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Power flow, grid tie system -Fundamental Question

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    #16
    Originally posted by ggunn View Post
    No, it's because the PV is producing 10kW that has to go somewhere. If not to the house loads, then where? It cannot flow out to the grid at the same time your house is drawing current from the grid; current cannot flow two directions at once in the same wire.
    I was going to use a similar argument. In math we often prove things indirectly, by contradiction. You assume the opposite of what you are trying to prove is true, and find a contradiction somewhere. Its maybe a little unsatisfying, but you can do a similar thing with this question and get to power flowing in two directions at the same time, which cant happen so its just the way it is: the PV goes to the house loads "first".
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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      #17
      Originally posted by electrofelon View Post
      I was going to use a similar argument. In math we often prove things indirectly, by contradiction. You assume the opposite of what you are trying to prove is true, and find a contradiction somewhere. Its maybe a little unsatisfying, but you can do a similar thing with this question and get to power flowing in two directions at the same time, which cant happen so its just the way it is: the PV goes to the house loads "first".
      And even if power could flow both ways at once the net result would be the same.

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        #18
        Originally posted by ggunn View Post
        And even if power could flow both ways at once the net result would be the same.
        Right. So maybe it can flow both directions at the same time and we'll never know it
        Ethan Brush - East West Electric. NY, WA. MA

        "You can't generalize"

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          #19
          Originally posted by five.five-six View Post
          but we know that it does come back at the speed of light.
          But does the same electron come back? We speak of the Electron Flow, which implies that 'my' electron moves along down-stream, but another, identical, electron comes along from up-stream to take its place. Like stuffing ping-pong balls into a 2" pipe-- assuming the pipe is full, you stuff one electron in at one end, and one pops out at the other end.

          Even with AC-- depending on the load, 'my' electron may end up several atoms one way or the other from where it starts-- depending on imbalances in the positive/negative AC current.

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            #20
            If I put a HUGE resistor bank in front of the PV supply feeder then the house will take most of the power from the Grid and very little from the PV.

            So clearly it's because of varying impedance from the 2 supplies. (much less impedance from the PV)


            That's what I'm thinking

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              #21
              A huge resistor bank is just another name for a large load. It is correct that if you increase the load in the house to the point where the PV can't supply it all anymore, then the grid will supply the difference. No different that if the customer is exporting to the grid and then their air conditioner kicks on and then they are importing.

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                #22
                Originally posted by ggunn View Post
                And even if power could flow both ways at once the net result would be the same.

                Same as a perfectly balanced neutral.

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                  #23
                  Originally posted by Designer69 View Post
                  If I put a HUGE resistor bank in front of the PV supply feeder then the house will take most of the power from the Grid and very little from the PV.

                  So clearly it's because of varying impedance from the 2 supplies. (much less impedance from the PV)


                  That's what I'm thinking
                  You are still thinking of grid tied PV as a voltage source. It's not; it's a current source. The output of a GT PV system does not depend on the load it sees, or more correctly, where those loads are. The grid itself is a huge load.

                  If you put a resistance in series with a PV system, all you are doing is producing a voltage rise at the terminals of the inverter and throwing away some of its output as waste heat. At some point as you raise the resistance the voltage rise will push the voltage at the inverter out of its operating voltage and it will shut down, but up to that point its output is virtually constant.

                  I'll admit that the concept of a voltage clamped current source is challenging to wrap one's head around. It isn't like putting a load on a battery (a voltage source) where the output current of the battery is proportional to the magnitude of the load.

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