# OT: Ping EE's: Connecting to The Grid

Awl --
Suppose I have my own home-grown generator -- wind, solar, etc -- that I
want to connect to The Grid.
The first problem I see is voltage regulation, shunt current:
To feed the grid, you have to be at some voltage above grid voltage.
How do you dump all your current capacity to grid, without possibly
over-volting the grid locally?
And, if just incrementally above grid voltage, how do dump all your current
capacity to the grid, without having to lose it to a shunt? Does a voltage
regulator solve this, or does it still bleed excess off to shunt -- or to a
battery pack?
Even more of a problem would seem to be phase, discussed in
Wow....
So connecting to the grid is not a trivial matter, not DIY stuff, it
appears.
Unless grid voltage regulators/synchroscopes are standard market items.
How does one do this? Idears? Links? ngs?
All these commercially provided windmill generators must have their own
syncroscopes/regulators?
Mebbe Edison was right: Shoulda stuck with DC?
Also, if what I read recently is correct, it seems that that very
high-voltage long distance transmission is actually more efficient with DC!
And now that there are solid state inverters, a DC-based grid might be
feasible.
With zillions of DC inputs to a DC grid, all the ripple (inherent in
rotary-type systems), would statistically average out to a fairly straight
line, for even better DC!!
Until recently, poss. even still today, Con Ed in NYC provided high-power
DC, for things like elevators, etc. in some older buildings/locations. Man,
what a spark!
But you never see DC in anything constructed new, after the '60s, poss.
earlier.
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".
?file=HP109_pg116_QandA_5 --Winston
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
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 -------------------
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.
Just take a garden variety induction motor and dump it back by running the shaft speed at a higher frequency, say 10% above slip.
Fine if your only contributing to the existing grid since your still externally excited....but if for some reason you become the major or sole source then all bets are off--if shaft speed is not governed somehow you will overspped if lacking a sufficient load, causuing high voltages to be back fed intot any still-connected utility lines etc...conversely, in a small villiage with no external grid connection for instance, folks absolutely NEED to leave their lights on 24/7 or else the electricity needs dumped into heating water or air else the prime source needs regulated ans so on.
Anyways, there is a VERY good booklet on this shit that some African guy named Nigel Smith wrote I have a copy and it's suggested reading for anyone interested in the topic.
Lemmee see if I can find the isbn...
Close enough...why occurs to me that my Kubota PTO at say 840 shaft speed and pully driving a 25 hp garden variety 3ph induction motor at say 3900 rpms why it oughta just about run the shop just fine in a pinch eh...
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
Well, a lot depends on what the generator actually is.
Not quite that simple, nor as complicated as you might think. FOr a mechanical rotating SYNCHRONOUS machine, and a (relatively) stiff grid, power flow is largely controlled by trying to vary the speed (frequency), and var flow by trying to control the voltage.
Again, it depends on what the source technology is. A rotating machine will not have any shunt you "dump" to.
Sounds like you are assuming a DC source. For that you have to use an inverter to connect. One that your utility will consider suitable, will already have this handled, and will have built in all of the required over and under voltage, over and under frequency, ground fault detection, and anti islanding protection. IF you try to go an other route, then you would have to provide all of that separately.
jk
"Robert Swinney" wrote: . (clip)Grid-tie residential systems have a watt-hour-meter connected from
^^^^^^^^^^^^^^^^^^^^^ Why do you need an additional meter? If I understand correctly, when you are "selling" power back to the utility, your meter runs backwards.
BTW, a friend/neighbor has this kind of a system. His system reduces his electric bill, but PG & E does not pay him if he ever puts out enough in a given month to produce a negative balance. They "keep the change."
That might be against Federal Law.
I do recall many years ago with this started, perhaps the 70s, Congress enacted laws forcing power companies to buy back uploaded energy.
Leo,
Governing legislation may vary by state. Many years ago, Live Steam ran an article about an induction generator of the type your neighbor has. It featured an ordinary induction motor ran by a steam engine. It was connected through the (single) residential watt-hour-meter to the grid. According to the article it would cause the watt-hour-meter to slow and then run backwards as the load was picked up.
Bob Swinney
"Robert Swinney" wrote: . (clip)Grid-tie residential systems have a watt-hour-meter connected from
^^^^^^^^^^^^^^^^^^^^^ Why do you need an additional meter? If I understand correctly, when you are "selling" power back to the utility, your meter runs backwards.
BTW, a friend/neighbor has this kind of a system. His system reduces his electric bill, but PG & E does not pay him if he ever puts out enough in a given month to produce a negative balance. They "keep the change."
Keeping the change - hum m
I'd say they should at the very least then keep a credit. e.g. you build up over the winter time and use credit over the summer. Otherwise it is theft of product or service by themselves.
Martin
Robert Sw> Leo,
induction motor ran by a
watt-hour-meter to the grid.
run backwards as the

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