All about sulfur hexafluoride
Sulphur hexafluoride (SF6) has excellent insulating properties and effectively extinguishes electric arcs in equipment commonly used in the power industry. However, despite its advantages, SF6 has a significant drawback — it is a greenhouse gas with a very high Global Warming Potential (GWP). Therefore, ensuring the tightness of electrical power equipment and improving the qualifications of personnel working with sulphur hexafluoride are essential. Activities in this area have been regulated for years by the European Commission’s regulations, which aim to reduce SF6 emissions and promote good practices among professionals in the energy sector.
SF6
In the early 1970s, leading switchgear companies worldwide developed the production of high-voltage gas-insulated switchgear using sulfur hexafluoride (SF6), as well as high-voltage circuit breakers in which this gas served as the arc-quenching medium. Later, there was a rapid development of this switchgear technology, which can be explained by the enormous possibilities that opened up for switchgear designers after discovering the excellent properties of SF6.
Designers had long tried to build prefabricated gas-insulated switchgear for high voltages (above 110 kV) based on switchgear for low (up to 1 kV) and medium voltages (below 110 kV). Attempts to build switchgear with solid (resin), oil, or compressed-air insulation did not yield satisfactory results. Prefabricated switchgear components with solid insulation of the required dielectric strength were heavy and unreliable due to cracks in large resin castings. Oil-insulated switchgear was hazardous in terms of explosion and fire and also heavy. Air-insulated switchgear was tested, but this technique did not find wider application due to the need for high pressure (about 6 MPa) and the associated requirement for mechanically strong enclosures.
Only a combination of solid insulation as support elements and SF6 gas as the main insulation met designers’ expectations and enabled the widespread construction of compact switchgear for the highest and medium voltages, in which busbars and all apparatus were enclosed in sealed metal housings.
SF6 switchgear occupies 10–20 times less floor space than conventional switchgear. The difference in volume is even greater, which is particularly important for indoor installations. SF6 switchgear is unaffected by chemically polluted, dusty, saline, or acidic mist atmospheres. Lightning strikes pose no danger. They are safe in terms of explosion and fire. Operation and maintenance are simplified due to safe, grounded enclosures of live parts and many technical safety systems. Despite widespread global use, there is no literature reporting serious accidents causing death or serious injury to personnel.
The high safety of SF6 switchgear results from the fact that this type of equipment can operate without human intervention, and live parts are inaccessible. Poisoning of personnel by SF6 decomposition products is practically unlikely, mainly due to low concentrations of toxic compounds, their noticeable odor, and the natural reaction of personnel to leave the area in the event of a sudden enclosure leak.
The failure rate of SF6 gas-insulated switchgear is much lower than that of open switchgear. Literature indicates that the relative number of serious failures in gas-insulated switchgear is about six times lower than in conventional switchgear. Gas-insulated switchgear also causes significantly fewer operational issues than traditional switchgear. Failures can be divided into two groups: failures similar to conventional switchgear (independent of design, e.g., mechanical drive or control failures) and failures specific to SF6 switchgear (e.g., solid insulation issues, enclosure leaks). The occurrence of the first type is roughly the same in both types of switchgear. Statistics for failures specific to SF6 switchgear show very low relative numbers. However, repair of such failures can be difficult and time-consuming, making failure rate comparisons less favorable. Failures in SF6 switchgear have more serious consequences for power system operation due to complex disassembly, repair, and reassembly, leading to relatively long restoration times.
In early SF6 switchgear designs, due to uncertainty about seals, devices for automatic gas replenishment were used when losses fell below a set level. However, construction solutions and sealing systems soon ensured sufficient tightness, allowing the elimination of automatic replenishment. Today, annual gas losses do not exceed 0.1%.
The first SF6 switchgear appeared worldwide from 1965. By 1974, all manufacturers (estimated at 20–25 companies) produced about 2,000 units. Initially, high prices limited demand for this type of equipment (cost of novelty, research). Later, total investment costs of SF6 switchgear were competitive compared to conventional (indoor) solutions. Investors considered the reduced space requirements and the possibility of architectural integration into urban or industrial areas. SF6 switchgear with cable connections can be installed underground, beneath squares or plazas. Assembly time is reduced since switchgear is delivered in large units (e.g., bays). Operating costs are significantly reduced due to fewer maintenance actions, inspections, and lower staffing needs, while reliability is greatly increased.
