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The resurgence of direct current

David Cross

Advances in power electronic converters are making high voltage direct current (HVDC) a more feasible solution than alternating current for long-distance transmission infrastructure projects.

The ‘War of the Currents’ is the name given to an at times fierce battle between proponents of alternating and direct current power supply that played out in the US when electricity was first starting to develop commercially in the late 19th century.

The two main protagonists were Thomas Edison, in the direct current (DC) corner, and Nikola Tesla, in the alternating current (AC) corner.

A key factor in why AC won – and why all cities today are powered by AC-based infrastructure – is that transformers don’t work with DC.

This meant, at least in the days of early power generation and transmission, that the voltage within the distribution grid was the voltage at which it was generated. Even in the infancy of the power industry, and thanks to some highly publicised fires in New York associated with Edison’s prototype DC grid, this was soon recognised as impractical.

But today DC is enjoying something of a resurgence, especially when it comes to the transmission of large amounts of power over long distances.

Reduced power losses

It is essentially a balancing act. If you want to send power a long way, you need a high voltage. AC transmission is inefficient over long distances, with power loses typically between 35-40% compared to the power losses in DC transmission which are in the order of one tenth of that.

DC transmission infrastructure, however, is expensive to build, in particular because it requires big and complicated boxes of power electronics at either end to convert it to or from AC, and to step the voltage up and down. But DC transmission towers are smaller and carry fewer wires than AC equivalents, therefore being cheaper, requiring less right of way and have less visual impact. Accordingly, broadly speaking, the more power you are shifting, and the longer the distance, the better DC is as the transmission solution.

High voltage direct current (HVDC) transmission has in fact been around for nearly a century, but its use until very recently has been limited. It is advances in the electronics that underpin the converters at either end that are making HVDC increasingly the solution of choice.

Offshore windfarms and subsea cable are key drivers

The key innovation underpinning the resurgence of HVDC over the last 20 years is the development of power transistors known as insulated gate bipolar transistors (IGBTs), which provide a more efficient switch and converter design known as a voltage source converter (VSC). Converting from AC to DC and back to AC requires semiconductor switches and the introduction of IGBT transistors has made the converters much more controllable and needing a smaller civil footprint.

The offshore wind farm market, requiring undersea cable connections, is proving to be an important driver for VSC technology. Germany, for instance, has been a pioneer in this area – initially to reduce carbon emissions and then following the 2011 Fukushima nuclear disaster, which prompted it to find alternatives to the 17 nuclear reactors it has pledged to shut down by the end of 2022.

A historical problem, particularly for offshore windfarms, had been the size of the valve halls needed at either end of the cable, which made the offshore connection very expensive. Swedish technology company ABB developed a system called HVDC Lite, incorporating the latest VSC/IGBT switching equipment, and it provides quicker, faster and more efficient conversion with fewer losses. It is also a more compact HVDC convertor that you can install more easily on a platform out at sea. Other manufacturers quickly produced similar systems.

For subsea cables, as things presently stand, DC typically becomes economic where transmission lengths are over 70km; for overhead power lines the tipping point in favour of DC is around 150km. (It’s the physics resulting from the cable design required in subsea cables that makes the losses in AC cable transmission significantly worse than in overhead lines.)

Increasing market share

As both the power electronic and DC cable technology continues to develop, both VSC and line commutated converter (LCC) systems will find a growing market.

HVDC is well suited to the increasing trend for electricity to trade across international borders, particularly within Europe. A 2GW HVDC link using the traditional thyristor type LCC system was installed between England and France as early as 1986, which mainly brings French nuclear power into the UK. A new 1GW project with DC cable running through one of the channel rail tunnels is currently under design/construction.

Looking to the future, more long-distance interconnectors from Europe to UK are in the pipeline, for example, for connection to Scandinavia’s surplus hydropower. There is even talk of a link to Iceland to tap into its huge geothermal energy resource – it’s a phenomenally long way but advances in HVDC converter and cable technology mean it is now technically viable.

HVDC offers other advantages. It improves power system stability and can be used in an AC system to form a hybrid system. Embedded HVDC offers opportunities to strengthen grids with reduced environmental impact compared to upgrading AC transmission lines. And asynchronous HVDC connections are often the only practical way to connect systems operating at different frequencies or where it is not possible to make a direct synchronous connection.

As HVDC makes further inroads into the HVAC segment of the market, Thomas Edison, the greatest champion of direct current, is no doubt looking down on all this and smiling.

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