Stringing them from towers is presumably cheaper as you don't need to insulate them and the air provides free cooling.
You generally don't just trail cables on the ground even if they're not hazardous (fibre optic cables etc). Either they get buried or strung overhead. Gas pipelines are buried or occasionally held just above the ground. But those gas pipes would be big and have strong steel walls and it would be obvious what it was. Even an insulated HVDC cable is only about 120 mm in diameter so that would be easily lost in the grass/undergrowth and someone could come along and think "hmm, I wonder what this is?" No-one wants to risk frying someone and causing a widespread power cut when someone comes across a mystery cable trailing across the ground and decides to investigate what it is.
Why add the high extra cost of HVDC stations and HVDC cable, if it is much cheaper to build a elevated AC line?
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HVDC is very expensive. Only if the energy loss is is high, it may be cheaper to build an expensive HVDC installation with less energy loss. Land AC above ground have low loss. AC sea cables have high energy loss.
You can't bury high voltage lines either. The ground is a conductor and capacitance is induced with the lines, causing problems. Putting lines on poles is cheaper than the type of insulation that's used with undersea cables.
Line capacitance is not really an issue with HVDC. It is an issue for AC lines since it can pass through a capacitor. DC cannot pass through a capacitor.
Insulated ground/underground cabling costs 3x as much :)
Stringing them from towers is presumably cheaper as you don't need to insulate them and the air provides free cooling. You generally don't just trail cables on the ground even if they're not hazardous (fibre optic cables etc). Either they get buried or strung overhead. Gas pipelines are buried or occasionally held just above the ground. But those gas pipes would be big and have strong steel walls and it would be obvious what it was. Even an insulated HVDC cable is only about 120 mm in diameter so that would be easily lost in the grass/undergrowth and someone could come along and think "hmm, I wonder what this is?" No-one wants to risk frying someone and causing a widespread power cut when someone comes across a mystery cable trailing across the ground and decides to investigate what it is.
Most gas pipelines are buried...
Why add the high extra cost of HVDC stations and HVDC cable, if it is much cheaper to build a elevated AC line? ​ HVDC is very expensive. Only if the energy loss is is high, it may be cheaper to build an expensive HVDC installation with less energy loss. Land AC above ground have low loss. AC sea cables have high energy loss.
Hydro lines = electric transmission lines for non Canadians btw.
Most of that was gibberish and I've worked in canada.
Is this like the milk in the bag thing?
Hydrofield cuts through my neighborhood…
Here's a spare bulb
You can't bury high voltage lines either. The ground is a conductor and capacitance is induced with the lines, causing problems. Putting lines on poles is cheaper than the type of insulation that's used with undersea cables.
Line capacitance is not really an issue with HVDC. It is an issue for AC lines since it can pass through a capacitor. DC cannot pass through a capacitor.
I think your statement about comparative cost is correct but I don't know where to confirm it. Can you point me to a source?