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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Fundamentals: CHP and Demand Matching

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    • CommentAuthorbillt
    • CommentTimeDec 8th 2013
     
    I'd guess that RJ is getting confused with the SMA off grid system where the Sunny Island inverter controls the output of the associated PV inverters by modulating the system frequency. As the frequency increase the Sunny Boy inverter output decreases, starting at 51Hz and dropping to zero output at 52Hz. As the UK grid frequency has to be 49.5-50.5Hz that particular system wouldn't work when grid connected.

    No doubt it would be possible to design small inverters which would load shed with very small frequency increments, but it wouldn't be worth it as their grid contribution is trivial. Solar farms are a different issue, but I'd expect then to be under central grid control like any other reasonably large generator.

    Posted By: SteamyTea
    I would think that a local grid will lots of micro generation on it is a pretty low quality noisy place (electrically).

    Anyone know how well an inverter tracks grid frequency?


    They track grid frequency perfectly. If they don't they'll go bang, or at least smoke briefly.

    There's no reason why a local grid should be noisy. Good inverters have a low distortion output, and adding other inverters in a controlled fashion shouldn't add noise.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 8th 2013
     
    Posted By: Ed DaviesIt depends on the imbalance between generation and demand.
    , Yes, that is what I meant.

    Posted By: billtbut it wouldn't be worth it as their grid contribution is trivial.
    Any idea when it does become a problem?
  3.  
    RJ is not getting confused he is just trying to explain how the Germans who are so far ahead of us in terms of Solar and Wind integration into their national grid have got around the problem by using smart inverters which are capable of load shedding beyond preset frequency limits set by government.

    This explains better than me

    http://www.vde.com/en/fnn/pages/50-2-hz.aspx
    • CommentAuthorEd Davies
    • CommentTimeDec 8th 2013 edited
     
    Posted By: billt: “I'd guess that RJ is getting confused with the SMA off grid system where the Sunny Island inverter controls the output of the associated PV inverters by modulating the system frequency.â€Â

    Yes, there are those off-grid systems. However, RJ is right that in Germany they now have a system where grid-tied PV inverters back off their output in response to grid frequency as has just been described here:

    http://www.navitron.org.uk/forum/index.php/topic,21536.msg249026.html#msg249026

    Searching around a bit I _think_ that all new systems are supposed to do this but only old systems over 10 kW needed to be retrofitted to do it or to cut off at various frequencies around 50.2 Hz (so they don't all go off together).

    There's also the, perhaps more worrying, concern with lots of G83 generation that they'll all drop off at the same low frequency which is already indicating that the grid is in poor shape (demand exceeding immediately available generation) with the possible consequence of a cascading failure (as GavinA reminds us earlier in that Navitron thread).
  5.  
    Grid frequency is a NATIONAL thing related to generation:demand balance. We haven't got problems with this. Yet.

    Voltage is a LOCAL thing related to impedance of the network and the load on it. The network was designed as a one way device as has previously been described. Asking it to transmit power in two directions can be problematic, and exhibits itself as a voltage that dips too low under load (winter) and rises too high under local generation (summer). Inverters can't fix this; you need to upgrade the local distribution infrastructure not just the inverter.

    G83 written with a random element to the exact disconnect time/frequency would have been wise in hindsight.

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