Decarbonising Distribution

Dec 04, 2023

The summer heatwave gave the UK a taste of what to expect from climate change. The experience has shown that it's vital to decarbonise infrastructure. Stephen Gibbs explains how a new type of insulating gas for switchgear can help

According to Ofgem, around 200,000 switchgear units serving the UK grid contain sulphur hexafluoride (SF6), the insulating gas that ensures safe and predictable operations. SF6 is excellent as an insulator and resists breakdown even during electrical arcing.

The drawback for SF6 is that it is a potent greenhouse gas with global warming potential around 24,000 times higher than CO2. In addition, it can linger in the atmosphere for 3,200 years. Because of this, use of SF6 is severely restricted and monitored under the UK's F-gas regulations and it must be handled by certified technicians.

Although these measures minimise risk, trace amounts of gas can and do escape through tiny imperfections in switchgear tanks and small losses during handling. In 2019-20, the UK's distribution network operators (DNOs) recorded an average leakage rate of 0.27%, which equates to 866kg of the gas.

However, operators can now install alternatives to SF6 gas insulated switchgears (GIS) thanks to recent development in technology from manufacturers of electrical switchgear, especially for the medium voltage switchgear in distribution grids. This has created a new opportunity to decarbonise the grid.

For switchgear rated at up to 12kV, dry air is ideal as an insulating medium for switchgear. As its name suggests, it is extremely pure and clean air with no moisture content. It is inexpensive, plentiful and well-proven, having been used at 12kV for many years.

The challenge is that as the voltage increases, so does the required level of insulation. There are two possible solutions to this.

The first is to use dry air at high pressure, which increases its dielectric strength (and, therefore its effectiveness as an electrical insulator). However, this requires a change of mindset for switchgear design, especially for units rated at 24 or 36kV.

At 36kV, dry air would need to be pressurised to several bars. One bar pressure is equivalent to the experience of being under a column of water 10m high. Therefore, to accommodate this high internal pressure for dry air, switchgear designers need to make stronger tanks with reinforced walls. They might even use a similar approach to the cylindrical tanks used to protect the contacts in high-voltage switchgear for transmission networks.

This would create switchgear that is larger in size and that may not fit into the available footprint when installed as a replacement. It would also create an operational risk as any loss of pressure would immediately affect switching performance, asset availability and safety. DNOs would need to counter this by installing extensive pressure monitoring and alarms to provide the data

Therefore, although dry air itself is inexpensive and familiar, the resulting switchgear has disadvantages.

The second of the two solutions is to use an alternative insulating gas that has been developed to replace SF6 for switchgear rated above 12kV. This contains a fluoroketone compound that is combined with dry air in a 15% / 85% ratio to create a gas mix called AirPlus with similar dielectric strength to SF6.

It can be successfully deployed for 36kV switchgear at marginally above atmospheric pressure. As a result, manufacturers can use the gas without needing to significantly redesign switchgear.

Over the switchgear's life, it is similar to operating SF6-insulated switchgear but without the reporting requirements as it is not subject to the F-gas regulations. In the event of a loss of pressure, the impact on performance would be modest, enabling the operator to schedule maintenance rather than having to take it out of service on an emergency basis. This is known as running flat, similar to a car with run-flat tyres.

Like any new technology, operators need to learn from the practical experience of SF6-free switchgear on their networks before they can set a strategy for adoption. At present, multiple utilities have delivered pilot installations, which they are evaluating.

One example is Northern Powergrid, which recently installed two variants of ABB's SafePlus switchgear at substations in County Durham. This includes versions rated at 12kV and 24kV, which are respectively insulated with dry air and the AirPlus gas mix.

Speaking about the project, Joseph Helm, Policy and Standards manager at Northern Powergrid, says: "Until recently, there have been limited viable alternatives to SF6. This pilot underlines our commitment to exceeding business plan targets, embracing the latest innovations, and lowering emissions. We have already reduced SF6 leaks by 23% but still want to drive an extra 15% by 2028. Switching to eco-efficient switchgear will help us to push ahead with these ambitious carbon commitments."

Meanwhile, UK Power Networks is evaluating AirPlus-filled switchgear at a site near Dartford in Kent under a project delivered as part of the DNO's Environmental Action Plan. As part of this project, UK Power Networks specified a double busbar and SF6-free version of ABB's ZX2 switchgear – making the site the world's first installation of its type. By adopting the double busbar, the operator will benefit from greater flexibility and availability.

Looking ahead, the European Union has already proposed an update to its F-gas regulations that aims to cut emissions of SF6 by two-thirds from a 2014 baseline by 2030. It is likely that the UK will adopt similar measures.

This will prompt DNOs to take action and drive greater uptake of eco-alternative GIS. In turn, the coming legislation has prompted switchgear manufacturers to develop and refine SF6-free designs. Both dry air and alternatives, such as the fluoroketone-based gas mix, have a role to play. Recognising the importance of this, ABB has opened the patents on the basic use of fluoroketone gas so that the planet can benefit. As a result, it's likely that we’ll see the development of a wider array of SF6-free switchgear over time.

Stephen Gibbs is UK product director for Distribution Solutions at ABB