An oversimplified explanation is that AC has less transmission losses over medium distances while high voltage DC has less transmission losses over extremely long distances. I found this stack-exchange post that goes into some of the details better[1]. It also explains some other advantages of DC.
The difference is not really about distance. Your link is accurate though.
High voltage is always more efficient for power transmission over any distance than low voltage, regardless of AC vs DC, in any conductor whose resistance is not zero. This is because of I^2R losses.
For any specified value of [high] voltage and a specified wire and a specified distance, DC will be slightly more efficient than AC, for other reasons like skin effect.
But we don't want high voltage in our homes, so changing voltage down for consumption and up for transmission is essential.
Voltage changing was basically impossible with DC back when the grid was invented 120 years ago. But it was easy with AC. So our grid is AC.
Today we have power semiconductors which allow us to switch DC voltages easily. If the entire grid were being invented today -- from scratch -- it would be entirely DC.
1. Long lines become a "transmission line" in the sense that you get reflections. These have to Z matched and you also have to get the phase angle right.
2 Capacitance loss to ground. The lines are a massive capacitor to ground. The longer the line the more power shorts to ground
2a Because of 2. You dont use AC lines underwater. The increased dielectric means that you loose too much power from anything but the shortest runs.
You could change the frequency to a do the long runs at a lower f, and itd have its benefits, but its outweighed by the drawbacks (namely transformer saturation)
3. Skin effect becomes more of a factor. HVDC can use the whole cross section of the wire to carry power whereas AC only uses the surface. At longer distances, the material savings of thinner/lighter wires offset the higher equipmet costsat eitherend.
As the headline says, 3 grids that are not synchronized . You cannot interconnect them using AC. It would basically be a shortcut.
Edit: You could of course keep the DC step within one site and do all lines in AC. But as the other answers say DC transmission also has benefits, so doing it inside one site you'd need all the equipment but wouldn't get all the benefits.
> As the headline says, 3 grids that are not synchronized
But why? All of the US runs 60 Hz, it should be relatively trivial to synchronize their three phases and tie the grids together. Ukraine managed to do this during an active war.
First some background, sorry for the long comment but I can see you need a bit of a primer before we get to your question:
Transmission lines and generators such as solar panels and wind turbines work at very different voltages. Once you need to convert the extra step to convert from AC to DC or VV isn't a really big challenge. Solar panels output anywhere from 20 to 100V, these are ganged into strings and strings are then coupled to inverters to create relatively low voltage AC, or each panel has its own inverter (not very common in solar farms). Those inverters feed into a local parallel grid which is then stepped up to join the national grid using a feed line (typically 10 to 50 KV, depending on the size of the farm and the local grid). Very large solar farms can have their own local understation where the voltage is stepped up to long haul voltage.
Sometimes there is co-generation with another source (such as solar/wind, solar/natural gas or some other combination).
Wind Turbines usually have generators that output anywhere from 10KV to 50KV depending on the capacity and the manufacturer. This can be variable frequency current or, in a tightly grid coupled turbine it can be at the grid frequency (you can tell the difference from a distance because all of the turbines in a wind farm like that will move in lockstep with each other, this is a good indication that they are AC synchronized). At the base of every turbine you will find a an inverter and/or a step up transformer like with the solar farms. A typical turbine will do anything from 1 MW (which really is small these days, but which used to be state of the art not all that long ago) all the way up to 14 MW behemoths.
These are most impressive up close, to put it very mildly, think of an Eiffeltower but it rotates...
HVDC transmisison lines themselves are super high technology and you're definitely not going to find these running from every Wind Turbine to the grid, what you will most likely find is a local, intermediate AC network from a bunch of wind turbines and/or a number of solar farms to a concentration point and then a much higher voltage line from there to the national grid.
'intermediate' for shorter connections is anywhere from 10 KV to 50 KV, and for longer interconnects up to several 100 KV, all the way up to 800 KV for the longest and most power carrying lines. The engineering behind all this stuff is super impressive.
