1. Why SF6? Historical view
In the early seventies of the twentieth century, the worlds leading apparatus companies developed the production of high-voltage shielded switchgear with sulphur hexafluoride (SF6) insulation, and high-voltage circuit breakers in which this gas was an arc-extinguishing medium. Later, this apparatus technique developed rapidly, which can be explained by the enormous possibilities that opened up for the constructors of electrical apparatus after discovering the excellent properties of SF6.
The constructors had long tried to build prefabricated high voltage shielded switchgears (above 110 kV) similar to low voltage shielded switchgears (up to 1 kV) and medium voltage (below 110 kV). Attempts to build shielded switchgears with solid (resin), oil or compressed air insulation did not give satisfactory results. Prefabricated switchgear elements with solid insulation with the required dielectric strength were heavy and unreliable due to cracks in large-volume resin casts. Oil-insulated switchgear was explosion and fire hazardous and also heavy. Switchgear with compressed air insulation was in trial operation, but this technique was not widely used due to the need to use high pressure (approx. 6 MPa) and the related need for enclosures with great mechanical strength.
It was only the combination of solid insulation as supporting elements and gas insulation (SF6) as the main insulation that met the expectations of constructors and enabled the widespread construction of small-size switchgears for the highest and medium voltage, in which bus bars and all devices were closed in tight metal sheaths.
SF6 switchgears occupy an area 10-20 times smaller than conventional switchgears. The difference in occupied volumes is even greater – which is of particular importance when building a hall switchgear. For switchgear with SF6 it does not matter whether the atmosphere is chemically polluted, dusty, containing salts or acid mist. Lightning discharges are harmless to them. They are both explosive and fire safe. The operation and maintenance of these switchgears is simplified due to the use of safe, earthed conductor shields and many technical safety systems. Despite the use of a large number of SF6 shielded switchgears in the world, there was no information in the literature about major accidents that would result in death or serious injury to personnel.
The high operational safety of switchgears made of SF6 results from the fact that this type of device can operate without service, high voltage parts are not available. Personnel poisoning with SF6 decomposition products are in practice unlikely, mainly due to the low concentration of toxic compounds, the ease of identifying their presence by smell and the natural reflex of the personnel to leave the room in the event of a failure involving a sudden unsealing of the housing.
The failure rate of shielded switchgears made of SF6 is much lower than that of open switchgears. The literature on the subject states that the relative number of more serious failures in enclosed switchgears is about six times smaller than the corresponding data for conventional switchgears. Shielded switchgears are also much less troublesome than traditional switchgears. The failures of SF6 switchgears can be divided into two groups: failures of the same type as conventional switchgears, i.e. independent of solutions (e.g. mechanical damage to the apparatus drive or control) and failures characteristic of SF6 shielded switchgears (e.g. elements of permanent insulation) , unsealing of casings, etc.). The incidence of the first type of malfunctions is more or less similar in both types of switchgear. On the other hand, the available failure statistics specific to gas-insulated switchgear show that the relative numbers of failures are very low. However, it should be taken into account that the removal of damage in this type of switchgear may prove difficult and take so long that the comparison of failure rates will be less favourable. We must remember that the occurrence of a failure in the switchgear with SF6 has much more serious consequences for the operation of the power system than in conventional substations. The more complicated disassembly, repair and reassembly make the time to restart relatively long.
In the first solutions of SF6 shielded switchgears, due to the lack of certainty about the seals, devices were used to automatically fill up with gas in the event of losses below the designated level. Quickly, however, the design solutions and the applied sealing systems ensured a sufficient tightness, which made it possible to resign from self-filling. Currently, companies ensure annual gas losses not exceeding 0.1% ..
The first SF6 shielded switchgears began to appear in the world from 1965. By 1974, all manufacturers, whose number could be estimated at 20-25 companies, produced about 2000 bays. Initially high prices inhibited the demand for this type of switchgear (price of new products, cost of research). In later years, the total investment costs of SF6 switchgears were already competitive compared to conventional (hall) solutions. Investors took into account the reduction of the costs of the occupied land and the possibility of architectural adaptation of the facility to the surrounding buildings in cities or industrial plant sites. Switchgear made of SF6, using cable lines, can be installed in the basement of buildings, under squares or squares. This type of device reduces the time of assembly on site, as the switchgear is delivered in large sets (e.g. panels). The operating costs are significantly lower thanks to the reduction of maintenance procedures, inspections and the servicing intervals, and thanks to the increase in the reliability of operation of electrical power equipment to a large extent.
The dynamic increase in demand for electricity requires the expansion of power grids and forces the introduction of high-voltage lines deep into cities and industrial plants. In many cases, shielded switchgears with SF6 insulation are the only possible solution at a voltage of 123 kV and higher in specific installation conditions (more about switchgears in chapter 6)
Along with the advances in the technique of sulphur hexafluoride insulation and the construction of switchgear insulated with this gas, works on the use of SF6 in circuit breakers for electric arc extinguishing have progressed. It is hard to expect that in the coming years a better gas for use in electric apparatus as a medium to extinguish an electric arc than sulphur hexafluoride will be discovered. When using SF6, the design of the circuit breakers is significantly simplified compared to the pneumatic and low-oil ones. At the same time, this apparatus has increased reliability and extended service life. A characteristic of SF6 is that when switching off even small currents in the atmosphere of this gas, there is no sudden “breaking of the arc” and thus no dangerous overvoltage. A favourable phenomenon is also a rapid increase in the electric strength of the break after arc-extinguishing in SF6 and thus the possibility of breaking the circuit at high slopes of the recovery voltage. An important advantage of SF6 is that the necessary gas pressure to extinguish a high-voltage arc does not have to be as high as in pneumatic circuit breakers. Hence the possibility of using self-compressing or self-generating pressure switches in the arc area. The extinguishing chambers have a relatively simple structure (more about the switches in chapter 7)
Unfortunately, the SF6 technique also has disadvantages. SF6 gas is more expensive than compressed air, it requires shields with much greater tightness. It causes some difficulties when filling the device (high vacuum). The choice of insulation and construction materials for circuit breakers is hampered by the aggressiveness of gas decay products under the action of an electric arc and their compounds. Toxic compounds are formed in the switch, which pose a threat to people – especially during repairs and disassembly. The last disadvantage means that the SF6 technique cannot be considered completely safe. But it should not be overly exposed and especially emphasised by opponents of this technique.
The operation of power equipment with sulphur hexafluoride requires the use of the necessary general and individual safeguards..
Interest in this technique in Poland began in the early seventies at the Electrotechnical Institute. The undertaken research and design work led to the installation of a single-pole 123 kV switchgear and 1250A continuous current switchgear with a circuit breaker with a breaking current of 25 kA for operational tests (power station in Sulejówek). For trial operation, two 123 kV overhead circuit breakers were also installed (Mory, Gdańsk). Later, a prototype of a 123 kV circuit breaker with a breaking current of 31.5 kA was developed, performed and conducted. At the Electrotechnical Institute, a prototype 123 kV switchgear was also made – but it was not installed. In Poland, the focus was on the production of a license switch (EDF) and the purchase of switchboards from foreign producers. Many years of research and design work carried out at the Electrotechnical Institute paid off only with the extensive experience gained in the field of SF6 technology.
Many years of experience with the use of SF6 in switching devices have shown that there is no serious risk to people, however, provided that appropriate precautions are taken and the established procedures are followed in the entire scope of operation and decommissioning of devices.
Personnel working with SF6 must be thoroughly acquainted with the properties of gas decomposition products, be aware of the health hazards and advised of the necessary safety measures to be taken to minimise the risk.
2. Chemical and physical properties of SF6.
Sulphur hexafluoride is a synthetic gas obtained by treating sulphur with fluorine gas. The molecule is octahedral in shape with six fluorine atoms in the vertices and a sulphur atom in the centre. Sulphur in this compound has the greatest valence. This structure is the reason for the extraordinary stability of the gas and its exceptionally high chemical inertia, as it requires considerable energy to decompose it. The decomposition of SF6 under the influence of temperature does not actually start until around 50 ° C. However, in the presence of some metals, especially metals and their alloys containing silicon, decomposition can occur as early as 180-2000C. SF6 is difficult to dissolve in water, slightly easier in alcohol. Pure gas does not react with hydrogen or metals, but only with oxygen in the presence of electrical discharges. It is a colourless, non-toxic, odourless and non-flammable gas.
The molecular weight of SF6 is 146.06 and the density at 20 ° C and 1 bar pressure is 6.16 g / l, which is about 5 times the density of air. So it is one of the heaviest gases known.
The thermodynamic properties of SF6 result from the Molier diagram. The critical point of SF6
is at a pressure of 37.46 bar and a temperature of 45.580C, which allows it to be condensed by compression for transport and storage.
The use of SF6 in electric power apparatus results from its excellent electrical properties. It is known that the electric strength of gases depends on many factors: the free path of the molecule, its cross-section, the resulting inelastic collisions and the ability to bind electrons in these collisions and to store their energy. Electronegative gases like SF6 have the ability to bind electrons by forming negative ions, which greatly increases their electrical strength (slowing down mobile electrons).
The electric strength of SF6 exceeds 1.8 – 3.0 times the air resistance depending on the test conditions. In a homogeneous field it is about 2.4 times greater. At a pressure of approx. 3 bar, SF6 reaches 75% of the strength of an insulating oil in the case of a homogeneous field, and in the case of a heterogeneous field, it may even show better insulating properties than oil .
Sulphur hexafluoride cannot be the only insulating material used in the switchgear or circuit-breaker – support and bushing insulators must be made of solid insulating materials. These materials work in an SF6 atmosphere, so it is important to understand the effect of SF6 on solid insulating materials. Of course, it is necessary to distinguish between the requirements for insulating materials stressed only by voltage in a pure SF6 atmosphere, e.g. bus bar insulators, and materials working in extinguishing chambers, additionally exposed to the action of SF6 decomposition products. The surface strength of these insulators in an SF6 atmosphere is particularly important. It is known that most manufacturers use epoxy resins with special fillers for insulators. This material fully meets the requirements of electrical and mechanical strength. Tests of samples covered with a layer of material containing fluorine compounds (called Teflon) showed a significant increase in the surface discharge voltage in the gas. However, there are a number of other insulating materials that may come into contact with SF6 in electrical equipment. These materials can show quite significant differences in properties depending on the manufacturer, even with a very similar composition, therefore it can be concluded that each solid insulating material that is intended to be used in SF6 devices must be tested (the comment concerns e.g. repairs).
Much research has been done to establish the heat transfer capacity of SF6. The issue is interesting because the specific (molar) heat of SF6 is lower than that of air, but per unit volume of gas it is 3.7 times higher than the specific heat of air. The thermal conductivity of SF6 of 1.2610-4W / cmK is more than twice lower than the thermal conductivity of air (2.8610-4W / cmK), but taking into account convection, the heat transfer capacity of SF6 characterised by the transfer coefficient heat, is greater than the corresponding capacity of air and approaches the values achieved with helium or hydrogen. This results in the possibility of increasing the current density in the conductor (e.g. bus bars) located in the SF6 atmosphere in relation to the density in the conductor in air.
A separate issue is the thermal conductivity of SF6 at high temperatures, i.e. during arc quenching. The authors of the study, relying on the fact that the dissociation of SF6 is particularly intense at the temperature of 2000-2100 K and ends practically at approx. 4000 K, and therefore at this temperature, SF6 is dissociated into F and S with a small part of diatomic compounds, say that under these conditions the ratio of specific heat and thermal conductivity can be assumed to be constant. The tests carried out in these conditions have shown that SF6 has an intense heat dissipation from the arc, which leads to a decrease in its diameter and an increase in the arc resistance .
The use of SF6 in the circuit breaker extinguishing chambers is related to the exceptionally good properties of this gas as an arc extinguishing medium. Already the first tests (1954, USA) showed that with free power cut-off, the extinguishing capacity in SF6 exceeds the extinguishing capacity in air by about 100 times. It is known that switching off the alternating current, especially at a low power factor, depends much more on the rate of rise of the half-gap strength than on the electrical strength of the cold gas. The rate of growth of the gap strength, i.e. the rate of deionisation of the half-column, depends on the thermal and electrical parameters of the plasma. In particular, the following are important: thermal conductivity and temperature distribution in the arc, dissociation conditions, voltage, power and energy drop, and finally the arc time constant. Switching tests in SF6 show the advantages of this gas from the point of view of the above-mentioned features.
The basic quenching properties of SF6 are related to the dissociation course of this gas. Dissociation begins at relatively low temperatures, approx. 2000 K, and proceeds “in instalments” with varying ionisation energies, and at the same time, when the temperature drops, the electrical strength increases very quickly. The arc extinguishes well in gases where rapid temperature drops in the half-column can occur, leading to temperatures less than that at which the electron density is 109 / cm3. This temperature, called the quenching temperature, is in the order of 3000 K. The arc core is formed when the arc temperature exceeds the dissociation temperature of gas molecules, and therefore good quenching properties are demonstrated by gases whose dissociation temperature is lower than the quenching temperature. In these cases, the arc temperature outside the core is less than 3000 K. The core rapidly collapses on the extinction of the arc, the half-column is less than 3000 K, and the electron density is reduced to such an extent that re-ignition cannot occur. In SF6, the arc core conducts virtually all current; outside the core there are temperatures at which the electron density and thus the electrical conductivity are very low.
The dissociation temperature of SF6 is below 3000 K, the arc time constant is very low, so the gas must have good extinguishing properties.
