High-voltage direct current transmission enables continental and larger grids
Siemens DC Transformer/Converter
The future is electric and renewable energy will dominate the mix, but how will electricity get from wind farms in the North Sea to central Europe, from Nevada solar farms to San Diego or from the Three Gorges hydroelectric dams to Hong Kong? At present, the majority of high-voltage electricity transmission lines are alternating current, but innovations in the past decades and even years means that they are increasingly likely to be direct current with major advantages for everyone. The phrase and acronym to keep an eye out for are high-voltage direct current and HVDC.
Why will HVDC dominate transmission?
HVDC lines always deliver more of the power put into them regardless of the distance that the electricity travels, which is a significant factor in and of itself. But the big reason this is important is that it’s cheaper at longer distances over land and at very short distances underwater and underground. This means that it’s very useful for bringing electricity long distances from renewable locations, connecting islands to the mainland and even continents to one another potentially.
Over a certain distance, the so called ‘break-even distance’ (approx. 600–800 km), the HVDC alternative will always provide the lowest cost.
The break-even distance is much smaller for subsea cables (typically about 50 km) than for an overhead line transmission.
The diagram is interesting. DC terminals will always be more expensive than AC terminals simply because they have to have the components to transform DC voltage as well as convert the DC to AC. But the DC voltage conversion and circuit breakers have been dropping in price. The break-even price continues to drop.
Right now on modern grids, transmission losses are 7% to 15% with aboveground transmission. With DC transmission, they are a lot lower, and they remain low even if you run the cables underwater or underground.
The archipelago factor
So HVDC makes a lot of sense for longer runs over land and best place for renewables tends to be a good distance from major centers of population. But the other place this makes a difference is for populaces spread over islands. Indonesia is a good example. It has 261 million people living on 6,000 of its 17,500 islands. And a lot of those islands depend on oil and diesel generation.
Japan is similarly challenged. While most people think of it as a country, or if they know more think of it as three or four big islands, it actually has 6,852 islands with 430 of them populated. Japan is looking at two major HVDC power links to Asia to enable it to break free of its need to generate and manage all of its own electricity in a limited geographical area with significant terrain challenges.
And most people think of the British Isles as a few big islands, but it’s actually well over 6,000 islands with close to 200 permanently occupied. Denmark has a mainland, but also 1,000 islands with 70 of them inhabited. This pattern repeats itself globally. The world is much more physically fractal and fractured than is apparent from maps and globes.
Remember that HVDC doesn’t lose effectiveness when submerged, unlike AC transmission. Remember also that renewables are more and less effective in different places, often different places than the people who need the electricity. And remember that offshore wind energy has one of the highest capacity factors available, with 50% as a usual number and 60% not unusual.
When HVDC pays for itself with lines 50 km long, connecting more people and renewables on dispersed islands becomes more economical.
The NIMBY factor
A key element in the successful use of renewables is continent-scale grids. Energy has to flow from low-carbon hydro in areas with low biomass far north, far south, across deserts, from areas with lots of sun nearer the equator, and from wind offshore or in windy plain areas to large urban and industrial areas. In the USA, the best wind and sun resources are in places a long way from most of the people.
With alternating current, that means huge towers and lines crossing long distances. And that means a lot of people protesting because they hate change, they don’t want their views spoiled, they think that their land is somehow special and shouldn’t have a tower on it, or because they have an irrational fear of the electromagnetic spectrum. Much of what has been written about NIMBYs for wind energy applies to transmission projects as well.
HVDC holds the promise of being able to dodge this problem in a lot of places. Where it’s impossible to overcome local outrage at the thought of big metal towers, it is possible to bury the line for a few miles as it doesn’t have increased losses to nearly the same extent and HVDC can travel arbitrarily long distances underground. It’s a bit more expensive, but it’s a way to dodge a lot of the NIMBY concerns.
And the last thing you can do with HVDC that’s interesting is that you can string it on existing AC tower paths replacing the AC lines, effectively making your existing, accepted transmission route deliver a lot more electricity to highly populated areas. That avoids NIMBY complaints too.
So how does HVDC pull off these feats?
This is partially a Thomas Edison vs Nikola Tesla story. Edison was committed to direct current, but Tesla liked alternating current. Alternating current was easier to step up and down and direct current couldn’t be transformed reliably, so alternating current became the transmission and distribution standard for electricity. Edison did some ugly things to try to win the fight, but lost. Then he won economically anyway.