The growing demand for electricity requires the expansion of power networks and the introduction of high-voltage lines into urban areas and industrial plants. In many cases, SF6 gas-insulated switchgear is the only feasible solution for voltages of 123 kV and above in specific built environments.
Alongside advancements in SF6 insulation technology and switchgear construction, work progressed on using SF6 in circuit breakers for arc quenching. It is unlikely that a better gas will be found in the near future for use as an arc-quenching medium than sulfur hexafluoride. SF6 allows much simpler breaker construction compared to pneumatic and low-oil designs. At the same time, the apparatus has higher reliability and a longer service life. A characteristic of SF6 is that even when interrupting small currents, there is no sudden “arc blowout” and, consequently, no dangerous overvoltages. Another advantage is the rapid recovery of dielectric strength after arc extinction, allowing interruption under high voltage rise rates. SF6 also requires lower pressure than pneumatic breakers, allowing the use of self-pressurized or self-generating pressure breakers. Arc chambers are relatively simple in design.
However, SF6 technology also has drawbacks. SF6 is more expensive than compressed air, requires much tighter enclosures, and presents challenges during filling (high vacuum). Choice of insulating and construction materials for breakers is complicated by the aggressive decomposition products of SF6. Toxic compounds are formed in the breaker, posing a hazard to personnel, especially during maintenance and dismantling. These drawbacks mean SF6 technology cannot be considered completely safe, though the risks should not be overly emphasized by critics.
Operation of SF6 equipment requires general and personal safety measures.
Interest in this technology in Poland began in the early 1970s at the Institute of Electrical Engineering. Research and design work led to the installation of a single-bay 123 kV switchgear with a 1250 A continuous current and a 25 kA circuit breaker at the Sulejówek testing station. Two 123 kV outdoor circuit breakers were also installed for testing (Mory, Gdańsk). Later, a 123 kV 31.5 kA prototype breaker was developed and tested, along with a five-bay 123 kV prototype switchgear—though the latter was not installed. Poland focused on producing a licensed EDF breaker and purchasing switchgear from foreign manufacturers. Years of research at the Institute of Electrical Engineering provided extensive experience with SF6 technology.
Long-term experience with SF6 in switchgear has shown no serious hazard to humans, provided appropriate precautions and procedures are followed throughout the equipment’s lifecycle. Personnel working with SF6 must be thoroughly familiar with decomposition products, aware of health risks, and informed of necessary safety measures to minimize risk.
Sulfur hexafluoride (SF6) is a synthetic gas obtained by reacting sulfur with gaseous fluorine. The molecule has an octahedral shape, with six fluorine atoms at the vertices and a sulfur atom in the center. In this compound, sulfur has its highest valence. This structure is the reason for the gas’s extraordinary stability and exceptional chemical inertness, as significant energy is required for its decomposition. SF6 decomposition due to temperature generally begins only around 500°C. However, in the presence of certain metals, especially metals and their alloys containing silicon, decomposition can occur at 180–200°C. SF6 is poorly soluble in water, slightly more so in alcohol. Pure gas does not react with hydrogen or metals, and reacts with oxygen only in the presence of electrical discharges. It is colorless, non-toxic, odorless, and non-flammable.
The molecular weight of SF6 is 146.06, and its density at 20°C and 1 bar pressure is 6.16 g/l, approximately five times that of air. It is thus one of the heaviest known gases.
The thermodynamic properties of SF6 are described by the Mollier diagram. The critical point of SF6 is at 37.46 bar and 45.58°C, which allows it to be liquefied by compression for transport and storage.
The application of SF6 in electrical equipment is due to its excellent electrical properties. It is known that the dielectric strength of gases depends on several factors: the mean free path of molecules, their cross-section, the occurrence of inelastic collisions, and the ability to capture and store electrons during these collisions. Electronegative gases like SF6 can capture electrons by forming negative ions, significantly increasing dielectric strength by slowing down free electrons.
The dielectric strength of SF6 exceeds that of air by 1.8–3.0 times depending on testing conditions. In a uniform field, it is about 2.4 times higher. At a pressure of about 3 bar, SF6 reaches 75% of the dielectric strength of insulating oil in a uniform field, and in a non-uniform field, it may even show better insulating properties than oil.
