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Modern DC Transmission Options

HVDC and UHVDC for long distance, power electronics advances, switchmode converters, China's 800kV grid investments, undersea cable applications

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While AC power once triumphed due to the simplicity of transformers, modern power electronics and switchmode conversion are reviving DC as a highly efficient solution for long-distance transmission and undersea applications. Proponents highlight China’s massive 800kV grid investments and the ability of DC to carry more current over existing wires by eliminating the "skin effect," yet skeptics warn that DC remains economically impractical for complex radial distribution networks. The transition is further complicated by the extreme costs and technical difficulties of DC circuit breakers and coordination, leaving the industry to weigh the benefits of advanced electronic efficiency against the massive inertia of our established, mass-produced AC infrastructure.

17 comments tagged with this topic

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> practical voltage for a DC grid using early electrical machinery is around 2 kV. What is a current (pun!) practical limit? If a 100MW PV farm and a data center are separated by 1km (20 Olympic pools) - is there a way to avoid AC? I know there are future solutions [1] [1] https://techcrunch.com/2025/04/07/former-tesla-exec-drew-bag...
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The early limit was because high voltage DC required producing it at the generator, whereas you could produce high voltage AC by generating at a lower voltage and then stepping it up with a transformer for long distance transmission. The rules are changing because of switchmode voltage conversion, using transistors to switch the voltage at a high frequency, where the magnetics (transformers, inductors) can be much smaller and more efficient, then converting back to DC. This is how virtually all smaller power supplies have been made for years, the only question (which I don't know) being how far along we are at reaching the voltage levels of long distance transmission in this way. I'd think that hustling us towards DC with electronic voltage conversion would be a reasonable strategic goal for dealing with the transformer problem, worthy of support by a government.
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HVDC and UHVDC are used extensively for long distance transmission, notably for undersea cables and in China, which has made massive R&D investments in the technology in order to shift energy from West to East. Large solar, wind and hydro in the West. However, DC does not make sense for a radial power distribution network. The article is propagating nonsense.
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>>> However, DC does not make sense for a radial power distribution network. Why not? Pure geeky curiosity.
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Virtually all HVDC transmission currently operating is point to point mostly for control reasons. My understanding is it's very difficult to coordinate multiple converter stations - power flow in DC networks is fully determined by the control systems of the converters unlike AC networks which in general lack active control devices (see the FACTS family of devices for examples that can be used in AC networks to actively control power flow).
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Not an expert but every node in an UHVDC network would need expensive equipment Point to point is just two nodes, but scaling that outward would be very expensive AC transmission is relatively cheap in comparison
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Huge installed base of network elements, minimal efficiency improvements. Much better to invest in switch mode frequency stabilisation with batteries and soft open points (SOPs), which balance load between phases and distributors without needing a radial reconfiguration. Radial DC is anachronistic thinking based on misunderstandings perpetuated by C-suite level just so stories like this Bloomberg nonsense.
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HVDC transmission over 100kV lines are common now. https://www.emeranl.com/maritime-link/overview
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That link talks about 5MW 35kv AC / 800v DC converters.. completely different thing, they try to sell a single-source PV invertor-to-35KV AC solution first, then 35KV to 800V DC second, to have a sorta complete solution of PV-to-datacenter. And it's only 5MW. And only 35KV AC. For moving 100MW even over a few km you would need 110KV at least. I think. An overhead wire can handle about 600A of current, that's the physical limit and the reason for kilovolts there. Consider also that there is nothing existing in transmission and switching gear certified for HVDC it being rare one-off projects so far, while AC is ubiquitious, more-or-less mass-produced and many people are trained in its maintenance.
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I've been wondering for awhile about the economics of the AC vs DC grid thing. Historically, AC made a lot more sense because transformers are simple and relatively straightforward to make. But now we have amazing capabilities to handle enormous amounts of power with modern IGBTs and similar power-switching transistors. (A modern high-end EV motor controller, for instance, might be able to handle a megawatt of power. Not continuously, but still.) Is a DC-DC converter now more economically viable than an equivalent transformer? The former is more techincally complicated, but the latter is bulky and requires large quantities of expensive input materials like copper.
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Or, alternatively, you switch to DC to get more current capacity over existing wires. (At a given voltage, a wire can generally carry more DC current because it doesn't have the same "skin effect" that AC has.) Even if the hardware at the substation is more expensive, it might be cheaper than upgrading the transmission lines.
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well aside from being effectively 20 times more expensive over entire operation... DC switches (as in, just a power switch) are vastly more expensive because while in AC you have 100 breaks in current a second, DC is constant so it is far harder to break. So even if you had device that could use both (not hard with SMPS, they have rectification as first step), it's still essentially " replace everything".
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The protections side is a big problem - most HVDC has circuit breakers operating on the HVAC sides of the link so going to full DC transmission presumably wouldn't eliminate that equipment.
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China has the most sophisticated grid in the world, and is spending $100B a year on expanding and upgrading. They have a uniquely high share generated by renewables. It runs 800kV and will go higher after the upgrade. The first Small Modular Reactor will come online this year. If you think that’s all just being built in random factories without standards you’re very much mistaken.
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"Transformers are necessary to make the AC system work." This isn't quite wrong but the motivation is backwards: AC is necessary to make transformers work. 1. All grids need to move energy at high voltage and low current to minimize losses. 2. This requires a mechanism to step voltages up and down for transmission. 3. In 1890 the only such mechanism was the transformer. 4. Transformers only work on AC, not DC. Hence our legacy grid is AC. Nowadays we have an additional mechanism: Power electronics. Power electronics work on both AC and DC, so transformers with their huge requirements for copper and steel are no longer necessary. We need to accelerate the transition of our grid to DC because DC grids are simpler and cheaper than AC grids.
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Grid-scale power electronics are also extremely niche and expensive, perhaps moreso than transformers. HVDC is used where it has a significant advantage, but ease of conversion is not one of those.
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It might be easier for DC transmission components to be standardized. Sure, anything with complex controls has a lot more opportunity to fail to interoperate, but DC gear can often be configured for different voltage ratios and can much more directly control how much current flows where. Maybe the grid needs a multi-source agreement for equipment like the network industry has for optics.