OT: Ping EE's: Connecting to The Grid

The simplest way to think about it is that DC moves matter, well electrons, and AC moves motion. Which is sort of a derivative, and as you well know derivatives have huge leverage power. :) cheers T.Alan

To say nothing of Frequency and phase, which must be bang-on or you'll experience a few little problems, like a rotating generator rotating off of its mounts, melting conductors, and other drama you won't care to repeat.

AC has "zero-crossings" so arcs can self-extinguish, and transformers can be used to change voltages without significant energy losses.

I suspect (not having looked at all) that modern gear employs inverters synchronized to the grid freq and phase, and voltage and current regulation at the very least. Somehow, an agreement with the carrier would seem to be mandatory so that YOUR stuff doesn't break THEIR stuff. /mark

I have heard facsimiles to that effect, but ultimately I don't know if it holds water.

Perhaps a better analogy for AC / DC might be a reciprocating IC engine vs a rotary! Where, in power/cu in or per pound, the rotary wins!

Ultimately, AC moves the same electrons the same *net* distance as DC, just through a shorter end-to-end distance.

The fact that an actual electron (if there even is such a thing, QM'ly speaking) in DC will translate/locomote a farther actual distance may be ultimately moot, ito IR drop/power losses.

In fact, the capacitance/inductance effects may be part of the AC disadvantage, at high V. Not really sure, tho. What is f'sure, tho, is that capacitance/inductance is moot, for DC.

Still, how to connect to an AC grid??

The folks in alt.energy.renewable will tell you that you need to buy one each "Grid Tie Inverter".

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--Winston

There is a simple way to synchronize a local AC source with the grid: tie a light bulb between your gen and the utility power line, before you close the switch. If the frequencies are close, but bulb will go on an off at a "beat frequency." Throw your switch when the bulb is off. The impedance of the power grid is so much lower than your local system, that you can't possibly cause them any harm. If your voltage is higher than theirs before you throw the switch, it will drop to line voltage as soon as you make the connection. Current will flow into the grid, and the output impedance of your local gen will cause your voltage to drop and match theirs. Your frequency will also match theirs. If you try to fall behind, they will drive your generator faster. If you try to get ahead, the current flow into the grid will load your generator and slow it down.

We did all this in the EE lab when I was in school.

isn't the electric utility required by law to assis you with this, however grudgingly?

It's also important that your stuff doesn't kill their people. The power company service people sometimes get real PO'd (or real hurt, or real dead) when they think they're working on a dead line - because they've turned it off, or a tree limb or a windstorm has broken it, or whatever - only to find out that you're pumping juice into it from DOWNSTREAM of where they think it's cut off.

Call the power company, and find out what the rules are. They're required to buy any power you make beyond your own use; but there are some proper, and not so proper, ways to make that happen.

KG

Burnt bulbs are "light off". Dangerous method. Go with a "light on".

How are you going to do that? The phase mismatch causes a voltage difference between the generator in question and the utility voltage. When the phases are matched, the voltage difference is at its lowest point, and the light goes off.

Exactly. Not a good plan. Ask the guys who did that....oh no....you can't!! You can join them, but you can't ask.

If you are going to generate DC with your source, there are several manufactureres who sell the grid-tie inverter you will need, up to about

7KW. This thing will take care of all the problems. If you are going to generate AC in the range of 75Kw an up, there are lots of mfr's who have switching equipment for you. It is expensive, think $30K to $100K, but the genset is real expensive, too. If you want to generate AC in the 10Kw to 50KW area you are in trouble, just as I am. But I am working on it. Trouble is that most utilities will only allow UL approved equipment. There is some wiggle room in this, depending on where you live. If you want to know more, email me off list.

Pete Stanaitis

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Proctologically Violated©® wrote:

You hire a licensed engineer and electrician to design and install the requisite equipment.

If you already have your own home grown power system, this could be trickier than one purpose built to tie to the grid.

As a minimum, they'll require that you install a disconnect switch accessible to them that they can lock out while doing work on that part of the system.

"Brian Lawson" wrote: Burnt bulbs are "light off". Dangerous method. Go with a "light on". ^^^^^^^^^^^^^^^^^ The bulb goes from dark to very bright at a frequency = the error frequency (I referred to it earlier as "beat frequency." ) Your idea that a burnt out bulb could fool you is pretty far fetched. Your suggestion that all the people who have tried this are dead is preposterous. There were about 25 people in my lab class, and we ALL did it. The class was given every semester, and I have an idea that if there had been ONE accident, oh, well, what's the use arguing.

Don't argue with a fool. Before long people can't tell the difference.

Don't forget that the lamp has to be rated for the peak voltage, so if you are monitoring a 120 VAC line you need 240 volt bulbs.

As others have mentioned, since solar generates DC, that implies the use of a grid-tie inverter. They've also touched on the safety issues.

For wind and other small mechanical sources, which you only intend to run grid-tie, no batteries, the simplest way as I understand it is to start with an induction motor.

Use the grid to bring the motor (and with it, the generating equipment) up to its base speed (as determined by the grid frequency and number of poles, less slip). Now as you add additional mechanical power with your generating equipment by adjusting blade angles or opening water valves, or whatever, you will catch up to and pass the design speed, and enter a sort of reverse slip. "Leading" the power of the motor will now push power back into the grid. The implication here, as I see it, is if a motor is rated to run full power at say,

3425 rpm, or about 5% under 3600, you stand to run into trouble if you try to drive it more than 5% faster (3775 rpm) before you start running into overloaded bearings and overheated wires due to too much current. I suppose the easy way to manage that would be to use an ammeter and monitor your output current relative to the nameplate current.

Now the next question becomes, how do you design the mechanical side of your generating equipment not to overload the motor? I can see it being easier with a small hydro setup than wind, due to it being more of a steady-state sort of power source, though I suppose you could play with furling vanes and all that, too.

I'm sure there's an explanation somewhere out on the net somewhere that does better justice to the topic than that, but that will at least get you started.

--Glenn Lyford

Glenn sez:

" . . . For wind and other small mechanical sources, which you only intend to run grid-tie, no batteries, the simplest way as I understand it is to start with an induction motor. ..."

An induction generator, essentially an induction motor driven above it's design speed, is usu. used in wind turbine generation of "power". Electricity produced is metered via a watt-hour-meter on the premises and "sold" to the grid. Grid-tie residential systems have a watt-hour-meter connected from the wind turbine back to the grid. This meter is in addition to the regular residential meter. The home owner receives credit from the utility based on the amount of power generated.

Bob Swinney