Alternating current has transmission limitations
Most long-distance transmission today is built using high-voltage alternating current, but it has some interesting challenges.
It is limited to 765 kilovolts (kV) per line, which is more than enough to fry an egg by itself and you can string multiple lines. Due to the nature of alternating current, after that, voltage losses due to the electromagnetic field interacting with the line’s insulation and heating it makes AC uneconomic.
Most of the transmission is through elevated lines, in part due to the expense of burying lines and maintaining underground lines, but also because the heat buildup problem is worse underground and they hold more charge, which limits the distance they can transmit electricity to about 80 kilometers. The holding more charge point, capacitance, means that more energy has to be pushed into the line before it can be reversed. Overhead, high-voltage alternating current lines are kept a long way from one another and the earth, because the electromagnetic fields of individual lines interact, reducing total capacity. Underwater alternating current transmission has worse challenges than underground transmission.
You can’t make AC wires thicker than they are to get greater throughput because alternating current has a strong tendency to flow near the surface of metal conductors, so making them thicker adds a lot of weight and not much transmission capacity.
The last bit of the challenge is that frequency is fairly hard to change, so any alternating current transmission must be between two grids operating at the same frequency.
That’s a lot of engineering compromises to deal with, but alternating current is obviously economically viable for transmitting electricity long distances, so these aren’t deal breakers in most situations. But that doesn’t mean that a better solution won’t be more effective.
Direct current transmission eliminates many of the limitations
Until 1954, there was no real alternative to this set of compromises. That’s the year that the problem of reliably changing the voltage of direct current up and down was cracked. ABB, a major player in this space, built a submerged 96-kilometer HVDC transmission line between the Swedish mainland and an island.
Direct current doesn’t have most of the limitations of alternating current for transmission.
While it’s limited to about 800 kV, not dissimilar from AC, the way it’s constructed you effectively get double the voltage of AC.
Underground and underwater lines don’t lose effectiveness for transmission compared to aboveground lines. No electromagnetic field is created by direct current to interact with other wires, the ground, or water.
The wires can be arbitrarily thick because direct current doesn’t tend to flow along the surface.
Direct current has no frequency, so it’s easy to connect two grids at different frequencies and use electronics to match frequency when it’s converted back to AC. HVDC is sometimes referred to as asynchronous transmission for this reason.
Recent advances have reduced HVDC challenges
But direct current still had two limitations that prevented it from taking over the world, at least until recently.
The first is that the voltage converters were a lot more expensive than the simple, physical alternating voltage converters. DC converters are electronic in nature, and as pointed out in the the future is electric article, electronics outperform the physical. That’s true in this case too, with DC transformers plummeting in cost. If you look around, almost every electrical device you can see runs on DC internally and the power blocks convert AC from the plug to DC for the device. That’s been made cheaply possibly by electronics, and it’s true for transmission-scale transformers as well.
The second problem is that circuit breakers for high-voltage direct current were ineffective. Circuit breakers are components that protect electrical systems from excess current. If you have an older home, you probably have a fuse box. If you have a newer home, you probably have electronic circuit breakers, typically a row or two of black switches in a panel. Mechanical breakers for DC were too slow while semiconductor breakers were fast enough but had 30% power loss. This has been hard to overcome, but it’s been licked recently with a new generation of hybrid breakers.
So that’s HVDC and why the electricity you use will increasingly come through HVDC transmission. It allows more power to be delivered over longer distances, it is increasingly economically viable, it works better underground, it works better underwater, it can dodge NIMBY complaints and it reduces the challenge of variable renewable generation. It’s one of the top innovations in the world of electricity, and it’s coming soon to a grid near you if it isn’t already there.
The set of innovations that have led to HVDC transmission being increasingly competitive and effective in places where AC is less effective are going to enable greater growth of renewables globally.
China’s State Grid Corporation has seriously put forward the idea of building a global HVDC grid to tie all of the wind and solar power in the world together by 2050. Imagine wind energy from China at night powering the USA and Europe during the day. The future truly is electric.
Michael Barnard, Chief Strategist, TFIE Strategy Inc. Business and technical future-proofing. Top Writer Quora since 2013. CleanTechnica, Forbes, Quartz+ more. In 4 books.