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Building the supergrid


Published May 21, 2013

KTH Royal Institute of Technology researchers are helping to ensure that the power grids of the future meet the new demands created by renewable energy.

KTH research focused on the electricity networks of tomorrow will lead to major reductions in carbon emissions, and help to create further export opportunities. (Photo: Holger Ellgaard)

The modernised electricity networks of tomorrow – which are required in order for renewable energy sources to be used – are highly demanding in terms of technology. Today’s electricity networks are inadequate, because they lead to energy waste and create unnecessary emissions. Researchers at KTH have helped to optimise parts of the electricity networks of the future, resulting in reductions in carbon emissions of up to 200,000 metric tonnes per year.

Built in the 1950s, many of Europe’s electricity networks are outdated and in need of an upgrade. One way of carrying out this upgrade has been to switch from using current source converters (CSCs) to voltage source converters (VSCs).

The VSC has many important functional advantages that mean it can be used for various applications. The fact is that a VSC is required to feed in renewable energy from large off-shore wind farms on account of the power-supply variations caused by renewable electricity production. If you want to move on to a complete electricity network powered from direct current (DC), you will need VSC technology because CSC is adapted for point-to-point connections.

Still, more energy is wasted when using the VSC, and it has also been more expensive than CSC. However, researchers are attempting to address the two downsides by working intensively on technology development, and that’s where KTH comes in.

Energy is lost during the switch from DC – which is used to transport power across long distances – and alternating current (AC), which is used at the end of power cables by industry and ordinary users. Multilevel power converters are used to carry out the switch.

Hans-Peter Nee, Professor of Electrical Energy Conversion at KTH, has been working with his fellow researchers on a multilevel power converter invented by the Professor Rainer Marquardt from Universität der Bundeswehr München.

Their research has partly been based on improving the multilevel power converter to minimise energy loss and to manage the power more effectively.

Hans-Peter Nee, Professor of Electrical Energy Conversion at KTH.

“When DC is turned into AC, it’s chopped up into pieces, creating a square wave,” Nee says. “But what you want instead is a sine wave and to create this, the multilevel power converter contains a row of multi-circuit converters that transform the current in several stages – in other words, creating many small square waves.”

This is the process that Nee and the other KTH researchers have reviewed and refined, partly by using thyristors instead of transistors. The result is a reduction in power loss during conversion, from 1.7 per cent previously to 0.7 per cent today.

This difference may at first seem insignificant, but it certainly is not.

“A considerable amount of energy is lost over 20 years, for example, as a result of the conversions,” Nee says.

Fellow researcher Staffan Norrga, Associate Professor at the Department of Electric Machines and Power Electronics at KTH, says: “There are about 150 gigawatt-sized HVDC (high-voltage direct current) links in the world today, about 5GW of which are made up of VSC-based converters. The loss reduction we’re talking about here – from 1.7 per cent to 0.7 per cent – means a 200,000 tonne reduction of C02 emissions per year for these 5GW based on a reasonable estimate of power usage.”

There is, however, more than one way that the new, more efficient multilevel converters can benefit the environment. They are a prerequisite for the electricity networks of tomorrow.

“An increasing number of renewable-energy sources place ever-growing demands on the electricity network – especially as the electricity supply generated from wind and solar power fluctuates,” Nee says. “This is something the super-electricity networks of tomorrow can handle, while the current electricity networks can’t.”

Nee adds that certain countries such as Germany are investing heavily in overhauling their electricity networks, which is in line with their increased investment in renewable energy. Sweden’s electricity network is also due to be modernised and partly transferred to DC.

“I’m convinced that super-electricity networks will be built in Europe, Asia and the US,” Nee says. “The industry that produces these electricity networks is based in Sweden, among other places, which creates significant export opportunities.”

Nee concludes by stressing that the introduction of multilevel technology would have happened even without the involvement of KTH, but at the same time KTH researchers have been among the most prominent groups when it comes to analysing the technology and proposing improvements. Examples of this include the switch from transistors to thyristors, management improvements, proposals for new circuit solutions for sub-modules, and the reduction in the numbers of capacitors used.

“The technology we’ve worked on will be used in a few years; some of it possibly within a year, while other parts may not be used for another six or seven years,” Nee says.

Multilevel converters are a part of the HVDC market. This market is estimated to have sales volumes for HVDC-related technology of between USD 110 to 120 billion between 2012 and 2020, according to the High-Voltage Direct Current Transmission Systems study by Pike Research, 2012.

In addition to Nee, the other KTH researchers and postgraduate students who have contributed to this work are Antonios Antonopoulos, Tomas Modeer, Kalle Ilves, Noman Ahmed, Arman Hassan Poor Chaleshtari and Tomas Jonsson.

For more information, contact Hans-Peter Nee on 070-695 34 70 or at:

Peter Larsson