4 ms·

"Real electrical systems have to deal with issues of reactance and other exciting math-heavy constructs designed to drive you into some other field of study."

by juancn 1y ago

"Real electrical systems have to deal with issues of reactance and other exciting math-heavy constructs designed to drive you into some other field of study." They had me at this

SoftTalker 1y ago
Wonder how much complexity using AC adds to this? With DC you would not have to worry about frequency and phase matching. But then you need to convert the DC back to AC at some point.
- deleted 1y ago
  [deleted]
- Cthulhu_ 1y ago
  Some parts of the energy transmission grid are DC, but looks like it's mainly international, undersea, and long-range transmissions: https://en.wikipedia.org/wiki/High-voltage_direct_current https://en.wikipedia.org/wiki/High-voltage_direct_current
  - sam_lowry_ 1y ago
    And only because long-distance transmission is more efficient.
- raverbashing 1y ago
  > Wonder how much complexity using AC adds to this? Summarizing a couple of year of very annoying electrical studies: Yes You can't believe how much
  - formerly_proven 1y ago
    On the other hand, having a continent-scale AC grid is not just possible but common. Europe and North Africa have one, Russia has one, China has one, the US has two or three. Meanwhile multiterminal HVDC is basically experimental stuff that's only possible with real-time digital control systems.
    - raverbashing 1y ago
      Having a worldwide data network is also only possible with real-time digital control systems as well The issue is one of technology (having high powered switching components only took of in the last 30? yrs or something) and legacy (Edit: IGBTs are from the 80's though it seems the 1st generation wasn't that great, IGCTs from the late 90s https://mb-drive-services.com/experience-with-igct-based-vfd/ https://mb-drive-services.com/experience-with-igct-based-vfd...
- lazide 1y ago
  (Efficient) High voltage AC is ‘easy’ due to how well transformers work, how durable they are, and how simple they are. AC does have issues with inductive loss when buried (or near anything conductive), however. For the same reason Transformers work and are awesome. High voltage DC is hard as it requires solid state components which are expensive to make, and prone to blowing up (aka relatively fragile). AC to DC and vice versa also adds non-trivial losses. High voltage DC (to a first approximation) doesn’t suffer from inductive losses however, which makes it much more efficient when near conductive stuff like the ground or seawater. It’s also ‘simpler’ (doesn’t have things like phase or frequency) which is convenient if doing things like transferring power between two power grids with dissimilar frequencies or phase. They each have their place.
  - thmsths 1y ago
    I believe high voltage DC also does not suffer from skin effect. Meaning that the current is transmitted through the whole conductor, not just the surface, leading to less material needed for the same current rating.
    - crote 1y ago
      The skin effect is frequency-related, with higher frequencies resulting in a thinner "skin". The grid frequency is low enough that it doesn't really play a huge role: a 1000mm2 aluminum conductor has a 10% higher resistance at 60Hz AC compared to DC. One thing to remember is that it is common for the conductor to have a strengthening core: this allows operation at a higher temperature (which means a higher current), and it allows for a longer distance between support towers (which means lower construction costs). If you're going to put some poorly-conducting steel or carbon fiber in the center of the conductor anyways, why worry about the skin effect? Besides, those conductors are massive already. At the highest voltage levels, a single set of conductors is carrying multiple gigawatts of power. Even if it were technically viable, would you want to build a single 16GW connection? If it fails, you're going to have serious trouble compensating for that - it's probably wiser to build it as 4x 4GW connections. HVDC also has its own issues with converter losses and not being able to easily step down to lower voltages. It's very nice for distances in the hundreds of kilometers, but it's just not a viable alternative for short city-to-city links.
