What makes a grid tie inverter TURN ON?

Discussion in 'Photovoltaics' started by Chuck Yerkes, May 20, 2004.

  1. Chuck Yerkes

    Chuck Yerkes Guest

    many panels. bright sun. grid tie. No power.

    Understood. Accepted.

    What, on the inverter's grid side, happens to make the inverter turn ON
    and let power flow from the grid and from the panels into my happen
    little house?
     
    Chuck Yerkes, May 20, 2004
    #1
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  2. Chuck Yerkes wrote:
    > many panels. bright sun. grid tie. No power.
    >
    > Understood. Accepted.
    >
    > What, on the inverter's grid side, happens to make the inverter turn ON
    > and let power flow from the grid and from the panels into my happen
    > little house?


    Grid tied inverters don't control what power flows into your house.
    Your house is still plugged into the grid the same way it always was
    and when the grid power is restored your house would have power. The
    inverter simply looks at the power coming from the grid and makes
    sure it's stable before it starts pumping power back into it. In a
    way, the inverter is pumping power into the entire grid and it just
    happens that your house is also connected to the grid nearby.

    Anthony
     
    Anthony Matonak, May 20, 2004
    #2
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  3. Chuck Yerkes

    Chuck Yerkes Guest

    Anthony Matonak wrote:
    > Chuck Yerkes wrote:
    >> many panels. bright sun. grid tie. No power.
    >> Understood. Accepted.
    >> What, on the inverter's grid side, happens to make the inverter turn
    >> ON and let power flow from the grid and from the panels into my happen
    >> little house?

    >
    > Grid tied inverters don't control what power flows into your house.
    > Your house is still plugged into the grid the same way it always was
    > and when the grid power is restored your house would have power. The
    > inverter simply looks at the power coming from the grid and makes
    > sure it's stable before it starts pumping power back into it. In a
    > way, the inverter is pumping power into the entire grid and it just
    > happens that your house is also connected to the grid nearby.


    Okay, so how does the inverter define "stable"? Is there something more
    than just a token sine wave at a couple milli-amps?

    Where I'm trying to go with this is that for a grid tie system, in an
    outage during solar generation time - what is needed to 'trick' the
    inverter into allowing the solar power to flow (with grid cutoff via the
    kill switch of course).

    Now, granted, a big ol cloud can drop your solar output to a couple amps
    and that would harm things plugged in. But if I have something on the AC
    side of the inverter that can temporarily pick up the holes, then I
    should be set.

    What that SOMETHING is is what I'm trying to figure out. If I'm down to
    5 amps (lights off cause it's daytime, but perhaps just fridge and
    minimal stuff) then a 1000watt generator would do it. Or a *small*
    battery system powered by just a couple panels an the alternate small
    inverter.

    I'm just straining because to me I should be able to take a 500-1000
    watt battery backed system and use that to kick on a 4-5kw system.

    How minimal a system is needed for this? If 2 panels can charge up
    batteries that are used ONLY for backup, then you can have many
    batteries, they will charge eventually. Once charged, any "extra" can
    be used to the grid on house side of the kill switch.

    Lose power? Hit the kill switch, get the second inverter to go into
    generate mode and 4kw of roof solar flows. In darkness you have 1kw.
    In bright sun, you have 4kw + 1kw.


    What's wrong with this picture?
     
    Chuck Yerkes, May 20, 2004
    #3
  4. Chuck Yerkes wrote:
    > Anthony Matonak wrote:
    >>
    >> Grid tied inverters don't control what power flows into your house.
    >> Your house is still plugged into the grid the same way it always was
    >> and when the grid power is restored your house would have power. The
    >> inverter simply looks at the power coming from the grid and makes
    >> sure it's stable before it starts pumping power back into it.

    >
    > Okay, so how does the inverter define "stable"? Is there something more
    > than just a token sine wave at a couple milli-amps?


    Yes. I believe they typically measure a few different things and one
    of them is how much the input from the inverter affects the power on
    the grid. Since the grid generators are huge compared to a home sized
    inverter their output, and hence the grid waveform, isn't going to be
    affected much by the inverter.

    > Where I'm trying to go with this is that for a grid tie system, in an
    > outage during solar generation time - what is needed to 'trick' the
    > inverter into allowing the solar power to flow (with grid cutoff via the
    > kill switch of course).


    Since inverters are designed specifically to not do this then I'd say
    you would probably have to work hard to fool it. You would do better
    to simply get an inverter + battery combination that does what you
    want in the first place. That is, you want an inverter/charger/battery
    that will maintain a battery bank and provide your house with power
    automatically when the grid fails. Such things exist but they are
    a little more expensive than straight grid tied inverters.

    Anthony
     
    Anthony Matonak, May 20, 2004
    #4
  5. Chuck Yerkes <> wrote:
    >many panels. bright sun. grid tie. No power.


