Power flow, grid tie system -Fundamental Question

[jaggedben]

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.

 

[five.five-six]

And even if power could flow both ways at once the net result would be the same.

Same as a perfectly balanced neutral.

 

[ggunn]

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.