Homebrewed Electricity > Controls
Grid-Tie Principle
Frank S:
Jack think of a grid tie as a pool of water fed by a stream and a well and several dipping their cups in at the same time.
the stream is constantly filling the pool the well occasionally adds to this once the water is in the pool there is no way to tell which cup is filled with what water.
if you are pumping from your well and filling your cup at the same time you still get what is available. Be it from someone else's well or the grid it is all the same the only way to say I want to drink my own water only is to disconnect from the grid.
you can however be consoled with the knowledge that sat least some of the water or in this case electricity, was yours
boB:
--- Quote from: Flux on January 01, 2014, 11:07:40 AM --- It does this by altering the voltage ratio of the source and the grid.
--- End quote ---
Which in turn determines the direction of power and how much.
Note the words "voltage ratio"... That is the key.
Similar to a variac on the AC side of the inverter.
It's really just a simple variation of Ohms law... Power and sign and magnitude of current,
resistance of the grid, etc. If the grid impedance is high, the grid tie inverter selling back
will tend to raise the grid voltage.... And the opposite, too... Just as a high impedance
(weak) grid voltage tends to sag when loaded down a lot.
boB
OperaHouse:
Just remember, the sum of the currents at any node is always zero.
Caleb:
This link was posted at The Back Shed. I can't speak to how well it works, but it might be instructive.
http://www.thebackshed.com/forum/forum_posts.asp?TID=2103
jack11:
ok, so there is some sort of balancing act a grid-tie inverter does with the grid, by adjusting the ratio of its output AC voltage to the grid's voltage. That sounds like a voltage adjustment an off-grid inverter could also do depending on its load, except that typical inverter loads are passive, and the grid is an active load which behaves quite differently. I scanned thru the PhD dissertation Caleb provided, it does seem to have some explanation of this, but a lot of reading too.
Regarding the priority of use, Frank is right, I wouldn't care whose energy I use. Except that the utility meter is between the grid and the local grid-tie RE system/local loads. Given the asymmetrical rate structure, if I put my local energy into the grid first, and then use some grid energy to power my local loads, then I get cheated (my sell rate is much less than my buy rate). On the other hand, if I somehow force my local energy into my local loads first, and only sell the excess energy to the grid, then I don't have to buy from the grid at all, and at least I get something for what I sell. This of course would not be an issue if the sell and buy rates were symmetric. All this is assuming that the utility meter is in the place I think it is.
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