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PV inverters - how do they know to switch off when mains is lost?

Discussion in 'Renewable Energy' started by flatfootskier, Jun 8, 2014.

  1. flatfootskier

    flatfootskier Member

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    I have an inverter which will stop working upon loss of mains. Main reason is so that if the line is broken, the guy repairing it doesn't get fried with power from the 'wrong way'

    Makes sense so far.I assume it must look at its output voltage and current and ensure that it's driving into a load which is neither open nor short circuit.
    When you've got more than one inverter, or your neighbour has one too, how do they detect loss of mains? With a moderate local load, I can't work out how they couldn't occasionally fool each other into thinking that they're still n the grid and keep generating despite loss of grid connection.
     
  2. RCinFLA

    RCinFLA Well-Known Member

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    Think of the power grid as the 'immovable object' from a momentary voltage and phase perspective.

    The spec for frequency on grid tie inverter for frequency accuracy on the grid is +0.5 Hz and -0.7 Hz. There is rough voltage limit specs also. The main way the grid tie inverter tells if the grid has disappeared is periodically it tries to wiggle its phase sync. If the grid is present (immovable object) it will detect high loading because the current goes up or down as the inverter tries to slew its phase match to the grid.
     
  3. flatfootskier

    flatfootskier Member

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    Thanks rc
    So if there's banks of inverters, not synchronised, wouldn't that appear pretty immovable to any single one inverter trying to slide its phase briefly? I.e lots of inverters ( imagine a housing estate with many independent systems) would look just like the grid.
     
  4. dave

    Dave New Member

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  5. RCinFLA

    RCinFLA Well-Known Member

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    No, they would all overvoltage and shutdown.

    Grid tie inverter looks like an AC current source pushing at the grid based on available PV power.

    The grid sets the voltage. The inverters only have an overvoltage/undervoltage safety shutoff. They do not have capability to regulate a precise AC voltage.
     
  6. flatfootskier

    flatfootskier Member

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    Mostly understood, but consider this example.
    I have a friend with a bank of 10 inverters. If the local load and pv output are matched, grid current = zero. We could disconnect from grid and nobody would know about it whilst this state remains. One of the ten inverters then tries to wiggle its phase, the other 9 would look immovable to it, like the grid...
    How would any inverter know to switch off?
     
  7. RCinFLA

    RCinFLA Well-Known Member

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    The situation you paint will stay stable about as long as a pencil stays balanced on its point.
     
    • Agree Agree x 1

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