Fake out Grid Tie

CJW8CJW8 Registered Users Posts: 23
I have 8 KW of solar panels on my roof with a Sunny Boy SMA 7000 inverter and I am on grid tie. As per design, if the grid goes down my inverter shuts down. I understand the safety implications if why this happens.

Just for discussion, I was thinking worst case scenario, such as a total grid shutdown due to natural or manmade disaster where grid power could be lost for anywhere from weeks to forever.

Would it be possible to fake out my inverter with either a 240 volt generator or a battery bank with 240 volt inverter in order to get whatever power my panels produce during the daylight hours.

I am not suggesting I would or anyone should ever do this but desperate times sometimes call for desperate measures. I am an Industrial Instrument and Electrical Technician by trade and work on 24 volt process controls all the way up to 11KV power systems. I fully understand the danger of back feeding the grid when it is down.

Any idea how a 5kw generator would behave if it were the "grid" providing the 240 v sense voltage for the inverter? Might it work as long as my demand didn't exceed the generator maximum output plus the production of the solar system at the moment?


  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: Fake out Grid Tie

    You can setup a Grid Tied inverter to "back feed" an AC sine wave off-grid inverter--That is called AC coupled.

    There are systems like this Sunny Island 5048U 5000 Watt Battery Based one that is setup to setup a local AC grid with solar, generator, etc. power modules all on the AC off grid "mains".

    And, the Xantrex (Schneider) XW Hybrid Inverter with the latest firmware is rumored to be setup for interfacing with pure grid tied inverters too (no released information on that function yet that I have seen--but I have not been watching that closely either).

    The issue with the XW is that it is 6kW maximum and you have ~7kW of output--so that may be an issue when you have lots of sun and not much AC loads.

    The way the XW probably manages the power from a generic GT inverter is that it raises the 60Hz frequency to 61Hz or so when the battery bank is fully charged. When this happens, the GT inverter will shut down (it thinks the grid is not stable) and will attempt to reconnect in 5 minutes.

    With other TSW off-grid inverters--It is unclear what each brand/model will do (to me anyways)--You probably, to be safe, would need a battery voltage/charger monitor that would cut off the GT output (with a solid state relay for example) when the battery bank is fully charged.

    Otherwise, you just need a couple transfer switches to connect your protected loads plus GT inverter from the AC main to the off grid TSW inverter (which simulates the AC Mains for the GT inverter).

    This is not a trivial setup--Lots of issues to worry about. Plus few Off-Grid inverters are designed and rated for AC Coupling to GT inverters.

    We can talk about it more if you wish... It is doable--but I am not sure how many people on this forum have done it yet in their own homes (probably less than a hand full so far???).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Fake out Grid Tie

    The Sunny Island (SI) was designed from the start to support this sort of coupling with the sunny boy inverters. On this side of the pond, most installers equate the sunny island with AC coupling and SMA only manufacture a 40A DC charger - so they expect their clients to AC couple all medium-large off-grid setups. In fact some installers think it's the only way to use the sunny island!

    What I'm trying to say is that AC coupling might be supported by other inverter manufacturers, like through a very recent firmware update on the Xantrex - so I can understand that they'd be considered strange or risky setups. But the SI and SB were designed to work together in this way right from the start. The setup is simple and easy to configure - in fact the wiring would be simpler than for a DC setup as most of the connections are AC using standard breakers.

    AC coupling is inefficient at charging batteries compared to DC, so if you were looking for a pure off-grid system I think DC coupling would be better for 98% of the cases. But in your case, since you're grid feeding most of the time and just want a backup for the occasional blackout - or even extended blackout, the SI + SB combination seems an appropriate choice. 8kW of PV is plenty for off-grid. You may need 2 x SI for that much PV - check with SMA america.

    Some case studies of real-world AC coupling:
    - http://www.windandsun.co.uk/Projects/island-project-gallery.htm
    - (PDF) http://www.sma.de/en/products/off-grid-inverters/sunny-island-5048-5048-us/dlf/U1lTR1VJREUtQUVOMDgzMjEx/download.html?type=281&cHash=ac01fc7f835b328ed2ac9c5c55742c84
  • XRingerXRinger Solar Expert Posts: 529 ✭✭✭
    Re: Fake out Grid Tie

    I also have a 5kw gas generator.. But, it seems that an inverter can do the job.


    However, the GTIs being used are cheap Chinese units.
    If you have more discriminating GTIs, they might not like your AC source.
    I was kinda amazed that a modified-sine-wave inverter would work in a home made Grid Island..

  • RCinFLARCinFLA Solar Expert Posts: 1,453 ✭✭✭✭
    Re: Fake out Grid Tie

    Unless generator is inverter-generator it is unlikely it will be stable enough to provide the -0.7hz to +0.5hz frequency spec for grid-tie inverter to maintain connection.

    I would also not risk a possible backwash into a generator that may damage it.

    The XW inverter software can shed the grid-tie inverter if ACout loads / battery charging is insufficient to consume the PV grid-tie output. It does this by moving the XW freq outside the limits of acceptance to the grid-tie inverter.

    To Xantrex there are a number of risks to allowing this in an official software release as there is a great variety of grid-tie inverters out there and being somewhat at the mercy on how they might do things like UL1741 grid tie testing along with possible bad effects puts Xantrex on the line for any failures created.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Fake out Grid Tie

    Better option would be the new XW charge controller designed to handle the voltages of a GT system. This is the way I plan to go.

    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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