The efficiency gained by stepping up AC to higher voltage (and correspondingly lower current) is far greater than that lost to skin effect. Since there is no way to efficiently step up DC (modern DC-DC converters are still horribly lossy), AC transmission ends up being more efficient, by a long shot.
There are scenarios where HVDC can be more suitable than AC, in particular, very long distance links (e.g. to a remote hydraulic station). All of the substation equipment is much more expensive in HVDC but the benefit of eliminating line reactance can outweigh this if you're doing 1000+ km without any interconnections.
Also, interconnection between neighbouring AC grids is an important HVDC application since we don't have to worry about transient stability.
Neither AC or DC are superior. Different technologies for different applications.
Can you explain like I'm a physics major why HVDC becomes more efficient than AC over ~1000km? A scale analysis would be great - where does the distance D come into the equation?
I get why AC is better than DC on ~100km, but I don't understand how it changes again at larger scales.
Both systems have resistive loses proportional to the square of the current. However:
1. Total power transferred in a DC system is proportional to the voltage, whereas power transferred in an AC system is proportional to the RMS voltage (which is roughly 0.7 of nominal for a sine wave), so more energy is transmitted at the same current level in HVDC.
2. AC systems manifest impedance which has a resistive (aka DC) component as above as well as a reactive (aka AC) component, i.e. Z = R + jX. In DC systems X = 0. In a theoretical transmission line no energy is absorbed or supplied from line reactance, but in practice we have to transmit a certain amount of reactive power (VARs) to charge the line capacitance/inductance each AC cycle. This reduces the amount of our current capacity (limited by thermal constraints) that actually carries current that can be delivered to the load as active power (watts).
This effect is somewhat although not directly proportional to distance (characteristic impedance has no dependence on line length, but voltage drops along the line due to resistive effects meaning the variation from the optimal reactive power-minimizing voltage level increases).
The effect of (1) and (2) is that for any given conductor, at a given voltage level, more usable energy can be transmitted with DC than AC, and that differential increases with distance.
That being said, building DC converter and switching stations is much more expensive than AC. So for a shorter line, or one that has many switching stations, I could counter the above by simply generating 5-8% more power at the generating station and still come out ahead (because in real engineering everything is about $).
Therefore, DC is only more cost-effective ($/MVA of energy delivered) at long distances.
Reactive power is the control then. With AC, we have to increase reactive power to shove more real power along the line, but reactive power doesn't transmit very well.
Follow on: in a national grid, could we just distribute the production of reactive power with capacitor banks in each town / neighbourhood? Heavy flywheels spinning at 50hz?
> building DC converter and switching stations is much more expensive than AC
Is this intrinsic to the technology or is it more because we have economies of scale from building infrastructure around AC for 100 years?
'Depending on voltage level and construction details, HVDC transmission losses are quoted as about 3.5% per 1,000 km, which is less than typical losses in an AC transmission system.[16]'
At line-frequency skin effect is not significant for almost any practical conductor (ie. reasonably conductive and with reasonable cross-section). What gets significant for long distance transmission is that few thousand kilometer long power transmission line starts behaving like, well, transmission line even on frequencies as low as 50/60Hz. Also HVDC elegantly sidesteps problem of grid synchronization.
Oh, that is completely not true for commercial AC power transmission -- just look up at a big power pylon and see the multiple power conductors per phase.
I assumed that skin depth at 50/60Hz is on the order of 20mm and that 40mm diameter conductor is not entirely practical to install on pylons (or generally in longer lengths than few meters), obviously I was slightly off in that estimate.