Since SF6 is an electronegative gas, it should be additionally taken into account that due to the binding of free electrons with SF6 molecules, the electron density will be lower than it results only from the temperature dependencies. This effect is approximately equivalent to that if the gas temperature would be reduced by 500 K
The great advantage of SF6 is that the arc core disappears only when the current passes through zero and the difficulty of earlier destruction of the core by external influence. The breakdown of the arc column in SF6 is very rapid only 6 – 7 s before the current zero crosses. This proves the advantage of switching off in SF6 over switching in air and vacuum – in SF6 there are practically no over voltages even when switching off small currents (inductive and capacitive). The low arc energy with the vanishing current can be explained by the low recombination temperature, which covers the loss of electrical conductivity in a fairly large range below 2000 K, in which the previously free, easily ionising sulphur atoms are bound to fluorine again. In the temperature range below the values at which dissociation of SF6 occurs, this gas can be considered an almost ideal quenching environment. Of course, it greatly increases the ability to extinguish the arc gas blast. The fact that in SF6 the arc has a compact structure of the core, even with a sharp decrease in current, is interpreted in such a way that not thermo diffusion phenomena play the main role in the decay process, but the very short-lasting avalanche process of the formation of low-mobility negative ions in the cooled plasma. An additional advantage of SF6 is the low speed of sound – 136 m / s (much lower than in the air). Hence, the minimum flow velocity of the quenching in the supersonic nozzle is much less deformation of the arc than in the air. The basic deformation resulting from SF6 flow is the reduction of the diameter of the arc column near the zero current crossing, thanks to which the losses due to diffusion reach a value that allows the arc to be quenched .
It should be remembered that the correct operation of electrical equipment, especially switches, is influenced by the quality of gas. SF6 must meet certain requirements with regard to the content of impurities (Table 2.1), as these have an influence on the properties of the gas.
Tabel 2.1. Requirements for technical SF6 according to PN-EN IEC 60376
|SF6||> 98,5 % by volume|
|Powietrze||< 10 000 μl/l (1 % by volume)|
|CF4||< 4 000 μl/l (0,4 % by volume)|
|H2O||< 200 μl/l (200 ppmv)|
|Mineral oil||< 10 mg/kg (10 ppmw)|
|Total acidity||< 7 μl/l (7 ppmv)|
|ppmv = parts per million by volume
ppmw = parts per million by weight
The impurities in the gas must be limited to such quantities that, individually or in combination, they do not endanger the functioning of the equipment in which the gas is to be used. For example, water (moisture), acid contaminants, and oxygen (when combined) can corrode components, leading to equipment malfunction. Water in the presence of acidic contaminants may condense at low temperature and high pressure, which may endanger the electrical safety of the device. In general, the degree of gas contamination influences the amount and type of secondary chemicals formed during the thermal decomposition of SF6 (e.g. after an arc occurs).
It seems an obvious recommendation that in the operation of electrical power equipment with SF6, filling and refilling with gas of proven quality should be ensured – preferably from one supplier.
3. State of SF6 in power devices.
Most often, the condition of the SF6 in the device differs from that of the gas at the time of filling it. It contains impurities that appear at various stages of preparing the device for operation and operation.
Gas contamination in a device is caused to a varying degree by:
- Improper selection of construction materials for the device, which may cause the desorption of moisture into the gas or the formation of impurities after a secondary chemical reaction with decomposed SF6,
- Factory assembly errors,
- Assembly errors on site,
- Leaks in casings and errors in filling losses,
- Gas breakdown as a result of electrical discharges and switching arc,
- Chemical reactions occurring after discharges,
- Operation of the internal mechanisms of the device.
Of course, the state of gas in the operated device essentially depends on its utility function. It is different in closed compartments of enclosed switchgear and different in high-voltage circuit breaker and other switching apparatus (e.g. switch disconnector). Let us discuss these issues first of all on the basis of the typical “product life scheme” presented below, which is most often analysed according to the ecological procedures of “Cleaner Production” .
In such a “product life scheme”, attention is paid to the impact on the state of contamination and its hazards at all stages of the product’s existence: from production to disposal.
The constructor of the device can influence not only the usability of the designed device, but also the condition of the gas it contains. The correct selection of construction and insulation materials is of great importance. It is both about their chemical reaction with SF6 (especially with decomposition products) and about the elimination of porous materials – absorbing moisture and air before assembly and transferring these substances to SF6. Choosing the right seal design – with high efficiency and durability, reduces gas losses and the possibility of introducing contaminants during refilling.
Tabel 3.1. Product life phases and the state of SF6 contamination
In the case of switches, it is extremely important to optimise the extinguishing chamber. It is about shortening the arc time (reducing the energy supplied by the arc) and limiting the gas to the necessary amount. It is also important to choose the type and volume of the adsorbent – ensuring effective functioning throughout the service life. The given issues should be resolved during the design tests of the device.
The manufacturer should ensure the proper technology of making the elements (surface smoothness), as well as dry and clean assembly at all its stages. During assembly, contact of elements with atmospheric moisture should be minimised, elements prepared for assembly must be sealed in foil and stored in a dry room. At the final stage of the installation of switching devices, an adsorbent is installed in them – its quality has a decisive influence on the subsequent state of the gas. The gas condition, in all SF6 devices, is influenced by the perfect assembly tightness of the housings, the performance of a high vacuum (drying the interior) before filling, and compliance with the principles of gas filling.
The above-mentioned quality assurance conditions are basically not influenced by the user (apart from the suppliers choice). Its role begins with the assembly and commissioning of devices. Even if the supplier installs the devices on site, the recipient should ensure appropriate storage conditions (as short as possible) and supervise the assembly process. Gas supplementation (according to the rules) and acceptance tests (according to acceptance conditions) must be performed accordingly. It is important to follow the rules of the operating manual of a given device in further use. It is very important for the durability of the device and the content of impurities in the gas to properly compensate for the losses – i.e. to replenish the gas.
SF6 high voltage circuit breakers present a separate gas condition problem. Regardless of the technological impurities introduced into the circuit breaker, similarly to the switchgear compartments, gas decay products and their secondary chemical compounds appear. During the period of operation, it must be observed not to exceed breaking current limit values and the limit number of starts (according to the switching capacity diagram in the operating manual). The breakdown of gas by electrical discharges and arc is the primary cause of the formation of toxic compounds.
Table 3.2. Type and allowable amounts of pollutants occurring during
gas operation according to PN-EN IEC 60480
|SF6||> 97 % by volume|
|Air and/or CF4||< 30 000 μl/l (3 % by volume|
|H2O||< 200 μl/l (200 ppmv)|
|Minerals oils||< 10 mg/kg (10 ppmw)|
|Acidity||< 50 μl/l (50 ppmv)|
|ppmv = parts per million by volume
ppmw = parts per million by weight
If it is found during the inspection that the concentration of pollutants exceeds the permissible level, the gas exchange procedure must be performed.
The last phase – disassembly of the device after its full use or damage, is the most important stage in terms of risk to personnel and the environment – especially in the case of extra-high voltage circuit breakers. These works should be carried out by a specialised team in accordance with the appropriate regime.
Taking the circuit breaker out of service does not have to involve disassembling the poles in parts in the distribution station. It is always necessary to lower the gas pressure to a slight overpressure in relation to the atmospheric pressure. This treatment should be performed by pumping the gas into the cylinder.
Maintaining a slight overpressure in the poles of the circuit breaker ready for transport (e.g. to the manufacturer) is to prevent moisture from entering the interior. Moisture causes:
- change in the nature of powder products from sediments not bound to the ground, to products that are sticky and stick to the internal elements of the extinguishing chamber. In this case, there are also more hydrolysis products,
- formation of aggressive gaseous secondary reaction products, which are corrosive to structural elements and are highly toxic.
4. 4. Disintegration of SF6 due to arc and electric discharge.
Sulphur hexafluoride has the properties of an inert gas as long as it is not subjected to thermal action. It takes place as a result of normal operation of the switch (breaking the electric circuit, extinguishing the arc) and during emergency electric discharges.
Interruption of the high-voltage current of the electric circuit is always accompanied by the necessity to extinguish the arc. In an SF6 circuit breaker, this usually takes place in a stream of compressed gas. Due to the high arc temperature, degradation of SF6 is unavoidable.
The study of the electric conductivity of the arc plasma [1,6] shows its successive jumps: the first one around 2,000-2,100 K corresponds to the half dissociation of SF6 and the appearance of free sulphur, the second, at approx. 3,000 K, is attributed to the dissociation of SF2 and SF3. The third, in the 15,000-20,000 K zone, is associated with an increasing proportion of electrons. Practically, after exceeding approx. 4,000 K, SF6 is dissociated into F and S. In this situation, conditions arise for secondary chemical reactions to take place inside the extinguishing chamber.
Under the influence of the arc (and spark discharges), mainly the following permanent gas decomposition products may appear: S, F2, SF2, S2F2, SF4 and S2F10, with SF4 being the greatest. In the presence of traces of oxygen and water vapour (they also dissociate at this temperature), some decomposition products, eg SF4, cause the formation of SOF2 compounds, and in the presence of metals, metal fluorides may be formed . After the temperature drops below 1,000 K, the atoms intensively recombine to form various compounds, combining with metal atoms, compounds derived from plastics, etc. Gaseous and solid compounds: CuF2, AlF3, WF6, CF4, SF4 are called primary compounds and are formed during and immediately after arc discharge. After the arc is extinguished, the atoms: sulphur, fluorine, oxygen, hydrogen, nitrogen, metals and carbon recombine and form mainly SF6, but also other compounds, most often: SOF2, SO2, HF, CF4, SF4, SO2F4. After low energy discharges also S2F10 – a very toxic gas which is difficult to detect, but it is produced in small amounts .
Chemical compounds generated in the switches are largely absorbed by the adsorbents installed inside the chambers (Al2O3, molecular sieve, NaOH + CaO mixture). The mass of the adsorbent is selected so that all gaseous oxygen compounds and CF4 are absorbed, especially very reactive SF4 and WF6, formed during the switching cycles during the life of the contacts. Powder products (with a diameter of approx. 2 µm), depositing on the surfaces of the elements of the extinguishing chamber, are mainly metal fluorides (eg CuF2, WO3). The amount of decomposed SF6 and SOF2 produced was determined to be proportional to the arc energy. it was found that 1 kJ of energy decomposes approximately 2.7 cm3 of SF6 and produces approximately 1.5 cm3 of SOF2 .
No gas degradation should take place in the compartments of the shielded switchgear where switching processes do not take place. The only reason for the decay of SF6 may be incomplete corona discharges – caused by defects, insulation defects. They can occur locally in many parts of the switchgear at a very low energy level, but persistent.
Partial discharges break down SF6 mainly into two compounds – SF4 and F, which later on reacting with traces of oxygen (O2) and water (H2O) to form chemical compounds such as HF, SO2, SO4 and SO2F2. There are also, but in very small amounts, higher chemical molecular compounds such as S2F10, S2OF10 and S2O2F10 .
Due to the low energy, low intensity of discharges, the amount of decay products generated in the devices is very low, in the order of several dozen ppmV, at the SF6 filling pressure at the level of approx. 500 kPa (i.e. higher than used in switchboards). Under normal operating conditions and proper tightness of the enclosures, this does not pose a threat to the personnel.
The source of the largest amount of SF decay products in switchgears are internal arc faults, which are accompanied by the release of high energy from the arc to the gas in a closed space, until the protection is triggered and the short circuit is turned off. It is related to an increase in pressure, discharged by tearing out the protective diaphragm or through a melted hole in the housing. The occurring chemical phenomena are similar in this case to those occurring at the switching arc, however, additional reactions may occur due to the contact of hot, ionised gas with metals and other materials than those used in the extinguishing chambers of the switches. The gas escaping from the casing also reacts with the surrounding atmosphere, including water vapour, O2, and N2, among others. The type of the resulting chemical products and their concentration depend on the design and materials used, the current intensity, the arc burning time, and the time elapsed since the discharge.
The post-failure condition with the release of gas and its decomposition products into the atmosphere of the room poses the greatest threat to people and requires the application of an appropriate safety procedure.
To summarise this issue, Table 4.1 provides a summary and general characteristics of SF6 breakdown products produced under different circumstances. However, it should be remembered that the type of decay products and their concentration depend on many factors – difficult to quantify.
Table 4.1. Approximate characteristics of the majority of SF6 decay products generated in power devices [19, 8]
|Źródło produktów||Major decay products SF6||Toxicity (rated)||Susceptibility to react
|Switching arc at low breaking current||SOF2
|Switching arc with high cut-off current||SF4
|*) w zależności od materiału obudowy|
Up to 1500C, materials such as metals, glass, rubber, plastics are completely resistant to SF6. At the temperature of 400-6000C, SF6 reacts with metals. Below this temperature no decomposition products are yet present. The breakdown products that will appear in SF6 are definitely more corrosive than the gas itself, especially in the presence of moisture. Metals are heavily attacked by the compounds, but the susceptibility to corrosion is concentration-dependent and not particularly high. Some inorganic materials, e.g. glass, porcelain, insulating paper, are very susceptible to corrosion. Others, such as epoxy castings, PTFE (Teflon), PVC, are much more resistant. Moisture greatly accelerates corrosion. Hence the conclusion: do not leave parts dismantled from devices unclean and not dried..
