Homebrewed Electricity > Controls

Grid-Tie Principle

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Caleb:
I vaguely recall someone referring to grid tie as "injecting current" into the grid.  If you have a boost converter, it really doesn't care so much what the output voltage is.  The voltage will rise until current flows out of the inductor into the load.  Perhaps a similar principle is applied to grid tie.

dnix71:
jack11 is wondering if it matters if his RE injection is on his side of the meter or the utility side. It should be at the meter itself in which case it wouldn't matter.

The injection can't occur beyond the house meter, unless there are 2 meters, otherwise you would be giving the utility free electricity and then buying it from them. It could be inside, between the meter and the house main panel, that might be the easiest place to install it, but it should be injected at the point of service as an extra set of wires to the meter itself.

The illegal backfeed inverters sold on fleabay (originally diverted from legal use in Holland) inject inside the house, so as long as the injection is less than or equal to the whole house consumption, the meter never runs backwards and the utility company wouldn't know what you were doing.

Rob Beckers:
In an attempt to make this thread a little less vague:

There are two types of inverters, one is used for off-grid, turning battery DC into local grid AC. The other is the topic of this thread, a grid-tie inverter that takes DC (from PV, wind, or hydro) and turns that into grid AC. The two may seem similar, but have a very fundamental difference.

The first, the off-grid inverter that works off batteries, creates the Voltage and frequency, it controls it. It tries very hard to keep the grid Voltage that it produces constant, much like a battery would. It functions as a Voltage source.

The second, the grid-tie inverter, has its micro-controller set up to follow the grid Voltage and frequency. It does not make it, it merely follows what's there. It tries very hard to keep its output current constant (as dictated by the MPPT algorithm in there, trying to get the most out of the DC source). As such it doesn't 'care' what the Voltage is (within limits), and if it needs to raise its output Voltage a little to keep that current flowing it will do so.

The grid-tie inverter normally raises the grid Voltage (to be able to pump that current into the grid). Usually just a little, sometimes a lot; I've seen sites where pushing 10kW into the grid raised it by 15 Volt. It all depends on the resistance/impedance that inverter 'sees' looking into the grid.

Where that current (and power) goes is of no concern to the inverter; much is likely to be used locally, since that is simply the lowest resistance path. How it's distributed between your house, the neighbours, or the rest of the grid is entirely a matter of Ohm's and Kirchoff's laws (much like several water pumps supplying a set of hoses; where the water goes depends on the resistance to flow in each of the directions).

The hardware for off-grid (Voltage source) and grid-tie (current source) inverters is pretty much the same. The difference is almost entirely in the programming of the micro-controller that runs the show.

-RoB-

Mary B:
And when you think of how much power even a small town like mine with 120 houses is using that little bit of energy you are adding to the grid might equal the line losses in your neighborhood

jack11:
ok, I think I am homing in on better understanding.

The last posts from Caleb and Rob, and earlier from others, are pretty much in line with what that PhD dissertation says.
I made an error in assuming that a grid-tie inverter functions as a voltage source (like a off-grid inverter). In fact, there would be a problem with this (see the dissert), and it apparently needs to function as a current source.

I think the dissert says exactly as Rob explained, once the inverter synchronizes and validates the grid waveform, it works off a current feedback sensor to control its output current to follow some reference profile the dissert calls "demand current" (see the inverter's ATP model code in dissert - fig A.5).

One thing that's still unclear to me is, how is this "demand current" reference derived by the inverter to follow?
Is it simply the max amount of current the inverter can produce within its own limitations and under the current grid/load conditions (voltage, impedance), or in some other way?
What variables (analog or digital) are used to create this "demand current" reference, and how often does it need to be recalculated?

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