45.55°CCritical pressure 37.59 barsIn short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2°C andin which noise propagates badly. SF6 on the marketSF6 which is delivered in cylinders in liquid phase, contains impurities (within limits imposedby IEC standards No. 376) Carbon tetra fluoride (CF4) 0.03 % Oxygen + nitrogen (air) 0.03 % Water 15 ppm C02 traces HF 0.3 ppm SF6 is therefore 99.99 % pur.Chemical propertiesSF6 is a synthetic gas which is obtained as we have just explained by combination of six atomsof fluorine with one atom of sulfur: You can see therefore that this reaction is accompanied by an important release of heat.This approximately similar to coal combustion.
Given that the energy released during synthesis is the same as is needed in order to dissociatethe final element, it can immediately be seen that: - SF6 is a stable gas - 524 k. calories are necessary for molecular breakdown, we can there fore already expectthat it will be a powerful cooling agent: The dissociation products before interruption of the arcAt normal temperature, the gas is stable, and does not react with its environment. In contactwith the parts where electric currents circulate, the gas is heated to temperatures of around fourhundred degrees SF6 gives the following decomposition products: Thionyl fluoride SOF2 Sulfur fluoride SO2F2 Sulfur tetra fluoride SF4 Sulfur deca fluoride S2F10 Thionyl tetra fluoride SOF4SF6 also reacts with the materials that are found in its environment:With water (impurity in the gas), it gives hydrofluoric acid HF,With air dioxide (impurity in the gas), it gives sulfur dioxide SO2,With carbon dioxide (impurity in the gas), it gives carbon tetra fluoride CF4,With the araldite casings which are high in silicon dioxide, it gives silicon tetra fluoride SF4. The dissociation products after interruption of an arc.An electric are develops high temperatures which can reach 15000 °C.At these temperatures, many dissociation products that we have previously studied disappear. Itis thus that, besides the impurities of the gas (water, air, carbon, and dioxide), there onlyremain: Sulfur fluoride SO2F2 Carbon tetra fluoride CF4 Silicon tetra fluoride SIF4 Sulfurous anhydride SO2. You can therefore see that a large number of products have been dissociated by the electric arc.The importance of the remaining products may be lessened by adding a powder (aluminasilicate).All these gases are heavier than air, and May, under certain conditions is poisonous. SF6 Safety precautions:Today there is no known dielectric and breaking agent combined better than SF6 gas. Initial stateIn its initial state, before it has undergone thermal stress (usually the electric arc); SF6 isperfectly safe in normal conditions: - It is non-toxic, - It is uninflammable, - It will not explode.This does not mean that no precautions need to be taken: because of its lack of oxygen, this gaswill not support life.
However, the concentration of SF6 would have to be high, since the International electrotechnical Commission (IEC) has shown that five mice left for 24 hours in an atmosphere of 79% SF6 and 21 % oxygen will not only remain alive but will show no signs of abnormalbehavior.Man dies when the oxygen level of the gas he is breathing falls below 12 %. Precautions and hygiene The first recommendation is not to smoke when SF6 gas is around. The heat given off by thecigarette may decompose the gas. Your cigarette would then take on a very strange taste alsoavoid operating combustion engines in this gas.When the work positions are indoors, have ventilation and / or a system for detecting thishalogen placed at the lowest points of the installations.Remember that SF6 is a very heavy gas. This device will warn you any gas leaks. Post-breaking stateAs we seen at the beginning of this Chapter, the heat from the arc modifies the SF6.This createsgaseous and solid decomposition products.It is these products that need to be spoken about. Certain of these gases are medically defined asbeing violent irritants of the mucous membranes and of the lungs. In extreme cases, they maycause pulmonary edema.The solid decomposition products (whitish powder) an aggressive when the react with thehumidity of the mucous membranes and of the hands.Following this rather unpleasant description of the SF6 after breaking we may reassureourselves on two counts:- For reasons of quantity- For reasons of probability. Quantity.The volume of decomposed is microscopic. This means that dangerous thresholds are rarelyreached, thanks in part to the molecular sieve which regenerates the decomposition products toform pure SF6. This sieve is present in all extinguishing chambers. Regeneration time is short,but depends on the number of ampere being broken.The presence of hydrogen sulphide, noticeable through its sickening smell, makes an excellentalarm signal. The smell detection threshold is ten times lower than the toxic threshold (1 ppm isdetected by smell).