Sulfur hexafluoride cannot serve as the sole insulating material in a switchgear or circuit breaker — support and bushing insulators must be made of solid insulating materials. These materials operate in the SF6 atmosphere, so it is important to understand the effect of SF6 on solid insulating materials. It is necessary to differentiate between materials stressed only electrically in pure SF6 (e.g., busbar insulators) and those operating in breaker chambers, which are also exposed to SF6 decomposition products. Surface strength of these insulators in SF6 is particularly important. Most manufacturers use epoxy resins with special fillers for insulators, fully meeting electrical and mechanical strength requirements. Studies of samples coated with fluoropolymer layers (Teflon) showed a significant increase in surface flashover voltage in the gas. There are also other insulating materials that may come into contact with SF6 in electrical devices. These materials can show significant property variations depending on the manufacturer, even with similar compositions, so every solid insulating material intended for use in SF6 devices must be tested (especially during maintenance).
Many studies have been conducted to determine heat transfer capabilities in SF6. Although the molar specific heat of SF6 is lower than air, per unit volume, it is 3.7 times higher than air. The thermal conductivity of SF6, 1.26×10-4 W/cm×K, is over twice lower than that of air (2.86×10-4 W/cm×K), but considering convection, the heat transfer capability of SF6 is higher than air and approaches that of helium or hydrogen. This allows higher current density in conductors (e.g., busbars) in an SF6 atmosphere compared to air.
A separate issue is the thermal conductivity of SF6 at high temperatures during arc quenching. Studies show that SF6 dissociation intensifies at 2000–2100 K and completes around 4000 K, resulting in F and S atoms and a small fraction of diatomic compounds. Under these conditions, the ratio of specific heat to thermal conductivity can be considered constant. Tests under these conditions demonstrated effective heat removal from the arc, reducing its diameter and increasing arc resistance.
The use of SF6 in breaker chambers is due to its excellent arc-quenching properties. Early tests (1954, USA) showed that its quenching capability exceeds that of air by about 100 times. AC current interruption, especially at low power factors, depends more on the rate of recovery of dielectric strength than the cold gas dielectric strength. The rate of dielectric recovery depends on the thermal and electrical properties of the plasma, including thermal conductivity, temperature distribution, dissociation, voltage drop, arc power and energy, and the arc time constant. Studies in SF6 confirm the advantages of this gas regarding these factors.
The main quenching properties of SF6 are related to its dissociation. Dissociation starts at about 2000 K and occurs in stages at varying ionization energies, with dielectric strength increasing rapidly as temperature drops. The arc core conducts practically all current; outside the core, electron density and electrical conductivity are very low.
SF6 being electronegative also reduces free electron density, equivalent to lowering gas temperature by ~500 K.
A major advantage of SF6 is that the arc core disappears only at current zero, preventing premature core destruction. Arc column dissipation occurs abruptly 6–7 ms before current zero. This provides an advantage over air or vacuum breakers — there are virtually no overvoltages even when interrupting small currents.
SF6 must meet quality requirements regarding impurities (see Table 2.1), as these affect gas properties.
Table 2.1. Requirements for technical SF6 (PN-EN IEC 60376)
| Substance | Concentration |
| 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 |
|
Impurities must be limited so they do not jeopardize device operation. For example, water, acidic contaminants, and oxygen together may cause corrosion and improper operation. Water with acidic contaminants can condense at low temperatures and high operating pressures, threatening electrical safety. Impurity levels also affect the type and quantity of secondary chemical compounds formed during thermal SF6 decomposition (after an arc).
It is recommended to fill and top up devices only with verified SF6 gas, ideally from a single supplier.
Most often, the state of SF6 in the device differs from the state of the gas at the time of its filling. It contains impurities that appear at various stages of preparing the device for operation and use [34].
Impurities in the gas within the device are caused to varying degrees by:
- improper selection of the device’s construction materials, which may lead to desorption of moisture into the gas or the formation of impurities through secondary chemical reactions with decomposed SF6,
- factory assembly errors,
- errors during on-site installation,
- leakage of enclosures and errors in refilling losses,
- gas decomposition due to electrical discharges and switching arcs,
- chemical reactions occurring after discharges,
- operation of internal device mechanisms [31, 32].
Of course, the state of the gas in an operating device fundamentally depends on its intended function. It is different in the enclosed compartments of a shielded switchgear than in a high-voltage circuit breaker or another switching device (e.g., a disconnector). We will discuss these issues primarily based on the typical “product life cycle diagram” shown below, which is most often analyzed according to the ecological procedures of “Cleaner Production” [44].
