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DC grid conversion is much more expensive than even a large transformer.

Cold (aka "black") start is not a common occurrence. Vacuum cleaners are not much of an issue. Industrial consumers are usually mandated to sort out their apparent power factor so it's not too weird for the grid.

What does matter sometimes is phase/frequency trips. The grid frequency is an important coordination mechanism between generators. When it gets distorted by loads coming on/offline, sometimes this can cause other generators to trip out at 59/61Hz, and then you get the Spain blackout situation.

Batteries could solve this but the software/regulatory framework isn't entirely there yet. See e.g. the UK market for "fast frequency response".

> With a bunch of AC microgrids joined by a DC major grid, you can completely sidestep that problem.

Not necessarily. Big local consumers will be large relative to the microgrid, which will not have a lot inertia. This is one of the things that you really notice when you go 'off grid', your grid is essentially your house and whatever else you decide to power from it and unless there are a couple of beefy motors already running starting a new one has a high likelihood of tripping the inverter, even a very beefy one. Start-up currents for larger consumers can be really high and you need a lot of inertia in your grid to overcome that.

> Start-up currents for larger consumers can be really high and you need a lot of inertia in your grid to overcome that.

This is true of an AC grid as well. Big inductive loads will often have to buy special equipment before hooking up to the grid because of their impact. It'd be the same with a DC first grid. To overcome a large startup current they'd likely need to buy a bunch of capacitors. Which, funnily, is exactly what they'd have to do to run on straight AC.

I grew up understanding that one of Tesla’s big innovations was using AC to transmit power distances so that there weren’t tremendous losses and line meltings or something. Can someone help me reconcile the delta between this understanding and the above comment? Was this not actually a thing? Or have we overcome it somehow?

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.

> 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.

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.

A nice demonstration:

https://www.youtube.com/watch?v=Zez2r1RPpWY

A more detailed explanation: https://www.youtube.com/watch?v=mQpzwR7wLeo

I take it welding is DC?

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.

AC/DC hybrid transmission infrastructure defeats important fault handling and inertia characteristics you get for free out of a fully connected AC grid. HVDC converter stations cannot handle remotely the same amount of fault current that synchronized machines in an AC grid can. It's maybe 1/10th the capacity. You also don't get the same guarantees of zero crossing in a fault scenario with HVDC. If you never cross zero, it's possible some circuit breakers cannot function anymore.

Do you have first hand experience re-energizing an entire grid, section of the power system, or individual generators after an outage?

Not a lot of people do, but you can generally read regulatory plans for how to do it. Here's the UK's: https://www.neso.energy/industry-information/balancing-servi...

There is also the new distributed restart plan: https://www.neso.energy/about/our-projects/distributed-resta...

I've done it. The larger the grid, the more difficult it is. But as long as you have fuel and an adequately maintained grid, its not as hard as some in the comments make it out to be. Better regulation would make it easier. For instance, in Singapore emergency diesel or some other method for black start is a requirement for most generation stations. The rest of the world likely has more lax requirements.

I’ve done small systems less than 2 MW with 2-3 generators, a mix of hydro and diesel, and only distribution level voltages.

It seems like you would black start some plant using its backup diesel generator to power pumps and controls, probably synchronize at least two units at the plant, then pick up some part of the transmission system. Perhaps the capacitance of the grid is and issue when it is unloaded, so the trick is to make sure enough real and reactive capacity is online for each subsequent transmission or load step. Probably a lot of transmission steps come with a significant amount of load too - not just picking up unloaded transmission lines. Sounds really fun, banging all the governors and exciters with once in a decade load steps