Panel discussion: The pros and cons of using SF6 gas in switchgear

July 14th, 2015, Published in Articles: Energize

Sulphur hexaflouride (SF6) gas has been used in MV and HV switchgear for many years. It is said to be highly toxic and a potent greenhouse gas. It is used because of its excellent electrical insulation properties, but recently some switchgear companies have suggested other solutions. In our virtual panel discussion we asked some specialists to comment on the risks and benefits associated with the use of SF6 in MV and HV switchgear, and to suggest alternatives, with their advantages and disadvantages.

Geoff Slacke, Marketing manager, Malaysian Switchgear Distributors.

Geoff Slacke, Marketing manager,
Malaysian Switchgear Distributors.

Does your company use SF6 in the switchgear it manufactures?

Geoff Slacke, Marketing manager, Malaysian Switchgear Distributors: Worldwide SF6 is extensively used in the switchgear industry as a both insulating and interruption media owing to its excellent dielectric strength and arc quenching capabilities. We however, restrict the use of SF6 only as an insulation media in our ring main units (RMU) and 11 kV to 33 kV gas insulated switchgear GIS) range for compact foot print. For arc interruption we rely only on vacuum technology which is much cleaner and relatively maintenance free when compared to SF6.

Brighton Mwarehwa, Technical manager, Schneider Electric: Schneider Electric is a manufacturer of medium voltage (MV) switchgear. The company manufactures switchgear in the MV range up to 52 kV. There are various types of switchgear that are manufactured using gas insulation where sulfur hexafluoride (SF6) is used. SF6 is mainly used in MV prefabricated metal-enclosed switchgear and in MV circuit breakers. In electrical switchgear technologies, SF6 is used for either or both: current breaking and insulation between conductors or between conductors and earth.

The SF6 pressures that are used in the MV switchgear sealed compartments are very close to the atmospheric pressure. Most of the switchgears are sealed and tested in a factory-controlled environment, and no gas handling is done onsite during both installation and expected life. GIS technology offers many advantages, such as: increased operator safety and excellent service continuity. It is also efficient, as it delivers more power in less space and can in this way get closer to end users. In addition, the GIS-enclosed compartments are maintenance free and independent of the environment.

What precautions are needed when handling SF6 gas?

Geoff Slacke: SF6 handling involves the following scenarios: filing of switchgear, maintenance operations, recycling of used gas, decommissioning of SF6-filled equipment. SF6 is one of the heaviest known gases with a density of almost five times that of air. Therefore if released into the atmosphere, the gas tends to accumulate initially in low-lying places where there is no natural ventilation resulting in suffocation. Therefore filling of SF6 requires protective clothing and equipment such as respirators, oxygen detectors etc. Personnel undertaking routine maintenance including the testing of SF6 gas which generally does not require internal access to the SF6 compartments do not need any protective clothing or equipment. Recycling of used gas and decommissioning of SF6-filled equipment may involve contact with SF6 decomposition products thereby making protective clothing and equipment necessary. In our switchgear, gas filling is not required at site. Further since we use SF6 only for insulation, decomposition products are rarely formed and therefore use of protective clothing and equipment can be minimised in normal conditions.

Brighton Mwarehwa, Technical manager, Schneider Electric.

Brighton Mwarehwa, Technical manager, Schneider Electric.

Brighton Mwarehwa: The SF6 gas is handled in filling of the switchgear, maintenance operations, recycling of used gas, decommissioning of filled equipment, and more. The manufacturer’s internal design, type test and routine tests process should be in accordance with international standards and recommendations. Furthermore, installation and commissioning should also follow international standards and technical reports, which stipulate the handling of SF6.

SF6 gas is considered as a greenhouse gas, consequently release of the gas into the atmospheres should be minimised or avoided. At the end of switchgear life, the gas should be recycled and switchgears should be designed to ensure leaks are minimised as far as possible. Appropriate designs are required for easy filling and collection. During equipment operation the level of the gas pressure or density should be continuously monitored so that leakages are detected at an early stage.

Can SF6 gas leak out of the switch if it is mechanically damaged?

Geoff Slacke: Yes, but if severely damaged; forming physical leak, as mentioned earlier the gas will accumulate in low-lying places if natural ventilation is not available. This being said, the chance of such occurrences in most switchgear is negligible since the gas-tanks are manufactured from thick stainless steel sheets which are robotically welded to enhance the mechanical integrity of the structure.

