4 ms·

From what I know, you will need a sync check relay, ie a relay that only closes when you're synced up. Those are generally available for 1MW and upwards (one ma

by zaarn 8y ago

From what I know, you will need a sync check relay, ie a relay that only closes when you're synced up. Those are generally available for 1MW and upwards (one manual notes a minimum constant load on the internal grid of 500 kW or it won't work), look like about the size of half of a car battery, at a price of "contact our sales team". Could it be made cheaper? Probably. If you get it wrong, the grid probably doesn't care but you'll briefly pump about 500W into the device that is supposed to have 500W going out of it. The reason these are big and expensive is that it requires significant safety gear so nothing explodes even in the worst case. And that safety gear is expensive. So you sell it to people who not only can afford it but also really really need it (ie, 1MW and upwards where you enter the domain of "can fry small section of grid")

TheSpiceIsLife 8y ago
I used to work with a few (16 odd) Siemens 200kVA UPS that did the syncing. There’s no technical reason the requisite electronics can’t be built on a much smaller scale. Home grid-tie solar inverters are clearly capable of syncing, so the electronics are already present.
- zaarn 8y ago
  grid-tie inverters are capable of syncing to a present signal, what they can't do is provide their own waveform and sync that to the grid when it comes back online.
  - Dylan16807 8y ago
    Providing a 60Hz waveform and syncing it to the grid the easy part. You could make a standalone device that does it out of a twenty cent microcontroller.
    - zaarn 8y ago
      And then you'd still have to switch over and you'd need a sync check relay for safety (if you don't and the microcontroller is off because you forgot a comma somewhere, your inverter explodes). Additionally producing the clean sinewave that you'd need for this is not that easy, atleast not at the quality levels you want for this (if your DAC that produces the wave is off by 1% then at a 2kW load you're going to burn up 20W somewhere that doesn't like 20W being burned up)
      - Dylan16807 8y ago
        Redundancy and fail-safes are important but can also be inexpensive. Most of the expense comes from being a niche product right now. How is being off by 1% going to hurt anything? I'm quite sure my mains voltage is already more than 1% off. If I get solar it had better not explode every time my air conditioner kicks on.
        zaarn 8y ago
        Your mains voltage cannot be 1% off it's own phase since it's the primary phase here and in terms of voltage a 1% difference doesn't matter. However, if you have your own generator it matters a lot. If your phase is off by 1 degree then that 1 degree will burn roughly .2% of the incoming power of the grid at the inverter (which is unlikely designed to handle this). If you're off by 1% you burn 20 Watts on a device not designed for it. If your voltage is off relative to the grid by 1% then you burn the difference, at 2kW that's about 20 Watts. And that's per volt. You'd be burning somewhere around 300 W if you happen to have the grid on the higher end of the tolerance and yours on the lower. A grid-tie inverter gets around this by simply following along the sine wave of the grid, this can be done relatively cheaply and safely with analog components so the error can be much smaller than 1% and deep into random noise territory. If you generate your own sine wave and compare it to an existing one it's much more difficult since you have to match amplitude and phase almost perfectly. So with grid-tie nothing will explode. With an autotransfer nothing explodes either. Wanting to seamlessly couple back in requires a lot of care and expensive components.
        Dylan16807 8y ago
        Oh, you're talking about when it syncs back up. I don't really see the problem. Next to your offline-capable sine wave generator, put a copy of that cheap safe analog circuit. Once you get almost in sync, crossfade over to the analog circuit. (If that's even necessary. It might be just fine to swap to it at a zero-crossing.) Now you're completely in sync. Reconnect to the mains.
      - TheSpiceIsLife 8y ago
stephen_g 8y ago
Any old IGBT or even just a highly spec’d MOSFET paired with an optoisolator (a couple of dollars of parts) could do that for a small system, based on input from the inverter’s existing controller. The specialist devices for large installations you’re talking about are only expensive because you need more expensive parts for the far larger amounts of current you’re handling (and probably because they’re made in lower volumes than commodity inverters), not because they’re doing anything particularly difficult.

by zaarn 8y ago

TheSpiceIsLife 8y ago

I used to work with a few (16 odd) Siemens 200kVA UPS that did the syncing. There’s no technical reason the requisite electronics can’t be built on a much smaller scale. Home grid-tie solar inverters are clearly capable of syncing, so the electronics are already present.

zaarn 8y ago
grid-tie inverters are capable of syncing to a present signal, what they can't do is provide their own waveform and sync that to the grid when it comes back online.
- Dylan16807 8y ago
  Providing a 60Hz waveform and syncing it to the grid the easy part. You could make a standalone device that does it out of a twenty cent microcontroller.
  - zaarn 8y ago
    And then you'd still have to switch over and you'd need a sync check relay for safety (if you don't and the microcontroller is off because you forgot a comma somewhere, your inverter explodes). Additionally producing the clean sinewave that you'd need for this is not that easy, atleast not at the quality levels you want for this (if your DAC that produces the wave is off by 1% then at a 2kW load you're going to burn up 20W somewhere that doesn't like 20W being burned up)
    - Dylan16807 8y ago
      Redundancy and fail-safes are important but can also be inexpensive. Most of the expense comes from being a niche product right now. How is being off by 1% going to hurt anything? I'm quite sure my mains voltage is already more than 1% off. If I get solar it had better not explode every time my air conditioner kicks on.
      - zaarn 8y ago
        Your mains voltage cannot be 1% off it's own phase since it's the primary phase here and in terms of voltage a 1% difference doesn't matter. However, if you have your own generator it matters a lot. If your phase is off by 1 degree then that 1 degree will burn roughly .2% of the incoming power of the grid at the inverter (which is unlikely designed to handle this). If you're off by 1% you burn 20 Watts on a device not designed for it. If your voltage is off relative to the grid by 1% then you burn the difference, at 2kW that's about 20 Watts. And that's per volt. You'd be burning somewhere around 300 W if you happen to have the grid on the higher end of the tolerance and yours on the lower. A grid-tie inverter gets around this by simply following along the sine wave of the grid, this can be done relatively cheaply and safely with analog components so the error can be much smaller than 1% and deep into random noise territory. If you generate your own sine wave and compare it to an existing one it's much more difficult since you have to match amplitude and phase almost perfectly. So with grid-tie nothing will explode. With an autotransfer nothing explodes either. Wanting to seamlessly couple back in requires a lot of care and expensive components.
        Dylan16807 8y ago
        Oh, you're talking about when it syncs back up. I don't really see the problem. Next to your offline-capable sine wave generator, put a copy of that cheap safe analog circuit. Once you get almost in sync, crossfade over to the analog circuit. (If that's even necessary. It might be just fine to swap to it at a zero-crossing.) Now you're completely in sync. Reconnect to the mains.
    - TheSpiceIsLife 8y ago

stephen_g 8y ago

Any old IGBT or even just a highly spec’d MOSFET paired with an optoisolator (a couple of dollars of parts) could do that for a small system, based on input from the inverter’s existing controller. The specialist devices for large installations you’re talking about are only expensive because you need more expensive parts for the far larger amounts of current you’re handling (and probably because they’re made in lower volumes than commodity inverters), not because they’re doing anything particularly difficult.