In such a “product life cycle diagram,” attention is paid to how the state of impurities and associated hazards may be affected at all stages of the product’s existence: from production to disposal.
The device designer can influence not only the functional properties of the designed device but also the condition of the gas contained within it. Proper selection of construction and insulating materials is crucial. This involves both their chemical reactivity with SF6 (especially with decomposition products) and the elimination of porous materials – which absorb moisture and air before assembly and release these substances into SF6. Choosing an appropriate sealing design – with high efficiency and durability – limits gas losses and the possibility of introducing impurities during refilling [33, 34].
Table 3.1. Product life cycle phases and the state of SF6 impurities
| Design Phase |
|
| Production Phase |
|
| Operation |
|
| Disposal |
|
In the case of circuit breakers, optimizing the quenching chamber is extremely important. This involves reducing arc duration (decreasing the energy supplied by the arc) and limiting the gas to the necessary amount. The selection of the type and volume of adsorbent is also important – ensuring effective performance throughout the operational period. These issues should be resolved during the design tests of the device.
The manufacturer should ensure proper technology for producing components (surface smoothness) and dry, clean assembly at all stages. During assembly, contact of components with atmospheric moisture should be minimized, components prepared for assembly must be sealed in foil and stored in a dry room. At the final stage of assembling switching devices, the adsorbent is installed – its quality has a decisive impact on the subsequent state of the gas. In all SF6 devices, the state of the gas depends on excellent assembly tightness, achieving a high vacuum (drying the interior) before filling, and adherence to gas filling procedures.
The user has virtually no influence on the above quality assurance conditions (except for selecting the supplier). Their role begins with the installation and commissioning of the devices. Even if the installation is performed on-site by the supplier, the recipient should ensure proper storage conditions (as brief as possible) and supervise the assembly process. Gas refilling (according to procedures) and acceptance tests (according to acceptance conditions) must be performed correctly. Following the operating instructions of the device is crucial during further use. Proper compensation of gas losses – i.e., refilling – is very important for the durability of the device and the level of impurities in the gas.
High-voltage SF6 circuit breakers present a separate issue regarding the state of the gas. Regardless of technological impurities introduced into the interior of the circuit breaker, as with switchgear compartments, decomposition products and their secondary chemical compounds appear. During operation, it is necessary to ensure that the maximum breaking current and the maximum number of operations (according to the switching capability diagram in the operating instructions) are not exceeded. Gas decomposition due to electrical discharges and arcs is the main cause of toxic compound formation [35].
The primary method for checking the state of the gas – especially in circuit breakers – is to take an SF6 sample and perform diagnostics, preferably chromatographic. The inspection frequency should be determined by the device manufacturer. In principle, gas condition inspection is carried out every 1 to 5 years during periodic maintenance.
Monitoring the gas condition during operation should confirm that impurity levels do not exceed permissible values.
Table 3.2. Types and permissible amounts of impurities during gas operation according to PN-EN IEC 60480 [41]
| Substance | Concentration |
| SF6 | > 97 % by volume |
| Air and/or CF4 | < 30,000 μl/l (3 % by volume) |
| 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 |
|
If, during inspection, it is found that the impurity concentration exceeds the permissible level, a gas replacement procedure should be carried out.
The final phase – dismantling the device after full operation or damage – is the most critical stage in terms of personnel and environmental hazards, especially for the highest-voltage circuit breakers. This work should be performed by a specialized team under proper procedures.
Decommissioning a circuit breaker does not necessarily require dismantling the poles into parts at the substation. It is always necessary to reduce the gas pressure to a slight overpressure relative to atmospheric pressure. This should be done by pumping the gas into cylinders.
Maintaining slight overpressure in the circuit breaker poles prepared for transport (e.g., to the manufacturer) aims to prevent moisture ingress. Moisture causes:
- change of the nature of powdery residues from unbound deposits to sticky products adhering to the internal elements of the quenching chamber. In this case, more hydrolysis products also appear,
- formation of aggressive gaseous secondary reaction products that are corrosive to structural elements and highly toxic.
Sulfur hexafluoride retains its properties as an inert gas until it is exposed to thermal effects. This occurs during the normal operation of the switch (interrupting an electrical circuit, extinguishing the arc) and during emergency electrical discharges.
