5 ms·

With the advent of solar and HVDC, is there a potential future where AC stops being the backbone of the grid, letting us drop the whole notion of grid-scale fre

by kibwen 3y ago

With the advent of solar and HVDC, is there a potential future where AC stops being the backbone of the grid, letting us drop the whole notion of grid-scale frequency? It sure sounds simpler not to have do the delicate phase matching dance, among other things.

Synaesthesia 3y ago
But transforming high voltage to low voltage or vice versa is really trivial with AC.
GaggiX 3y ago
I believe that with AC is easier and cheaper to change the voltage using transformers compare to DC.
- nick7376182 3y ago
  Very much so. That is why it seems we won't have DC distribution (ever? Not sure what it would take for DC to make more sense)
  - bee_rider 3y ago
    For DC, you don’t need to synchronize the grids. For some reason HVDC (specifically HVDC, not just DC, most DC devices use low voltages, far below the voltages that would be efficient for transmission), is apparently more power efficient than AC. I’m not sure why, though.
    - markus92 3y ago
      At certain power levels, reactive effects take over. Takes more energy to charge up the transmission lines 50 or 60 times a second, than it actually can transmit. Also, no skin effect.
    - amoshebb 3y ago
      In a DC circuit, a capacitor looks like an open circuit because after it fills up it just sits still. In an AC circuit, a capacitor looks like a resistor, it takes work to fill and empty it every cycle. Transmission lines over the earth behave not just like wires, but also like capacitors. Higher voltage reduces resistive losses, but in AC they’re penalized by these “fill and empty a capacitor” ones, that DC doesn’t.
    - vlovich123 3y ago
      > there is no need to support three phases and there is no skin effect. AC systems use a higher peak voltage for the same power, increasing insulator costs > This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power. > Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end. Another factor that reduces the useful current-carrying ability of AC lines is the skin effect, which causes a nonuniform distribution of current over the cross-sectional area of the conductor. Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses (or heating effect), a given conductor can carry more power to the load when operating with HVDC than AC Basically, while heat losses are the same, the AC system requires extra power to constantly be switching the electron flow whereas in a DC system there’s only active power to move the electrons and the only loss is heat loss. Additionally, for a given conductor, HVDC can be transferring at the peak rated voltage for the wire whereas AC can only transfer that voltage at the peak which means it’s 71% less power (although that’s more a cost savings thing).
  - nick7376182 3y ago
    I was referring to distribution (local) rather than transmission (between grids or regions, I believe HVDC is limited to transmission so far)
crmd 3y ago
The grid needs to run at multiple voltages - transmission, distribution, industrial/commercial/residential service - which is complex on a DC grid. Synchronizing the phases is an easier problem.
- jnsaff2 3y ago
  Synchronization is a one time event. Once the networks are coupled the physics itself keeps it in sync. The problem is that physics also dictates that the interconnection links need to be big enough to handle the power imbalance between the different parts of the networks. So grid management is mostly about balancing production and demand as a whole and in sub-grids.
vegetablepotpie 3y ago
This is unlikely. While transmission lines may go DC, all of the distribution, the lines that goes from a substation to peoples houses, in the US is AC. Although it’s possible to wire a house for DC, and people have done that, many of the appliances we use, use AC power. Although AC phase matching is a delicate technical problem, it’s one we’ve solved for over a hundred years. DC presents other engineering challenges that are non-trivial. For example, circuit breakers for AC power are designed to “break” when the AC curve hits zero volts. This eliminates the chance of arcing and makes breakers smaller and cheaper to manufacture. A DC breaker has a chance of arcing and it may be necessary to make them larger, or use exotic gasses with high dielectric values to prevent this from occurring. Either of these increase costs for homeowners.
- stavros 3y ago
  You're assuming that, in the future, we'll still have huge, centralized plants. With solar, it's very possible that we'll have small stations in each town or area, and only need to balance between them infrequently, greatly reducing the amount of energy we need to carry across large distances.
  - dgfitz 3y ago
    > You're assuming that, in the future, we'll still have huge, centralized plants. You're assuming that we won't. > With solar, it's very possible that ... How nice the world would be we could all lean on "well because it is possible it must eventually happen."
    - stavros 3y ago
      - Things will be the same, except for X. - Maybe other things will be different too. - Oh, so everything will be different?!
    - remram 3y ago
      > You're assuming that we won't. That's really not how I read that comment.
avianlyric 3y ago
Probably not. Spinning inertia and grid frequency are a core component of running a stable grid. Gives you a large store of energy that can absorb sudden spikes in demand or drops in supply, so systems have time to react before really bad things happen. The grid frequency is an incredibly useful communication tool that allows any piece of equipment to easily and accurately measure the current health the overall grid, and automatically make adjustments to help balance and improve the health the of the grid (either by increasing or decreasing load/supply). Because the frequency is set by physically large spinning turbines it means it’s also a direct and inseparable measure of total grid health, not something that’s dependent on another system to monitor and communicate grid health. It’s hard to overstate how much of our electricity grids depend on grid frequency, and one having thousands of systems monitoring and adapting to grid frequency, to remain as robust and stable as they are. In a DC world you don’t get that anymore, and keeping a grid balanced becomes substantially more complex requiring potentially unreliable side-channel communication to allow equipment on the grid to coordinate themselves. Its really hard to beat a system where one of it core fundamental attributes (frequency) needed for power transmission, is also the perfect attribute for distributed coordination of load and supply.
- deleted 3y ago
  [deleted]
- marcosdumay 3y ago
  > Gives you a large store of energy that can absorb sudden spikes in demand or drops in supply, so systems have time to react before really bad things happen. Capacitors do the same for DC. They are also more efficient and reliable. The thing about a communication channel is true. But it will become true for AC after almost all of the generation becomes free of rotational inertia too (PV, modern wind, and batteries). And you need side-channel communication to decide what generator will take over what load right now.
  - akira2501 3y ago
    > Capacitors do the same for DC. They are also more efficient and reliable. Yea.. but their failure mode can sometimes be a dead short. Engineering around this in a power delivery application is a severe hassle.
reverendjames 3y ago
No.
rapjr9 3y ago
Vermont is considering installing Tesla Powerwalls for every home in the state to create distributed storage of power which they can then control to optimize the grid. Theoretically those Powerwalls could be fed with DC instead of AC and use their inverters to power each home with no need for grid synchronization. Each home would have its own AC frequency. One issue might be devices which use the grid frequency to run clocks. If the frequency in each home is different or drifts then clocks across the state might not tell the same time or get out of sync. I'm not sure how common grid synced clocks are any more, most clocks these days use a quartz crystal as a time reference instead of the grid. Industry will have to remain on the AC grid and they probably do use grid frequency for a variety of things, but there aren't inverters and batteries big enough to power the largest industries anyway. So perhaps a hybrid grid will develop, legacy AC for industry and DC for homes to remove one stage of conversion inefficiencies (both to the home and in feeding power back to the grid from the batteries). Seems like it would be easy to test this on a small scale to start with on a small group of homes.