Brighton Mwarehwa: The switchgear filled with SF6 goes through rigorous quality checks in the factory. The checks include leak detection and there is a requirement in accordance to the IEC, which stipulates that the leakage rate per annum should not exceed a certain value. The transportation of the switchgear is also controlled to reduce the probability of damage. Only trained and certified personnel are used for installation of gas-insulated switchgear, which also reduces the probability of damage. In the unlikely event of a mechanical damage the gas may leak into the environment; cases recorded so far have been very few in the switchgear industry. As SF6 gas is heavier than air, it will be easy for the gas to leak quickly when the damage is on the bottom. In general, the leakage rate is slow and it also takes some appreciable time for the SF6 to disperse from the source of the leak.

What risks could be associated with such a leak?

Geoff Slacke: While pure SF6 gas used for switchgear insulation is non-toxic, it can exclude oxygen and hence cause asphyxiation which is major threat associated with leak in Gas Insulated Switchgear (GIS). Whereas when SF6 is used as an arc-extinguishing medium, some permanent by-products can be formed, particularly in presence of oxygen making the effects of a leak far more severe.

S + O + 2F     →     SOF2
SF4 + O         →     SOF2 + 2F
SF3 + O         →     SOF2 + F
SF5 + O         →     SOF4 + F

The oxygen involved in the above reactions may remain as result of the evacuation process or may be released by the electrode materials during arcing. The by-products cause irritation of the skin, eyes and mucous membranes, such as in the respiratory tract and in high concentrations can cause pulmonary oedema, given sufficient time of exposure.

Brighton Mwarehwa: The SF6 gas is listed in the famous Kyoto protocol as one of the potential greenhouse gases that contribute to the global warming potential of our earth. SF6 is always used in gas tight compartments, which greatly minimise leaks. As the quantities that leak from the switchgear are low, the contribution to the greenhouse effect can be considered extremely minimal to affect climate changes. Nevertheless, there are toxic by-products that are produced in SFduring arcing. Hence SF6 leakages can become dangerous when leaked in large concentrations. However, if leaks happen in an open space, the concentration will be diluted and effects will be negligible. Also, even in enclosed substation rooms, normal ventilation will dilute the concentration and effects become negligible as well.  Therefore, it can be stated that the risk for leakages is very minimal and next to negligible in most of the cases, especially in MV switchgear where the gas pressures are next to the atmospheric pressure.

What alternatives to SF6 exist?

Geoff Slacke: If follows from the previous responses that use of SF6 as an insulation medium is virtually harmless to operating personnel if the gas is handled under a controlled environment. The real threat is posed by using SF6 for arc extinction in puffer-type circuit breakers and therefore alternate arc-control devices must be implemented. The most promising medium among the available alternatives is vacuum.

Brighton Mwarehwa: Electrical switchgear always needs insulation for current breaking, and insulation between conductors and between conductors and earth. There is no technically and economically viable alternative with equivalent properties to SF6, which is being used on switchgear systems as a dielectric for conductors. Technologies that are used as an alternative to the use of SF6 do not have the comparable extraordinary electrical, physical, chemical and thermal properties that are exhibited by SF6. In the MV range of switchgears for current breaking, the alternative available technologies used are oil, air and vacuum. To isolate conductors, the same technologies are used as an alternative, and solid insulation is another common technology used.

What are the advantages of using an alternative to SF6?

Geoff Slacke: Use of vacuum for arc control and interrupting medium has many advantages, the foremost being the absence of any arcing by-products. The vacuum interrupters are hermetically sealed for life requiring no maintenance through-out its working life. The Vacuum Interrupters are easily replaceable at the end of its operational life and the disposal of used bottles is relatively far less complicated when compared to disposal of SF6 gas and associated decomposition products. Coming to operational capabilities, vacuum has an excellent dielectric strength, which is sufficient for switching of most currents in distribution networks, substations and process plants.

Brighton Mwarehwa: Most of the alternative technologies used on the MV switchgear, except vacuum, can be maintained regularly during the lifecycle of the switchgear. That is, there are options for extensive maintenance during the lifecycle of the switchgear. The use of solid dielectric for busbar systems, instead of SF6, has the advantage that it enables the SF6-based units to require no gas handling onsite during installation, commissioning and operation of the GIS equipment. Another advantage with the use of other alternatives is less impact on the environment and on the material used for the electrical insulation. However, the overall direct and indirect environmental impacts of alternatives whole system may be more than the SF6 insulation based system.