Interrupting a high-voltage electrical circuit is always accompanied by the need to extinguish the arc. In an SF6 switch, this usually occurs in a stream of compressed gas. Due to the high temperature of the arc, the decomposition of SF6 is unavoidable.
Studies of the electrical conductivity of arc plasma [1,6] show successive spikes: the first around 2,000–2,100 K corresponds to partial dissociation of SF6 and the appearance of free sulfur; the second, around 3,000 K, is attributed to dissociation of SF2 and SF3; the third, in the range 15,000–20,000 K, is associated with an increasing share of electrons. Practically, after exceeding about 4,000 K, SF6 is dissociated into F and S. This creates conditions for secondary chemical reactions inside the interrupter chamber.
Under the influence of the arc (and spark discharges), the main stable decomposition products of the gas may include: S, F2, SF2, S2F2, SF4, and S2F10, with SF4 being the most abundant. In the presence of traces of oxygen and water vapor (which also dissociate at this temperature), some decomposition products, e.g., SF4, form compounds like SOF2, and in the presence of metals, metal fluorides may form [40]. After the temperature drops below 1,000 K, atoms recombine intensively, forming various compounds, combining with metal atoms, plastics, etc. Gaseous and solid compounds such as CuF2, AlF3, WF6, CF4, SF4 are called primary compounds and form during and immediately after the arc discharge. After the arc is extinguished, atoms of sulfur, fluorine, oxygen, hydrogen, nitrogen, metals, and carbon recombine, forming mainly SF6 but also other compounds, most commonly: SOF2, SO2, HF, CF4, SF4, SO2F4. Low-energy discharges also produce S2F10 – a very toxic and hard-to-detect gas, though in small amounts [21].
Chemical compounds formed in switches are largely absorbed by internal chamber adsorbents (Al2O3, molecular sieves, NaOH + CaO mixture). The mass of the adsorbent is chosen to absorb all gaseous oxygen compounds and CF4, especially highly reactive SF4 and WF6 formed during switching cycles over the contact’s lifetime. Powdery products (about 2 mm in diameter), depositing on chamber surfaces, are mainly metal fluorides (e.g., CuF2, WO3). It has been determined that the amount of decomposed SF6 and generated SOF2 is proportional to the arc energy: 1 kJ of energy decomposes approximately 2.7 cm3 of SF6 and produces about 1.5 cm3 of SOF2 [15].
In compartments of a gas-insulated switchgear where switching processes do not occur, gas degradation should not happen. The only reason for SF6 decomposition here may be partial corona discharges caused by defects or insulation faults. They can occur locally in many parts of the switchgear at very low energy levels but over prolonged periods.
Partial discharges decompose SF6 mainly into two compounds – SF4 and F – which later react with traces of oxygen (O2) and water (H2O) to form compounds such as HF, SO2, SO4, and SO2F2. Higher molecular compounds such as S2F10, S2OF10, and S2O2F10 are also formed, but in very small amounts [21].
Due to the low energy and low intensity of discharges, the amount of decomposition products formed in devices is very low, on the order of tens of ppmV, at an SF6 filling pressure of about 500 kPa (higher than used in switchgears). Under normal operating conditions and with properly sealed housings, this does not pose a risk to personnel.
The largest source of SF6 decomposition products in switchgear is internal arc faults, accompanied by the release of large energy into the gas in a confined space until protection devices operate and interrupt the fault. This results in increased pressure, often causing release through a protective membrane or a melted hole in the housing. The chemical phenomena are similar to those in switching arcs but may involve additional reactions due to contact of hot ionized gas with metals and other materials not used in interrupter chambers. Gas escaping to the atmosphere also reacts with surrounding air containing water vapor, O2, and N2. The type and concentration of chemical products depend on construction, materials used, current intensity, arc duration, and time elapsed since discharge [3].
Post-fault conditions with gas and decomposition products released into the room pose the greatest risk to humans and require proper safety procedures.
To summarize, Table 4.1 provides an overview and general characteristics of SF6 decomposition products formed under different circumstances. It should be noted that the type and concentration of decomposition products depend on many factors – difficult to quantify [4].
