Dynamic & variable AC power diversion on off-grid Outback inverters

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Dynamic & variable AC power diversion on off-grid Outback inverters

Post by kco »

The background for this questions is:
  • I'm fully off-grid with 2.3kw of hydro (Midnite Classic 250) and 3kw solar (FM80) with a FlexPower Two 2x VFX3048E. All electric, no gas etc.
  • I currently dump excess power from the hydro to prevent them overspeeding. This feels like a waste. DC dumping into the HWC isn't really a simple option, as when it gets too hot it'll cut out, and hence I'll have no load.
  • Our HWC is a 2kw element, and sometimes during a really low rainfall summer we drop down to e.g. 1.2kw from the hydro. I'm not super keen to cycle the batteries too much (or monitor it manually). The new solar helps, but it seems like a waste to dump 1.2kw because instead of somehow using it in the 2kw element.
  • The 'obvious' solution is AC power diversion to my HWC - when there's excess power, it diverts the excess (i.e. not 2kw) to the HWC. I don't need it to be perfect or instantaneous.
  • I'm planning on getting the info I need (i.e. when there's excess and how much) by scraping the info from the modbus protocols of my various devices, though maybe there's a better way. PV currently only kicks in when more power is needed than the turbines can supply, so often I don't know how much power I could get from the PV - but I'm hoping the voltage will give me a rough idea.
The specific question - does anyone know of a method of variable (i.e. not on-off) and non-manual power diversion that won't hurt my inverters (or anything else)? This can be done in grid-tie solar diversion easily, because the grid cares less about choppy wave forms on such a small scale. A choppy wave form of 2kw on my 6kw (continuous) inverter might be a different matter.

I see two options:
  • "bursting" - if I want 50% power, I only use every second cycle, etc. With zero-crossing, this means there's no massive instantaneous changes in voltage, and i'm still asking for sinusoidal-ish (just not every cycle) ... but I still feel like the inverters may not like it? I don't care if there's a hum, but if it causes too much flickering (though we're LED so less of an effect) or results in a poor supply for other devices, that might not be great.
  • phase angle - this gives the most control, but given the voltage ends (start?) instantaneously and not at zero-crossing, it feels like bad things can happen - and apparently you do need some filtering (capacitors/inductors?) to handle it. I've heard of one case of someone trying it (not on an Outback I believe - see http://scoraigwind.co.uk/tristar-follow ... c-heaters/) but I believe the result was the inverters "growling" - though they haven't died yet as far as I know. I'm not enough of an electric engineer to know how to do the filtering, or how hard/expensive it'd be.
Are they any I've missed that might work? If not, can anyone comment on the likely impact on my inverter (or anything else)? FYI I did send a support ticket to Outback about 6 (?) months ago, but haven't heard back.

The only other relevant thing I can think to add is there is one possible commercial option - the AC Thor https://www.my-pv.com/en/info/compatible-manufacturers. It seems to support off-grid as it e.g. supports Victron inverters off-grid ... though there's also no support for Outback! (Maybe because it seems to work by the inverter upping the frequency when the battery is charged, and the Outback doesn't do that?) I also don't know how it does the variable diversion, and whether it's any better than the options above .. and at NZD4k, I'm not willing to try it out.

Note that clever heaters (e.g. a sequence of 20x 100W loads) would be nice and such, but I really want to focus on the HWC element for now, especially as in summer (when I really want to divert partial loads) I don't care about heaters, but I do care about hot water! Unless someone can think of a way to get 20x 100W loads (or even 4x 500W loads) into an HWC. (Ours is a dual, so I can get 2x elements in ... and the lowest I can find is 800W, which would be OK as generally I can just switch that on/off without worrying about trying to divert partial power.)
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