It should be emphasised that the issue of material resistance was particularly important in the early design and production period of SF6 devices. For example, special porcelain (based on Al2O3) has been developed for overhead circuit breakers, which is fully resistant to SF6 decay products even without coating (inside the chamber). A separate programme also concerned the development of appropriate rubber for gaskets. Each material research programme took into account the fact that SF6 equipment is expected to last 20, 30 or more years.
5. SF6 and the environment
The widespread use of SF6 in electric power apparatus in the world often raises concerns to what extent this gas and its decomposition products threaten the global environment. The literature on the issues of the impact of SF6 on the natural environment [19, 20] explains many issues.
Two issues are analysed in the most detail, bearing in mind the impact of SF6 on the natural environment:
- how the use of SF6 contributes to the greenhouse effect,
- how much the use of SF6 contributes to the depletion of the ozone layer in the stratosphere
In the analysis of this impact [18, 19, 20] it was taken into account that:
- about 80% of the annual production of SF6 is intended for the electrical industry, so the question of the impact on the atmosphere of SF6 used in the power industry is justified,
- SF6 used in the power industry is stored in closed vessels (switchboards, circuit breakers) and in cylinders,
- Emissions caused by SF6 power equipment are only caused by operating errors or leakage due to equipment leaks. These reasons are minimized due to staff training and high tightness of the devices.
It was found that SF6 gas does not participate in the stratospheric ozone decomposition – it does not undergo photolytic activity, because it does not contain chlorine atoms.
However, SF6, like many other gases, e.g. CO2 or CFCs, absorb infrared radiation in the region of the atmosphere where this radiation spectrum occurs, its presence in the atmosphere can contribute to the so-called secondary artificial infrared radiation, returning to the lower atmosphere, causing the greenhouse effect.
It should be emphasised, however, that the greenhouse effect discussed above is caused artificially, increased by human activity, as opposed to natural warming caused by water vapour, CO2, etc.
The impact of SF6 on global warming depends on:
- its concentration in the atmosphere, which in turn is determined by the amount of gas released into the atmosphere, and by how long SF6 retains its properties in the atmosphere,
- its absorption properties – in the area with the infrared spectrum.
The issue of the introduction of SF6 decomposition products into the environment and their impact on them remains to be discussed. Well, while SF6 is itself a chemically very stable gas and remains in the atmosphere for a very long time, because it does not enter into any reaction that would lead to the degradation of this gas, compounds formed as a result of the decomposition of SF6, which can be produced during partial, spark discharges They are environmentally friendly as they are highly reactive and are transformed extremely quickly into environmentally harmless end products. In addition, there is a significant adsorption of decay products and their secondary compounds in the filters of the devices in which they are formed and only a small amount of them enters the atmosphere due to leakage. Of course, it is so provided that the conscious evacuation of gas from devices by man is eliminated. Failures of the arc melting of enclosures, with the uncontrolled ejection of gas and its decomposition products into the atmosphere, are extremely rare – SF6 devices are very reliable.
Opponents of the use of SF6 due to its impact on the environment, including gas decomposition products, assume that all of the SF6 produced will eventually be released into the atmosphere. However, unlike other man-made gases, SF6 used in power equipment is properly stored, and the operation of plant and auxiliary equipment ensures that SF6 cannot be released into the atmosphere. Such a thesis is justified by the implementation of SF6 regeneration – the process of restoring the ability to use SF6 in devices
It should be emphasised that in the past SF6 regeneration has not been widely practiced for the following reasons:
- SF6 producers and users were not fully aware of environmental protection issues,
- procedures and technologies of regeneration have not been clearly and clearly defined,
- no standards (procedures) have been developed for SF6 recovered at the installation site of power equipment with SF6 or at the factory,
- SF6 leakage into the atmosphere in the past has not been sufficiently analysed.
All of the above reasons are no longer relevant. Recent surveys conducted by CIGRE show that the majority of users of SF6 power equipment are aware of the need to protect the natural environment. They avoid the release of SF6 to the atmosphere and have started systematic SF6 recovery at the installation site. On the other hand, the PN-EN IEC 60480 standard clearly defines the processes of recovery, regeneration and certification of SF6 gas.
So what should we do if we consciously want to continue to use SF6 in electrical apparatus:
- SF6 cannot be deliberately released into the atmosphere,
- SF6 losses from electrical equipment are reduced thanks to design improvements and should be further minimised through proper assembly and correct maintenance procedures,
- SF6 should be regenerated,
- the standards for SF6 regeneration and purity procedures should be strictly followed
The implementation of these postulates, of course, depends on the awareness of the issues of SF6 application – from the management of power stations to the technical staff.
6. SF6 insulated switchgear
Shielded high voltage switchgear is the main reason for the use of sulphur hexafluoride in power devices. As previously mentioned, they determined the good insulating properties of this gas. With the use of this gas, it became possible to construct high-voltage switchgear in enclosures – similar to low voltage switchgears – often called hooded ones.
The technology based on SF6 has several significant advantages :
- thanks to the good insulating and switching capacity of sulphur hexafluoride, the dimensions of the devices have been significantly reduced. This in turn allows you to:
- reduce the area occupied by the installation and improve the layout of the substation,
- significantly reduce the number of component parts, which is associated with a reduction in the consumption of raw materials and energy in production, technological process, processing, use and disposal,
- hermetic housing of high-voltage bus bars in earthed sheaths makes the system based on the use of SF6 independent of atmospheric pollutants and degradation processes, and enables:
- significant extension of equipment reliability time,
- significantly reduced requirements for maintenance, inspections and repairs, as a result greater reliability, durability and availability – i.e. constant readiness for operation,
- reduction of energy losses and fire risk.
The above statements show, apart from the excellent technical parameters, the operational certainty and economic aspects that SF6 insulation has no alternative solutions that would outweigh it from an ecological point of view. This allows the use of the most advantageous solutions if we take into account the environmental circumstances occurring during the entire life cycle, and the total costs 
The construction of shielded high-voltage switchgear began with the construction of 123 kV switchgears. The tightly enclosed SF6 gas-insulated high-voltage switchgear looks like a conglomerate of metal pipes and tanks with considerable dimensions.
In order to be able to assess the entirety of such a structure, it is necessary to introduce a number of specific classification criteria, not generally used in other versions of switchgear. These classification criteria fall within a few of the most important groups of issues. Leaving aside the secondary aspects, the distribution of switchboards can be represented as follows:
- due to the method of insulating the bus bars:
- switchboards with single-pole insulated bus bars,
- switchboards with three-pole insulated bus bars,
- due to the bus bar system:
- single bus bar system,
- double bus bar system,
- due to the ability to connect the connector used:
- circuit-breaker switchgears (bays),
- switchgears (bays),
- due to the way the switches are set:
- switchboards with horizontal switches,
- switchboards with vertical circuit breakers,
- due to the supporting structure:
- switchgears with a separate supporting structure,
- “self-supporting” switchgears (covers are also a load-bearing structure),
- complex structure, special support structure and field housing on it,
- due to the place of installation of the switchgear:
- indoor switchgears,
- outdoor switchgears.
- due to the type of switch:
- circuit breakers with SF6 extinguishing agent,
- vacuum circuit breakers
Particular manufacturers designed the individual components (elements) of switchgears in such a way that it was possible to have different switchgear compositions, different switching devices, different systems and connections (cables, overhead lines. It is estimated that, due to the switchgear design, the most advantageous is the cable routing downwards, in which case the bus bars are placed at the top.
The enclosed switchgear has the same sets of apparatus as conventional switchgear, but of a different construction – suitable for use in a closed enclosure and for SF6 insulation. Therefore, the main advantage of shielded and insulated SF6 switchgears is their much smaller size.
When analysing the switchgear structures of different manufacturers, some “architectural” differences in the structure are noticed, but this does not have a significant impact on their functioning. We note the use of stainless and non-magnetic steel, rolled aluminium, and cast aluminium. Manufacturers usually emphasise the superiority of the selected type of housing.
Of course, manufacturers did not stop at building 123 kV switchgear. Later, leading companies installed 245 kV, 300 kV and 525 kV switchgear. The most important thing is that with the increase of the rated voltage, the ratio of switchgear sizes in the traditional solution to the dimensions of the shielded switchgear increases. As a result, the area occupied by the switchgear is reduced many times and the overall economic effect of the installation is visible thanks to the reduction of land costs.
In the case of high voltage shielded switchgear, it is particularly important that they operate with high reliability (low failure rate), as the risk of external factors (pollution, lightning discharges, cracking of insulators, birds, etc.) is eliminated. In order to familiarise the reader with some details of the design solutions inside the enclosed switchgear, we choose as an example a switchgear already installed in Poland, ie switchgear from ABB (Fig. 6.1).
Figure 6.1. Cross-section of the circuit-breaker bay with the cable line of the 123 kV switchgear type ELK-0 (ABB), 1 – double bus bar system, 2 – circuit breaker, 3 – current transformer, 4 – voltage transformer, 5 – cable connection chamber, disconnector and earthing switch, 6 – drive, disconnector – earthing switch, 7 – control cabinet
Figure 6.1. Cross-section of the circuit-breaker bay with the cable line of the 123 kV switchgear type ELK-0 (ABB), 1 – double bus bar system, 2 – circuit breaker, 3 – current transformer, 4 – voltage transformer, 5 – cable connection chamber, disconnector and earthing switch, 6 – drive, disconnector – earthing switch, 7 – control cabinet 
Basic parameters of the presented switchgear:
- rated voltage: 72.5 – 170 kV,
- rated current: 1250 – 3150 A,
- SF6 pressure beyond the switch (absolute): 420 kPa,
- breaking capacity of the circuit breaker: 25 / 31.5 / 40 kA,
- SF6 pressure in the switch: 600 kPa.
In order to appreciate the advantages of miniaturisation of this type of switchgear, attention should be paid to the dimensions of the bay and the minimum space necessary for it, as shown in Fig. 6.2.
Figure 6.2 shows the top and side view of a five-pole switchgear in H configuration (made by ABB), including:
- in the bays of F1 and F4 supply lines:
- disconnector – earthing switch on both sides of the circuit breaker,
- earthing switch on the side of supply lines,
- transformers: current and voltage,
- in the F3 coupling area:
- circuit breaker with a current transformer,
- disconnector – earthing switch on both sides of the circuit breaker,
- and in outgoing bays F2 and F5:
- disconnector – earthing switch on the side of busbars,
- downstream earthing switch,
- voltage transformer.
On request, the switchgear can have disconnectors or circuit breakers in the outgoing lines.
Anyone who knows what a conventional five-pole switchgear looks like and how much space it takes up must admit that an enclosed – insulated SF6 switchgear takes up
Figure 6.2. The dimensions of the ELK-0 switchgear (H layout) and the minimum dimensions of the room 
From the user’s point of view, it should be noted that in the presented figures 6.1 and 6.2, you can see the correct installation of the so-called ejectors (membranes), i.e. correctly directed upwards. The membrane has a cover in the form of a steel mushroom, protecting against external damage to the membrane and its uncontrolled ejection during pressure discharge in the switchgear or switch compartment. It is unacceptable, for the safety of personnel, for the ejectors to be directed in such a way that in the event of gas blowout, it will be directed towards the person in the room.
Wherever a mistake was made consisting in improper directing of the ejectors – with the risk of hitting a membrane or gas stream at a person, special protective covers should be made.
As another detail of the switchgear interior, the ELK-0 switchgear switchgear will be discussed (Fig. 6.3).
Figure 6.3. Sectional view of switch disconnector ELK-O (ABB); 1 – partition insulator, 2 – driving insulator, 3 – cylinder. 4 – compression space, 5 – continuous current contacts, 6 – arcing contacts, 7 – safety diaphragm (ejector) with a cover
The disconnector is designed to switch off the operating current, and the arc is extinguished by a blast of SF6 gas, automatically compressed in the cylinder. During opening, the insulating cable drive (2) transfers the movement to the movable contact (5) and cylinder (2). This causes the arcing contacts (6) to open and the gas compressed in the cylinder in the compression space (4) to be blown into the nozzle (the piston is stationary). Epoxy disc insulators (1) tightly separate the disconnector space from other switchgear compartments and constitute the current path supports. Ensuring tightness between the disconnector (similarly the circuit breaker) and the switchgear is important due to the elimination of the possibility of gas contamination in the switchgear with decay products from the disconnector (circuit breaker). The diaphragm (7) is torn off when the gas pressure in the tank exceeds the permissible value.
Necessary devices of the switchgear are the disconnector and earthing switches. Most often, both functions are included in one apparatus. The basic design of this ANSALDO apparatus is shown in Fig. 6.4. Moving contacts of the disconnector (vertical) and earthing switch (horizontal) have separate drives. Of course, there is a mutual blockade of the drives, preventing the simultaneous closing of both cameras..
Figure 6.4. Switchgear unit: disconnector – earthing switch; of ANSALDO (Italy)
In the initial period of installing enclosed switchgear, there was a fairly widespread concern about meeting the requirement for a visible disconnect break. It was not possible to meet this condition in covered switchgear. The disconnector status (position) is indicated by the indicator. And only it is used to visually assess the state of closure – opening of the apparatus. Disconnectors of modern switchgears made of SF6 are devices with very high operational reliability, and the electric break in the gas ensures sufficient durability. The French power industry has set exceptionally stringent requirements for the safety of the disconnection break.
Manufacturers (eg Delle-Alsthom) had to use insulating screens in their disconnectors, which, in the open state of the disconnector, slid between the contacts, ensuring the coordination of the break insulation. Currently, the solution of a disconnector with an insulating screen has not found any imitators – it was considered as unnecessarily complicating the structure.