Table 4.1. Approximate characteristics of most SF6 decomposition products in electrical devices [6, 8, 19]
| Source of products | Main SF6 decomposition products | Toxicity (estimated) | Reactivity with atmospheric moisture | ||
| Chemical formula | State | Amount | |||
| Hot contacts | SOF2 SO2F2 SO2 |
gas gas gas |
low low low |
high low medium |
medium low low |
| Partial discharges | SOF2 SF4 HF SO2 SOF4 S2F10 |
gas gas gas gas gas gas |
low low very low very low very low very low |
high medium medium medium high high |
medium low low low low low |
| Switching arc at low interrupting current | SOF2 SOF4 SO2F2 |
gas gas gas |
low low low |
high high low |
medium medium low |
| Switching arc at high interrupting current | SF4 WF6 SOF2 CF4 HF CuF2 WO3 |
gas gas gas gas gas solid solid |
medium medium medium medium low medium medium |
medium high high non-toxic medium non-toxic non-toxic |
high high medium none low none none |
| Internal arc | HF SF4 CF4 ACF3* FeF3* |
gas gas gas solid solid |
medium high medium high high |
medium medium non-toxic medium non-toxic |
low high none medium none |
| *depending on the housing material |
Up to 150◦C, materials such as metals, glass, rubber, and plastics are completely resistant to SF6. At 400–600◦C, SF6 reacts with metals. Below this temperature, decomposition products do not yet form. Decomposition products formed in SF6 are significantly more corrosive than the gas itself, especially in the presence of moisture. Metals are intensively attacked by these compounds, but corrosion susceptibility depends on concentration and is not particularly high. Some inorganic materials, e.g., glass, porcelain, insulating paper, are very susceptible to corrosion. Others, e.g., epoxy castings, PTFE (Teflon), PVC, are significantly more resistant. Moisture greatly accelerates corrosion. Therefore, disassembled parts from devices should not be left uncleaned or un-dried [8].
It should be emphasized that material resistance was particularly important in the early period of designing and manufacturing SF6 devices. For example, special porcelain (based on Al2O3) was developed for air-insulated switchgear, fully resistant to SF6 decomposition products even without glazing (inside the chamber). A separate program was also carried out to develop suitable rubber for seals. Every material research program considered that SF6 devices are expected to operate for 20, 30, or more years [36].
The widespread use of SF6 in electrical power equipment worldwide often raises concerns about the extent to which this gas and its decomposition products pose a threat to the global environment. The literature devoted to the impact of SF6 on the natural environment [19, 20] clarifies many issues.
Two main issues are analyzed in the most detail regarding the impact of SF6 on the natural environment:
- how the use of SF6 contributes to the greenhouse effect,
- to what extent the use of SF6 contributes to the depletion of the ozone layer in the stratosphere.
In analyzing 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 power engineering is justified,
- SF6 used in the electrical industry is stored in sealed containers (switchgear, circuit breakers) and in cylinders,
- the causes of emissions from electrical devices containing SF6 are only operational errors or leaks due to equipment tightness. These causes are minimized through personnel training and the high tightness of the equipment.
It is stated that SF6 does not participate in the stratospheric ozone depletion effect – it is not photolytically active because it does not contain chlorine atoms.
However, SF6, like many other gases such as CO2 or CFCs, absorbs infrared radiation in the area of the atmosphere where this radiation spectrum occurs; its presence in the atmosphere can contribute to so-called secondary artificial infrared radiation, returning to the lower layers of the atmosphere, causing the greenhouse effect.
It should be emphasized that the greenhouse effect discussed above is artificially induced, amplified by human activity, as opposed to natural warming caused by the release of water vapor, CO2, etc.
The impact of SF6 on global warming depends on:
- its concentration in the atmosphere, which in turn is determined by how much gas was released into the atmosphere and how long SF6 retains its properties in the atmosphere,
- its absorption properties – in the area where the infrared radiation spectrum occurs.
There remains the issue of introducing SF6 decomposition products into the environment and their effects. While SF6 itself is a very chemically stable gas and remains in the atmosphere for a long time because it does not enter any reactions leading to its degradation, the compounds formed during the decomposition of SF6, which can be produced during partial, spark, and arc discharges, are harmless to the environment as they are highly reactive and are quickly converted into environmentally harmless end products. Additionally, there is significant adsorption of decomposition products and their secondary compounds in the filters of the devices in which they are produced, and only a small amount enters the atmosphere due to leaks. Of course, this applies provided deliberate evacuation of the gas from devices by humans is avoided. Arc-furnace casing failures, with uncontrolled gas and decomposition product release into the atmosphere, are extremely rare – SF6 devices are very reliable [39].