AC suffers from something called the skin effect, it essentially means that only a small part of the cross section of a powerline carries current, effectively limiting the carrying capacity of the line to a fraction of its theoretical DC limit. So by using DC rather than AC for very long connections line losses can be minimized and much more power can be transferred through a line because those losses translate into heat generated in the line. So HVDC makes very good sense for the long haul links coupling remote areas. They might even make sense intercontinentally (though I'm a bit more skeptical about this after the pipeline attack on the NS pipelines, HVDC lines would be quite fragile and very difficult to repair after an attack).
Note that you always have these losses, but the overhead of the AC->DC->AC conversion is such that it only makes sense for longer lines or lines carrying a very large amount of power. But even the shortest AC line suffers from that skin effect.
I’m sure you’ve heard something like “alternating current is more efficient than direct current for transmitting power”, but this is only true in specific conditions. At very high A/C voltages, the magnetic field created by the alternating current “pushes” elections away from the core of their conductor. This has the same effect as using a smaller diameter conductor, increasing heat which increases resistance and reduces efficiency. At sufficiently high voltages and distances, DC can have less loss and also has the added bonus of not needing to consider differences in A/C frequency when transmitting power between areas that have dissimilar electrical grids.
DC is more efficient at basically any voltage. AC won because its voltage is easily stepped up and down, and high voltage is superior for long range transport vs low voltage.
But high voltage DC for huge, inter-state transport lines makes a lot of sense.
AC is never more efficient than DC. AC was easier to step up and down than DC 120 years ago. That's the only reason we use it*, despite worse efficiency.
* Motors are the other reason. Big electric motors in 1900 ran better on sinusoidal AC than DC. We have much better motor technology today, plus power semiconductors to control them. So motors are no longer a reason for the grid to be AC.
"At very high A/C voltages, the magnetic field created by the alternating current “pushes” elections away from the core of their conductor. "
Thats a frequency effect and is addressed in power-lines. The line cores are of steel cable to provide strength, but is a poor conductor. The inner core supports the Al "shell" that actually carries the current.
Or as we learned in electromag physics, why stranded wire can carry more amperage than solid (because it has more surface area).
Note: Generalizing and making a lot of assumptions. This statement is not encouragement to ignore AWG markings or limits in any way, as the effect isn't that large.
miteyironpaw|3 years ago
[1] https://engineering.stackexchange.com/questions/19758/transm...
dreamcompiler|3 years ago
High voltage is always more efficient for power transmission over any distance than low voltage, regardless of AC vs DC, in any conductor whose resistance is not zero. This is because of I^2R losses.
For any specified value of [high] voltage and a specified wire and a specified distance, DC will be slightly more efficient than AC, for other reasons like skin effect.
But we don't want high voltage in our homes, so changing voltage down for consumption and up for transmission is essential.
Voltage changing was basically impossible with DC back when the grid was invented 120 years ago. But it was easy with AC. So our grid is AC.
Today we have power semiconductors which allow us to switch DC voltages easily. If the entire grid were being invented today -- from scratch -- it would be entirely DC.
sbaiddn|3 years ago
1. Long lines become a "transmission line" in the sense that you get reflections. These have to Z matched and you also have to get the phase angle right.
2 Capacitance loss to ground. The lines are a massive capacitor to ground. The longer the line the more power shorts to ground
2a Because of 2. You dont use AC lines underwater. The increased dielectric means that you loose too much power from anything but the shortest runs.
You could change the frequency to a do the long runs at a lower f, and itd have its benefits, but its outweighed by the drawbacks (namely transformer saturation)
andromeduck|3 years ago
usr1106|3 years ago
Edit: You could of course keep the DC step within one site and do all lines in AC. But as the other answers say DC transmission also has benefits, so doing it inside one site you'd need all the equipment but wouldn't get all the benefits.
mschuster91|3 years ago
But why? All of the US runs 60 Hz, it should be relatively trivial to synchronize their three phases and tie the grids together. Ukraine managed to do this during an active war.