by kibwen 3y ago

Synaesthesia 3y ago

But transforming high voltage to low voltage or vice versa is really trivial with AC.

GaggiX 3y ago

I believe that with AC is easier and cheaper to change the voltage using transformers compare to DC.

nick7376182 3y ago
Very much so. That is why it seems we won't have DC distribution (ever? Not sure what it would take for DC to make more sense)
- bee_rider 3y ago
  For DC, you don’t need to synchronize the grids. For some reason HVDC (specifically HVDC, not just DC, most DC devices use low voltages, far below the voltages that would be efficient for transmission), is apparently more power efficient than AC. I’m not sure why, though.
  - markus92 3y ago
    At certain power levels, reactive effects take over. Takes more energy to charge up the transmission lines 50 or 60 times a second, than it actually can transmit. Also, no skin effect.
  - amoshebb 3y ago
    In a DC circuit, a capacitor looks like an open circuit because after it fills up it just sits still. In an AC circuit, a capacitor looks like a resistor, it takes work to fill and empty it every cycle. Transmission lines over the earth behave not just like wires, but also like capacitors. Higher voltage reduces resistive losses, but in AC they’re penalized by these “fill and empty a capacitor” ones, that DC doesn’t.
  - vlovich123 3y ago
    > there is no need to support three phases and there is no skin effect. AC systems use a higher peak voltage for the same power, increasing insulator costs > This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power. > Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end. Another factor that reduces the useful current-carrying ability of AC lines is the skin effect, which causes a nonuniform distribution of current over the cross-sectional area of the conductor. Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses (or heating effect), a given conductor can carry more power to the load when operating with HVDC than AC Basically, while heat losses are the same, the AC system requires extra power to constantly be switching the electron flow whereas in a DC system there’s only active power to move the electrons and the only loss is heat loss. Additionally, for a given conductor, HVDC can be transferring at the peak rated voltage for the wire whereas AC can only transfer that voltage at the peak which means it’s 71% less power (although that’s more a cost savings thing).
- nick7376182 3y ago
  I was referring to distribution (local) rather than transmission (between grids or regions, I believe HVDC is limited to transmission so far)

crmd 3y ago

The grid needs to run at multiple voltages - transmission, distribution, industrial/commercial/residential service - which is complex on a DC grid. Synchronizing the phases is an easier problem.

