SULPHUR HEXAFLUORIDE GAS (SF6)
Fig1.1
Sulphur hexafluoride is a nontoxic, inert, insulating and cooling gas of high dielectric
strength and thermal stability. It is particularly suitable for application in both high-voltage and
medium-high voltage power circuit breakers as well as in high-voltage cables, transformers,
transducers, particle and electron accelerators, X-ray equipment and UHF transmission
systems and as an etching and chamber cleaning gas in the semiconductor industry. The
construction of new equipment with higher capacity and improved performance has been
made possible by the excellent electrical, thermal and chemical properties of SF6. Changing
from conventional dielectrics to sulphur hexafluoride – a non-flammable, chemically-inactive
and non-toxic heavy gas – results in considerable space and weight savings and improvements
in the operational safety of converted equipment. SF6 is also used in medical technology: for
example as a contrast agent in ultrasonic examinations as well as in ophthalmology,
pneumonectomy and diseases of the middle ear, e.g. treating loss of hearing in middle ear
infections.
Areas of application for sulphur
hexafluoride
Electrical engineering The use of sulphur hexafluoride in place of solid and liquid insulators
offers a number of important advantages: High dielectric strength at lower cost When
pressurized, sulphur hexafluoride can exhibit the same dielectric strength as liquid insulators.
However, the per-unit volume cost of SF6 is only a fraction of that of liquid dielectrics.
Regeneration capacity Following a breakdown, sulphur hexafluoride regenerates itself. Its
original strength is spontaneously restored. Low pressure-increase in the case of breakdown
SULPHUR HEXAFLUORIDE GAS (SF6)
Due to the very low adiabatic coefficient of sulphur hexafluoride, the pressure rise as a result
of thermal expansion following dielectric breakdowns is less than that with other gases and
very considerably less than is the case with liquid dielectrics.
Fig 2.1 fig 2.2
High-voltage switchgear and switching control panels
The excellent arc-quenching and insulating properties of sulphur hexafluoride have permitted
the construction of completely new types of high-voltage circuit breakers and switching
stations with outstanding features: compact and space-saving design, low noise-levels,
protection against accidental contact of live parts, against intrusion of foreign matter through
the metal cladding and elimination of the fire hazard. Substations using sulphur hexafluoride
for insulation purposes are particularly in demand where, on account of limited space, a
compact design is required. These substations occupy only 10 –15 % of the space required by
conventional air-insulated units. New SF6-filled equipment can thus be installed at distribution
points in densely-populated areas where site costs would prohibit the use of traditional
methods. Thanks to their insensitivity to polluted air, enclosed outdoor versions of SF6-
insulated substations are installed in the chemical industry, in desert regions and in coastal
areas. SF6 is used as a quenching agent both in power circuit breakers for enclosed substations
and in circuit breakers for open outdoor substations.
GIL
Gas insulated transmission line (GIL) Gas insulated transmission lines are particularly well
suited for high power transmission. Conventional designs are filled with pure SF6, and have
been operating safely and reliably in all parts of the world for more than 20-years. Second-
generation gas-insulated lines for high power transmission are the best option where
environmental or structural considerations rule out the use of overhead transmission lines.
SULPHUR HEXAFLUORIDE GAS (SF6)
The outstanding features of a GIL system are its high transmission capacity (up to 3,700 MVA),
superior electromagnetic compatibility (EMC) to any other transmission system, low losses,
high safety and flexible installation options. GIL systems at a rated voltage up to 550 KV can be
laid above ground, installed in tunnels or buried directly in the soil, depending on individual
requirements. For long distances the replacement of pure SF6 with more economical SF6 / N2
mixtures has been researched because the arc quenching properties of SF6 are not relevant in
insulating applications. Today the overall optimization of gas mixtures, gas pressure and
dimensions of GIL mean this technology is a highly competitive transmission medium in a broad
range of applications. Our research and application department supports this application with
their knowhow about the initial production of SF6/ N2 mixtures through to the separation of
SF6/N2 mixtures at end of service life or whenever required. For this long time lasting
technology Solvay has developed steps in filling, mixing and separation technology.
LOW VOLTAGE ELECTRICAL SYSTEMS
APPLICATION OF SF6 GAS
The merits of SF6 technology, in particular its excellent arc-quenching capacity, are also put
to good use in circuit breakers for the 10 – 40 kV range. They replace conventional, low-oil-
volume circuit breakers and also satisfy heavy-duty requirements such as those occurring
under short-circuit conditions and repeated switch-off under load. As with high-voltage
circuit breakers, medium-voltage switchgear requires little maintenance and are particularly
suitable for locations where oil-filled equipment is undesirable.
SULPHUR HEXAFLUORIDE GAS (SF6)
Electrical Transformers
Fig 3.1
Its excellent heat-transfer capacity, nonflammability and non-toxicity have also promoted the
use of sulphur hexafluoride in the construction of transformers. On account of their high
operational safety, SF6-gas transformers are installed in mines and department stores. Their
relatively light weight, compact design and low noise levels are decisive advantages.
Other high-voltage applications of sf6 gas
The use of sulphur hexafluoride has also established itself in the insulation of super-voltage
generators in particle-accelerating machines, such as in Van de Graaf accelerators, beta-trons
, neutron generators and other such equipment used for radiation applications in scientific
institutions, medicine and industry. By virtue of the high dielectric strength of the gas, pressure
vessels can be constructed in considerably lighter fashion. The use of SF6 in older units,
previously insulated with mixtures of air and carbon dioxide, has resulted in a marked increase
in efficiency SF6 plays a similar function in voltage stabilizers for electron microscopes and in
X-ray equipment used in production control and the non-destructive testing of materials.
