Power and renewables

SF6 replacements: switching how we think about power transmission

DNV GL auf dem 24. Dresdner Fernwärme-Kolloquium

The effort to replace this potent greenhouse gas in switchgear could also open the door to a new reliable, cost-effective and greener option for electricity transmission.

In a post last year, I discussed how the T&D industry can combine economic opportunities and environmental benefits through new technology. As an example of this, I mentioned the search for greener alternatives to sulphur hexafluoride (SF6) for use in gas-insulated switchgear (GIS) and circuit breakers.

Technically, SF6 is perhaps the ideal choice for such applications, with excellent insulating and arc interruption properties. But it is also the most potent greenhouse gas currently known. Hence, the effort the power sector has invested in finding suitable green alternatives.

That effort is now coming to fruition, and we are starting to see a number of more environmentally friendly solutions coming to market. These include vacuum switching combined with air insulation or replacing the SF6 with mixtures of carbon dioxide and fluorinated-organic gases such as C5 perfluoroketone (C5-PFK) or iso-C4 perfluoronitrile (C4-PFN).

Such solutions typically target GIS applications up to 145 kV, with non-SF6 circuit breakers and substation components already undergoing pilot testing on the market. Indeed, various SF6-free GIS designs were already displayed at last summer’s CIGRE exhibition. Proponents claim that, for these applications, the new, greener options offer almost the same insulation and arc interruption performance as SF6, but these claims are still to be independently verified.

Ensuring quality for greener switchgear 

From a certification perspective, switchgear based on these solutions need to be held to the same high levels of quality and reliability as SF6-insulated components. Essentially this means performing the same types of tests – but potentially with certain important modifications.

For example, high-voltage vacuum-based components are best tested in a 3-phase configuration instead of the single-phase configuration typically used today. In addition, “direct” 3-phase testing where power is provided from a single source eliminates any concerns over the equivalence between test and real service conditions. This kind of testing requires facilities such as the KEMA High-Power Laboratory in Arnhem, the Netherlands and KEMA Laboratories – USA in Chalfont, Pennsylvania that can deliver the extremely high power levels necessary.

One potential issue around the certification of such components is that many of the relevant international standards for testing GIS focus on SF6 and may not be directly transferable to these emerging green alternatives. For example, IEC 60376 and IEC 60480 provide guidelines for purity and other technical specifications for “new” and “reused” SF6 respectively. There is no analogous guidance for any other insulating gas that could be used in GIS. Moreover, whereas SF6 molecules recombine after switching, many of these new alternatives are consumed during switching. This could lead to new lifetime issues.

Industry body CIGRE last year launched a new working group (B3.45) to investigate whether these requirements need to be adapted or new standards need to be developed. This year, a second new CIGRE working group (D1.67) is being established to specifically focus on the dielectric performance of non-SF6 gases and gas mixtures with fluorinated components. DNV is proud to be bringing the extensive testing expertise of our KEMA Laboratories to both of these working groups.

One issue that clearly needs to be addressed is the standardization of certain solutions since mixtures of gases are more difficult to handle than single-molecule gases like SF6. Furthermore, interoperability between components is an absolute must in the power sector, so the current variety of SF6 alternatives is not a welcome prospect.

A new option for transmission

The establishment of these two working groups by a renowned industry organization such as CIGRE shows the importance the power sector places on finding greener alternatives to SF6 for GIS. But the emergence of these alternatives also opens up intriguing possibilities elsewhere in power systems.

For example, in many countries there is a strong desire to replace or supplement existing overhead power lines with underground cables. Such a move could both reduce the environmental impact of transmission and provide an additional means of mass power transport over long distances. Until now, however, these efforts have been held back by concerns over cost and reliability.

Standard cables would need to use considerably more copper than overhead lines to handle the required currents, making them expensive. In addition, a huge number of joints would be required could have a serious impact on reliability. A recent analysis of many years of results from testing cable assemblies at KEMA Laboratories has shown that joints are the most unreliable part of any cable assembly.

This problem could be overcome by using gas-insulated lines (GIL). However, the long distances covered by transmission networks (potentially the length of the country) would require large volumes of insulating gas. If that gas was SF6, the potential climate impact of the enormous amount of banked gas would be unacceptable. However, effective and environmentally friendly alternatives to SF6 make green GIL possible too – and examples of this were also on show at the CIGRE exhibition last year.

With greener GIL options, electricity could be carried with fewer underground transmission systems, and those systems wouldn’t need to be compartmentalized for environmental protection. Thus, gases originally developed for use in switchgear, could enable a new reliable, cost-effective and environmentally friendly means for mass transit of electricity.