AC coupling to existing minigrid

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rtribble
rtribble Solar Expert Posts: 28 ✭
in Off Grid Solar & Battery Systems #1
I currently have two trace 5548 supplying power in 240V split phase to a minigrid of devices:

I recently added an AC unit that uses about 16AAC per leg, works fine most of the time, every 5th time or so the inverters (one leg or other) trips for over current, on startup of the ac compressor, I am assuming that the Locked rotor amp draw is in excess of 30 amps.

I would like to add an AC coupled inverter to the mini grid, powered from the dc battery bank, suggestions on design, make, model , etc. ??:confused:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: AC coupling to existing minigrid
    rtribble wrote: »
    I currently have two trace 5548 supplying power in 240V split phase to a minigrid of devices:

    I recently added an AC unit that uses about 16AAC per leg, works fine most of the time, every 5th time or so the inverters (one leg or other) trips for over current, on startup of the ac compressor, I am assuming that the Locked rotor amp draw is in excess of 30 amps.

    I would like to add an AC coupled inverter to the mini grid, powered from the dc battery bank, suggestions on design, make, model , etc. ??:confused:

    "16 Amps AC per leg"? If it's a 240 VAC device it draws 16 Amps; the power simply goes from one L to the other L.

    You want to increase your AC Watt capacity of the two Trace 5548's. In order to do this you'd need an inverter that will stack with the Traces. Good luck finding another 5548, because that's basically the best way to go.

    But you're looking at it as needing just a bit more, so why not add on another small inverter right? Because the chances of it synching to the existing inverters is slim to none. This becomes different if you use a GTI because that will synch, but will the Trace inverters be able to handle the power it produces when no load is available? Good question. They weren't designed to so I wouldn't expect them to. Worst case here is that one of your inverters fries. Best case it will back-feed and charge batteries with no regulation.

    16 Amps @ 240 VAC would be over 3kW. Maybe the solution here is a separate inverter dedicated to running the AC. And keep in mind that the battery bank would have to be able to manage the full capacity, which is looking like a very large total load.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #3
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    Re: AC coupling to existing minigrid

    From his signature he has 4 - 5548s I guess he has only 2 paired, I thought you could stack them 2x2? for a single unit... of course if one goes down...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #4
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    Re: AC coupling to existing minigrid

    You may be better of putting your effort into reducing the start-up load of the compressor.

    I am available for custom hardware/firmware development

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  • rtribble
    rtribble Solar Expert Posts: 28 ✭
    #5
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    Re: AC coupling to existing minigrid

    How do the micro inverters sync to an existing grid ? maybe use a stack of these ?
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  • rtribble
    rtribble Solar Expert Posts: 28 ✭
    #6
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    Re: AC coupling to existing minigrid

    From his signature he has 4 - 5548s I guess he has only 2 paired, I thought you could stack them 2x2? for a single unit... of course if one goes down...

    is this true can I do 2 x 2 stacking ?? giving me 22kw across two legs ??
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #7
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    Re: AC coupling to existing minigrid
    jonr wrote: »
    You may be better of putting your effort into reducing the start-up load of the compressor.
    I agree, here is a thread that has such a product. It's been around a few years. It's seems your right on the edge and this a CHEAP fix.

    http://forum.solar-electric.com/showthread.php?23451-Soft-Start-for-Motors&p=190677#post190677
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: AC coupling to existing minigrid
    rtribble wrote: »
    How do the micro inverters sync to an existing grid ? maybe use a stack of these ?

    The same way that any GTI syncs; checks Voltage and frequency and aligns output to match. As I mentioned you can't just attach GTI to any inverter and expect it to function.

    And at roughly $400 for a panel and inverter plus wiring to get about 200 Watts it's just as BC04 said; right on the edge where improving the load characteristics becomes more feasible.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    #9
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    Re: AC coupling to existing minigrid

    There should be a label on the side of the compressor with the locked rotor amperage. There is a common misimpression that LRA is the maximum a motor will draw. The LRA rating is at minimum working voltage, probably 205 vac for a 240 vac nominal run system.

    Maximum starting will be 15-20% higher for 240vac. The other factor is how long the surge last which is dependent on type of compressor and even how much Freon charge is in the system.

    I have a Lenox with a scroll type compressor which is one of the worst for starting current. My compressor runs at 13.5 amps at 240vac and has a starting surge of 120 A rms (that is 170 amps peak) that last for 400 msec on a normal 240vac grid connection.

    The popular reed value compressors are commonly 5 to 7 x run current for startup surge.

    The normal SW5548 is pretty limiting on its surge level and duration. It pretty much pukes out if surge goes above 78 amps. I don't remember the max time duration for the 78 amps but it is in ballpark of 100 msec max.

    The SW5548plus models have their software allow a much higher surge with variable time length tolerance based on how high the surge current. I have two SW5548plus'es series stacked that starts my Lenox central AC. The SW+ will perform a soft start effect so it dips in output voltage during the AC startup surge.

    You cannot easily AC couple a GT inverter to output of SW as SW will shunt down on any detected back feed current into ACout. Error will show AC input wired to AC output. You would have to ensure the load always consumes at least the PV GT generated power. Not very practical to expect to control.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #10
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    Re: AC coupling to existing minigrid
    RCinFLA wrote: »
    SW will shunt down on any detected back feed current into ACout.

    It would be interesting to know the exact algorithm. It has to tolerate some back feed of current because this is what reactive loads do. Ie, for part of the waveform, any capacitor or inductor acts as a generator.

    I am available for custom hardware/firmware development

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