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AC coupling 'using grid-tie array during utility failure'

Discussion in 'Energy, Environment, and Policy' started by nwdiver, Sep 20, 2014.

  1. nwdiver

    nwdiver Active Member

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    #1 nwdiver, Sep 20, 2014
    Last edited by a moderator: Mar 13, 2016
    I 'discovered' this a few months ago at a conference. I know a lot of people would like the ability to use their solar array in the event of a power outage. There are currently 3 'off-grid capable' inverters that can sync to a grid-tie inverter and trick it into thinking the grid is back. These inverters then couple with the grid-tie inverter and regulate it's output by sending excess energy to the battery or dump load. If the battery is full they increase frequency to 60.6Hz which won't effect anything being powered but will cause the grid-tie inverter to trip off-line.

    The AWESOME thing about AC coupling is that it can be used with virtually ANY grid-tie system... Micro-inverters, SMA, Fronius... doesn't matter.



    The 3 AC coupling inverters I'm aware of are;
    - Magnum Energy PAE series
    - Outback Radian
    - SMA sunny island (must be stacked to get 240vac)
     
  2. Lloyd

    Lloyd Active Member

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    This is very cool!! I was waiting for someone to come out with this equipment!!
     
  3. FreeOfPge

    FreeOfPge Member

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    Interesting idea. We have SMA sunny boy inverters, so guess we're out of luck. Often wondered if the system could be tricked by plunging power into a socket via a generator. Don't want to try it, then end up killing or injuring a lineman working on the power lines. I understand some systems have a switch that will isolate the home from the grid but AFAIK, in California its against code (or pg&e's specs) to have a the switch on grid tied solar arrays.
     
  4. nwdiver

    nwdiver Active Member

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    Nope... you're IN luck :smile: You'd need to isolate your home from the grid when the grid goes down but those AC coupling inverters 'act' like the grid. You can use SMA sunny boy inverters in parallel with the AC coupled inverters. I've got a Magnum Energy 4448 inverter on order that I plan to couple to a SMA SB8000 in the event of a power outage... or zombie apocalypse. More to follow :wink:

    To clear any confusion... those AC coupled inverters are the 'off-grid' side. ANY UL 1741 grid-tie inverter should work.
     
  5. mitch672

    mitch672 Active Member

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    #5 mitch672, Sep 20, 2014
    Last edited: Sep 20, 2014
    FreeOfPge, you can get the SMA Sunny Island with their home-manager and a dummy load, or they can change the inverter frequency to be slightly off, and the Sunnyboy would shutdown (as would almost all grid-tied inverters). There is a thread on MNL where a creative person did it, and in fact he's using salvaged Nissan Leaf pack modules for the storage on the Sunny Island, he even has a BMS that communicates via CAN bus to the Sunny Island

    Checkout this thread on MNL: My Nissan Leaf Forum View topic - Use of Nissan Leaf battery in Solar installation.

    Here is a link from his post, about the Sunny Island 6.0H, which I believe is for Europe (230V, 1 leg): http://www.tritec-energy.com/images/content/D_264_SMA_SI6H_BRO_enu.pdf
     
  6. mspohr

    mspohr Active Member

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    Good to see progress in this area.
    I just installed a grid-tied system with Enphase microinverters.
    It's nice to know that I will have options for future battery/off-grid operation.
     
  7. FreeOfPge

    FreeOfPge Member

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    Did some digging and read more about this, appears to be a great setup indeed. Thanks for posting!
     
  8. nwdiver

    nwdiver Active Member

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    I got my geek on today... took the house off the grid and used my new Magnum Energy 4448 to simulate the grid. The advantage of a DC coupled system vs an AC coupled system can be summed up in one word.... Harmonics. Everything worked but my grid-tie SMA inverter was making some unusual sounds and all my LED bulbs were oscillating in brightness. Voltage appeared to be 'hunting' between 248 and 252. I'd love to hook up an O-scope and really see what's going on.

    Well... those small things are certainly better than having 8kW available but unusable if there's a prolonged grid failure. I'll still be able to charge my car and use my A/C :biggrin:
     
  9. Cosmacelf

    Cosmacelf Active Member

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    So the Magnum is an AC coupled system??
     
  10. Lloyd

    Lloyd Active Member

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    Let me know what you find.... I am would like to attempt this system.


     
  11. nwdiver

    nwdiver Active Member

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    Yes; I paralleled a Magnum Energy 4448PAE with a SB8000 with the main breaker open. I did this in the evening so I wouldn't overload the 4448. Peak output was ~4kW. It was kinda cool to watch battery amperage fluctuate to buffer power since there were clouds this evening. You should be able to use the FULL output of the grid-tie inverter if the power has a place to go but the back-feed could damage the 4448. The voltage fluctuations are probably inevitable but nothing seemed to care too much. My MS charged just fine, the AC ran great, the TV didn't flicker. The only thing that was kind of annoying was the lightbulb flickering but you don't really need lighting when the grid-tie is on-line anyway... it usually... er... ALWAYS daytime... :smile:
     
  12. Brian T

    Brian T New Member

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    Be careful nwdiver. You are doing a couple of things that are dangerous and could damage your equipment.

    First off, the Magnum 4448-PAE can only handle an AC-Coupled PV array of up to 4400W. In fact you should limit that to 4000W to be safe. If you want to do this as apermanent installation, you should swap out your SMA 8000W inverter for two 4000W inverters, leaving one as a straight grid-tie, and using the other one AC-Coupled with the Magnum.

    The SMA inverter used with the Magnum should be connected through a circuit breaker located in a separate "critical loads" sub-panel, NOT in the main utility service panel. The "AC Output" of the Magnum is also connecd in this separate sub-panel. The "AC Input" of the Magnum is them conneced to a breaker in the utility main panel. It's all explained in Magnums AC-Coupling literature here:'

    http://magnumenergy.com/wp-content/uploads/2013/06/AN-0002-AC-Coupling-Rev-5-10.pdf

    and here:

    http://magnumenergy.com/wp-content/uploads/2013/06/Magnum-AC-Coupled-Line-Diagram-1-May-2010.pdf

    You also have to use some sort of battery charge control to keep from overcharging your backup batteries. The Magnum can't control this in an AC-Coupled setup. Magnum shows using a diversion controller and DC dump load, but you could also use a DC voltage-controlled relay to eith turn on an AC load (needs to be large enough to consume the full 4000W of PV), of just use the DC relay to disconnect the AC connection to the SMA inverter, which will turn it off.

    FYI - I have been working in the solar eklectric industry for 30 years, and am currently an applications engineer for a solar equipment wholesale distribution company. I do not own a Tesla, but wish I did.

     

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