Probability.In normal operation, electric Switchgear using SF6 has a leak rate guaranteed to be less than 1% of the mass per year. This makes any danger impossible in normal operation.The abnormal situation is the risk of an appliance exploding. This is fortunately extremelyinfrequent. And if by chance such an incident accrued, the putrid smell would make us aware ofit immediately. Precaution and hygiene.If you were to find yourself in contact with decomposed SF6 gas, you must leave your post andensure that the gas is eliminated by means of powerful ventilation.Once the polluted gas has disappeared (when the smell becomes bearable) you are still incontact with solid decomposition products.Operations on the equipment must be carried out with a gas mask, gloves and appropriateclothing. All this - together with the powders themselves - shall be sent to a factory for dealingwith dangerous products.Any damage to the hands caused by these powders can be neutralized by limewater. ConclusionIt is important to point out that sulfur hexafluoride does not bring about an increase in the risksentailed in the work stations. This lack of specific danger is furthermore confirmed by the factthat we have not had to record any accident since 1960, the year in which SF6 was first used asa breaking agent. As a matter of interest SF6 does not harm the ozone layer. This is partly dueto its weight. The electric arc The creation of an arcEveryone has noticed that, when placing one’s hand near to a television screen, one feels a forcewhich attracts.There exists, in fact, in this apparatus, what one calls an electric field. The latter is the source ofan electric current, for it is this that displaces the electrons in the conductors.An electric field appears at the separation of the live contacts. Such a field of a very greatintensity will draw electrons at the hot points of contacts.The electric arc has been born. If its own energy is not sufficient, the arc will extinguish rapidlyitself. If, on the other hand, it is crossed by a strong current, it draws throughout its own energy,which ensures the survival of the arc. The electric arc:We have seen that the electric field was at the origin of the displacement of electrons. When thecontacts separate, the electric field draws electrons to the hot points. These electrons are goingto circulate in surroundings which are not conductive, which one calls dielectric, and will causethe temperature of the surroundings to increase, if they are in sufficient number.All bodies, under the influence of temperature, end up by reaching their threshold of ionicdissociation. At this moment, it parts with electrons, and becomes conductive. These electronsthemselves, and for the same reasons, will create others. We have an avalanche, that is to say,creation of electrons, which will accelerate. One can reach temperature of 15000 °C. The valueof the thermal power can be 10MW.The electric arc is thus going to follow the variations of alternating current, and thus, at regular
intervals, the arc will disappear and reappear immediately, if the electrons have not beeneliminated because in this case, the surroundings remain conductive.In order to eliminate these electrons, one could:- Rid oneself of them by some physical means, like blow-out for example, - use dielectric with a very high speed of recuperation (the case of SF6) - use a process to reduce the temperature of the element (decompression, blow-out,etc.) Out-off a currentIf we perfect a system which allows cooling the arc (turning arc, magnetic blow-out, mechanicalor thermodynamic blow-out, etc ...). One can well understand that the arc increasing totemperatures of 1500°C.Under the effect of current passing through it, will see a temperature decrease as soon as thealternating current starts its descent towards 0. The temperature will decrease all the more rapidly as:- SF6 has two states of conduction, and appearance of the resistive arc will bring about a fall in the intensity, and thus its temperature,- SF6, as we have seen in its physical properties, is a gas which Absorbs large quantities ofenergy when it dissociates.The blow out of the arc will thus (mean) evacuate a large quantity of energy.This lowering of temperature will make the ionic recombination of the bodies and the dielectricwill recover its insulating properties which thus ensure interruption of the current.Lastly the hydrofluoric acids attack all metals giving metallic fluorides which are all veryhydroscopic insulating powders. Fig (1) Disruptive voltage versus pressure
Fig (2) SF6 absolute pressure versus temperature with constant volume mass (density) Rajneesh Budania