- mannykannot 1y ago
  I imagine AC makes things considerably more difficult. Reactance is a consequence of inductance and capacitance, which cause current and voltage changes to be out of phase with each other. While the current on a DC transmission line will vary, it will not be reversing every ~10 milliseconds, and I suppose the voltage will be very steady except during startup and shutdown. On an AC transmission line, I suppose any corona discharge going on shuts off and restarts every time the voltage reverses. They make a characteristic buzz in humid conditions.
- idiotsecant 1y ago
  That's easy enough to say but big iron power electronics are orders of magnitude more expensive than copper windings and magnetic steel laminations , both in terms of capital cost and maintenance. It's also dramatically harder to extinguish DC arcs so switching gear and substations need more expensive designs as well. The grid could not exist at present, let alone 100 years ago, if we operated it all DC. Over time more and more of it will shift to DC, particularly for long haul, but AC has enough advantages that's it's unlikely to ever go away. I'm short, it's this way for a reason. The complexity is nessessary.
- thenobsta 1y ago
  All of it, I guess. That phasor is always complex.
  - codethief 1y ago
    Ha, I was waiting to see a comment with the obvious pun to be made here. :) For non-physicists: https://electronics.stackexchange.com/questions/128986/why-use-complex-numbers-to-represent-amplitude-and-phase-of-ac https://electronics.stackexchange.com/questions/128986/why-u...
- formerly_proven 1y ago
  I think the contrary is true. Frequency and phase matching is a feature, not a bug. Because AC gives you frequency, and frequency is not just easy to measure but also easy to have accurate references and very immune to noise, it is also not influenced by step-up/step-down transformers. This makes balancing an AC grid possible. Frequency below the set point? Increase power output. Frequency above? Reduce. As an added bonus, this happens to exactly match up with how synchronous generators work. So you get a fairly robust way for many distributed power plants to collectively coordinate power output without additional communication just by agreeing on the physical quantity which we can measure with the highest precision of any quantity very cheaply. That is not possible with DC.
  - sam_lowry_ 1y ago
    Bingo! So many people miss this.
- f1shy 1y ago
  DC comes with another 1000 problems. It wouldn’t be any better, really.
- api 1y ago
  Is this why some very high power transmission lines are HVDC perhaps?
belter 1y ago
Reactance is such a beautiful French word...It just reminds you of some frustrated romantic attempts in a Paris scenario. :-) But your electrical supplier will charge you for it. Had a electrical power teacher with a past employment at a power supplier, who sadly loved to brag how he used to terrorize small farms when their own power generators had a cos φ below 0.98. I think the rule for Portugal is cos φ above 0.97 and for Spain 0.95 also known as el coseno de phi... "Cos-phi compensation" - https://fortop.co.uk/knowledge/white-papers/cos-phi-compensation-cosinus/ https://fortop.co.uk/knowledge/white-papers/cos-phi-compensa...