In SF6 shielded switchboards, insulators and seals are the most critical elements.
Insulators – most often made as epoxy castings, must meet the requirements for:
- mechanical strength,
- surface and through-surface dielectric strength,
- resistance to SF6 breakdown products.
It should be emphasized that the fulfilment of the last two conditions depends to some extent on the cleanliness of the assembly and the filling of the switchgear. Compliance with the rules in this regard is particularly important in the event of any repairs requiring the dismantling of insulators. Reassembly must be performed with utmost cleanliness. Even trace contamination on the surface of insulators (e.g. sweat from the hands) may cause partial discharges and, consequently, the production of toxic S2F10 during operation. The seals of devices with SF6 ensure long-term operation without the need for refilling with gas. This eliminates or significantly reduces the possibility of external pollutants (air, moisture) entering the gas. In the event of disassembly and reassembly, the previously used gaskets must not be installed (only new – of the correct type) and the assembly must be particularly clean.
Good experiences with the use of SF6 as insulation in shielded high voltage switchgears resulted in the commencement of works on the construction of insulated switchgear for medium voltage, that is 12 kV; 17.5; kV; 24 kV; 36 kV etc.
In this voltage range, basically two types of construction have developed:
- “tubular” structures with a similar concept as 123 kV switchgears,
- “cabinet” constructions with a similar concept as conventional prefabricated switchgears.
In the first case (Fig. 6.5), the switchgear is basically constructed of apparatus adapted to the SF6 technique – i.e. designed for a given type of switchgear. The exception is the switch. It is most often a typical vacuum circuit-breaker, adapted to be installed in an SF6 switchgear. The choice of this type of circuit breaker for enclosed switchgear is to ensure high connection durability of the apparatus (appropriate for vacuum circuit breakers) without the need for maintenance inspections.
This switchgear has the same advantages as the high voltage shielded switchgear: reduced dimensions, reduced failure rate, operational safety (elimination of the risk of electric shock), etc. Automation of manoeuvring activities enables practically maintenance-free operation of the switchgear.
SF6 gas pressure in this type of switchgear is quite low, which helps to maintain tightness and limits gas losses from the enclosures.
This type of switchgear is generally adapted to work at temperatures ranging from -5◦C to 35◦C, i.e. in indoor conditions.
Figure 6.5. 1236 kV shielded switchgear with SF6 insulation (with a vacuum circuit breaker) in GEC ALSTHOM “tube” casings; 1 – double bus bar system, 2 – three-pole disconnector, 3 – vacuum circuit breaker compartment, 4 – cable joint, 5 – HV cable, 6 – current transformer, 7 – voltage transformer, 8 – overvoltage limiter, 9 – circuit breaker drive, 10 – disconnector drive, 11 – measurement and control board
The second design solution of SF6 insulated switchgears is based on the concept of prefabricated switchgears with the most common devices. Sulphur hexafluoride performs the task of earth and inter-pole insulation. Due to the better electrical resistance and thermal conductivity of SF6 than air, these switchgears can be of smaller dimensions compared to traditional ones.
Figure 6.6. Shielded insulated switchgear SF6 type ZV2 36 kV (ABB) (yellow – compartment filled with sulphur hexafluoride), A and B – bus bar and disconnector compartments, C – circuit breaker compartment, E – drives and control cabinet, 1 – current transformer, 2 – voltage transformer, 3 – cable gland, 4 – vacuum switch, 5 – bus bars, 6 – disconnector – bus bar earthing switch, 7 – bulbar disconnector
Fig. 6.6 shows, as an example of a “cabinet” construction, a switchgear type ZV2 with a double bus bar system, for voltage up to 36 kV (ABB company) and continuous current 1250 2500 A. The dimensions of such a panel are: height 2250 mm, width 750 mm, depth 1600 mm. SF6 pressure (absolute, at 200C): nominal – 1.2 bar, lower operating pressure 1.1 bar, withstand test voltage – 1.0 bar. Filling one field with gas requires approx. 10 kg of SF6.
The above-mentioned SF6 pressure values show that the overpressure of the gas compared to atmospheric pressure is negligible, which means that the tightness of the system is easy to obtain.
In both switchboards, shown as examples of design solutions, vacuum circuit breakers are installed. It may seem strange at first glance. However, it turned out that despite the use of SF6 insulation, it is justified to install vacuum circuit breakers in this type of switchgear. It results, as already mentioned, from the need for greater connection durability of the apparatus without the need for maintenance, and the possibility of obtaining smaller dimensions of the field.
The use of vacuum circuit-breakers in these switchgears has another very important meaning: practically no SF6 decay products (as in circuit breakers made of SF6) and even with gas leaks into the atmosphere in the room, there is no toxic hazard to humans. Especially that at this voltage it is difficult to expect partial discharges in the compartments filled with SF6.
Hence it can be seen that in the case of both types of switchgear, the hazard can only be from an internal arc and an explosion of the enclosure – and this is almost impossible at this voltage level and structural reliability.
7. 7. Circuit breakers with SF6.
The first circuit breakers using sulphur hexafluoride as an arc extinguishing agent were created about 10 years earlier than switchgear insulated with this gas. The results of the SF6 arc extinguishing research have become the starting point for the idea of constructing high voltage circuit breakers. The first circuit breaker was put into operation by Westinghouse in the first half of the 1950s (at 115 kV). It was a six-break switch with capacitive voltage distribution control. What does the progress in the design of circuit breakers with SF6 show
Figure 7.1 shows in the simplest way the physical arc extinguishing mechanism in the arc extinguishing chamber of the circuit breaker with SF6. We see that when the contacts are opened, the gas is simultaneously compressed in the moving cylinder. Due to the closing of the critical cross-section of the nozzle with a fixed contact and an arc, in the initial phase of opening the switch, the gas is compressed in the movable cylinder (item B). When the contacts spread to the distance at which the arc can be extinguished, the gas pressure is so great that it flows out strongly in the arc zone. Thus, arc quenching is based on the principle of self-compression and self-regulating gas flow to cool the arc. The arc column limits the gas flow and causes a temporary, additional increase in pressure, proportional to the value of the cut-off current. [16, 17].
Figure 7.1. The arc extinguishing process in the extinguishing chamber of the HV switch with SF6 (according to MAGRINI GALILEO): A – chamber in the closed position, B – switch-off start, C – compressed gas exhaust – arc extinguishing
When switching off the operating currents (below the rated current), the phenomenon of gas flow inhibition is small and the arc is quenched as soon as the contacts spread to the distance at which the dielectric strength of the break when the current passes through the zero value is sufficiently large.
Switching off the short-circuit current – especially with a value close to the breaking current, is associated with the blockage of the SF6 flow through the nozzle, which favours the pressure increase due to gas compression in the cylinder and its heating by an arc in the nozzle zone. Only the disappearance of the arc column in the area of the current crossing through the zero value allows gas outflow, cooling the arc column and restoration of the dielectric strength of the break. The process is very “smooth” – the arc is extinguished and the current is interrupted at its zero, which helps to reduce the so-called switching overvoltages.
In practice, three basic design solutions of the extinguishing chamber are used in terms of the way the gas flows, compressed in the cylinder during the opening of the contacts, into the arc zone (Fig. 7.2).
Figure 7.2. Models of extinguishing systems of SF6 circuit breaker chambers; a – single-stream chamber with an insulating nozzle, b – two-stream asymmetric chamber with an insulating nozzle, c – two-stream symmetrical chamber with conductive (metal, graphite)
It has been noticed that in the extinguishing chambers discussed above, it is not possible to fully use the compressed gas portion and the energy capacity of the drive. When switching off the low currents, the nozzle is not “clogged” by the arc and gas flows almost freely from the compression tank. Hence, the increase in gas pressure is relatively small. In order to switch off a circuit with a high slope of the recovery voltage, it is necessary to ensure an appropriate value of pressure. This requires a drive that guarantees a high speed of opening the circuit breaker. In contrast, when switching off a high short-circuit current (eg breaking current), the nozzle is “clogged” with an electric arc and the gas flow stops. The drive brakes because the high pressure of SF6 in the compression cylinder causes a high resistance to motion. If the dimensions of the compression cylinder and the drive energy are not properly selected, unfavourable for the shutdown process, the moving contact may be withdrawn (the so-called rebound) until the current crosses zero. Only in the zero current zone, gas flows through the nozzles and the drive is restored. Eliminating these “reflections” in the movement of contacts requires a sufficiently strong drive. The energy of such a drive is not efficiently used when switching off smaller currents. This disadvantage was minor when a hydraulic drive was used, and it was especially significant with spring drives.
Figure 7.3. Diagram of a self-compression quenching chamber with thermo-expansion; A – closed position, B – compression chamber (V2) operation, C – thermal expansion operation
The latest generation of SF6 circuit-breakers has a modified self-cushioning chamber so that the discussed operating disadvantages are eliminated.
For example, the companies: ABB, AEG and GEC Alsthom, used a two-stage gas compression system in their extinguishing chambers (Fig. 7.3). The compression chamber is here divided into two volumes V1 and V2, connected by valves. In the first phase of cylinder movement, when opening the switch, the gas is compressed in the volume V2 and forced through the open valves into the space V1 and further into the arc zone. Thus, the conditions for extinguishing small currents (e.g. working currents) were created. When the nozzle is “clogged” by an arc of high short-circuit current, a phenomenon of strong “thermo-expansion” occurs, i.e. gas pressure increase in the volume V1 as a result of its temperature increase (heating by the arc). The valves between volumes V1 and V2 close, preventing gas flow back. As the volume of V2 continues to decrease, the pressure of SF6 increases further – which could inhibit the movement. This is prevented by the opening of the “safety” valve in the piston and the release of gas from this volume. In this way, the pressure in the arc extinguishing zone is automatically regulated. A circuit breaker with such a chamber may be economical spring operated. .
Figure 7.4. ELK type extinguishing chamber by ABB: 1 – continuous current conduction contact, 2 – arcing contact (electrically conductive during switching off), 3 – nozzle (insulating), 4 – SF6 compression cylinder, 5 – gas space
In fig. 7.4 we can see the ELK (ABB) circuit breaker chamber in which a two-stage current switching off process is applied. Note that the extinguishing system of this chamber is very optimised – which is the result of many years of research by the designers. The chamber has the above-mentioned two compartments (V1 and V2) for compression using the processes of gas self-compression, thermo-expansion (i.e. pressure increase of SF6 due to heating by removing energy from the arc) and ablation, i.e. gas pressure increase as a result of gassing (evaporation) of the nozzle material . As long as there is gas pressure in the self-compressing c It depends to a small extent on the value of the cut-off current, then in the chamber using thermo-expansion and ablation, a clear dependence of the gas pressure in the nozzle on the value of the cut-off current is obtained
Since ablation (gassing of the nozzle material) is one of the factors of the increase in gas pressure during arc-extinguishing, a question may arise about the wear rate of the nozzle in multiple shutdown processes. The nozzle durability tests show that there is no excessive wear of nozzles in the range of the switching capacity assigned to the switch. This is due to two facts: proper selection of the material and exposure of the nozzle to ablation only when switching off high short-circuit currents.
It is easy to notice from the above descriptions of the operation of the extinguishing chambers, why in the gas of the circuit breakers, apart from the gas decay products, there are also compounds of these products depending on the material of the contacts and nozzles. Subsequent disconnection of the current involves another portion of chemical compounds as a result of the arc’s action on the gas, contact materials and nozzles. A significant part of these products is adsorbed in special filters consisting of Al2O3 adsorbing granules.
The company Magrini Galileo used in its circuit breaker type SB6 123 – 245 kV, a single-break tripping chamber with its own driving mechanism (Fig. 7.5). All its elements (extinguishing chamber, insulating column and drive) are filled with SF6 gas, which in this switch is an insulating, quenching and driving factor for moving contacts during switching on and off. The closing and opening of the circuit-breaker is performed by a double-acting piston connected directly with the movable contact.
The energy source for the drive is SF6 gas with the working pressure (higher), located in the column and the shutdown chamber. Actuating the opening or closing solenoid valve causes the control valve to move. The result is the opening of the master valve and the flow of gas from the top of the pole to the double-acting piston cylinder. In this way, the piston, under the pressure of the gas, moves and moves the moving contact of the extinguishing chamber. After the shifting operation is completed, the gas flows to the lower expansion chamber. The micro compressor, controlled by the differential relay of the pressure sensor, replenishes the gas volume in the chamber to the initial pressure. Maintaining the movable contact in the end positions – open or closed – is ensured by a two-position bistable spring mechanism, regardless of the presence of gas in the drive cylinder .
Figure 7.5. Column cross-section of the SB6 circuit breaker by Magrini Galileo: A) space with (higher) working pressure, B) low pressure expansion space, 1 – safety valve, HV connections, 3 – stationary arcing contact, 4 – nozzle, 5 – moving main contact, 6 – insulating cable, 7 – filter, 9 – closing valve assembly, 10 – auxiliary contact block, 11 – micro-compressor, 12 – valve for pumping air and gas filling, 13 – molecular sieve, 14 – main stationary contact, 15 – arcing contact movable, 16 – upper insulator, 17 – lower insulator, set of opening valves, 19 – double-action piston, bistable mechanism flat springs, 21 – plug-in socket for control circuits
Circuit breakers with SF6 in the voltage range 12-36 kV have a more varied design than those for high voltage. Various companies found their design – convenient to their own technology. However, the principle of operation of most of the extinguishing systems results from the known self-resilient systems. In the 1980s, also for this voltage range, rotating arc circuit breakers appeared, in which the influence of the magnetic field on the electric arc and the possibility of cooling the arc in SF6 as a result of its rapid movement were used.
