How common are long-distance DC lines and what are they used for?
My understanding is that the vast majority of power infrastructure is AC, and long distances are using High Voltage AC lines
My understanding is that the vast majority of power infrastructure is AC, and long distances are using High Voltage AC lines
As renewables (solar, wind, hydro) become more popular, so too does the need for reliable long-distance power transmission. With fossil fuel, fuels could be delivered to the point of generation - with solar, wind, and hydro, the location of the generator is non-negotiable and so the energy must be transported instead.
So we'll probably see more and more HVDC systems in coming years.
Similar effects happen with normal power lines as well, so it also has benefits on long-distance connections on land. Since you need conversion equipment on all ends, it's typically used only for point-to-point links, not for lines with branches in between - an example might be connection hydro plants in the wilderness somewhere to more populated areas.
For links between non-synchronized networks, since conversion is needed anyways to match between them.
Wikipedia: https://en.wikipedia.org/wiki/High-voltage_direct_current and https://en.wikipedia.org/wiki/List_of_HVDC_projects
The recent change is the fact that high-voltage semiconductors now make step-up/step-down efficient. In many cases, more efficient than purely using transformers (most semiconductor stuff still uses some form of inductive effect--they just do so at MUCH higher frequencies than in the past which allows them to use MUCH smaller components and get much higher efficiencies).
One of the big advantages of AC has to do with switching. If you flip a switch on AC, within about 10 milliseconds the voltage goes to zero as a natural consequence of the waveform irrespective of what current was flowing previously. A switch can interrupt that without much grief.
If you flip a switch on DC, it has to completely stop the current flow in the presence of a large current and/or voltage that will create a conductive plasma. That means a much larger switch for the same level of power.
There is a reason why your ISP has a huge switch and it is often encased in some inert gas. And also why it sounds like a bomb going off if it trips. And why it takes a hydraulic pump to reset.
Side note: The Pacific DC Intertie was the largest in the world until the 3 Gorges Dam interties--and it dated to the Kennedy administration and used vacuum tubes. https://en.wikipedia.org/wiki/Pacific_DC_Intertie
On a related note, there was a fascinating article on sf's dc grids by Spectrum https://www.google.com/amp/spectrum.ieee.org/tech-history/da... (pardon the amp link)
My office building taps into it to power the old school loading elevator!
[0]: http://www.eib.org/infocentre/stories/all/2015-february-02/c...
https://www.economist.com/blogs/graphicdetail/2017/01/daily-...
https://en.wikipedia.org/wiki/HVDC_Three_Gorges_%E2%80%93_Gu...