    No output from your grid-tied inverter, if there's no grid.

    >Understood. Accepted.


    If you want power when the grid is out, get a solar power system
    that'll sell power back to the grid when the batteries are full (like
    Outback). Tricking a UL-???? inverter into turning on when the grid
    is out is _supposed_ to be a hard problem.

    --
    William Smith
    ComputerSmiths Consulting, Inc. www.compusmiths.com
     
    William P.N. Smith, May 20, 2004
    #5
  6. booboo <booboo> wrote:
    >You could (theoretically and practically too I think) take a grid tie
    >only inverter (sunny boy, ST, PVPowered etc.) and parallel that with a
    >sine wave battery powered inverter. The battery inverter being the
    >"grid".


    Theoretically, you could, but in the real world there aren't any
    four-quadrant battery inverters like that available to the RE market.
    There's probably some lab equipment that could do it, but it's not
    going to sell for a buck a watt...

    Just as a zeroth problem, what will you do when the solar power input
    exceeds the house load?

    --
    William Smith
    ComputerSmiths Consulting, Inc. www.compusmiths.com
     
    William P.N. Smith, May 20, 2004
    #6
  7. Chuck Yerkes

    Guest

    ----- The following addresses had permanent fatal errors -----
    <>
    (reason: 550 User unknown)

    Greetings Mr. Guppy,

    >UL1741 defines a number of parameters , Frequency , Voltage (both single
    >cycle and multicycle values , which are different tolerances)...


    I was about to order a copy until I discovered it was $795, or a mere $335
    in pdf form :) I gather it's 145 pages, and the last rev was in 1/17/01.
    Do you have a copy? Would you send me the parameters and limits in
    a sentence or two or fax me a page or two listing them?

    I'd like to help make a small grid-tie inverter with a 350W 90%-efficient
    Veco "filtered MSW" inverter with a 2-year guarantee ($49 at Wal-Mart, who
    also sell its $69 700 W bigger brother) and a standard $5 600 W lamp dimmer
    and a $5 current limiting choke and a $10 PIC for MPPT (with an analog output
    for the dimmer) and anti-islanding and writing a "Small Guerilla PV" story
    about it, in an effort to inspire some company to get a small UL-1741
    inverter back on the US market, which might be reasonably simple repackaging
    of the one in the story. I'm thinking the lamp dimmer filter should meet
    the rms distortion requirement (if not, why not, ie why should utilities
    be more concerned about source than load distortions, unless they are
    trying to give grid-tie a hard time?)

    This might be an interesting small system, with a single 12 V battery and
    a way to get some 120 V during a power outage...

    Thanks for your help, if possible...

    Nick

    Nicholson L. Pine System design and consulting
    Pine Associates, Ltd. (610) 489-1475
    821 Collegeville Road Fax: (610) 831-9533
    Collegeville, PA 19426 Email:

    Computer simulation and modeling. High performance solar heating and
    cogeneration system design. BSEE, MSEE, Sr. Member, IEEE. Registered
    US Patent Agent. Web site: http://www.ece.villanova.edu/~nick
     
    , May 20, 2004
    #7
  8. "Solar Guppy" <> wrote:
    >If there isn't a large enough load for the Sunnyboy (or any battery-less GT
    >inverter) to put its power into the grid , the voltage will simply rise and
    >the UL1741 high line will trip the inverter off-line.


    And then the battery inverter will stabilize the voltage, and (after
    some timeout) the grid-tie will come back on, rinse, lather, repeat.
    Sizing the battery inverter, the grid-tie inverter, and the house
    loads becomes another non-trivial engineering task. Not that it can't
    be done, but IMHO it's another piece of complexity you don't
    need/want.

    --
    William Smith
    ComputerSmiths Consulting, Inc. www.compusmiths.com
     
    William P.N. Smith, May 21, 2004
    #8
  9. booboo <booboo> wrote:
    >And at that point, you wouldn't need the extra support of the extra
    >grid tie inverter until some more AC loads came on.


    Sure you would (might), unless the battery inverter can carry the
    whole house load, and then you might as well have done the Outback
    "full solar but sell the excess to the grid" route.

    --
    William Smith
    ComputerSmiths Consulting, Inc. www.compusmiths.com
     
    William P.N. Smith, May 21, 2004
    #9
  10. "Solar Guppy" <> wrote:
    >I am not suggesting this is the best implementation , rather , you indicated
    >it is only done in the lab and I was informing the group SMA has guided
    >actual installations to use this typology


    Fair enough, though my brain hurts when I try to imagine a stable
    control configuration that would work properly. Do they tie the two
    units together in some way so the battery inverter can try to balance
    the load, battery charger, and instantaneous solar power input?

    --
    William Smith
    ComputerSmiths Consulting, Inc. www.compusmiths.com
     
    William P.N. Smith, May 21, 2004
    #10
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