An example of a self-compressing switch is the HC switch (made by ABB) shown in Fig. 7.6. Each pole of the switch is enclosed in a separate epoxy housing. In order to eliminate wear of the movement seal, a different method of sealing the operating lever was used than in other types of switches. This type of circuit breaker, rated at 630 A to 2500 A, has a breaking current of up to 25 kA at a voltage of up to 24 kV.
Figure 7.6. Cross-section of the pole of the self-compressing circuit breaker type HC (ABB) in the open state: 1 – fixed contact unit, 2 – continuous current contacts, 3 – fixed arcing contact, 4 – movable arcing contact, 5 – insulating nozzle, 6 – extinguishing gas exhaust space, 7 – shaft (common for 3 fields) 8 – movement seal, 9 – drive shaft, 10 – drive lever 
ABB has introduced a rotating arc medium-high voltage circuit breaker (Fig. 7.7) .
Figure 7.7. Pole cross section of a HB type rotating arc switch (ABB): 1 – connection, 2 – cylindrical coil, 3 – contact lamellae, 4 – rotating arc “race”, 5 – movable contact, 6 – extinguishing chamber, 7 – gas evacuation area hot, 8 – drive lever, 9 – drive shaft, 10 – auxiliary compression piston.
When opening the circuit breaker, the electric arc ignites between the arcing contacts: the fixed one in the form of a disc and the movable, tubular one. The intermittent current then flows through the cylindrical coil, creating a magnetic field affecting the arc (like a current conductor). As a result, there is a force transversely directed to the axis of the arc, causing it to rotate. During this movement, the arc cools down so efficiently that the current is interrupted when the current passes through the zero value.
Circuit breakers with self-generating SF6 pressure chambers are also manufactured for medium voltage. Characteristic for this circuit breaker is the complete separation of the path for conducting continuous current and during tripping. When the circuit breaker is opened, the “disconnector”, ie the continuously carrying contact, first opens. The current is commutated to an arcing contact. The arcing contact then opens and the arc ignites. As long as the movable arcing contact is within the space closed with a special insulating nozzle, the burning arc heats up a certain portion of the gas. Its pressure increases due to thermo-expansion and ablation. As the contact extends further from the nozzle, the gas is blown into the expansion space and the arc is extinguished (while the current passes through the zero value). The switching parameters of this circuit breaker, together with its arc extinguishing technique, clearly confirm the excellent extinguishing properties of SF6
8. 8. Other apparatus with SF6.
Sulphur hexafluoride, due to its excellent dielectric properties, is also used in other power devices (not only in switchgear compartments and switches).
High-voltage busducts with SF6 insulation are increasingly used in applications where safe energy supply is concerned, even over long distances – resistant to the action of a polluted atmosphere (e.g. chemical exposure zones) (Fig. 26).
Figure 26. High voltage bus bar filled with SF6 and SF6 air bushing;
1 – porcelain insulator, 2 – current path, 3 – SF6 space, 4 – internal insulator, 5 – screen controlling electric field distribution, 6 – internal SF6 space, 7 – pressure gauge
Interestingly, following the SF6 current and voltage transformers used in shielded switchgears (Fig. 27), there are also transformers insulated with this gas, instead of oil, for use in conventional switchgears.
Figure 27. Transformers of shielded switchgears: on the left – a current transformer; 1 – barrier insulator, 2 – bushing insulator (secondary circuits), 3 – winding body, 4 – winding, 5 – secondary circuit terminal block, voltage transformer on the right: 1 – barrier insulator, 2 – magnetic core, 3 – primary winding, 4 – secondary winding, 5 – secondary winding terminals, 6 – connection, 7 – ejector diaphragm.
9. Principles of safe work with devices with SF6.
Over fifty years of experience in using SF6 in electrical power devices has shown that there are no serious problems in the field of health and safety, however, provided that certain precautions are taken and the established procedures are followed. Numerous studies carried out on an international scale emphasize that in all cases when the operating personnel suffered any injuries, safety procedures were not followed or the personnel was not equipped with appropriate protective equipment. It can be concluded that not working with SF6 equipment is dangerous, but not when following the procedures.
Workers working with devices containing SF6 should be trained in:
- information on the properties of primary SF6, the formation of decomposition products and their impact on the human body,
- rules of safe work with devices with SF6,
- use of auxiliary equipment to work with devices containing SF6,
- use of general and personal protection equipment as well as hygiene rules,
- handling contaminated gas and solid decomposition products to eliminate risks to people and the environment,
- giving first aid.
Switchgear service should be provided with:
- a separate room for changing clothes, storing protective clothing and personal protective equipment,
- access to a washroom with hot and cold water,
- airtight containers for storing used clothing and materials that have come into contact with SF6 decomposition products,
- a separate room for storing personal clothes, eating and relaxing.
When working with sulfur hexafluoride, there may be the following five cases with varying degrees of danger to personnel :
- work with primary gas,
- SF6 leakage during normal operation of the working equipment,
- work with the risk of SF6 decomposition products: maintenance or service work, expansion of a shielded switchgear,
- emergency operation of the switching device: e.g. internal short circuit or fire outside the device, causing the casing to become unsealed,
- decommissioning of switchgear, removal of gas and powder contamination
Due to the specific nature of the threat and the procedures to be followed, each case will be discussed separately
Table 9.1. Reactivity and toxicity of the gaseous decomposition products of SF6 
|Decay product||Chemical stability In air||Stable re action products||Degree of Toxocity||Threshold value||Odour|
|S2F2||Decomposes quickly||S, HF, SO2||0,5||–||Acrid, sour|
|SOF2||Slow decomposition||SO2F2, HF||0,6¸1||1,0¸5||Rotten eggs|
|SOF4||Decomposes quickly||jw.||0,5||–||Like HF|
|WF6||Decomposes quickly||WO3, HF||0,1||–||Like HF|
|CF4||Durable||CF4||Non toxic||–||No odour|
When determining the concentration limits of compounds for the above-mentioned situations that may arise during operation, the so-called Threshold limit value (TLV), i.e. the maximum allowable concentration that does not adversely affect people working eight hours a day being of working age.
9.1 Working with Primary SF6.
Sulphur hexafluoride is supplied in liquid form in pressure cylinders of various volumes. Primary SF6 must meet the requirements of PN-EN IEC 60576 and the supplier should attach a gas quality certificate.
When working with pure SF6, the acceptable TLV level in the room is 1000 ppmV (SF6)  – which corresponds to a concentration of 6000 mg / m3. Clean gas in the atmosphere of the room may appear as a result of leakages from leaky devices containing this gas, from cylinders with open valves, or due to improper handling. Natural ventilation is sufficient to reduce the concentration of pure SF6 or completely remove it from the room. In rooms where there is a probability of pure SF6 accumulation, it is not allowed to use devices with an open heater, do not weld or use devices with a temperature exceeding 2000C. Smoking is not allowed in the rooms!
Rooms where SF6 equipment is installed, gas containers are stored, or work is performed should be marked:
- in front of the entrance a plate with the inscription: “SF6“,
- inside there are boards with the inscription: “No smoking” and “No open fire”.
During work performed in closed rooms (e.g. inside the switchgear building) connected with filling the devices with gas, the gas may accumulate in the air for a certain period of time. In the absence of ventilation, the permissible concentration level may be exceeded. Therefore, it is recommended to develop a (written) procedure specifying the method and procedure for the periodic measurement of SF6 concentration in the air. It must be adapted to the measuring devices used in the given conditions. If the measurements show that the level of 1000 ppmV has been exceeded, ventilate the room until the concentration drops below the limit value.
Particular care should be taken in low-lying rooms (cable ducts, basements) where gas heavier than air may accumulate. When working in these rooms, ventilation that forces air flow should be used.
When work is carried out in the open air, natural “ventilation” prevents gas accumulation. Special precautions in these conditions should be kept in the very close vicinity of the devices.
Transport and storage of SF6 cylinders with users requires the following rules:
- SF6 gas cylinders must be transported and stored with valves closed and caps screwed on. The cylinder must not be thrown or overturned, and must be protected against mechanical impact (e.g. impacts). Transport should be carried out on special trolleys with cylinders attached,
- gas should be stored in separate, ventilated rooms, separate from other types of gases, away from heat sources, flammable and explosive materials. The room must not contain any heaters with open elements with a temperature above 2000C. Gas losses should be prevented from flowing to other rooms (e.g. basements) where people work,
- Solar operation on gas cylinders is forbidden. Like any compressed gas, SF6 may cause the cylinder to explode when overheated,
- it is unacceptable to unscrew or loosen the reducer screwed on the cylinder connection when the valve of the pressurised gas cylinder is not closed,
- it is inadvisable to release gas from the cylinder with a large stream – your hand may freeze to the valve,
- cylinders emptied of gas should be separated, with valves closed and protective caps screwed on,
- protective gloves must be worn during transport and when manipulating the cylinder valves.
Gas recovered from operated devices should be in separately marked cylinders – preferably in rooms other than the primary gas.
Basic work with primary SF6 is filling new or renovated equipment with gas. Primary gas, when filling the switchgear devices, must be pushed from tanks containing gas (e.g. cylinders) under high pressure to the apparatus housings, the rated pressure of which is much lower. Accordingly, the following recommendations must be observed when filling:
- use equipment adapted to this activity for filling,
- pipe connections must be adequately protected against mechanical damage,
- valves and reducer must always be operational,
- pressure gauges (manometers) should be regularly calibrated,
- pipes and valves connected to them, used for pumping gas, should be connected in such a way that they can be removed from the filled casing without fear of gas contamination.
The vast majority of devices are so-called sealed pressure systems, which means that after filling them in the factory, during the entire period of operation (from installation to decommissioning), they do not need to be refilled with gas.
Some devices are so-called “closed pressure systems”. Such devices must be filled or refilled at the place of installation.
The procedures used in the process of filling (topping up) with gas when putting the device into service should be defined so that four fundamental requirements are met:
- personnel operating the devices must not be exposed to unjustified risk,
- gas leakage into the atmosphere must be minimal,
- leakage from the casing after filling cannot exceed the permissible value,
- after filling, the enclosures should contain gas of the required quality (specified by the manufacturer).
The most common method of filling the enclosures of switchgear devices is:
- pumping out the air from the selected closed compartment of the switchgear with a vacuum pump to the residual pressure – specified by the manufacturer of the apparatus,
- checking the tightness (under vacuum) in accordance with the manufacturers recommendations. Such a procedure also promotes the internal evaporation of moisture and its evacuation when the vacuum pump is turned on again,
- gas is slowly filled to the rated pressure. Do not fill the switchgear above the rated pressure – specified by the manufacturer (e.g. the SF6 pressure sensor may be damaged),
- checking the leak tightness of the device installed or filled with gas at the place of installation. The manufacturers instructions should specify the method of checking for leakage and specify the equipment needed for this.
liminating leaks in switchgear is essential for three fundamental reasons:
- due to the operating conditions of the devices; the switching capacity and insulation capacity decrease when the gas density decreases as a consequence of leakage,
- personnel safety. Gas losses mean the possibility of gas accumulation in the room and an increase in gas concentration in the atmosphere. The risk increases when leakage occurs from the circuit breakers,
- negative impact on the climate.
The most important conditions that must be met when correctly filling (and refilling) the devices with gas are:
- high vacuum before filling,
- guaranteeing the purity of filling and thus guaranteeing good gas quality,
- filling to the correct gas pressure (taking into account the gas temperature),
- minimisation of gas leakage into the atmosphere.
Gas should be released from the cylinder in a small stream. During too rapid gas flow (lack of a reducer or its improper adjustment), the cylinder valve may freeze due to adiabatic expansion. To prevent sticking of the hand opening the cylinder valve, perform this operation in a protective glove.
When transferring SF6 from the liquid state, the two shut-off valves in the SF6 liquid batch connection must not be closed at the same time, as the temperature rise may lead to an explosion.
Before starting the process of filling (refilling) the device with SF6 gas, measure the ambient temperature and determine the value of the rated pressure at the given temperature on the gas state diagram (eg Fig. 9.1).
The filling pressure value of a given device should be checked after the gas temperature has equalised with the ambient temperature – waiting for about 1 hour.
Some instructions give a table of SF6 pressure values for individual densities (lines a, b, c) depending on the temperature. In practice, you should always use these arrays. If there is no such table of pressure corrections, the diagram should be used (Fig. 9.1).
The switching capacity of the apparatus and the electric strength of the gas depend, among others, on gas density. In the case of SF6, the gas pressure at a given density depends to a large extent on the temperature – e.g. when looking at the line and we see that at the temperature of -20C we have a pressure of approx. 0.34 MPa (3.4 bar) and it increases to approx. 0.54 MPa (5.3 bar) at 40 ° C. Therefore, each time refilling gas in the device, it is necessary to check the correct pressure value at a given temperature.
Figure 9.1. The dependence of SF6 pressure on temperature: A – condensation line (liquid-gas state line), B – gas density line, a – density line at the time of filling, b – signalling density line, c – blocking density line
The rated pressure of the switch is 500 kPa (i.e. 0.5 MPA at 200C). What should be the pressure when filling the losses at the temperature of 30C, and what should be at -300C?
We look at the line “a” and find that:
- at a temperature of 300C, the pressure is 0.52 Mpa (5.2 bar),
- at a temperature of -300C, the pressure is 0.4 Mpa (4 bar).
