Direct Current Renaissance

The closing years of the nineteenth century saw a fierce battle to establish the best method for supplying electricity to consumers, with DC on one side (promoted by Thomas Edison) and AC on the other (backed by Nikola Tesla). DC lost, and the world has been dominated by AC ever since.

The story might have ended there but for two things. First, DC is remarkably efficient for long-distance bulk power transfer – indeed, it has been used in this role for decades. Second, more and more of the electrical devices we use are natively DC – everything from your mobile phone to LED lights and electric cars.

All of this is leading to a reappraisal of DC for transmission, distribution and even final consumption by electricity users. So how might this work in practice?

DC transmission

Transmission is the bulk transfer of electrical energy, typically over long distances. This is achieved using overhead transmission lines or underground (or subsea) cables. Using high-voltage DC (HVDC) for transmission instead of high-voltage AC has a number of advantages.

First, less material is needed. This is because DC requires only two conductors (AC needs three). Second, electrical losses are lower with DC because only active power is transferred (by contrast, AC transfers both active and reactive power). Third, the possible length of transmission links is much greater with DC thanks to the absence of reactive power.

HVDC is a proven technology – and it is getting better all the time. Recent developments include voltage source converters (VSCs) and improved transmission capacity for cables. This is achieved with higher voltages, higher operating temperatures, bigger conductor cross sections and the introduction of extruded technology. All of this means that the footprint and cost of HVDC projects is falling relative to the energy transferred. In short, HVDC transmission is becoming much more competitive.

A bright future for HVDC

Two important market trends are driving increased interest in HVDC transmission. The first is the growing demand for electricity interconnectors. These span oceans and link the grids of nations and regions. The second driver is subsea export cables for the growing number of offshore wind farms.

To date, some 15,000 km of HVDC submarine cables have been installed, using both MI (mass impregnated) and XLPE (extruded) cable technology. An additional 20,000 km of HVDC interconnectors are expected to be deployed by the beginning of 2030, not including offshore wind farm export cables. The installed base of extruded cables is expected to increase and equal the length of mass-impregnated cables by the end of this decade. Manufacturers of HVDC submarine cables are positioning themselves to capture the market by investing in more production and installation capacity.[1]

[1] https://www.4coffshore.com/

Could DC be used for distribution as well?

Medium voltage (MV) and low voltage (LV) distribution networks, and power distribution within buildings, have long been dominated by AC. But a progressive shift to DC – achieved through the development of LV and MV microgrids – could bring energy savings, improved interoperability, easier renewable energy integration and greater sustainability.

Interest in DC microgrids is being driven by fundamental changes in the way that electricity is generated, stored and consumed.

First, power generation is becoming less and less centralised and moving closer to sources of demand. Rooftop solar photovoltaics and small wind turbines are examples. Solar photovoltaics are natively DC, as are some micro wind turbines.

Second, battery storage is becoming widespread. Uninterruptible power supplies (UPSs) are one example. These are used by businesses, such as data centres, to maintain supply security. There are also growing deployments of battery energy storage systems (BESSs) for grid balancing. On top of this, home energy storage systems are now becoming available. Last but not least, electric vehicle batteries have grid integration potential. A key point about battery storage is that most of it is distributed rather than centralised, and all of it is natively DC.

Third, on the consumption side, DC devices are now widespread and uptake is accelerating. As noted earlier, many commonly-used devices, from phones to LED lighting and electric vehicles, are natively DC. Today, all of these devices depend on adaptors to convert AC to DC.

All of this is creating an environment that is ripe for DC microgrids with generation and consumption in the same grid, backed up with battery storage – including electric vehicle batteries. One of the beauties of the DC microgrid model is that it removes the need to convert AC to DC, eliminating the need for adaptors – an energy saving in its own right.

How is Nexans enabling DC?

Nexans is a leader in the submarine HVDC market and the company continues to invest in growing its manufacturing and deployment capacity. In 2021, we launched Nexans Aurora, the world’s most advanced cable laying vessel. Nexans is well positioned to support the future needs of both transmission system operators and wind farm developers.

With DC deployments growing in the high-voltage transmission sector, the next step could be medium and low-voltage DC microgrids. These will need to utilise optimised cables, accessories and connectors to be technically viable. They will also need to be reliable and to meet the requirements for energy efficiency, sustainability and safety.