8 ms·

> If we could magically pick a frequency and voltage for electrical systems to use (without sunk costs), what would it be? > What's the most efficient for mode

> If we could magically pick a frequency and voltage for electrical systems to use (without sunk costs), what would it be? > What's the most efficient for modern grids and electronics? I do not think it is possible to answer the question as posed. It is a trade-off. Higher frequencies permit smaller transformers in distribution equipment and smaller filtering capacitors at point of use. On the other hand, the skin effect increases transmission losses at higher frequencies. If you want minimum losses in the transmission network, especially a very long distance transmission network, then low frequencies are better. If you want to minimize the size and cost of transformers, higher frequencies might be better. Maybe the generator is close to the user so transmission loss is less important. If you want smaller end-user devices, high frequency or DC might be more desirable. You have to define some kind of objective function before the question becomes answerable.

im3w1l 2y ago
Wouldn't it make sense to do both then? Low frequency or even dc long distance transmission that gets converted to standard frequency closer to the user?
- deleted 2y ago
  [deleted]
- connicpu 2y ago
  There's considerable losses involved when you want to convert between frequencies. DC also has considerable losses over long distance, so there's a lower bound on the frequency before the efficiency starts to go down again.
  - manwe150 2y ago
    Right, with modern technology my understanding is HVDC (essentially 0HZ?) is the way to go now (high voltage to minimize resistive loss, DC to minimize skin effect) if we were building a new grid with the same wires, but not economical to retrofit an existing system that is already working, since it is already working
  - somat 2y ago
    It is not that DC has more losses. it is that transforming DC voltage is non-trivial. With AC you just need a couple of magnetically coupled inductors, no moving parts, easy to build, efficient, reliable. With DC this does not work, you need to convert it to AC first do the transform then convert it back. Nowdays we can achieve pretty good efficiency doing this with modern semiconducting switches. But historically you needed to use something like a motor-generator and the efficiency losses were enough that just transmitting in ac was the clear winner. The losses over distance thing is the fundamental conflict between desired properties. For transmission you want as high a voltage as possible, but high voltage is both very dangerous and tricky to contain. So for residential use you want a much lower voltage. we picked ~ 200 volts as fit for purpose for this task. but 200 volts has high loses during long distance transmit. So having a way to transform the current into voltage is critical. Some of our highest voltage most efficient long distance transmission lines are DC, but this is only possible due to modern semiconducting switches. https://en.wikipedia.org/wiki/High-voltage_direct_current https://en.wikipedia.org/wiki/High-voltage_direct_current
  - mlyons1340 2y ago
    Power line losses are proportional to I^2R so whether its DC or AC isn't really the concern. V=IR so assuming R is constant, a higher transmission voltage results in exponentially lower power losses. DC is actually whats currently used for long distances to achieve lowest power line losses (HVDC).
    - choilive 2y ago
      The skin effect is an important difference (and advantage in favor of HVDC) so it is in fact a concern of AC vs DC.
      - lazide 2y ago
        Also inductive losses, which are only a thing in AC.
      - mlyons1340 2y ago
        True, skin effect limits the conductor size (~22mm in Al @60Hz) but overhead transmission already uses bundled conductors to address that as well as to improve mechanical strength, cooling, and reactance. The advantage of HVDC is in the lower dielectric and reactive losses while skin effect is minimal.
    - ahartmetz 2y ago
      >exponentially quadratically
willis936 2y ago
I think the question could be constrained as "what frequency uses the minimum amount of copper to remake the electrical distribution network that exists today?" This would be a pretty good approximation of the ratio of transmission lines to transformers.
- Panzer04 2y ago
  You could build the lot with DC and massively reduce transformers, but transformers are probably a lot more reliable than switching converters everywhere. Not sure which would be cheaper tbh.
  - mgiampapa 2y ago
    Sure, the transformers would be smaller, but your transmission lines would be thicc.
    - Mistletoe 2y ago
      Isn’t it the other way around? >Generally, for long-distance power transmission, DC lines can be thinner than AC lines because of the "skin effect" in AC, which concentrates current flow near the surface of the conductor, making thicker wires less efficient; therefore, for the same power transmission, a DC line can be smaller in diameter than an AC line
      - lazide 2y ago
        The issue is actually that DC voltage conversion is much harder than AC, because AC can use transformers, and DC can’t. This is especially a problem at high voltages and currents. Also, DC arcs don’t self extinguish as well as AC arcs do, so DC arcs are a lot more dangerous and destructive. It’s why HVDC lines are still relatively rare (and capital expensive), and typically used for long haul or under salt water, where the inductive loss from AC would cost more than the higher capital costs required for DC voltage conversion and stability.
- TheSpiceIsLife 2y ago
osigurdson 2y ago
Objective function: Minimize operational cost Decision variable(s): Electrical system frequency Scope: Global electrical grid
- deleted 2y ago
  [deleted]
- Brian_K_White 2y ago
  2 meaningless statements. For instance, I would say that the scope of the global electrical grid includes every phone charger. Not just because the last foot devices are techically connected, but because they are the reason the rest even exists in the first place. So nothing that serves either the long haul or the local at the expense of the other can be called "minimal operational cost". So trains use their own 25hz or even lower because that's good for long haul. But that would mean phone chargers are undesirably large and heavy. Or maybe it would mean that every house has it's own mini power station that converts the 25hz utility to something actually usable locally. Meanwhile planes use 400hz 200v 3-phase for some mix of reasons I don't know but it will be a balance of factors that really only applies on planes. Things like not only the power to weight but also the fact that there is no such thing as mile long run on a plane, the greater importance to avoid wires getting hot from high current, etc. Simply saying "the objective function is 'what is best?' and the scope is 'global'" doesn't turn an undefined scope and objective into defined ones.
  - osigurdson 2y ago
    Not the original commenter, but here is another angle: If the original grid designers had a time machine and spent 2 decades studying electrical engineering in modern times before going back, what frequency would they have chosen? Does this help you understand the original commenter's question?
    - Brian_K_White 2y ago
      In this imaginary world, does every house and business have it's own power station? Are small electronics powered by dc or ac? Do perhaps power strips incorporate something like a power supply that takes the long haul power and proivide something locally more usable, like how many power strips today incorporate usb power supplies? Is it worth making the grid slightly less efficient for houshold/office usage in trade for making it more efficient for EV charging at every parking spot, or is there something totally different like wireless power in the roads all along the roads... You're asking how high is up.