The advantage of using vacuum in place of SF6 for current breaking is that less space is utilised for the current breaking module. The VCB breaking module is normally smaller compared to SF6. If vacuum is used for the conductor system insulation it will be very delicate and expensive to maintain.  In the case of indoor metalclad SF6, the switchgear comes as a modular solution and will require a complete replacement of the MV panel in case of breaker, earth switch or disconnector failure. With the use of alternatives, such as air insulated switchgear breakers, earth switch or disconnector components can be replaced individually without the need to replace the whole panel.

What are the disadvantages of using an alternative to SF6?

Geoff Slacke: The main reason SF6 quickly rose to prominence in the switchgear industry is because of its dielectric properties due to electronegative character of its molecules. Because of its low dissociation temperature and high dissociation energy, SF6 is an excellent arc quenching medium. When an electric arc cools in SF6, it remains conductive to a relatively low temperature, thus minimising current chopping before current zero, thereby avoiding high over-voltages that could result in re-strikes. Also SF6 performs better in terms of capacitor switching performance These switching characteristics are the biggest advantages of SF6 over vacuum. However, it must be noted that most of these difficulties are overcome by correct application of surge suppression devices such as lightning arrestors which limit the over-voltages making vacuum a suitable interrupting medium in all medium voltage applications.

Brighton Mwarehwa: In the case where air is used for breaking the circuit breakers, the circuit breakers will be huge. This is because the principle of breaking is based on a large expansion of the arc. Furthermore, there will be large noise because of breaking in the air. Significant maintenance is required and, for that reason, many switchgear of the air type, such as in the case of metal enclosed switchgear, will be of the withdrawable versions. Where air is used as the main dielectric for the conductors, large space is also required as air is a less superior dielectric than SF6.

Oil circuit breakers came as a panacea to reduce the footprint of air circuit breakers. However, even though they offered a huge reduction of footprint, they are maintenance intensive and require the changing of oil after some operations. Additionally, oil based circuit breakers are not very safe to operate because of the fire risk. The oil circuit breaker failures can easily result in fatalities among operators and also the public.

Use of vacuum breaking only without the SF6 insulation, can come in fixed or withdrawable pattern switchgears. However, it will always be coupled with the use of other technologies for insulation between conductors. Though the VCB is very reliable and compact, the accompanying insulation to complete the switchgear assembly, if it is not screened versions, will pose a reliability risk.

Other comments:

Geoff Slacke: My team and I , like many others are doing in preparation for this virtual panel discussion regarding SF6 versus other insulating methods, could sift through the many articles and papers available on the internet and reference/design books in print. However, as the distributors of Tamco medium voltage switchgear in South Africa, we do not profess to be experts on SF6 that has been used as an insulating medium for over half a century. Malaysian Switchgear’s expertise lies in the layout design, wiring, testing, installation, commissioning and maintenance of the Tamco range of medium voltage switchgear panels up to 36 kV, as well as dry ice cleaning of compromised sub stations.

Brighton Mwarehwa: The use of SF6 in MV switchgear has brought advantages in performance, size, weight, total cost of ownership (TCO), safety, and reliability. The TCO, which includes maintenance costs, is much lower than for alternatives.

The term GIS (gas insulated switchgear) in MV is commonly used also in hybrid solutions, which are partially SF6 based switchgear systems. It is used where, for example, the busbars are solid dielectric and only the circuit breaker, disconnectors and earthing switch are in SF6 gas. GIS is today a proven technology with more than 30 years of experience, resulting in it gaining greater investor trust and economic justification. GIS is highly recommended for customers, as it offers minimum space usage. Customers should however consider GIS options that do not require gas handling when the switchgears leave the factory environment and comply with the requirements of IEC standards. A huge advantage for customers is that due to development in manufacturing processes and the increased demand of the technology, the TCO of GIS switchgear is now significantly lower to the alternatives.

In the MV range of switchgear – in the case that compact switchgear is required – SF6 is recommended for conductor insulation and vacuum circuit breakers for current breaking. This is considered as the most suitable solution for optimum switchgear.

Send your comments to:energize@ee.co.za

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