I ask as they specifically cite the need for transmission from west to east (to power hungry cities).
I have also wondered how efficient can those transmission towers be.
Anyone have any knowledge? Is it reasonable to expect power to be generated on one side of China, and have it used on the other side?
Edit: qualified maximum distance to economically feasible maximum distance
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.
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.
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?
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.
> Since 2006, it’s built 19 of these multi-billion-dollar lines, stretching almost 30,000 kilometres and supplying 4% of national electricity demand. For comparison, no other country has a single UHV line in full commercial operation.
> But China’s enthusiasm for UHV is waning. The technology is beset by conflicts of interest between grid companies and central and local governments. The lines themselves are underperforming, and more recent projects are coming online amid a period of electricity generation overcapacity.
> This means that approvals for new lines have slowed, and grid companies are unlikely to meet their targets for new ones.
https://www.chinadialogue.net/article/show/single/en/10376-S...
https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity...
So basically the distance is determined by economics. If China could install solar and cables cheaply enough, then they could compete with domestic European and American electricity. I find that doubtful though.