im3w1l 2y ago

Wouldn't it make sense to do both then? Low frequency or even dc long distance transmission that gets converted to standard frequency closer to the user?

deleted 2y ago
[deleted]
connicpu 2y ago
There's considerable losses involved when you want to convert between frequencies. DC also has considerable losses over long distance, so there's a lower bound on the frequency before the efficiency starts to go down again.
- manwe150 2y ago
  Right, with modern technology my understanding is HVDC (essentially 0HZ?) is the way to go now (high voltage to minimize resistive loss, DC to minimize skin effect) if we were building a new grid with the same wires, but not economical to retrofit an existing system that is already working, since it is already working
- somat 2y ago
  It is not that DC has more losses. it is that transforming DC voltage is non-trivial. With AC you just need a couple of magnetically coupled inductors, no moving parts, easy to build, efficient, reliable. With DC this does not work, you need to convert it to AC first do the transform then convert it back. Nowdays we can achieve pretty good efficiency doing this with modern semiconducting switches. But historically you needed to use something like a motor-generator and the efficiency losses were enough that just transmitting in ac was the clear winner. The losses over distance thing is the fundamental conflict between desired properties. For transmission you want as high a voltage as possible, but high voltage is both very dangerous and tricky to contain. So for residential use you want a much lower voltage. we picked ~ 200 volts as fit for purpose for this task. but 200 volts has high loses during long distance transmit. So having a way to transform the current into voltage is critical. Some of our highest voltage most efficient long distance transmission lines are DC, but this is only possible due to modern semiconducting switches. https://en.wikipedia.org/wiki/High-voltage_direct_current https://en.wikipedia.org/wiki/High-voltage_direct_current
- mlyons1340 2y ago
  Power line losses are proportional to I^2R so whether its DC or AC isn't really the concern. V=IR so assuming R is constant, a higher transmission voltage results in exponentially lower power losses. DC is actually whats currently used for long distances to achieve lowest power line losses (HVDC).
  - choilive 2y ago
    The skin effect is an important difference (and advantage in favor of HVDC) so it is in fact a concern of AC vs DC.
    - lazide 2y ago
      Also inductive losses, which are only a thing in AC.
    - mlyons1340 2y ago
      True, skin effect limits the conductor size (~22mm in Al @60Hz) but overhead transmission already uses bundled conductors to address that as well as to improve mechanical strength, cooling, and reactance. The advantage of HVDC is in the lower dielectric and reactive losses while skin effect is minimal.
  - ahartmetz 2y ago
    >exponentially quadratically