Opponents of SF6 use, due to its impact on the environment, including the decomposition products of the gas, assume that all produced SF6 will eventually be released into the atmosphere. However, unlike other human-made gases, SF6 used in electrical devices is properly stored, and the operation of the installation and auxiliary equipment ensures that releasing SF6 into the atmosphere is impossible. This assumption is supported by the implementation of SF6 regeneration – the process of restoring SF6 for use in equipment [30].
It should be emphasized that in the past, SF6 regeneration was not widely practiced for the following reasons:
- manufacturers and users of SF6 were not fully aware of environmental protection,
- regeneration procedures and technologies were not clearly defined,
- standards (procedures) for SF6 recovered on-site from installed electrical equipment or in the factory were not developed,
- the release of SF6 into the atmosphere in the past was not sufficiently analyzed [39].
All the above reasons have lost their significance. Recent surveys conducted by CIGRE show that most users of SF6 electrical devices are aware of the need to protect the natural environment. They avoid releasing SF6 into the atmosphere and have started systematic on-site recovery of SF6. 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 using SF6 in electrical devices:
- SF6 must not be deliberately released into the atmosphere,
- losses of SF6 from electrical devices are reduced through design improvements and should be further minimized through proper installation and correct handling procedures,
- SF6 should be subjected to regeneration,
- standards regarding SF6 recovery procedures and purity should be strictly followed [39].
Implementing these measures, of course, depends on awareness of SF6 use – from the management of power stations to technical personnel.
“High-voltage enclosed switchgear is the primary reason for using sulfur hexafluoride in power equipment. As previously mentioned, this decision was driven by the excellent insulating properties of this gas. With the introduction of this gas, it became possible to design high-voltage switchgear in enclosures – similar to low-voltage switchgear – often called cubicle switchgear.
The technology based on SF6 has several significant advantages [20]:
- thanks to the excellent insulating and interrupting capabilities of sulfur hexafluoride, the dimensions of the devices have been significantly reduced. This, in turn, allows for:
- reduction of the area occupied by the installation and improved layout of the power station,
- significant reduction in the number of components, which is associated with decreased consumption of raw materials and energy during production, technological processing, operation, and disposal,
- a hermetic enclosure of high-voltage busbars in grounded covers ensures that the SF6-based system is independent of atmospheric contamination and degradation processes, and also allows for:
- significant extension of equipment reliability,
- greatly reduced requirements for maintenance, inspections, and repairs, resulting in higher reliability, durability, and availability – i.e., constant readiness for operation,
- reduction of energy losses and fire hazards.
These statements show that, aside from excellent technical parameters, operational certainty, and economic aspects, SF6 insulation has no alternative solutions that surpass it from an environmental perspective. This allows the use of the most advantageous solutions when considering environmental circumstances throughout the entire lifecycle and total costs [20].
The design of enclosed high-voltage switchgear began with 123 kV units. A fully enclosed high-voltage switchgear with SF6 insulation resembles a conglomerate of metal pipes and tanks of significant size [10, 11].
To assess the overall design, a set of specific classification criteria must be introduced, which are not usually applied to other types of switchgear. These classification criteria fall into several main groups of issues. Ignoring minor aspects, the classification of switchgear can be presented as follows:
- based on the method of busbar insulation:
- switchgear with single-phase insulated busbars,
- switchgear with three-phase insulated busbars,
- based on the busbar arrangement:
- single busbar system,
- double busbar system,
- based on the type of switch installed:
- circuit-breaker switchgear (fields),
- disconnect switchgear (fields),
- based on the orientation of the circuit-breakers:
- switchgear with horizontal breakers,
- switchgear with vertical breakers,
- based on supporting structure:
- switchgear with separate supporting structure,
- “self-supporting” switchgear (the enclosure itself serves as the supporting structure),
- complex design with special support structure and field enclosures mounted on it,
- based on installation location:
- indoor switchgear,
- outdoor switchgear,
- based on the type of circuit-breaker:
- SF6 gas circuit-breakers,
- vacuum circuit-breakers [9].
Manufacturers designed individual components of switchgear to allow various combinations, different switching devices, layouts, and connections (cables, overhead lines). In enclosed high-voltage switchgear, field composition is typically used. They are usually installed indoors. For optimal design, the cable is usually routed downward, with busbars located at the top. In large stations, a double busbar system is most often used, and cables are always routed downward.
