High-voltage direct current (HVDC) electric power transmission – or as some say, the “electrical superhighway” – has become the technology of choice when it comes to transmitting power from renewable energy sources to the wider grid.
As offshore wind projects have proliferated and expanded, with locations further from shore, developers have been hard pressed to fit an ever-increasing amount of transmission equipment onto offshore HVDC converter platforms.
Recent developments with DC gas-insulated switchgear (DC GIS) from Siemens Energy have not only overcome the size challenge but have increased the voltage range for this HVDC technology.
Dr Maria Kosse, who has over 9 years’ deep understanding of HVDC gas-insulated systems within the energy sector, is currently Lead Engineer for Grid Access applications at Siemens Energy. Before taking on her latest position, she was responsible for the development of high-voltage DC GIS assemblies for up to ±550 kV. Energy Northern Perspective spoke with Dr Kosse about DC GIS technology and Siemens Energy’s work with its partners in their efforts to demonstrate the technology and its supporting DC cables.
The DC comeback
Dr Kosse explains that historically, AC has dominated electricity transmission, “but DC has been coming back in the last decades.”
The reason? Renewables.
“We see digitalisation, connectivity, globalisation, electrification, which are related to the current growing demand for energy, especially ‘clean’ and decarbonised energy, so to speak. And in the context of the necessary reduction of the greenhouse gas emissions, this now means the use of electricity generated by renewable sources,” says Dr Kosse.
And it’s the distances from these renewable sources that have driven the uptake of HVDC, Dr Kosse relates, “The technology has a special role to play to enable integration of wind energy, which generally occurs at great distance from the shore. This also ensures the exchange of large amounts of electricity over long distances in a very efficient manner.”
Old tech meets new
As Dr Kosse describes it, the location of a DC GIS evokes transportation that hails from long before the advent of modern superhighways. “We use the phrase ‘switchyard’, and there we have different functionalities. There’s a disconnector at the heart – and we have earthing switches, for example, to ensure everything is safe for maintenance requirements. Also, we have devices for measuring current and voltage and for handling overvoltages, so there’s a lot of functionality that needs to be reflected in a small space. And this is combined to the so-called ‘switchyard’.”
When asked about the advantages of using gas for insulation, Dr Kosse replies, “The switchyard can be insulated with different gases, and the simplest one to use is the ambient air we breathe. But atmospheric air is quite poor in terms of insulation performance compared with other insulating materials. So, what we are using in our gas-insulated switchgear are technical gases kept under pressure. For example, a pressure increase of factor five will also increase the insulation performance by nearly factor five and hence reducing the space needed for insulating the voltage.”
“For the development of our DC GIS, we took our proven AC GIS technology that has been in service for more than 50 years now,” she says. “We have very good knowledge of this technology, and we worked to identify how to adapt it to the DC world.”
But applying lessons learned from AC switchgear was not without its challenges, notes Dr Kosse: “So, it’s basically all about the control of the electric field. Therefore, we designed a new solid insulator. The gas-insulated switchgear, whether it’s for AC or for DC, consists of the insulating gas under pressure and the solid insulation – because you need a solid partition insulator ensemble that holds the conductor in its place. And this insulating system controls the electric field. In terms of AC or shortly after energising with DC, the electric field is controlled by the permittivity – the epsilon – of the insulating materials.”
“For a continuous DC application, this is a completely different story, because the electric field is controlled by their conductivities, which are extremely temperature dependent. And when you’re transmitting power, the current leads to a temperature gradient in your equipment. This means different temperatures lead to different conductivities, which lead to different electric fields,” Dr Kosse explains. “And this is what we understood from our in-depth investigation – and in what we reflected in the design of the new insulator. So, the enclosure of the gas-insulated switchgear is the same as we use for AC, and one part of the insulating system inside – the gas – is also the same, so basically the solid insulator has been adapted to secure the electric field of the DC voltage stress.”
