CO2 and methane are the best-known greenhouse gases, but many industrial gases also show significant global warming potential (GWP). The most potent is SF6, used extensively in gas-insulated switchgear (GIS) and gas-insulated lines (GIL) for electricity distribution (see also box, right).
The EU has proposed a two-stage ban on SF6 in new ‘medium-voltage’ (MV) electrical installations: its use in equipment up to 24kV would be prohibited from 2026, up to 52 kV from 2030. But it points out that this is only achievable if suitable solutions are available.
Mark Waldron is technical leader for National Grid’s Net Zero Asset Strategy, responsible for elements of the high-voltage (HV) network, which operates at between 66-420kV, and for some MV substations. He says: “For me, 33kV and down are medium voltage.” The sites he deals with use SF6 extensively: “it’s an insulation medium, and the interruption medium in the circuit breakers.”
At such high voltages, solid insulation is not practical, and old-fashioned oil-filled switches give off toxic gases. Air can be used as an insulator, if space is not an issue. But in the UK that is rarely an option: with SF6, says Waldron, “we can build metal-enclosed switchgear in a relatively small space. We still build air-insulated substations, but there we are dealing with clearances of 3 or 4m; using pressurised SF6 we can get them down to half a metre. SF6 is the de facto standard: for the last 35 to 40 years no-one’s produced anything else.”
National Grid has about 916 tonnes of SF6 installed at the moment – most applications use pure SF6, although some simpler installations use an SF6-nitrogen mix purely for insulation.
RELEASES
How does SF6 get released into the environment? “Generally it’s in sealed systems, but they are not truly sealed: the leak rate might be 0.2-0.3% per year.” However, “the leak rates for older kit might be higher than that; last regulatory year we lost approaching 10 tonnes from our 900-tonne inventory.” However, National Grid is reducing leakage year-on-year: “We have a target to bring it down to 50% of the 2019 rate by 2030.”
There are two technological approaches to replacing SF6: finding a replacement gas, or using a combination of vacuum-filled interruptors and highly-pressurised dry air as an insulator. At medium voltages, the latter approach is common already, and Waldron says the outlook is about “rolling those out in large numbers of relatively small assets.” But for HV equipment, “we are using relatively huge quantities of gas, in much larger kit.”
Hitachi Energy, GE and ABB are approaching the MV and HV markets using a replacement gas mix based on C4FN, a fluoronitrile which is marketed as Novec 4710 insulating gas by 3M. Mixed in small percentages with CO2 and oxygen, it delivers similar performance to SF6 at similar pressures and in similar dimensions. Pure C4FN has a GWP of 2,240, but the mixture gives an overall 99% reduction in GWP compared with SF6.
Hitachi Energy uses the brand name EconiQ for this technology, while GE calls it g3 and ABB refers to it as AirPlus (confusingly, at lower voltages, ABB also offers the SafePlus Air switchgear – Italian example pictured below – which uses pressurised air as an insulator).
Siemens’ Blue ‘Clean Air’ technology combines vacuum-switching with gas insulation using a formula ‘which consists exclusively of components of the natural ambient air’. Siemens says the dimensions and operation of the system are unchanged, and ‘The complete absence of fluorine gases… means that our customers are well-prepared with regard to possible future regulations’.
Liechtenstein’s power utility (LKW) was one of the first operators to adopt Siemens Blue switchgear, and head of energy management Armand Jehle says: “The advantage is that there is no need for controlled handling… which reduces the effort required all around and has a positive impact on lifecycle costs.”
Siemens plans to extend this technology to higher voltage ranges, up to the 420kV of the main grid.
Back in the UK, Mark Waldron observes: “The use of pressurised air for insulation at this level is quite challenging.”
Waldron emphasises that National Grid shows no favouritism for any of the competing SF6-free technologies: “We are being technology-agnostic,” he says, “because we need to do stuff quickly. We need to be working with all the technologies available, given the amount of work we’ve got coming up over the next 30 years.”
Retrofitting is an option, but “there are limitations of where you can use existing hardware and substitute the gas”. One project where National Grid has been able to replace SF6 is the 400kV Richborough Substation in Kent, where it worked with Hitachi Energy to install EconiQ gas.
“All of the SF6 we use is in closed pressure systems, but they are accessible by filling valves,” says Waldron. The pressure may range from 3.5 to 7.5 bar (above 7.5 bar the gas would liquefy at -25°C). “We extract SF6 from obsolete equipment and store it. Historically, it was destroyed, but now we are trying to close the cycle and reuse as much as we can.”
“Generally speaking, to eliminate SF6 from older equipment is going to involve replacing equipment, and optimised replacement programmes over a long period to reduce our inventory. SF6 inventory in itself is not a problem – emissions is a problem.”
Waldron remarks that while National Grid has a stated ambition to remove SF6 from the network by 2050, this is not a detailed plan: “The transmission network in the UK is going to go through revolutionary change in the next 20-30 years. We are going to have to replace large portions of the network anyway, so we’re looking at plans that integrate the renewal of the network, the reinforcement of the network, and the elimination of SF6.”
BOX: What is SF6?
Sulphur hexafluoride is a synthetic, colourless, odourless gas which is extremely stable and inert. It is non-flammable, non-toxic (it has been used in eye surgery procedures) and five times as dense as air. Its properties, particularly its high dielectric strength, make it an ideal insulation and interruption gas (IIG) in high-voltage switchgear, and around 80% of the world’s consumption of SF6 is used this way. Its density and inertness also make it useful as a tracer gas and to prevent oxidation in magnesium casting.
However, like other fluorinated gases – including the hydrofluorocarbons (HFCs) commonly used as refrigerants – SF6 applications and disposal are controlled under the EU’s F-gas regulations. This is because SF6 is a greenhouse gas (GHG): in the atmosphere it has a similar insulating effect to CO2, trapping infrared radiation and warming the ground below.
But SF6 is the most potent industrial GHG known, with a global warming potential (GWP) of 25,200. This means that one tonne of SF6 has the equivalent effect as 25,200 tonnes of CO2, or around 200 million km of internal-combustion car travel.
GWP is calculated over a 100-year period, and SF6 is extremely long-lived: it typically stays in the atmosphere for 3,000 years (compared with 12 years for methane) so over longer periods its relative GWP is even worse.
Despite measures to reduce the production of SF6, around 9,000 tonnes is emitted into the atmosphere each year (mainly from leaking or failing switchgear), representing a real issue for the climate.