by juancn 1y ago

"Real electrical systems have to deal with issues of reactance and other exciting math-heavy constructs designed to drive you into some other field of study." They had me at this

SoftTalker 1y ago

Wonder how much complexity using AC adds to this? With DC you would not have to worry about frequency and phase matching. But then you need to convert the DC back to AC at some point.

deleted 1y ago
[deleted]
Cthulhu_ 1y ago
Some parts of the energy transmission grid are DC, but looks like it's mainly international, undersea, and long-range transmissions: https://en.wikipedia.org/wiki/High-voltage_direct_current https://en.wikipedia.org/wiki/High-voltage_direct_current
- sam_lowry_ 1y ago
  And only because long-distance transmission is more efficient.
raverbashing 1y ago
> Wonder how much complexity using AC adds to this? Summarizing a couple of year of very annoying electrical studies: Yes You can't believe how much
- formerly_proven 1y ago
  On the other hand, having a continent-scale AC grid is not just possible but common. Europe and North Africa have one, Russia has one, China has one, the US has two or three. Meanwhile multiterminal HVDC is basically experimental stuff that's only possible with real-time digital control systems.
  - raverbashing 1y ago
    Having a worldwide data network is also only possible with real-time digital control systems as well The issue is one of technology (having high powered switching components only took of in the last 30? yrs or something) and legacy (Edit: IGBTs are from the 80's though it seems the 1st generation wasn't that great, IGCTs from the late 90s https://mb-drive-services.com/experience-with-igct-based-vfd/ https://mb-drive-services.com/experience-with-igct-based-vfd...
lazide 1y ago
(Efficient) High voltage AC is ‘easy’ due to how well transformers work, how durable they are, and how simple they are. AC does have issues with inductive loss when buried (or near anything conductive), however. For the same reason Transformers work and are awesome. High voltage DC is hard as it requires solid state components which are expensive to make, and prone to blowing up (aka relatively fragile). AC to DC and vice versa also adds non-trivial losses. High voltage DC (to a first approximation) doesn’t suffer from inductive losses however, which makes it much more efficient when near conductive stuff like the ground or seawater. It’s also ‘simpler’ (doesn’t have things like phase or frequency) which is convenient if doing things like transferring power between two power grids with dissimilar frequencies or phase. They each have their place.
- thmsths 1y ago
  I believe high voltage DC also does not suffer from skin effect. Meaning that the current is transmitted through the whole conductor, not just the surface, leading to less material needed for the same current rating.
  - crote 1y ago
    The skin effect is frequency-related, with higher frequencies resulting in a thinner "skin". The grid frequency is low enough that it doesn't really play a huge role: a 1000mm2 aluminum conductor has a 10% higher resistance at 60Hz AC compared to DC. One thing to remember is that it is common for the conductor to have a strengthening core: this allows operation at a higher temperature (which means a higher current), and it allows for a longer distance between support towers (which means lower construction costs). If you're going to put some poorly-conducting steel or carbon fiber in the center of the conductor anyways, why worry about the skin effect? Besides, those conductors are massive already. At the highest voltage levels, a single set of conductors is carrying multiple gigawatts of power. Even if it were technically viable, would you want to build a single 16GW connection? If it fails, you're going to have serious trouble compensating for that - it's probably wiser to build it as 4x 4GW connections. HVDC also has its own issues with converter losses and not being able to easily step down to lower voltages. It's very nice for distances in the hundreds of kilometers, but it's just not a viable alternative for short city-to-city links.
mannykannot 1y ago
I imagine AC makes things considerably more difficult. Reactance is a consequence of inductance and capacitance, which cause current and voltage changes to be out of phase with each other. While the current on a DC transmission line will vary, it will not be reversing every ~10 milliseconds, and I suppose the voltage will be very steady except during startup and shutdown. On an AC transmission line, I suppose any corona discharge going on shuts off and restarts every time the voltage reverses. They make a characteristic buzz in humid conditions.
idiotsecant 1y ago
That's easy enough to say but big iron power electronics are orders of magnitude more expensive than copper windings and magnetic steel laminations , both in terms of capital cost and maintenance. It's also dramatically harder to extinguish DC arcs so switching gear and substations need more expensive designs as well. The grid could not exist at present, let alone 100 years ago, if we operated it all DC. Over time more and more of it will shift to DC, particularly for long haul, but AC has enough advantages that's it's unlikely to ever go away. I'm short, it's this way for a reason. The complexity is nessessary.
thenobsta 1y ago
All of it, I guess. That phasor is always complex.
- codethief 1y ago
  Ha, I was waiting to see a comment with the obvious pun to be made here. :) For non-physicists: https://electronics.stackexchange.com/questions/128986/why-use-complex-numbers-to-represent-amplitude-and-phase-of-ac https://electronics.stackexchange.com/questions/128986/why-u...
formerly_proven 1y ago
I think the contrary is true. Frequency and phase matching is a feature, not a bug. Because AC gives you frequency, and frequency is not just easy to measure but also easy to have accurate references and very immune to noise, it is also not influenced by step-up/step-down transformers. This makes balancing an AC grid possible. Frequency below the set point? Increase power output. Frequency above? Reduce. As an added bonus, this happens to exactly match up with how synchronous generators work. So you get a fairly robust way for many distributed power plants to collectively coordinate power output without additional communication just by agreeing on the physical quantity which we can measure with the highest precision of any quantity very cheaply. That is not possible with DC.
- sam_lowry_ 1y ago
  Bingo! So many people miss this.
f1shy 1y ago
DC comes with another 1000 problems. It wouldn’t be any better, really.
api 1y ago
Is this why some very high power transmission lines are HVDC perhaps?

belter 1y ago

Reactance is such a beautiful French word...It just reminds you of some frustrated romantic attempts in a Paris scenario. :-) But your electrical supplier will charge you for it. Had a electrical power teacher with a past employment at a power supplier, who sadly loved to brag how he used to terrorize small farms when their own power generators had a cos φ below 0.98. I think the rule for Portugal is cos φ above 0.97 and for Spain 0.95 also known as el coseno de phi... "Cos-phi compensation" - https://fortop.co.uk/knowledge/white-papers/cos-phi-compensation-cosinus/ https://fortop.co.uk/knowledge/white-papers/cos-phi-compensa...