Parallel to the development of SF6 plant technology in the high-voltage sector, SF6-insulated,
high-voltage measuring instruments and calibrated power sources have also been produced.
SF6-fillings are also used in instrument transformers, pressurized gas capacitors and surge
arresters for super voltages.
SF6 as a process gas in the semiconductor industry
SULPHUR HEXAFLUORIDE GAS (SF6)
Sulphur hexafluoride is more and more used for the manufacturing of semiconductor devices
such as IC (integrated circuits), flat panels, photovoltaic panels and MEMS (Micro-Electro-
Mechanical-Systems). It is used to produce chips (processors and memories) on mono-
crystalline silicon wafers, or solar cells on poly-crystalline wafers. On a different substrate, e.g.
glass, those silicon circuits are manufactured on thin deposited films of different silicon
materials and take the more familiar form of solar panels or flat screen TVs. Fluorinated gases,
such as sulphur hexafluoride (SF6), play two very important roles in the manufacturing of
silicon devices. Firstly, they are used to selectively remove materials through a process which
is called ‘etching’. The fluorinated gases are activated in plasma and the fluorine radicals that
are generated react with the silicon giving volatile species. In this way they pattern the different
elements of the circuit on the base. SF6 is especially efficient in the production of MEMS (Micro
Electro Mechanical Systems), because the high energy and reactivity of its ions allow an
unbeatable etching rate. SF6 is employed in the production of integrated circuits for the so
called ‘isotropic etching’, because it reacts in any direction and can pattern round cavities.
Secondly, fluorinated gases are used to clean the production tools from unwanted Si-based
layers and particles. SF6 is an optimum gas to generate the F radicals that etch and thus
efficiently remove the deposits on the walls of the plasma chamber. This process is known as
‘chamber cleaning’. Gas purity is of key importance in this market segment, because any
possibility of contamination has to be minimized. For this purpose Solvay offers N5.0 material.
ELECTRICAL PROPERTIES
Electron affinity The excellent insulating properties of sulphur hexafluoride are attributable to
the strong electron affinity (electronegativity) of the SF6 molecule. This is based mainly on two
mechanisms, resonance capture and dissociative attachment of electrons, in accordance with
the equations: SF6 + e– —> SF6 – (1) SF6 + e– —> SF5 – + F (2) The process represented
by equation (1) applies to electron energies of 0.1 eV with an energy range of 0.05 eV, and that
represented by equation (2) applies to an energy range of 0.1 eV[3].
Dielectric strength
The strong interaction of high-energy electrons with the polyatomic SF6 molecule causes their
rapid deceleration to the lower energy of electron capture and dissociative attachment. SF6-
breakdown is therefore only possible at relatively high field strengths. The breakdown voltages
at 50 Hz and 1 bar in a homogenous field are thus 2.5 to 3 times higher than the corresponding
values for air or nitrogen. The breakdown strength of air is dramatically increased by the
addition of small quantities of SF6. In contrast, air has only a limited influence on the
breakdown strength of sulphur hexafluoride. The addition of 10 % of air by volume reduces the
SULPHUR HEXAFLUORIDE GAS (SF6)
breakdown voltage of SF6 by about 3 %, the addition of 30 % air by about 10 %. The breakdown
voltage of SF6 reaches that of transformer oil at a pressure of only 3 bar . The behaviour of
sulphur hexafluoride conforms over a wide range of pressures to Paschen‘s Law: at higher
pressures, however, deviations have been observed under certain conditions [5,6,7]. The
breakdown strength of SF6 is independent of frequency: it is there fore an ideal insulating gas
for UHF equipment . The Corona-onset voltage using SF6 in non-homogeneous fields is also
considerably higher than that using air. Figures 26 and 27 show the respective dependence on
pressure and radius of curvature of the electrodes in the case of SF6 and air in a point-to-plane
electrode system.
Arc-quenching capacity
On account of its thermal properties and low ionisation temperature, sulphur hexafluoride
exhibits outstanding characteristics for the extinguishing of electric arcs. All other conditions
being equal, the arc-quenching time using SF6 is about 100 times less than that using air
Loss factor
The loss factor, tan ∂ of sulphur hexafluoride is extremely low (less than 2.0 · 10-7). A value of
tan ∂ < 10-3 was determined for liquid SF6 at –50 °C. Diagrams and data pertinent to the
electrical properties of sulphur hexafluoride may be found in the Milek „
Other properties of sf6 gas
Sulphur hexafluoride data sheets“ Sulphur hexafluoride is a colourless, odourless, non-toxic
and non-flammable gas. With a molecular weight of 146.05, SF6 is about 5 times heavier than
air and one of the heaviest known gases.
SULPHUR HEXAFLUORIDE GAS (SF6)
Fig 4.1
Storage of sf6 gas
Sulphur hexafluoride is transported as a pressurized liquified gas. The local safety precautions
and handling practices i.e. TRG in Germany and KGS in Korea must be met. The packaging
should not be exposed to direct sunlight and must be secured against overturning or rolling.
Storage and work areas must be well ventilated. In particular, ventilation must be effective at
ground level on account of the fact that SF6 vapour is heavier than air. If the gas is stored
underground, appropriate forced ventilation must be provided. Wherever SF6 is handled,
there must be no open flames (e.g. welding flames) or hot metal surfaces (e.g. infrared
equipment). Eating, drinking and smoking whilst working with SF6 is strictly forbidden.
Although SF6 is recognized as being physiologically safe, certain precautions have to be taken
in order to guarantee a safe handling of this substance. An important precondition is a strict
adherence to the threshold limit value (TLV). Wherever this cannot be achieved pertinent
safety measures must be selected according to the degree of potential danger. For safety
precautions and additional information in handling SF6 please see the safety data sheet