Small, but scalable
“From our experience over the last 50 years, we know that gas-insulated technology saves a lot of space, and space is money for example in terms of applications offshore or in densely populated areas,” she says. “And this benefit we also wanted to have for our DC application, and that’s why we developed the DC GIS.”
The space savings also lends itself to modularity, with the benefit of added scalability. “In terms of it being more a modular station, there would be no limitations, so it depends on the functionalities you would like to reflect in the switchyard. If you need to build up a complex arrangement, then you can just add more modules, more disconnectors, more passive components, and arrange the busbars to make all the connections,” Dr Kosse relates. “So, there is simply no limitation in terms of the modules – especially when it comes to a multi-terminal approach and different power transmission media, such as overhead lines or HVDC cables.”
Type approval testing
This summer, Siemens Energy, along with partners Sumitomo and Südkabel, performed a world-first type approval test for the 525-kV direct-current (DC) cable system and DC GISGIS at Mannheim laboratories of KEMA Labs, a CESI brand in Germany. “We have type tested according to the latest international cabling system requirements, and this is also very important for the upcoming projects where we were expecting a collection of DC cable lines for offshore converter platforms as well as for onshore transition stations,” explains Dr Kosse. “The cables that we used for the tests have been specially developed by Sumitomo with a most advanced XLPE compound.”
“The energy sector was waiting for tests combining DC GIS and DC cable for the 525 kV level,” says Dr Kosse, describing how the type tests paved the way for future development. “We have test requirements for the DC cable reflected in current standards, but there is nothing written about how to test the DC GIS and the DC cable together. Now, we at Siemens Energy, along with Sumitomo and Südkabel, have tested for the nominal voltage of 525 kV, and there has been a push from the energy sector to have this as a role model for projects that use such combinations.”
“We are also looking forward to the future, because in addition to our requirements for a voltage level of 525 kV, we have performed subsequent polarity reversal operations for the voltage level of 640 kV, Dr Kosse continues. “And now that this has passed, it ensures the integrity of the system towards higher voltage levels, which is the way forward. In the future, transmitting power will mean higher voltages and higher currents and this is what we expect with the growing demand for electrical energy in the coming years.”
“We are really proud that we could reach this goal and to make the technology available to the community and the energy sector,” she adds.
Milestones toward the future
“The DC GIS already has a very high rating and is capable not only for 550 kV high voltage, but also for 5,000 amps of current, and this is already a look towards the future of power,” explains Dr Kosse, describing how the recent tests confirm the way forward. “The gas-insulated technology is capable of high currents as well, which is also a very big advantage. In terms of space, it goes hand-in-hand with the insulating materials used, which is why we worked to develop the combination of DC GIS and DC cable.”
“Together, both are making the most out of the insulation performance, and by combining these technologies we receive the greatest advantage in terms of space savings,” she says.
“One example is the offshore converter platform, where the goal is to have the smallest footprint you can get, because a greater size and weight are costly. And here the DC GIS, in combination with the DC cable, will be the solution, especially in terms of high voltage. The higher the voltage is, the more space you need with air-insulated technology – and that’s why the gas-related technology is proving to be much more beneficial.”
“Thinking towards the future, we should be expecting higher voltages, in projects with applications from 525 kV up to 2,000 megawatts for transmitting power,” Dr Kosse continues. “We also envision a multi-terminal approach in the DC world. So rather than the current point-to-point DC connections, we are expecting in the future to have the multi-terminal approach so that in a case when one of the lines is not in service, the operator can switch over to a different DC line and route the energy, for example, over a different wind farm.”
And the future is not so far away, Dr Kosse notes.
“And we can expect these developments in the next 5 to 10 years. We are already developing these concepts – not only on paper – but in the way we are working to apply them.”
Just at the beginning of this year, transmission system operator TenneT announced an innovation partnership with Siemens Energy and other suppliers to develop the first 2 GW offshore HVDC grid connection in the Netherlands.