Aloha|3 years ago
HVDC is desirable because it allows non-synchronous connection of different synchronized grids.
pengaru|3 years ago
Primarily it's simply more efficient over long distances than AC.
https://en.wikipedia.org/wiki/High-voltage_direct_current#Ad...
michael1999|3 years ago
barney54|3 years ago
jacquesm|3 years ago
Transmission lines and generators such as solar panels and wind turbines work at very different voltages. Once you need to convert the extra step to convert from AC to DC or VV isn't a really big challenge. Solar panels output anywhere from 20 to 100V, these are ganged into strings and strings are then coupled to inverters to create relatively low voltage AC, or each panel has its own inverter (not very common in solar farms). Those inverters feed into a local parallel grid which is then stepped up to join the national grid using a feed line (typically 10 to 50 KV, depending on the size of the farm and the local grid). Very large solar farms can have their own local understation where the voltage is stepped up to long haul voltage.
Sometimes there is co-generation with another source (such as solar/wind, solar/natural gas or some other combination).
https://group.vattenfall.com/press-and-media/newsroom/2019/v...
Wind Turbines usually have generators that output anywhere from 10KV to 50KV depending on the capacity and the manufacturer. This can be variable frequency current or, in a tightly grid coupled turbine it can be at the grid frequency (you can tell the difference from a distance because all of the turbines in a wind farm like that will move in lockstep with each other, this is a good indication that they are AC synchronized). At the base of every turbine you will find a an inverter and/or a step up transformer like with the solar farms. A typical turbine will do anything from 1 MW (which really is small these days, but which used to be state of the art not all that long ago) all the way up to 14 MW behemoths.
https://www.ge.com/renewableenergy/wind-energy/offshore-wind...
These are most impressive up close, to put it very mildly, think of an Eiffeltower but it rotates...
HVDC transmisison lines themselves are super high technology and you're definitely not going to find these running from every Wind Turbine to the grid, what you will most likely find is a local, intermediate AC network from a bunch of wind turbines and/or a number of solar farms to a concentration point and then a much higher voltage line from there to the national grid.
'intermediate' for shorter connections is anywhere from 10 KV to 50 KV, and for longer interconnects up to several 100 KV, all the way up to 800 KV for the longest and most power carrying lines. The engineering behind all this stuff is super impressive.
https://en.wikipedia.org/wiki/High-voltage_direct_current
Then, to answer your question:
AC suffers from something called the skin effect, it essentially means that only a small part of the cross section of a powerline carries current, effectively limiting the carrying capacity of the line to a fraction of its theoretical DC limit. So by using DC rather than AC for very long connections line losses can be minimized and much more power can be transferred through a line because those losses translate into heat generated in the line. So HVDC makes very good sense for the long haul links coupling remote areas. They might even make sense intercontinentally (though I'm a bit more skeptical about this after the pipeline attack on the NS pipelines, HVDC lines would be quite fragile and very difficult to repair after an attack).
Note that you always have these losses, but the overhead of the AC->DC->AC conversion is such that it only makes sense for longer lines or lines carrying a very large amount of power. But even the shortest AC line suffers from that skin effect.
I hope this answers your question.
xnyan|3 years ago
newZWhoDis|3 years ago
But high voltage DC for huge, inter-state transport lines makes a lot of sense.
dreamcompiler|3 years ago
* Motors are the other reason. Big electric motors in 1900 ran better on sinusoidal AC than DC. We have much better motor technology today, plus power semiconductors to control them. So motors are no longer a reason for the grid to be AC.
sbaiddn|3 years ago
Thats a frequency effect and is addressed in power-lines. The line cores are of steel cable to provide strength, but is a poor conductor. The inner core supports the Al "shell" that actually carries the current.
ethbr0|3 years ago
Note: Generalizing and making a lot of assumptions. This statement is not encouragement to ignore AWG markings or limits in any way, as the effect isn't that large.