Enclosed switchgear has the same sets of devices as conventional switchgear but with a different construction – suitable for closed enclosures and SF6 insulation. Therefore, the primary advantage of SF6-insulated enclosed switchgear is its significantly smaller size.
Analyzing designs from different manufacturers, some “architectural” differences in construction can be observed, but they do not significantly affect functionality. Enclosures may be made from stainless and non-magnetic steel, rolled aluminum, or cast aluminum. Manufacturers often emphasize the superiority of their chosen enclosure type.
Manufacturers did not stop at 123 kV switchgear. Leading companies later installed switchgear for 245 kV, 300 kV, and 525 kV. The key point is that as the rated voltage increases, the ratio of traditional switchgear dimensions to enclosed switchgear dimensions increases, significantly reducing the area occupied by the switchgear and highlighting the total economic effect through reduced land costs.
In high-voltage enclosed switchgear, operational reliability (low failure rate) is particularly important, as external factors (pollution, lightning, insulator breakage, birds, etc.) are eliminated. To illustrate construction details, we choose a switchgear already installed in Poland, i.e., ABB switchgear (Fig. 6.1) [38].
Fig. 6.1. Section of a 123 kV ELK-0 (ABB) cable line circuit-breaker field; 1 – double busbar system, 2 – circuit-breaker, 3 – current transformer, 4 – voltage transformer, 5 – cable connection chamber, disconnect switch, and grounding switch, 6 – disconnect–ground drive, 7 – control cabinet [38]
Basic parameters of the presented switchgear:
- Rated voltage: 72.5 – 170 kV,
- Rated current: 1250 – 3150 A,
- SF6 pressure outside the circuit-breaker (absolute): 420 kPa,
- Breaker interrupting current: 25/31.5/40 kA,
- SF6 pressure inside the breaker: 600 kPa.
To appreciate the miniaturization advantages of such switchgear, attention should be paid to the field dimensions and the required minimum room dimensions, as shown in Fig. 6.2 [38].
Fig. 6.2. Dimensions of a five-field ELK-0 switchgear (H configuration) and minimum room dimensions [38]
The enclosed high-voltage switchgear is made up of individual fields, each with its own enclosure. The layout ensures minimal interference between fields, optimal access for maintenance, and safety for personnel. The compact design significantly reduces the space needed compared to traditional open-type switchgear.
Each field may include:
- Circuit-breaker compartments,
- Busbar compartments,
- Instrument transformer compartments (current and voltage transformers),
- Cable or line connection compartments,
- Control and protection compartments.
The key advantage of modular field construction is flexibility: fields can be added, replaced, or reconfigured without major redesigns of the entire switchgear.
Enclosed switchgear also ensures:
- High operational reliability due to SF6 insulation and hermetic sealing,
- Reduced maintenance requirements because of the protected environment,
- Minimal environmental impact during operation,
- Safety of personnel, as live parts are not exposed.
For higher voltage levels (245 kV, 300 kV, 525 kV), the same principles apply, but design challenges increase due to the higher insulation distances, mechanical forces during switching, and thermal management. Despite this, the overall footprint remains smaller than that of traditional air-insulated switchgear.
An example of a high-voltage enclosed switchgear for 245 kV and above includes additional features:
- Enhanced mechanical support for larger circuit-breakers,
- Advanced gas monitoring and leakage detection systems,
- Integrated sensors for real-time condition monitoring,
- Improved grounding and surge protection systems.
SF6 Advantages Recap:
- Excellent dielectric strength allows compact designs,
- Effective arc-quenching properties ensure safe interruption of high currents,
- Stable chemical properties provide long-term operational reliability,
- Non-flammable, reducing fire hazards,
- Environmentally sealed system minimizes pollution and contamination effects.
Summary of the Enclosed Switchgear Concept:
Enclosed switchgear, especially when SF6-insulated, represents a major evolution in high-voltage equipment design. Its main advantages are:
- Space-saving due to compact modular construction,
- High operational reliability with minimal maintenance,
- Enhanced safety for personnel,
- Environmental resilience due to hermetic enclosures,
- Flexibility in system configuration and expansion.
This makes SF6-insulated enclosed switchgear a preferred choice for modern power stations, urban substations, and critical infrastructure where reliability, compactness, and safety are paramount.