Hvdc can go a very very long way; thousands of miles economically.
500kv a.c. @ 60z starts to become impractical above 1000 miles if I recall correctly. I'll not sure how 50hz power is more or less affected on transmission, but it requires larger transformers, making each conversion step more expensive from a fixed cost perspective.
Edit:
I didn't recall correctly, looks like hvac is not very effective above 700mi, and in underground and underwater applications, far less.
First, because you need to turn it into AC eventually and it doesn't have a frequency, it makes connecting grids a snap. You can hook a 50Hz grid to a 60Hz grid or even just two 60Hz grids with different phases.
The lack of alternating means you can bury the line. HVAC will lose power when it is close to conductors (ground, water) due to the capacitive effect. It technically still happens when in the air, just to a much smaller extent.
HVAC's benefits are that it is cheap to make transformers for. It is relatively easy to kick it up to 2MV then down to 240V. The often cited reason AC won over DC is that you can transmit it for cheaper, which isn't totally true, the reason AC won is because you can transform it easier. It was much easier to generate power at 40V, bump it up to 1kV, then down to 240V. Higher voltage == less line loss.
Wikipedia said: "The electrical grid in Japan is isolated, with no international connections, and consists of two wide area synchronous grids which run at different frequencies and are connected by HVDC connections. This considerably limits the amount of electricity that can be transmitted between the north and south of the country."
That side benefit of the cheap HVAC transformers makes HVAC practical for transmission where you have many cities that you are passing by and need to distribute power to.
HVDC shines when you have a hydro plant a long distance away, with nothing in between.
If you want to pass by a bunch of cities in between and distribute power to them, it would get expensive to put a small HVDC substation “spur” at each.
Can you mix HVAC and HVDC on the same infrastructure?
Yeah, nothing really stops you from intermixing, you just need an inverter at every point where the two join.
The problem is that inverters are expensive enough that you really don't want to do it a lot. This is why HVDC is usually used for grid interconnection or underwater transmission.
If you can drop a bunch of lines down, then with AC there isn't to much of a reason not to just pump the voltage up to insane levels. The main drawback is that the higher the voltage you push out, the more AC power will want to "leak". That effectively means you need larger pole and you need a larger distance between the AC lines. This is why you can easily pick out high voltage AC lines from regular lines, they are larger towers with bigger distances between lines.
Would this mean that the future of long distance energy transmission will be underground HVDC? And we'll see the slow deprecation (over time) of overhead HVAC transmission lines through right of ways?
Doubt it. HVDC is still HV. While you could bury the lines without a bunch of loss, you also have to start thinking about insulating the lines to keep the electricity contained but also to make sure that if someone digs up the line they don't cause a situation where you'd have to kill a large portion of the grid.
A benefit of aerials is that, all things considered, they are pretty cheap to put up over land. You don't have to insulate the lines, the air does that. You don't have to worry about people digging them up, they are too high. And if the voltage is high enough, you don't generally have to worry about something bridging the lines and causing a fire.
About the only thing you need to worry about is some natural disaster taking out the tower.
What I see HVDC being used for is joining the various grids of the world to allow for power from Texas to supplement power in Idaho. A national grid would improve efficiency, especially of renewables across the time zones.
Perhaps we'll see it used more in places like California, where the land itself isn't so valuable where HVAC runs, but the risk of combustion and cost of remediation is so high it's cheaper to run HVDC underground where it can't start costly fires that bankrupt utilities.
500kv a.c. @ 60z starts to become impractical above 1000 miles if I recall correctly. I'll not sure how 50hz power is more or less affected on transmission, but it requires larger transformers, making each conversion step more expensive from a fixed cost perspective.
Edit: I didn't recall correctly, looks like hvac is not very effective above 700mi, and in underground and underwater applications, far less.