ACDC

Long Distance Power Transmission

Years ago, I was struck by the magnificent object on which a colleague’s coffee rested. It was a coaster made from the cross-section of an electrical cable. Through its transparent resin coating, you could see its structure had much more intricacies than most of us would attribute to a simple cable. The coaster was signed by ABB and when I confessed to my colleague that I would love to have one like it, he replied that he was not going to give it to me, and that with that cross-section, of which the copper core was only a small part, over 1 GW of power could be transferred using HVDC (High Voltage Direct Current) technology.

My colleague was the chief ideologist of an ambitious project to transmit 600 MW from a remote peninsula in South Australia to the urban core of Adelaide and the next two years were spent conducting technical studies with ABB to optimise the complex grid connection process.

This technology made a large wind farm economically viable. It is hard to believe that this project was just a small sample of many others whose main metrics are extraordinary.

Scale economies

Everything to do with HVDC is gigantic and about 70% of the global market is shared between two companies: Hitachi (which recently acquired ABB Power Grids) and Siemens. The rest is held by General Electric and other large companies. This oligopoly is due to the insurmountable barriers to entry for HVDC technology, due to its high logistical and technological complexity, and is only beginning to be modified by increasing competition from Chinese companies such as State Grid Corporation of China (SGCC).

A ‘typical’ HVDC project would be defined by the following magnitudes: 2 GW, 500 kV, 1000 km, 2 M€/km, but you don’t realise the magnitudes until you see it up close.

In Sweden, I visited the ABB facility in the city of Ludvika [2], which directly employs about 3000 people (more than 10% of the population). Ludvika is a city surrounded by lakes and nature, and its identity is inseparable from the industry that inhabits it.

The main entrance leads into a huge hall. Its perimeter, and that of the corridors leading from it, is decorated with diagrams, black-and-white photos of engineers with bushy beards and glass cases displaying incomprehensible gadgets. A detailed chronology refers to the major technological disruptions that have been unfolding for a century. Each meeting room is named after an inventor.

The manufacturing areas have direct access to the rail network. A train enters the facility to load transformers the size of multi-storey buildings, ready to be shipped all over the world.

This immersion in the Ludvika facility allowed me to understand why the development of HVDC technology is limited to so few players: it requires a unique combination of vision, expertise, specialised infrastructure and a complete industrial ecosystem housed in an idyllic city. The current market, valued at $10-12 billion, with 250-300 GW of projects spread across very few companies, reflects this reality. To better understand why these figures and the complexity behind them, it is worth examining the technical underpinnings that make this technology unique.

Technical aspects

HVDC technology allows the transmission of electrical power under clearly advantageous conditions over alternating current transmission above a certain threshold. This is where the technical draw is made in the historic current war match. Siemens sets this threshold at around 600 km [3] to transmit 1 GW of power (only 50 km for submarine transmission).

In general, an HVDC system interconnects two AC systems separated by a large distance. At the transmission terminals are the converters, which contain power electronics, filter stages, transformers and control systems. Even if they are drawn in a small box, this equipment occupies entire industrial halls.

The first part of the acronym (‘HV’) indicates that very high voltages (from 100 kV to 1100 kV) are used. Voltage and resistive losses have an inverse and quadratic relationship, and therefore very high voltages are used, so much so that the huge distances required to isolate hundreds of kilovolts give many of these devices the aesthetics of mid-century science fiction films.

The choice of voltage level corresponds to an economic optimisation: the higher the voltage, the more complex the equipment on both sides of the DC cable. The higher voltage range is reserved for links of thousands of kilometres.

The second part (‘DC’) indicates that the transmission is DC, and this has important benefits. The first is that, because there is no reactive current, better use is made of the conductor capacity and there is no need for reactive compensation or intermediate transformers to achieve a uniform voltage profile along the link.

In addition, the ‘film effect’ [4], whereby the current density tends to concentrate near the surface of the conductor, disappears when the current is continuous and the current is distributed evenly over its cross-section, resulting in better utilisation of the cable. DC also reduces the ‘corona effect’ [5] (which causes the crackling that can be heard in high-voltage cables, due to air ionisation) and with it the losses, voltage drops and insulation fatigue.

On a practical level, this translates into significant savings in the complexity of cabling and supporting structures, and a significant reduction in losses. For all these reasons [10], the cost of DC transmission is approximately one third lower than AC transmission.

In addition to these advantages, power electronics allow interconnection of AC systems of different frequencies and independent reactive power management at each terminal. Modern HVDC can provide almost any type of ancillary service imaginable to the interconnecting AC systems.

But to benefit from the advantages of HVDC transmission, some of the more complex electrical engineering problems have to be solved, mainly those arising from the very high voltage.

For example, protection systems must be able to interrupt a current that never passes through zero. In power electronics, the voltage drop must be equally distributed and the firing of hundreds of semiconductors in multilevel converters must be coordinated to the nanosecond.

HVDC transformers [6] must withstand extremely high voltages and operate under very demanding conditions, requiring specialised design and manufacturing.

Moreover, the entire system must operate in a coordinated manner within a few milliseconds, even if its terminals are thousands of kilometres apart.

For these reasons, the cost of the terminals represents a very high initial investment, which is only compensated by increasing the length of the link.

Relationship with renewable energy

The massive integration of renewables without Grid-Forming capacity and hybridised storage would not have been feasible in many parts of Europe without the incorporation of HVDC technology. This explains why we see so many connections with countries like Norway, Sweden or the UK, which has gone from generating 40% of its power from thermal plants to less than 2% in a decade.

There are uninhabited places with immense photovoltaic resources. The countless advances in each and every element related to HVDC technology justify the conception of projects of inconceivable magnitude, such as the direct connection between the United Kingdom and Morocco [8] and between Australia and Singapore [9], with transmission distances of the order of 4000 km.

Another notable area of application is off-shore wind farms, where this technology enables cost savings, energy evacuation with minimal losses and first-class grid services. It is worth reviewing references such as Dogger Bank in the UK [12].

Conclusions

Understanding the magnitude and transformative potential of HVDC technology requires a vision that transcends conventional figures.

HVDC technology represents one of the key pillars for the global energy transition. Its ability to transmit large amounts of energy over long distances, combined with its efficiency and flexibility, make it an ideal solution for large-scale renewable energy integration. The mega-projects currently under development, with transmission distances in excess of 4000 km, are testament to its transformative potential.

My visit to ABB’s Ludvika site, where it all began with a fascination with a coaster showing the cross-section of an HVDC cable, showed me the real scale of this technology. Although our 600 MW project in South Australia did not materialise – a decision that the subsequent blackout proved right – the experience made me realise that behind every ‘simple cable’ there is a century of innovation, thousands of engineers and a technology that is redefining the boundaries of power transmission.

I have to say that in Ludvika I got a coaster, but it was made of plastic and didn’t hold a real section of cable… I got rid of it out of pure envy.

REFERENCES

Cover Photo of a Cobalt-Erythrite shutter, by Ala.