The above-mentioned issue of determining corrections of SF6 pressure values does not apply to pressure sensors installed in devices. They are designed in such a way that they are stimulated by a change in gas density.
Refilling with SF6 of devices with a pressure drop detected (e.g. sensor stage 1 triggered) is basically pure gas operation. This is so provided that when connecting (or disconnecting) the connecting line to the switch, we do not leak an unnecessary portion of contaminated gas from it.
When filling (topping up) SF6 devices, the following should be used:
- protective gloves,
- glasses (chemical type).
9.1.1. Collection and testing of SF6 samples.
Equipment manufacturers instructions should specify how often and when it is necessary to test a sample of SF6 gas, specify the method and equipment, and the limit values of contamination . The purpose of this check is to confirm that the gas condition ensures correct operation of the devices.
It is essential to obtain a sample representative for the gas in the device, therefore care should be taken not to take the gas sample through filters that may be installed in the device and to ensure that enough gas is transfused to obtain a representative sample.
Testing the state of gas from a power device can be performed using two methods:
- directly at the device – the so-called field technique, involving the use of portable analysers, it is a cheaper but less accurate method,
- laboratory method, with taking a sample to an intermediate vessel, allowing full examination of the content of impurities.
The dimensions of the intermediate cylinder should correspond to the volume of SF6 required for the laboratory test. It should be assumed that this volume should be 2 to 3 litres, as this amount is needed to perform a complete gas test, i.e. purity, composition, moisture content and acidity tests.
Pay particular attention to the cleanliness of the vessel and the sampling process. The contamination of the intermediate tank will add to the contaminants already present in the equipment being inspected. Therefore, the sampling cylinders cannot be used for substances other than SF6. They must be completely emptied and defecated after each use.
Taking gas samples for diagnostics from operating devices, especially switches, is treated as work with a limited risk of SF6 decomposition products. Use the necessary protective equipment for these activities and ensure effective ventilation of the room.
9.2. SF6 emission during normal operation of the power device
With the current leakage levels of devices – even if installed in a room, you should not be afraid of gas concentration in the air reaching the level of danger to personnel (1000 ppm). A general conclusion, also confirmed in international studies, states that the risk to staff health, even when the leakage is much greater than the permissible leak, is very small.
The standards define three types of SF6 gas-filled equipment:
- a) a controlled system that is automatically topped up from an external source (rarely used),
- b) closed system, which is periodically topped up from an external source by the service,
- c) sealed system, no refilling required, fully assembled and filled and factory tested.
Equipment leakage can in principle be considered only in cases a and b (mainly b), as the leakage of the c system apparatus leads to the apparatus malfunction and the necessity to withdraw it from use.
We already know that gas leaks should be limited for the following fundamental reasons:
- due to the operating conditions of the devices; the switching capacity and insulation properties decrease when the gas density decreases as a consequence of leakage,
- personnel safety,
- negative impact on the climate.
For these reasons, manufacturers are improving their sealing systems and now the tightness of SF6 equipment is determined by the number of years 10, 20 or 30 – which corresponds to a tightness of 0.1%. Medium voltage devices have a gas working pressure not much higher than the atmospheric pressure and here the leaks are definitely limited.
The hazard to personnel posed by SF6 leaks depends mainly on the state of the gas, its concentration in the atmosphere and the place where the equipment is installed.
Gas leaking from devices in which it has not been subject to thermal decomposition is not contaminated (most often in switchboards and bus bars) and there is practically little risk of exceeding the hazardous state. On the other hand, leaks from devices with contaminated gas (e.g. switches) pose a certain risk. Fortunately, SF6 contaminants are perfectly smelled (hydrogen sulphide – the smell of rotten eggs), already at the level of 1 ¸ 5 ppmV, i.e. below the TLV for this situation.
Under the contamination conditions of SF6, its concentration in the room, determined by the TLV coefficient, is 1.6 ppmV for SOF2 and 200 ppmV for SF6.
TLV concentration is for long-term operation. Therefore, it is worth knowing that the temporary exposure of SOF2 at a concentration of up to 500 ppmV does not pose a health risk. This means that manipulation can be performed under these conditions, but not longer work.
If the operator notices a perceptible smell of hydrogen sulphide, do not perform any work in the room until it is effectively ventilated. Then find the place of the leak and remove it if possible.
In principle, ventilation should be provided in switchgear buildings, although experience has shown that natural ventilation prevents the accumulation of gas escaping from the equipment due to normal (but not emergency) leakages.
Gas may accumulate in rooms below the switching station – in cable ducts, cellars, etc. Before entering them, they should be ventilated intensively as a precaution.
Of course, the gas concentration values in air given above can only appear in the case of indoor units. Overhead devices pose practically no toxic hazards to the personnel during normal operation, due to the dilution of the gas in the atmosphere. It does not mean, however, that under these conditions gas leakages can be allowed at a higher level than from devices installed indoors. It is a threat to the climate (see Chapter 5) due to the long life of SF6 in the atmosphere.
Summing up, it can be stated that operation in conditions of normal gas leakage from SF6 devices does not pose a threat to the operator – even if it is a leak from devices in which gas decomposition occurs (e.g. switches).
9.3.Working with contaminated SF6.
Working with contaminated SF6 takes place when it is carried out in contact with a gas that has been inside the housing of the distribution device for some time and therefore may be partially decomposed or contain impurities. It is necessary to work with contaminated gas in the following situations:
- gas filling a closed pressure system in which electrical discharges took place,
- when taking samples from devices as above,
- removal and replacement of gas during maintenance, repair or expansion of switchgear equipment,
- total or partial gas emission in an emergency,
- disassembly of electrical power equipment associated with their decommissioning.
Work with contaminated gas may only be carried out by personnel thoroughly acquainted with the properties of SF6 decomposition products, aware of the health hazards and informed of the necessary safety measures to be taken to minimize the risk. They must be trained in first aid.
When there are significant amounts of SF6 decomposition products in the atmosphere, this can be recognised by an unpleasant odour (hydrogen sulphide) and irritation of the upper respiratory tract. These symptoms may occur within seconds after contact occurs (e.g. leakage). In such a situation, the personnel should immediately leave the room and wait, preferably in the fresh air, after taking appropriate actions – e.g. airing the room, starting ventilation and cleaning the atmosphere.
When SF6 breakdown solids and adsorbents or a vacuum cleaner bag are removed, workers must be aware that they contain adsorbed gaseous products that may be released from them, and therefore take appropriate precautions against this.
When the housing of a device containing SF6, which has been operating for a certain period of time in the energy system, is opened, we should remember that it contains powdered decomposition products (metal fluorides).
When contact with SF6 decomposition products is unavoidable, use personal protective equipment!
The Regulation of the Minister of Development and Finance of December 7, 2017 on the minimum technical equipment appropriate for the performance of activities covered by the certificate for personnel in the field of fluorinated greenhouse gases and controlled substances, specifies the following required personal protective equipment for personnel:
- a protective suit covered with a waterproof layer, without pockets, with a clasp on the wrists and legs,
- covers for footwear (PVC or neoprene) or safety boots,
- disposable (nitrile or neoprene) or industrial gloves,
- industrial protective glasses (chemical type),
- half masks or protective masks equipped with FFP2 dust filters and FFE 1P2 acid compounds absorbers, in accordance with the standard introducing the PN-EN 14387 + A1 standard and the standard introducing the PN-EN 149 standard, used for short-term inspection,
- protective equipment used in the event of opening the equipment with fluorinated greenhouse gas SF6 and removing from its interior powder products of decomposition of fluorinated greenhouse gases SF6:
- a high-performance industrial vacuum cleaner, designed to collect non-explosive dusts posing a health hazard, equipped with a filter adapted to catch particles with a size of 1 µm and a hose ending with a non-metallic nozzle, and automatic closure of the container after its filling,
- double-layer plastic bags for storing used vacuum cleaner bags and used disposable personal protective equipment,
- preparations for the neutralisation of powder decomposition products of SF6 fluorinated greenhouse gases, containing sodium carbonate, sodium bicarbonate or slaked lime,
- plastic containers for the storage of harmful waste, including bags.
Personnel working in conditions of contact with contaminated SF6 must have access to a wash basin (shower) with hot and cold water. Prerequisites are needed for the preparation of appropriate neutralising solutions. The area where the works are performed should be marked. Information that smoking is prohibited is mandatory.
Certain precautions must be taken when opening the enclosures outdoors. You have to take into account that:
- wind can cause solid decay products (in the form of light, loose powders) to float around equipment,
- rain or high air humidity can accelerate the hydrolysis of compounds which can lead to the formation of HF.
Therefore, the powders should be removed (with a special vacuum cleaner) immediately when opening the housing. For example, when opening the circuit breaker, you should first unscrew the bolts of the lower flange of the chamber, slowly lift the cover with a crane and simultaneously collect the volatile powder. Working in the rain is highly advisable!
Work in rooms should be carried out with effective ventilation turned on, so that the permissible concentration of SF6 and its decomposition products in the air is not exceeded. The concentration level should be monitored all the time. Always use appropriate respiratory protection equipment when opening the case.
In the event that no gas diagnostics has been performed before opening the housing, it should be expected that the state of the gas depends on the type of switchgear, energy and type of electrical discharges (arc, spark discharges, partial discharges) and the concentration of contaminants in SF6 is as follows:
- enclosure without extinguishing chambers and not connected with other enclosures of switches (disconnectors), gas – zero to low concentration, solid products – no or a small amount,
- casing containing or connected to the circuit breaker extinguishing chamber, gas – average concentration, solid products – the number depends on the number of connections and the current value,
- casing inside which an internal arc was created, interrupted by switching off the short-circuit by the switch (the safety diaphragm has not worked – the so-called ejector), gas – high concentration difficult to determine, solid products – a very large amount, composition depends on the materials that are in the casing,
- internal arc with diaphragm activation, gas – a mixture of air and gaseous products, may be hydrolysis, which leads to a large amount of acid compounds, solid products – a large amount, some of them may have been blown out and polluted the air.
When organising the work of disassembling the SF6 device, you should be prepared for gas evacuation from the enclosures. Before removing the used SF6 from the power equipment, it is recommended to take a gas sample and carry out tests to determine the degree of contamination (content of moisture, oxygen, acid compounds and hydrated fluorine compounds). This is to properly organise further activities and decide on the further fate of the gas (reuse, filtering, disposal).
One of the basic issues that arises for the switchgear personnel when it is necessary to evacuate SF6 from the device is the technology of this procedure. Currently, international recommendations say that the primary form in such cases is recycling and remanufacturing. In short, it comes down to recovering the gas from the device into the cylinder, assessing its condition, and then – depending on the condition – recycling it or reclaiming it and reusing it, or finally disposing of it.
Despite the proper use of protective equipment, there is some danger to workers when working with contaminated SF6, and especially to bystanders. Therefore, one must observe the principle that: “WHEN WORKING WITH CONTAMINATED SF6, ONLY PERSONS NECESSARY TO PERFORM A SPECIFIC ACTIVITY ARE ALLOWED IN THE ROOM!”.
The effects of SF6 breakdown products can cause various symptoms in people exposed to them. The susceptibility to irritation is to some extent an individual matter. The extent of the irritation will always depend on the exposure time and the concentration of pollutants in the air. The decomposition products of SF6 may cause irritation of the skin, eyes, mucous membranes of the respiratory tract, and at high concentration and long exposure – emphysema.
For multi-component gas mixtures, the toxicology distinguishes between three cases :
- each component acts in a different way or on a different organ, the effects of each factor are considered separately,
- factors affecting the same organs in a similar way, their influence is cumulative,
- the influence of one factor far outweighs the influence of the others, only this factor is considered.
For SF6, SOF2, its amounts and health effects are considered to assess overall toxicity. However, this compound is hydrolySed, therefore:
- for long exposure times, the products of this reaction should be taken into account, i.e. HF and SO2,
- for short times or when strong ventilation is turned on, the hydrolysis phenomenon can be avoided.
Since the cumulative effect of SOF2, HF and SO2 is a greater threat, the advisability of effective ventilation of rooms can be seen from the last point of view. The TLV threshold limit for SOF2 is 1.6 ppmV (5.66 mg / m3).
During low energy discharges (e.g. partial discharges), the most toxic decay product of SF6 is S2F10, therefore, despite its small amounts, its effect on health is considered. Other relationships that may arise under these conditions are not considered. Unfortunately, the influence of this compound on the human body has not been fully studied so far.
Workers carrying out work involving exposure to gas decomposition products must be trained in first aid. Especially in the following cases:
- skin irritation – anyone suffering from such symptoms should be immediately removed from the room where the work is taking place, take off the overalls and outer clothing and wash the irritated areas with cold running water, if irritation persists, consult a doctor,
- eye irritation – a person suffering from such symptoms should be immediately removed from the room where the work is taking place, immediately rinse the eyes with clean water for at least 15 minutes (possibly with an eye wash) and immediately consult a doctor, informing about the cause of the irritation, in acute cases, call an ambulance,
- breathing problems – a person suffering from such symptoms should be immediately removed from the room where the work is taking place, take off overalls and outer clothing, cover with a blanket and be constantly observed, call an ambulance, if breathing becomes weak, start artificial respiration.
9.4. SF6 emissions to the atmosphere
By emission of SF6 gas to the atmosphere we mean its leakage during a failure in the following circumstances:
- large leak, caused by mechanical damage to components or seals,
- internal short circuit with an arc accompanied by actuation of the safety diaphragm or melting of the device cover,
- an external fire that may lead to the leakage of the casing (damage to the seals, actuation of the diaphragm due to gas heating). the probability of this happening, with compliance with fire regulations, is very low.
