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Grid-Tie Principle

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jack11:
I've been trying to understand the principle by which a grid-tie inverter pushes the energy into the grid. A good way would be via some mathematical representation from AC circuits, maybe energy/power flow using phasors.

So, can someone explain or quote references, what exactly a grid-tie inverter has to do to push the energy into the grid. Maybe create some small voltage differential, or via some other means. Maybe all this is given in the grid-tie standards from UL and IEEE, I haven't got my hands on them yet.

The example below may help with this.

Lets say we have two constant-voltage AC sources connected together. Source1 is the grid transformer. Source2 is a RE system inverter.
These two sources are perfectly synchronized in frequency, phase, and voltage waveform. They are of the same type, ex. single-phase, split-phase, etc. All enabling conditions have been validated by source2 to sell energy to source1.
For now assume there are no AC loads attached to source1 and/or source2, just source2 feeding source1.

In practice source1, that's typically used to supply energy to the loads, is bidirectional and can also act as a sink, that is energy can also flow into it.
Source2 can only act as a source of energy, as you can't push the energy into the inverter's AC output.
Since we are dealing with two constant-voltage AC sources, the key thing for energy flow seems to be the direction of the average current flow (if it's flowing at all).

And so on …………

Mary B:
Not voltage differential, think current differential. If your house is using less current than what your grid tie inverter(GTI) is putting out the rest flows onto the local grid. That reduces demand further upstream and the utility compensates theoretically. In reality your neighbor probably uses the little you generate as excess and the local grid doesn't even see it as current flow back from the house.

OperaHouse:
Think of it as a flywheel that is kicked occasionally to keep it spinning. You are thinking of this elaborate phasing.  Most of these are like MSW inverters, just dumping as much current as the can into the line.   Once the grid tie sees something like 60V it just dumps in as much current as it can.  The inverter can easily adjust current tens of thousands times a second.  Like saussage, you don't want to see how it is made.

Flux:
Another way to think of it.

Regard the grid as infinite, nothing your inverter will do to alter it. If both voltages are the same nothing happens but as soon as your inverter volts exceed the grid current will flow into the grid. this current will be limited by the source impedance of the inverter supply and will alow the available current from your wind or solar to feed the grid.

Normally you need to match the available power to keep your panels or wind turbine on optimum production and that is where the inverter curve adjusts the power to suit the load to get the best match. It does this by altering the voltage ratio of the source and the grid.

Flux

jack11:
ok, thank you all.

thinking about it some more, no specific voltage differential seems needed, just a distributed power source sitting on the grid, matching it's voltage waveform, and ready to pump current into the grid if the grid loads somewhere down the line demand it.

This brings up another question, about the priority of who gets to use what power.
Say, I now have some local loads in the house that's equipped with this grid-tie system.
If a local load turns on, is it guaranteed to use the energy from the local grid-tie RE system first, or from the grid first, or half and half, or what?
If it uses the energy from the local grid-tie system first, and then uses the grid to supply the overage only, then what is the mechanism that allows it to prioritize usage this way ???
This has a big impact on the financial benefits from a grid-tie system, in case of the unsymmetrical utility rate structure (where they pay you much less per kWh than you have to pay them).

Still. it would be nice to have some math model for it all, and be able to show via a simple computer simulation that it works the way you think it does. I just need to see that sausage being made to convince myself I can eat it ;-)

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