willis936 2y ago

I think the question could be constrained as "what frequency uses the minimum amount of copper to remake the electrical distribution network that exists today?" This would be a pretty good approximation of the ratio of transmission lines to transformers.

Panzer04 2y ago
You could build the lot with DC and massively reduce transformers, but transformers are probably a lot more reliable than switching converters everywhere. Not sure which would be cheaper tbh.
- mgiampapa 2y ago
  Sure, the transformers would be smaller, but your transmission lines would be thicc.
  - Mistletoe 2y ago
    Isn’t it the other way around? >Generally, for long-distance power transmission, DC lines can be thinner than AC lines because of the "skin effect" in AC, which concentrates current flow near the surface of the conductor, making thicker wires less efficient; therefore, for the same power transmission, a DC line can be smaller in diameter than an AC line
    - lazide 2y ago
      The issue is actually that DC voltage conversion is much harder than AC, because AC can use transformers, and DC can’t. This is especially a problem at high voltages and currents. Also, DC arcs don’t self extinguish as well as AC arcs do, so DC arcs are a lot more dangerous and destructive. It’s why HVDC lines are still relatively rare (and capital expensive), and typically used for long haul or under salt water, where the inductive loss from AC would cost more than the higher capital costs required for DC voltage conversion and stability.
TheSpiceIsLife 2y ago

osigurdson 2y ago

Objective function: Minimize operational cost Decision variable(s): Electrical system frequency Scope: Global electrical grid

deleted 2y ago
[deleted]
Brian_K_White 2y ago
2 meaningless statements. For instance, I would say that the scope of the global electrical grid includes every phone charger. Not just because the last foot devices are techically connected, but because they are the reason the rest even exists in the first place. So nothing that serves either the long haul or the local at the expense of the other can be called "minimal operational cost". So trains use their own 25hz or even lower because that's good for long haul. But that would mean phone chargers are undesirably large and heavy. Or maybe it would mean that every house has it's own mini power station that converts the 25hz utility to something actually usable locally. Meanwhile planes use 400hz 200v 3-phase for some mix of reasons I don't know but it will be a balance of factors that really only applies on planes. Things like not only the power to weight but also the fact that there is no such thing as mile long run on a plane, the greater importance to avoid wires getting hot from high current, etc. Simply saying "the objective function is 'what is best?' and the scope is 'global'" doesn't turn an undefined scope and objective into defined ones.
- osigurdson 2y ago
  Not the original commenter, but here is another angle: If the original grid designers had a time machine and spent 2 decades studying electrical engineering in modern times before going back, what frequency would they have chosen? Does this help you understand the original commenter's question?
  - Brian_K_White 2y ago
    In this imaginary world, does every house and business have it's own power station? Are small electronics powered by dc or ac? Do perhaps power strips incorporate something like a power supply that takes the long haul power and proivide something locally more usable, like how many power strips today incorporate usb power supplies? Is it worth making the grid slightly less efficient for houshold/office usage in trade for making it more efficient for EV charging at every parking spot, or is there something totally different like wireless power in the roads all along the roads... You're asking how high is up.