A leak causing a significant outflow of gas may result in reaching the permissible limit value of pollutants concentration in the air, but it will only occur when the leakage level is exceeded several times compared to the permissible one.
The occurrence of such a leak may be indicated by:
- installed alarm devices,
- portable detector with high sensitivity, meters installed on the device,
- characteristic strong unpleasant odour (hydrogen sulphide).
Before taking any preventive measures, check that the permissible concentrations are not exceeded! Preferably by direct measurement of the concentration of the breakdown products, or indirect method – by measuring the concentration of SF6.
A greater risk is posed by the rapid outflow of SF6 together with its decomposition products during an internal fault. The causes of an internal short circuit are:
- electrical damage to solid insulation,
- mechanical damage to the insulation, causing e.g. a change in the field distribution,
- incorrect switching operation.
An internal short circuit causes a large increase in pressure in the housing. The value of this pressure depends on the amperage, arc voltage and duration of the short circuit, as well as the volume of the enclosure. The resulting pressure causes the diaphragm (ejector) to activate and large amounts of contaminated gas to flow out into the atmosphere. In rare cases, the local casing can be melted and gas can flow out through this hole.
Such a condition will cause serious threats to the personnel. They should leave the room immediately, as high concentrations of SF6 decomposition products and secondary compounds are immediately formed. Additionally, toxic compounds such as metal vapours, organic compounds, etc. are formed.
Such a situation does not necessarily imply a very high risk to health, as long as the exposure time is kept to a minimum.
In indoor switching stations, the staff should clean the room, using appropriate personal protective equipment:
- collect settled powders with a vacuum cleaner,
- then wash the surfaces using weak alkaline solutions,
- start ventilating the room.
It is recommended to measure the concentration of gases before starting renovation works.
A situation similar to an internal short circuit may arise due to a fire. The effects will depend on the intensity, duration and extent of the fire and, consequently, the extent of the damage to SF6 equipment.
A fire in the switchgear room can increase the risk of decomposition of SF6 in the flames, so the clean gas escaping from the equipment immediately becomes a decomposing gas. Use personal protective equipment when extinguishing a fire! If the fire brigade is called to extinguish the fire, it should be notified at the time of the call about the risk of toxic chemicals.
After the fire is extinguished, the actions described for the case of an internal short circuit are performed. Appropriate measures are taken as appropriate to the situation that arises.
The final hazard of decomposition products is the decommissioning of equipment filled with SF6. In such a case, measures related to environmental protection should be applied.
The condition of the gas in the device and the amount of powders depend on the accumulated energy of the arc (or discharges). It depends on the place of installation in the power grid and on the “connection history” (number, frequency, value of currents). In most cases, even in circuit breakers, the amount of powders is small. According to , a typical MV circuit breaker after 10 years of operation contained gas contaminated by the following compounds:
- air: single ppmV,
- CF4: 40 60 ppmV,
- SOF2: trace amount,
- SO2F2: trace amount.
The reasons for the low concentrations of pollutants are as follows:
- high currents (especially boundary currents) are very rarely connected in operation,
- adsorbents installed in the device fulfil their function. Optional measures may be taken when decommissioning equipment containing SF6:
- work is entirely performed by a specialised subcontractor (contractor), usually this method is applied to small-sized devices, safety rules must be followed during transport,
- gas is removed by the user. Further works are carried out by a subcontractor, they are used for large devices where it is necessary to reduce the gas pressure or partially disassemble for transport. After delivery to a specialized company, the device is renovated or decommissioned in accordance with the principles of environmental protection,
- work carried out entirely by the user, the user is fully responsible for organizing the appropriate work technology from the point of view of personnel safety and environmental protection.
Table 9.4.1 Quantities of SF6 breakdown products expected 
|Distribution device||Expected degree of contamination (gas, powders)|
|Bus bars||Small, zero to tenths of a percent; minimum amount of powders|
|Cable connections||As above|
|MV disconnector, MV switchgear for a ring network, MV circuit breaker or HV circuit breaker||medium up to several percent,
|The housing in which the electric arc was created||high (more than 5%)
a large amount of powders
When starting the disassembly, you should have a room equipped with:
- exhaust ventilation at the floor and supply ventilation at the top, ensuring complete air exchange in the room within an hour,
- gas recovery equipment,
- gas cylinder,
- chemical type industrial vacuum cleaner,
- plastic containers for parts and used materials (filters, cleaning cloth, etc.),
- neutralising fluid,
- a source of running cold and hot water,
- a changing room for employees performing work,
- boards informing about the ban on smoking and open fire.
Workers disassembling devices must use appropriate personal protective equipment (chapter 9.3), especially to protect the skin and eyes against high concentration powders and liquids, and respiratory tract.
Compressed air must not be used to remove gaseous and powdered SF6 decomposition products – they will spread and float.
Dismantling begins with the housing. Its further handling depends on the type of device and the amount of decomposition products:
- low concentration (e.g. bus bars, switchgear bus bars), no special measures are required, elements are segregated for reuse and scrapping,
- medium concentration (e.g. MV and HV switchgear without a switch), elements of the housing should be neutralised within 1 hour, then rinsed with running water, dried and closed in plastic bags,
- high concentration (e.g. switches, disconnectors), when slowly lifting the housings it is necessary to collect the powders with a vacuum cleaner, neutralise the housing within 1 hour, then rinse with running water, dry and close in plastic bags.
Adsorbents and bags from vacuum cleaners must be put into appropriate containers and subjected to the neutralisation process in time T2 (table 9.4.2).
Sorbents and vacuum cleaner bags must not be removed by incineration (harmful dusts, vapours and fumes are released).
Parts removed from the inside of the device should be immersed in a neutralising solution for 1 hour (with low concentration of decomposition products) or for T2 time (with high pollution), and then carefully cleaned, rinsed, dried and sealed in foil.
After the work is finished, the clothes should be immersed in the solution for 1 hour, then washed and dried (or removed).
These clothes (including shoes) and tools should only be used for such work (with decomposition products). They cannot be used for other purposes.
Tools should be washed in a solution, rinsed and dried.
For the neutralisation of SF6 breakdown solids, aqueous solutions meeting the following recommendations should be used:
- neutralising liquid should not cause corrosion,
- the liquid should be sufficiently alkaline so that there are no unneutralised acid compounds,
- should not be excessively alkaline so that it is not difficult to dispose of in accordance with the regulations.
In PN-EN IEC 60480, three different compounds are given for neutralisation and washing, with slightly different properties (see table below).
Table 9.4.2. Compounds and their concentrations for the neutralisation of SF6 decomposition products according to PN-EN IEC 60480
|Relationship||Chemical formula||Concentration kg/100 l||T1 [h]||T2 [h]|
|* Be careful and avoid contact with skin and eyes.
** Recommended for skin cleansing.
Neutralised powder decomposition products (vacuum cleaner bags, collected from the surface of the elements) and liquids after neutralisation should be disposed of in accordance with the local regulations in force, i.e. handed over to the institution authorised to collect this type of waste.
10. Recycling and regeneration SF6.
The issues of recycling and regeneration of SF6 gas are discussed in this chapter based on PN-EN IEC 60480 “Requirements for sulphur hexafluoride (SF6) and its mixtures for re-use in electrical equipment” and Regulation (EU) No. 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases.
In the past, minimising equipment leakage was paramount. Progress in the field of tightness of SF6 power apparatus resulted in limiting leakages to a value of 0.1% per year. There are also hermetically sealed devices that do not require any maintenance (topping up) during the entire period of operation. Now, the most important issue is to prevent the deliberate release of SF6 from equipment into the atmosphere – during maintenance, servicing or decommissioning of equipment, and the ecological management of closed-circuit SF6 gas, which includes gas recycling and regeneration.
The term regeneration should be understood as full cleaning of SF6 gas in order to restore its original gas parameters, which is confirmed by laboratory tests and a gas quality certificate. This procedure must be in accordance with the standards, procedures and involve the use of appropriate equipment. The benefits are as follows:
- lowering the cost of using the device,
- compliance with public policy in the world to avoid greenhouse gas emissions,
- demonstration of a voluntary pro-ecological effort,
- activities in line with the standards.
In recent years, the need to recycle and regenerate SF6 has become recognised, thanks to the creation of clear standards and regulations containing a wide range of recommendations and procedures for the recovery, recycling and regeneration processes. There are also realistic recommendations for cleanliness standards for reclaimed SF6 to be reused in a power plant.
To outline the recommendations for recycling and remanufacturing procedures, it is necessary to define a few terms used in the text:
- recovery – collection and storage of fluorinated greenhouse gases from products, including containers, and equipment during maintenance or servicing or before disposal of products or equipment,
- primary substance – a substance that has not been used previously,
- recycling – means the reuse of a recovered fluorinated greenhouse gas following a basic cleaning process,
- regeneration – means the reprocessing of a recovered fluorinated greenhouse gas in order to achieve the operational properties of the original substance, taking into account the intended use,
- destruction – means the process by which all or most of a fluorinated greenhouse gas is permanently transformed or decomposed into one or more stable substances that are not fluorinated greenhouse gases,
- regenerated SF6 – sulphur hexafluoride, which has undergone a regeneration process,
- reuse – the use of reclaimed gas to refill the SF6 power equipment.
Many design features of SF6 equipment contribute to the successful application of gas recycling and reclamation:
- removal (adsorption) of decomposition products and moisture by internal absorption devices, keeps gas contaminants at low levels, which greatly facilitates gas purification during recycling and regeneration,
- minimising gas volume and pressure reduces the amount of gas to be recycled and regenerated,
- division of the device into sealed compartments, reduces the amount of gas that must be recycled and regenerated, especially in the case of internal arcs – when you need to deal with severely contaminated, contaminated SF6,
- special fittings in the SF6 gas system and self-sealing valves prevent gas losses or air contamination caused by improper handling of the device.
10.1. Gas pollution and its effects
The issue of the formation of contaminants in sulphur hexafluoride has already been discussed in chapters 3 and 4. There, attention was paid mainly to gas decomposition as a result of normal (switches) or emergency operation of devices, and then to the threat that these pollutants pose to the operator. Since the state of the gas is critical to the recycling process, it is worth discussing briefly
SF6 pollutants that can be generated in power equipment come from six main sources, namely:
- gas service,
- contaminated surfaces of: housings, structural elements, adsorbing devices,
- SF6 decay products as a result of electrical discharges,
- secondary chemical reactions,
- mechanical generation of dust particles in the device.
Air left in the sheath during the vacuum operation prior to SF6 filling and introduced during equipment filling and refilling, as it remains in the lines and valves, may be involuntarily added to SF6. This is usually the result of operating errors or the use of malfunctioning equipment, and can be of high contamination.
The amount of air (and even dust) that is introduced in this way can be reduced by:
- appropriate design of pipes and valves,
- following the appropriate operating procedure,
- Thoroughly pumping air out of the enclosure before filling the SF6 equipment (e.g. below 1 mbar).
Gas contamination by leakage is the result of air and moisture penetrating (diffusing) into the pressure shields from the outside, because the partial pressure of air and moisture (water vapour) outside the shield is higher than inside. The main leak paths can be: sheath porosity (osmosis), seals of moving parts, and O-rings. Osmosis through metal and insulating elements is in practice insignificant because the air and water vapour diffusion coefficients in these materials are very low. Of greater importance are the “paths” through the seals.
Significant amounts of contamination can be expected in devices in which, as a rule, there is a switching arc. In circuit breakers, the arc during high current shutdown causes erosion of contact materials and nozzles as a result of the action of the hot arc plasma.
The main cause of decomposition, decomposition of SF6, is the reaction of these material erosion products with fragments of thermally dissociated SF6 and other trace gases such as air and water vapour. The most important of these reactions are described by the following summary formulas:
Cu + SF6 → CuF2 + SF4
W + 3SF6 → WF6 + 3SF4
CF2 + SF6 → CF4 + SF4
In switchgear, adsorption in filters (for this purpose) is the dominant mechanism for removing decay products. The other two processes are relatively immaterial from a quantitative point of view. Due to adsorbents, only in rare cases, when often very high currents are switched off, a high concentration of pollutants can occur in a short time. After a few hours or days, the condition improves as determined by the duration and effectiveness of the adsorption. Internal arcs are the result of breakdown of solid insulation or incorrect tripping by switchgear and are extremely rare.
Moisture and air are adsorbed on the inside surfaces of the housings and on the surfaces of the components before assembly, especially as residue after cleaning. Polymeric materials contain moisture inside and turn out to be the most important source of moisture in the gas. Adsorptive devices that are not properly handled (activated) can contain both moisture and air and adsorb SF6 breakdown products, which in turn can be released during the evacuation process (vacuum) or at elevated temperatures. The quantities of adsorbed substances are difficult to estimate as they depend on specific materials use, production methods, quality control, and installation and filling, maintenance and handling procedures.
The contamination of SF6, due to electrical discharges, was discussed in detail in Chapter 4. So now, only a few sentences to summarise this issue. We already know that SF6 is partially broken down by electrical discharges, which can be grouped into four main types:
- incomplete corona discharges,
- spark discharges,
- arcs when switching off currents,
- internal arches.
