UTILITY GRID-TIE with Back-up GENERATOR, how does it work? will it work?

ChanChan Registered Users Posts: 45 ✭✭
I have grid-tie system, is it possible to have the grid-tie system work should the grid goes out, but the GENERATOR is turned on?


  • BB.BB. Super Moderators, Administrators Posts: 32,019 admin
    "Normal" Grid Tied systems (solar panel + GT inverter, no battery bank), generally do not work with a genset.

    Normally, you setup a "protected AC panel" that can draw power from either AC mains or a genset from behind a transfer switch.

    The GT inverters would be connected to the AC mains only.

    Several reasons... One is many gensets do not hold AC frequency (60/50 Hz, +/- 0.5 Hz) accuratly enough.

    For those gensets that do, then you have the issue that most GT Inverters fee 100% of the solar array output back to the AC mains. If ou the AC loads are less than the GT Inverter output, then the "excess power" wants to go backwards through the genset--Not a good idea.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ChanChan Registered Users Posts: 45 ✭✭
    So... there is no way to make a GTI to work with any genseT?
  • BB.BB. Super Moderators, Administrators Posts: 32,019 admin
    The short answer is there is no "easy" way to do it.

    The longer answer is that newer inverters are coming out that will (probably) have "adjustable" output power. Whether this is done by a utility (to limit GT solar energy when there is too much), or to better integrate with backup/off grid gensets (smaller system for homes and such)--I don't know.

    If you where looking to power a hospital with a LARGE Genset--Possibly you could find a manufacture (like Cummings or Cat) that can integrate with solar power to reduce fuel usage.

    But you are really bumping up against competing requirements:
    • GT Solar--Originally, harvest as much energy as possible and dump to the "infinite capacity" utility grid (much like charging a giant AC battery bank).
    • GT Solar--Over size by 2-3x your average day time load (collect excess energy during daytime, spin meter backwards, then by back power at night).
    • Gensets--Designed to generate power (somebody has to set the 230 VAC 50 Hz-Or whatever--Voltage and Frequency)
    • GT Solar--Not designed to supply starting surge power (well pump, etc.). Battery bank or genset is.
    • Gensets--Generally need to run at 40 to 60% of rated power for fuel efficiency and reduced carbon buildup/wet stack. Using GT power with a genset running very low output power still costs fuel and is not good for the diesel (and gasoline gensets do not use much less fuel at less than 50% of rated output--So a GT inverter would not save fuel when running a genset with less than 1/2 loading).
    • To get GT+Genset to play together--Need genset with accurate frequency control, or programmable GT inverter to accept +/- 5 Hz variation. Need adjustable GT output so that light loads and genset running is not swamped by GT Solar power.
    Today--The way to get this all to work, and is done in some places in Africa (last I read)--Is to do "Hybrid" Generator power. Basically--A battery bank+AC Inverter runs nighttime and middle of the day loads--And a Genset fires up for heavy evening loads (cooking, people home doing house work, lighting, etc.) and charging the battery bank. Once loads drop (and battery charging is done), the battery+AC inverter take over again.

    And you can do this today with many inverter-charger systems + solar panels + genset... But, it does cost money. Battery banks are not cheap, and need replacement every 5-10 years or so (plus maintenance).

    As of today, with standard demand loads (factory, office, etc.)--A battery is going to be in the system somewhere. About the only loads that work well with direct solar power (no batteries) is well pumping to a cistern/storage tank/pond. The pump can speed up/slow down based on available solar energy--And the "product" is cheaply/easily stored (water in tank/pond).

    Is it possible to do this with other types of loads--Yes, I could see this being done for cold storage/food processor room (run the cooling from solar, store "cold" in ice, cold plate, etc.). It has also been looked at to help building A/C efficiency (pre-cool ice tank/cold sink with cheap over night power, then cold sink to reduce A/C costs during the day.

    What you ask for is doable--But, for the most part, the battery bank is the costly/non-economic part of the system today. If/when a new battery technology becomes cost effective, you may see this roll out rather quickly (with or without solar panels).

    Any way--My observations.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mpcjaxmpcjax Registered Users, Users Awaiting Email Confirmation Posts: 1
    Chan said:
    So... there is no way to make a GTI to work with any genseT?
    Yes, there is and I did it during hurricane Irma. I have a small solar array ,1080 watts, with 2 Chinese made grid-tie inverters "WVC-600". With a partially clouded sky I was gettng 700 watts out of my array which reduced my load on my generator, Briggs&Stratton Storm Responder 6250, by 11%. Disconnected then reconnected several times to confirm what I was seeing on generator's load meter. Since my house load is always more than what my array will produce, I had no concern about back feeding into the generator. Expecting that you could run your house 100% off your array is a little crazy. :I guess you could partially cover your array to reduce the output to a safe level. I am sure this is not recommended by neither the generator or grid-tied manufacturers,
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    I've seen boxes that people have built that measure the direction of power flow and modulate a dump load so that a grid-tie inverter never back-feeds power into a source (the grid or a generator).   But I haven't seen commercial versions.

    Not clear to me exactly what would happen if mpcjax's house load went below 700 watts.  Perhaps a generator over-speed which would cause a too high frequency and knock the GTI offline?

    I am available for custom hardware/firmware development

  • mvasmvas Registered Users Posts: 384 ✭✭✭
    How does the Outback VFXR3048E Grid-Tie Inverter use an AC Generator power & Battery Bank power to supply AC House Loads, when the Grid is down ? 
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