Posted by on May 5, 2009, 12:56 am
Suppose I have a windmill that can crank out 40 A at 240 V, or aprox. 10 kW.
Suppose I am using that full 10 kW, for boiling water, welding, whatever.
Then, I disconnect that load, and feed the grid *only*.
How much power will I be actually supplying to the grid?
I imagine the answer is that there is no way to really anticipate this by
measurement, as the grid will always be at 240 V, so you can't use Ohm's law
or anything like that.
If everyone's windmill is set at 240.00 V, then your particular windmill
will contribute energy to the grid "catch as catch can", possibly according
to a randomized "Thevenin's Theorem" -- which appears in wiki.
But what if your windmill is a little off?
Suppose it's cranking power at 239 V.
Then, you will *never* be able to sell power back to the grid! Correct?
Or, if it's set at 241V, you might be selling an "unfair" amount of power to
Or, a windmill owner could cheat, ant set his 'mill at 250 V, trying to pump
his windmill's whole capacity into the grid -- in this case, all 10 kW,
24/7, for boucou income.
How does The Grid protect itself against this?
Mae West (yer fav Congressman) to the Gangster (yer fav Lobbyist):
Hey, Big Boy, is that a wad (of cash) in yer pocket, or are you just
glad to see me??
Posted by Don Young on May 5, 2009, 1:24 am
Any small generator can and will furnish its entire output capacity to the
grid if it is properly set up. When connected, the generator operates at the
grid frequency and voltage regardless of any adjustments to the generator.
The generator voltage regulator will have no effect on the grid connected
voltage but if not properly set up can cause reactive circulating current
(VARS) to flow in the generator. The real power (WATTS) supplied by the
generator is a function of the driving power available. The generator can
produce power for the grid, just coast along, or take power from the grid
while acting as a motor, depending on the power/torque applied to the
The generator voltage and frequency have to be adjusted to be the same as
the grid before it can be connected. In your case, if there is adequate wind
(power) to generate 10KW then 10KW will be furnished to the grid. If you
want to learn about this in more detail, study generator synchronizing and
Posted by Tim Jackson on May 5, 2009, 12:24 pm
Proctologically Violated©® wrote:
The grid won't notice, it's your little generator that will go bang if
you connect it directly to the grid, and so needs protecting. In
practice you'll blow the fuse(s).
Without some control gear a generator generates neither constant
voltage, constant current or constant frequency. In order to feed a
load alone it needs at least a voltage regulator to ensure that it
maintains its voltage over load (and wind) variations. In order to feed
into the grid a different sort of control is needed, basically a
mechanism for bringing the generator frequency into synchronisation with
the grid on start up, and regulating the *current* to achieve optimal
loading of the generator.
In a simple alternator setup, it is the field coil current that would be
adjusted to produce the either the rated output voltage (stand alone),
the generator's rated current (grid tie with excess wind), or the
maximum available current (grid tie with lower winds).
The amount of power you feed into the grid depends on the setting of
this current regulator. In an environment where micro-generation
provides a small proportion of the load, then it is harmless for the
regulator to be set to the maximum the generator will supply - the
supply company's regulating generator will automatically back off
accordingly - hence saving them the money to pay you with.
However if such unregulated (by the grid) generators provided a majority
of the current, some more sophisticated control is needed to prevent the
supply company's regulating generator from going to zero. In the real
world of big generators this is done by grid frequency control as we
discussed here recently. The result of oversupply is that the
generators speed up and the grid frequency rises slightly.
Grid-regulated generators automatically reduce their output current as
the frequency increases (over a narrow range) to bring it back to its
Posted by harry on May 7, 2009, 9:11 pm
If you could synchronise your generator too the same voltage and
frequency as the grid (and it can be done with a couple of light
bulbs) you can connect it. You would need to be able to control the
speed pf on your generator. Ideally you would be able to independently
adjust the strength of the fixed magnetic field (usually on the
rotating part). Once connected ,subject to there being no current
overload, it would be locked onto the grid voltage and frequency and
could run at no other speed. If the torque were low it would draw
current, if the torque were increased it would put current back into
The same effect can be achieved with a simple induction motor a lot
more easily. Check out "assynchronous generators"
Your electricity meter (if it were the old fashioned "spinning disk"
type) would run backwards if you were putting power back in ,ie the
disk would go the "wrong way round". The full output of your
generator could be put into the grid
No-one would know, unless your meter ended up reading less than your
previous reading (Heh Heh)
Here in the UK you can legitimately do this & they will pay you for
the power you generate. (But at a much less rate than they charge!)
You have to have an "import/export meter"
Devices are avialable that do all of the above automatically.
There is another technology that takes any DC volts and turns it into
the correct AC volts & frequency & automatically synchronises &
connects up & everything. Bunch of electronic tricks! This is the way
forward. They cost a lots of money.
Info here;- http://www.windandsun.co.uk/Inverters/windy_boy_inverters.htm
Windmills can be made more efficient generating DC.
Posted by Mike on May 8, 2009, 1:22 pm
On Thu, 7 May 2009 14:11:18 -0700 (PDT), harry
UK domestic, commercial and industrial meters haven't been reversible
for many years, in addition electronic ones will indicate in an
unambiguous manner any attempt to wind back the reading by illegal
customer connected generation.
No, if you select the appropriate supply company and for either
wind,hydro or chp you will get paid for electricity you export AND you
also get paid for ALL the electricity you generate even if you use it
yourself. 5p/kWh for the former, 9.2p/kWh for the latter giving a
total of 14.2p/kWh. The current price for import is 12.63p/KWh
The same company also pays 28p/kWh for solar generated export.