HVDC is a miracle of modern engineering that could not have been done in the days of Tesla. It removes several sources of losses that otherwise would have turned valuable power into heat. That said, it isn't without drawbacks: the cables are quite expensive, harder to repair and somewhat fragile, and 'local stepdown' which otherwise would just be a properly rated (capacity and insulation) transformer now turns into a much higher technology exercise. HVDC is for now relegated to a long haul role not unlike oil pipelines compared to the AC network which is far more interconnected and wide spread. You are unlikely to see HVDC used for lower level distribution in the next decade, just as you are unlikely to see your local gas station hooked up to an oil pipeline.
DC is also much harder to switch than AC; the latter has zero-crossings which tend to extinguish any arcs that form, but DC will just keep going. Look at the DC vs AC ratings on switches and you'll see a huge difference.
It can be either AC or DC. Aluminum TIG welding uses AC, whereas you'd use electrode-negative DC for steel or copper. As I understand it, with aluminum you need the electrode-negative part of the waveform to transfer heat to the work piece, but you need the electrode-positive part of the waveform to clear out the crud that accumulates in the electrode-negative part. Often you set a lopsided duty cycle and use different frequencies depending on how deep you want the weld to penetrate.
If you go to 100% electrode positive you tend to heat the metal rather poorly, but can turn the end of your tungsten electrode into a molten blob -- which is usually not desirable.
> the cables are quite expensive, harder to repair and somewhat fragile
Nope, HVDC uses the same style of cable as AC. I'm not sure why you'd think they'd be different.
The HVDC cables that can be expensive are meant to be submerged. A feat that only HVDC can do. HVAC can't be submerged due to the capacative effect.
But otherwise I agree. It's more a pipedream for me that HVDC becomes more common place as I believe it'd make grids ultimately more stable and resilient.
Hm, yes, you are right, I must have been reading on submerged cables, but it's a while ago.
The devil is in the details here, AC tri-phase cabling can not easily be re-purposed for HVDC purposes because you only have a pair of conductors rather than three 120 degree out of phase lines. So while technically the cable itself can be the same the carrying capacity of a triple of conductors would be reduced and one of the conductors would be idle, so if this is an in-ground or overhead cable not specifically made for DC that is a lot of wasted carrying capacity.
