SF6 Gas Properties

Introduction

SF6 is a combination of sulfur and fluorine its first synthesis was realized in 1900 by French researchers of the Pharmaceutical Faculty 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 extinguishing the
electric arc. This aptitude is quite remarkable.

Physical properties

It 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 mixture consisted of 79 % SF6 and 24 % oxygen): five mice were then immersed in this atmosphere for 24 hours, without feeling any ill effects.

It is a gas which the speed of sound propagation is about three times less than in air, at atmospheric 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, it can be seen that the dielectric strength also increases and than around 3.5 bar of relative pressure, SF6 has the same strength as fresh oil.

 The principal characteristics of the gas are as follows:

Molar mass                                   146.078

Critical temperature                     45.55C

Critical pressure                           37.59 bars

In short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2C and in which noise propagates badly.

SF6 on the market

SF6 which is delivered in cylinders in liquid phase, contains impurities (within limits imposed by 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 properties

SF6 is a synthetic gas which is obtained as we have just explained by combination of six atoms of 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 dissociate the 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 expect that it will be a powerful cooling agent:
                                   

The dissociation products before interruption of the arc

At normal temperature, the gas is stable, and does not react with its environment. In contact with the parts where electric currents circulate, the gas is heated to temperatures of around four hundred degrees SF6 gives the following decomposition products:

Thionyl fluoride               SOF2

Sulfur fluoride                 SO2F2

Sulfur tetra fluoride          SF4

Sulfur deca fluoride         S2F10

Thionyl tetra fluoride       SOF4

SF6 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. It is thus that, besides the impurities of the gas (water, air, carbon, and dioxide), there only remain:

   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 (alumina silicate).

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 state

In its initial state, before it has undergone thermal stress (usually the electric arc); SF6 is perfectly 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 gas will not support life.

However, the concentration of SF6 would have to be high, since the International electro technical 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 abnormal behavior.

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 the cigarette may decompose the gas. Your cigarette would then take on a very strange taste also avoid operating combustion engines in this gas.

When the work positions are indoors, have ventilation and / or a system for detecting this halogen 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 state

As we seen at the beginning of this Chapter, the heat from the arc modifies the SF6.This creates gaseous and solid decomposition products.

It is these products that need to be spoken about. Certain of these gases are medically defined as being violent irritants of the mucous membranes and of the lungs. In extreme cases, they may cause pulmonary edema.

 

The solid decomposition products (whitish powder) an aggressive when the react with the humidity of the mucous membranes and of the hands.

Following this rather unpleasant description of the SF6 after breaking we may reassure ourselves on two counts:

- For reasons of quantity

- For reasons of probability.

   Quantity.

The volume of decomposed is microscopic. This means that dangerous thresholds are rarely reached, thanks in part to the molecular sieve which regenerates the decomposition products to form 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 excellent alarm signal. The smell detection threshold is ten times lower than the toxic threshold (1 ppm is detected 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 extremely infrequent. And if by chance such an incident accrued, the putrid smell would make us aware of it immediately.

  Precaution and hygiene.

If you were to find yourself in contact with decomposed SF6 gas, you must leave your post and ensure that the gas is eliminated by means of powerful ventilation.

Once the polluted gas has disappeared (when the smell becomes bearable) you are still in contact with solid decomposition products.

Operations on the equipment must be carried out with a gas mask, gloves and appropriate clothing. All this - together with the powders themselves - shall be sent to a factory for dealing with dangerous products.

Any damage to the hands caused by these powders can be neutralized by limewater.

Conclusion


It is important to point out that sulfur hexafluoride does not bring about an increase in the risks entailed in the work stations. This lack of specific danger is furthermore confirmed by the fact that we have not had to record any accident since 1960, the year in which SF6 was first used as a breaking agent. As a matter of interest SF6 does not harm the ozone layer. This is partly due to its weight.

 

The electric arc

  The creation of an arc

Everyone has noticed that, when placing ones hand near to a television screen, one feels a force which attracts.

There exists, in fact, in this apparatus, what one calls an electric field. The latter is the source of an 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 great intensity 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 rapidly itself. 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 the contacts separate, the electric field draws electrons to the hot points. These electrons are going to circulate in surroundings which are not conductive, which one calls dielectric, and will cause the 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 ionic dissociation. At this moment, it parts with electrons, and becomes conductive. These electrons themselves, 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 value of 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 been eliminated 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 current

If we perfect a system which allows cooling the arc (turning arc, magnetic blow-out, mechanical or thermodynamic blow-out, etc ...). One can well understand that the arc increasing to temperatures of 1500C.

Under the effect of current passing through it, will see a temperature decrease as soon as the alternating 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 of energy 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 dielectric will recover its insulating properties which thus ensure interruption of the current.

Lastly the hydrofluoric acids attack all metals giving metallic fluorides which are all very hydroscopic insulating powders.

Fig (1) Disruptive voltage versus pressure

Fig (2) SF6 absolute pressure versus temperature with constant volume mass (density)