525 kV XLPE DC Cable Systems – a key contributor in the green transformation

By Michael C. Hjorth, Executive Vice President, Chief Commercial Officer, HV Solutions, NKT:

Climate change and environmental degradation are an existential threat to Europe and the world. To overcome these challenges, Europe needs a new growth strategy that transforms the Union into a modern, resource-efficient and competitive economy where there are no net emissions of greenhouse gases by 2050. The European Green Deal is the roadmap for making the EU's economy sustainable and based on renewable energy. The decarbonizing of the EU’s energy systems is critical to reach the climate objective and an upgraded power grid is essential for success.

An interconnected power grid of highly efficient power cables is a key component in paving the way for efficient integration of renewable energy across Europe. This requires connection of remote green power generation, interconnections between countries and markets as well as efficient power transmission with low losses over long distances. Here, high-voltage direct current (HVDC) power cables are the most efficient way for transporting energy and several large HVDC (high voltage direct current) interconnectors are in use, under development or being planned to contribute to achieving the target of carbon neutrality in Europe by 2050, for example with the large DC corridor projects in Germany.

In May 2020, NKT won the first commercial order for a 525 kV extruded HVDC cable system with XLPE insulation for the 2 GW project SuedOstLink in Germany. The 525 kV cable system is a key element in developing a DC grid that can utilize renewable energy generation more efficiently. A single pair of 525 kV extruded HVDC cables can transmit enough power from large offshore wind farms to supply approximately two million households. The power can be transmitted over longer distances than previously possible while keeping the low transmission losses. The innovative and proven XLPE insulation has a low DC conductivity. This is a key property in order to avoid electrical failure from a thermal runaway process at high electric stress.

Over the years, we have built a long track record of manufacturing and installing around 6 500 km of efficient, reliable and powerful high voltage XLPE cable systems for submarine and underground DC cable applications, often used for long interconnectors, for example Murray Link Australia (in operation since 2002), Cross Sound (in operation since 2003), Gotlight, (in operation since 1999). We have also more recent projects like Caithness-Murray UK, EWIP UK – Ireland, Estlink Finland - Estonia, and NordBalt, Sweden - Lithuania. And latest for the SuedOstLink. For the total DC cable system market, there are around 12 000 km DC XLPE cables installed or under construction.

Since its introduction in 1998, XLPE DC cables systems, together with the converter technology, have provided a major platform for HVDC power transmission. The development and commercial introduction of XLPE HVDC cable systems starting at 80 kV level in late 1990’s grew very rapidly to 150 kV and was followed by 200 kV and 320 kV, which inherently also raised the transmission capacity towards and above 1 GW. All these systems are based on the same technological platform. In order to reach higher voltages, and thereby transmission capacity, a continuous improvement of the XLPE insulation material was needed. The material system together with NKT’s know how in production and installation of DC cables and advanced field grading technology provided an even better platform for reaching far higher voltage levels.

In August 2014, we introduced this improved technology to the market by launching a complete HVDC cable system for both land and sea applications at the record-breaking level 525 kV. The cable system, also including a number of newly developed accessories, enables electricity transmissions at significantly higher power levels than previously possible. It is proven in line with the qualification process according to international standards and recommendations.

NKT’s latest world record 640 kV XLPE DC cable system launched in 2017 is the result of our intensive investment in research and development in the field of HVDC transmission technology. This development has been made in continuation of the 525 kV cable system that was launched in 2014.

The environmental footprint of any high voltage cable system is primarily determined by the losses aggregated during the lifetime of the system. When using XLPE HVDC for transmission of renewable power the losses are very low. This means that almost all the power sent in to the cable also reach the end of the cable, ensuring a higher output of electricity to be fed to the transmission networks over the system lifetime. This ensures that power generated from wind, hydro or other renewable sources are used efficiently.

At NKT, we ensure that the cable system is manufactured with low carbon footprint and we thereby run both our high-voltage factories on 100 % green electricity. During the past years, we have also successfully industrialized the process of recycling XLPE in close collaboration with AXJO Plastic which is now manufacturing robust cable drums from the recycled XLPE material.

If we shall reach the target of being carbon neutral in Europe by 2050, all parties must contribute. At NKT, we are committed to do our part in making Europe a carbon neutral continent with manufacturing and installation of robust and green DC cable systems.