SF6 in converter construction
The gas sulphur hexafluoride (SF6) is used in electrical power engineering as an insulating and extinguishing medium, particularly in switchgear and its components. In addition to its many positive properties, SF6 has the disadvantage of a very high global warming potential (GWP). According to the Intergovernmental Panel on Climate Change (IPCC), the GWP value is 23,500. The GWP value is basically the CO2 equivalent of a chemical compound and is a measure of its relative contribution to the greenhouse effect, i.e. its average warming effect on the earth’s atmosphere over a certain period of time (usually 100 years). It thus indicates how much a certain mass of a greenhouse gas contributes to global warming compared to the same mass of CO2. The categorisation of SF6 as one of the six greenhouse gases in the Kyoto Protocol of 1997 was the impetus for a discussion of emission-reducing measures in its use as an insulating and extinguishing gas in electrical equipment. In 2017, around 8,000 tonnes of SF6 were produced worldwide. This quantity corresponds to the equivalent of 190 million tonnes of CO2. This, in turn, is around 0.53% of global CO2 emissions in 2017. However, this conversion does not take into account the longer residence time of SF6 in the atmosphere. While CO2 is said to remain in the atmosphere for 4 to 120 years, SF6 lasts an impressive 3,200 years. (1) In Germany, traders delivered around 976 tonnes in 2017. Around 80 per cent is processed in the electrical industry. The gas is not released directly into the atmosphere, but is usually filled into closed components. However, leaks and improper maintenance can release the gas into the atmosphere. This means that around 15 per cent of the quantity used is released into the atmosphere. (1) Even if the gas can be reused, disposal has not yet been resolved and the majority is ultimately released into the atmosphere.
In order to further limit the concentration of SF6 in the atmosphere, political decision-makers have issued various bans.
bans have been issued. For example, an EU regulation prohibits the gas from being filled into the studs of shoe soles. Car tyres, tennis balls
and double-glazed windows have also been manufactured without SF6 for several years. Only in switchgear and its components remains
use remains permitted. (1)
Switchgear can generally also be designed as an air-insulated system without SF6. Nevertheless, individual components
contain SF6 here too. The following table provides an overview of this.
Air-insulated systems are generally much larger due to their lower dielectric strength. Especially in
SF6 systems are therefore often favoured in confined spaces. For this reason, renewable energies are also
SF6 systems are also increasingly used for this reason. (2)
Even though many companies are currently researching an environmentally friendly replacement for SF6, there are still relatively few
alternatives that have so far been used on a large scale in the electrical industry. The following figure provides an overview of the insulation materials used.
The dielectric strength often depends on many parameters such as manufacturing tolerances of field-influencing parts,
contamination of the insulating material, ageing of the material as well as the homogeneity of the electric field and the voltage stress (transients or harmonics).
voltage stress (transients or harmonics). The following applies to all insulating materials: the greater the insulation distance
the breakdown voltage and thus the maximum possible operating voltage also increases. For this reason, the
system size increases with the operating voltage. Gases such as SF6 have an almost linear increase, while solids, oil and vacuum have a kind of saturation.
vacuum a kind of saturation occurs.
Transformer construction in medium and high voltage also utilises the good insulation properties of SF6. In the medium voltage
pressures of just over 1 bar are usually used, and up to approx. 8 bar in high voltage. In medium voltage, it is often
voltage transformers that are moulded in an SF6 atmosphere in order to comply with the standard partial discharge levels.
The materials used in production are often not sufficient to build standard-compliant devices. Insulation distances are too short,
non-optimised field curves and contamination of the epoxy resin can lead to undesirable partial discharge levels.
In and around the primary coil of the voltage transformer, higher partial discharge levels that no longer conform to standards can occur again and again for the reasons mentioned above.
Figure 2: Sectional drawing – active part of an MV voltage transformer
In the voltage transformer, the entire operating voltage from the primary connection to the earth connection is isolated. This
voltage is transformed to the secondary winding with the appropriate transformation ratio. Due to
Due to contamination and very low cast resin wall thicknesses, partial discharges can occur in the core window, leading to failure of the device.
failure of the device. If the device is moulded in an SF6 atmosphere, these partial discharges can be completely suppressed. Also
partial discharges can also occur within the primary coil itself, where hardly any epoxy resin enters.
The layer insulation must have a corresponding dielectric strength. The maximum voltage applied to the layer insulation
can be reduced by dimensioning the primary coil. In addition, improved insulation materials can
can lead to a reduction in the partial discharge level. If the insulation is weak, encapsulation under SF6
can also be carried out here, which can reduce the partial discharge levels.
As in the switchgear, however, the SF6 gas can escape from the voltage transformer over time through various leakages.
The insulation strength is reduced over the years. A total failure in the measuring field is nevertheless unlikely, as the high
normative test voltage levels are not reached in practice. In the future, however, an increasing number of voltage transients and higher
of voltage transients and higher harmonic amplitudes in the medium voltage, which can significantly reduce the service life of electrical components.
which can significantly reduce the service life of electrical components. (3)
MBS AG has been manufacturing the complete range of medium-voltage SF6-free products for three years. Thanks to improved
insulation materials in production and optimised control of the field curves, the failure rate for partial discharge
partial discharge measurement, which is part of the routine test, is nevertheless extremely low. The customer receives an SF6-free transformer
with outstanding insulation properties over its entire service life. The CO2 balance of the energy supply companies is not negatively affected.