3 ms·

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

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

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