The former, which occur only in the case of defects and faults in the insulation, introduces relatively small amounts of contamination. The second type of discharges occurs with a large number of insulation defects or during switching operations. A similar type of decay product is then produced as in corona discharges, but their number and composition are different. For example, in disconnector compartments they are very small because these units rarely operate and break only small capacitive currents. The higher amounts can only be accumulated with severe insulation deficiencies resulting in permanent spark discharges and where the compartment with such a deficiency is not equipped with an adsorber.
Table10.1.1 Pollution SF6
|Contamination||Main source||Destructive effect on||Tolerable level of contamination in the device|
|Service, arc extinction||switching off,
|Moisture||absorption from the surface of casings and polymers||surface insulation by liquid condensation||200 ppmV|
|SF4, WF6, SOF4, HF SOF2, SO2, SO2F2||arc discharge, partial discharge, secondary reactions,||insulation surface, toxicity||50 ppmV,|
|CuF2, WO3, WO2F2, WOF4, AlF3||contact erosion in switchgear, internal discharges,||toxity||Non critical*|
|Coal, metal dust||carbonisation of polymers,
|insulation surface, gas insulation||low*|
|Oil||Pumps, lubricants||insulation surface||10 ppmv|
|* cannot be quantified|
In these cases, the arc most often ignites between metal parts that are not arc resistant, such as aluminium, copper and steel. The materials are subject to very high arc erosion. The concentration of SF6 contaminating products in such cases can reach high levels (up to several percent of the gas volume).
The mechanical generation of dust particles is mainly from contacts. In a properly designed fastener, the abrasion metal particles should fall into an area where they have no effect on the condition of the insulation. However, if they fall into the area of a high voltage electric field, such as the surface of an insulating barrier, they may cause a jump over the insulator surface and ultimately lead to an arc discharge. These particles must always be removed efficiently in the recycling process.
The effects of the SF6 impurities discussed below can be summarized in the following points:
- risk to health and the environment,
- corrosion of materials,
- deterioration of the insulating strength of the contact gap,
- deterioration of the surface strength of the insulation,
- deterioration of the switching capacity of apparatus,
- change of heat dissipation.
Table 10.1.1. presents, according to , the overall picture of the most important pollutants, their sources and destructive effects.
10.2. Recycling and regeneration of SF6.
As defined in Regulation (EU) No 517/2014 of the European Parliament and of the Council, recycling of fluorinated greenhouse gas is a basic treatment process, while regeneration is a complete process consisting in restoring the original properties of a given substance. In practice, the basic differences between the recycling process and the regeneration process are presented in Table 10.2.1.
Table 10.2.1. Recycling and regeneration
|basic purification process||full purification process, restoration of the original properties of SF6|
|performed in the place where the device is installed||performed in a specialised company|
|requires the use of only replaceable filters||requires the use of replaceable filters and cryogenic technology to remove non-reactive gases|
|no need to check gas quality||gas quality confirmed by a quality certificate|
|the possibility of using recycled gas only in the same device from which it was recovered||possibility of using gas after regeneration in any device|
|it allows only moisture, solids and decomposition products to be removed from the gas||it allows moisture, solids, decomposition products and non-reactive gases to be removed from the gas|
|can be carried out by certified personnel servicing electrical power devices||it is carried out only by certified and qualified personnel in the field of cryogenic processes as well as technologists and lab technicians testing the gas in the analytical laboratory|
Pumps and filters (Table 10.2.1) for the recycling of SF6 are commercially available and have been used for a long time. They are attainable, obtainable and mobile. Currently, their costs are decreasing, the method of operation is improved, and the dimensions are reduced down to portable equipment, capable of handling smaller amounts of gas. With their help, it is also possible to treat heavily contaminated gas with decomposition products. The quality of recycled SF6 can be checked by commercially available equipment measuring SF6 purity, moisture content and some breakdown products and admixtures. It should be remembered that the gas after the recycling process can only be used in the same device from which it was recovered.
Table 10.2.1. Types of filters used for recycling and regeneration of SF6
|Particulate filter||Removes decay products and other solid contaminants.||Candle filter with large filter surface, 100% filtration is achieved for particles ≥ 1.0 μm.|
|Moisture filter||removes moisture.||Filter composed of Al2O3 aluminium oxide with a pore diameter of 20-50 Å, grain size 2-5 mm and a molecular sieve with a pore diameter of 4 Å, achieves dew point temperatures below -50 ° C after one drying process, can absorb up to 160 g of water .|
|Filter for gaseous decomposition products||Removes gaseous decomposition products||Filter composed of Al2O3 aluminium oxide with a pore diameter of 20-50 Å, a grain size of 2-5 mm and a molecular sieve with a pore diameter of 4 Å, the absorption capacity of the cartridge depends on the absorbed substance, for sulphur oxide (SO2) and thionyl fluoride (SOF2) it is equal to about 5-7 wt.%, meaning 30-40 g for each filter cartridge.|
|Oil filter||Removes oil||Oil adsorption with an activated carbon filter|
The regeneration process, in addition to mobile filters, requires the use of cryogenic technology, which is the only currently known way to remove non-reactive gases such as air and CF4 from SF6 gas. It is currently not possible to carry out cryogenic SF6 regeneration in the place where the device is installed, it is necessary to transport the gas to a specialized company and to carry out the regeneration by qualified personnel. After the regeneration process, the gas quality is checked in an analytical laboratory. Regenerated SF6 can be reused in any electrical device. It is an expression of ecological awareness and socially responsible management of SF6 gas with a closed circuit.
Refilling the device with gas after the recycling or regeneration process should take place after removing the air from the power device with the use of a vacuum pump (in accordance with the instructions of the manufacturer of the device). Then refill this device with gas from a cylinder or tank, using suitable compressors and hoses. Separate use of recovery and refilling hoses is to avoid additional contamination of the gas.
10.3. The required purity of SF6 after recycling and regeneration
The purity requirements that SF6 gas must meet after the regeneration and recycling process so that it can be reused are defined in PN-EN IEC 60480 “Requirements for sulfur hexafluoride (SF6) and its mixtures for reuse in electrical equipment”. The quantities are expressed (Table 10.3.1) in percent by volume (% V, ppmV) as these units have become customary for SF6 insulated electrical power equipment. These cleanliness requirements can be related to three different critical levels, namely:
- maximum levels of contamination in primary SF6,
- maximum levels of contamination in SF6 after recycling and regeneration,
- maximum tolerated levels of contamination in power devices.
The primary contamination levels of SF6 are given in the standard PN-EN IEC 60376 “Requirements for technical sulphur hexafluoride (SF6) and make-up gases for its mixtures used in electrical equipment”. The levels of SF6 contamination after recycling or regeneration are specified in the standard PN-EN IEC 60480 “Requirements for sulphur hexafluoride (SF6) and its mixtures for reuse in electrical equipment”. The maximum tolerated levels of contamination in devices are those above which the performance of SF6 insulated electrical power equipment may start to deteriorate, or above which health risks must already be taken into account.
Pollutants can be measured with inexpensive, portable measuring equipment. The measuring instruments in use can be equipped with sensors measuring SF6 purity, moisture, SO2, HF, CO and H2S content.
It should be emphasized that the parameters of SF6 gas can also be measured with much greater accuracy in an analytical laboratory by using technologies such as gas chromatography, ion chromatography, infrared absorption, gravimetry and photometry. They are not possible to use in the place of installation of the device, as they are relatively expensive and require highly qualified service. The parameters of the regenerated gas must be confirmed by tests in an analytical laboratory.
The required purities that have been determined for the reclaimed gas to be reused are defined in PN-EN IEC 60480 and are given in Table 10.3.1.
Table 10.3.1. Required parameters for a regenerated SF6 according to PN-EN IEC 60480
|SF6||> 97 % objętościowo|
|Air and/or CF4||< 30 000 μl/l (3 % objętościowo)|
|H2O||< 200 μl/l (200 ppmv)|
|Mineral oil||< 10 mg/kg (10 ppmw)|
|Acidity||< 50 μl/l (50 ppmv)|
|ppmv = parts per million by volume
ppmw = parts per million by weight
The required parameters for reclaimed SF6 are less restrictive than for virgin SF6 (table 10.3.2), however, for the moisture content and mineral oil, the permissible values are the same. Regenerated SF6 can, however, and is recommended to meet the parameters of the primary gas standard.
Table 10.3.2. Requirements for reclaimed SF6 according to PN-EN IEC 60376
|SF6||> 98,5 % by volume|
|Air||< 10 000 μl/l (1 % by volume)|
|CF4||< 4 000 μl/l (0,4 % by volume)|
|H2O||< 200 μl/l (200 ppmv)|
|Mineral oil||< 10 mg/kg (10 ppmw)|
|Total acidity||< 7 μl/l (7 ppmv)|
|ppmv = parts per million by volume
ppmw = parts per million by weight
10.4. Basic issues ofSF6 SF6 reuse
From an ecological and economic point of view, it is desirable to keep the contaminants in SF6 low so that multiple gas regenerations can be performed.
The same portion of SF6 should be used in product testing, installation, maintenance and repair. When the limits specified in Table 10.3.1 are reached in the equipment, the gas should be withdrawn from service. After regeneration, it should be handed over for use in a newly installed device. So it should be in constant use. Such continuous use of gas is possible thanks to the guarantee of a high level of its quality, so that it can fulfil its functions many times. This can only be achieved by proper gas handling and regeneration, confirmed by laboratory testing.
The regenerated gas must be quality checked before being recirculated to the electrical equipment so that the pollutants do not exceed the values specified in Table 10.3.1 and preferably the values specified for the primary gas specified in Table 10.3.2. Four levels of contamination must be checked, namely:
- purity of SF6,
- total level of gaseous non-reactive pollutants (air and CF4),
- moisture content (H2O),
- total acidity level.
In addition, PN-EN IEC 60480 indicates that the content of potential pollutants, such as H2O and CO, does not have to be tested due to the lack of sufficient tests and data at present. The content of mineral oil also does not need to be monitored due to the fact that only oil-free SF6 gas management devices are available on the market, hence it is impossible to contaminate the gas with this substance.
There are four basic ways to perform a gas quality control in a power appliance:
- constant monitoring of all pollutant levels in the gas tank or in the gas stream in the pipe and alarm if one of them exceeds the specified limit of purity requirements,
- periodic control of contamination levels in the storage tank, using portable or permanently installed sensors,
- gas check after refilling the device – verification of cleanliness requirements after refilling the electric power device with gas. However, with a post-fill check there is some risk that too much contamination, if any, may only emerge when it has caused some damage to the power device. A gas containing too much moisture may excessively wear adsorbent devices, and moisture may condense on the inside surface of the enclosure,
- gas sampling and laboratory analysis – this is the best and most reliable method for detecting contamination levels, but the drawback of this method is a significant delay in time, as well as the need to precisely collect the required amount of gas to obtain a representative sample.
10.5. Transport regulations for SF6.
The regenerated sulphur hexafluoride can be stored at the regeneration site or it can also be transported to other locations for reuse. This requires appropriate regulations for the storage and transport of contaminated and reclaimed gas.
The classification of SF6 gas transport depends on its history – whether it is a primary gas, recovered from equipment or after a regeneration or recycling process.
Primary SF6 – Tanks for the storage and transport of primary gas must comply with national legislation on pressure vessels (ADR 2.2).
Regenerated SF6 or recycled and suitable for re-use in power equipment – for this category of gas, the contamination levels must comply with the purity requirements for re-use in power equipment (PN-EN IEC 60480). It can be stored and transported like virgin SF6, but cylinders should be labelled “Reclaimed / Recycled SF6 for reuse in electrical equipment”. Proposed transport category as for primary SF6.
Recovered SF6 – recovered SF6 is treated as hazardous waste (waste code 16 05 04 *) and transported in this way. It must be classified as toxic and assigned to one of the dangerous groups defined in the regulations. The cylinder must be labelled “Recovered SF6, only transported for analysis, destruction, remanufacturing or recycling” (ADR 2.3 and 8).
10.6. Final liquidation of SF6.
In the event that SF6 cannot be regenerated or recycled, it can be destroyed by thermal processes.
As defined in Regulation (EU) No 517/2014 of the European Parliament and of the Council, decommissioning is a process by which all or most of the fluorinated greenhouse gas is permanently transformed or decomposed into one or more stable substances that are not fluorinated greenhouse gases.
SF6 heated to a temperature above 1000 ° C begins to dissociate, decomposition of gas molecules into sulphur and fluorine atoms, which then undergo ionization. Thus, SF6 gas can be destroyed when thermal processes take place at temperatures above 1000 ° C.
10.7. Recommendations for handling SF6.
A careful reading of this chapter suggests general recommendations for manufacturers and users of electrical power equipment:
- the deliberate release of SF6 into the atmosphere should be avoided,
- SF6 must be recycled and reclaimed (circular economy).
The producers should be expected to implement the following demands:
- manufacturers should inform about the possibility of using recycling and regeneration of SF6 and other materials used in the production of the device,
- producers should define the conditions under which gas can be reused,
- the manufacturer should encourage, as far as possible, the re-use of gas and provide users with appropriate instructions,
- very high sensitivity leak monitoring devices should be used,
- Records should be kept of gas sales, supplies from a gas producer, shipments to customers, and gas returned to the factory from customers and specialist recycling companies.
Recommendations for users of electrical power devices can be summarised in the following points:
- users should conclude agreements with specialist gas regeneration companies,
- SF6 power equipment should be operated and maintained in accordance with the manufacturers’ instructions and the rules of environmental protection,
- SF6 power equipment should be repaired if the gas leakage exceeds the permissible values,
- Records should be kept of all gas related work (including the amounts of SF6 used in the individual work).
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