jnsaff2 3y ago
Synchronization is a one time event. Once the networks are coupled the physics itself keeps it in sync. The problem is that physics also dictates that the interconnection links need to be big enough to handle the power imbalance between the different parts of the networks. So grid management is mostly about balancing production and demand as a whole and in sub-grids.

vegetablepotpie 3y ago

This is unlikely. While transmission lines may go DC, all of the distribution, the lines that goes from a substation to peoples houses, in the US is AC. Although it’s possible to wire a house for DC, and people have done that, many of the appliances we use, use AC power. Although AC phase matching is a delicate technical problem, it’s one we’ve solved for over a hundred years. DC presents other engineering challenges that are non-trivial. For example, circuit breakers for AC power are designed to “break” when the AC curve hits zero volts. This eliminates the chance of arcing and makes breakers smaller and cheaper to manufacture. A DC breaker has a chance of arcing and it may be necessary to make them larger, or use exotic gasses with high dielectric values to prevent this from occurring. Either of these increase costs for homeowners.

stavros 3y ago
You're assuming that, in the future, we'll still have huge, centralized plants. With solar, it's very possible that we'll have small stations in each town or area, and only need to balance between them infrequently, greatly reducing the amount of energy we need to carry across large distances.
- dgfitz 3y ago
  > You're assuming that, in the future, we'll still have huge, centralized plants. You're assuming that we won't. > With solar, it's very possible that ... How nice the world would be we could all lean on "well because it is possible it must eventually happen."
  - stavros 3y ago
    - Things will be the same, except for X. - Maybe other things will be different too. - Oh, so everything will be different?!
  - remram 3y ago
    > You're assuming that we won't. That's really not how I read that comment.

avianlyric 3y ago

Probably not. Spinning inertia and grid frequency are a core component of running a stable grid. Gives you a large store of energy that can absorb sudden spikes in demand or drops in supply, so systems have time to react before really bad things happen. The grid frequency is an incredibly useful communication tool that allows any piece of equipment to easily and accurately measure the current health the overall grid, and automatically make adjustments to help balance and improve the health the of the grid (either by increasing or decreasing load/supply). Because the frequency is set by physically large spinning turbines it means it’s also a direct and inseparable measure of total grid health, not something that’s dependent on another system to monitor and communicate grid health. It’s hard to overstate how much of our electricity grids depend on grid frequency, and one having thousands of systems monitoring and adapting to grid frequency, to remain as robust and stable as they are. In a DC world you don’t get that anymore, and keeping a grid balanced becomes substantially more complex requiring potentially unreliable side-channel communication to allow equipment on the grid to coordinate themselves. Its really hard to beat a system where one of it core fundamental attributes (frequency) needed for power transmission, is also the perfect attribute for distributed coordination of load and supply.

deleted 3y ago
[deleted]
marcosdumay 3y ago
> Gives you a large store of energy that can absorb sudden spikes in demand or drops in supply, so systems have time to react before really bad things happen. Capacitors do the same for DC. They are also more efficient and reliable. The thing about a communication channel is true. But it will become true for AC after almost all of the generation becomes free of rotational inertia too (PV, modern wind, and batteries). And you need side-channel communication to decide what generator will take over what load right now.
- akira2501 3y ago
  > Capacitors do the same for DC. They are also more efficient and reliable. Yea.. but their failure mode can sometimes be a dead short. Engineering around this in a power delivery application is a severe hassle.

reverendjames 3y ago

No.

rapjr9 3y ago

Vermont is considering installing Tesla Powerwalls for every home in the state to create distributed storage of power which they can then control to optimize the grid. Theoretically those Powerwalls could be fed with DC instead of AC and use their inverters to power each home with no need for grid synchronization. Each home would have its own AC frequency. One issue might be devices which use the grid frequency to run clocks. If the frequency in each home is different or drifts then clocks across the state might not tell the same time or get out of sync. I'm not sure how common grid synced clocks are any more, most clocks these days use a quartz crystal as a time reference instead of the grid. Industry will have to remain on the AC grid and they probably do use grid frequency for a variety of things, but there aren't inverters and batteries big enough to power the largest industries anyway. So perhaps a hybrid grid will develop, legacy AC for industry and DC for homes to remove one stage of conversion inefficiencies (both to the home and in feeding power back to the grid from the batteries). Seems like it would be easy to test this on a small scale to start with on a small group of homes.