How is power fed back into the grid?

[Sterling Martin]

I'm a bit confused as to how power is fed back into the grid. For instance, if I have a A.C. generator, how do I get it phased right? Also, how would the D.C. output from solar cells have to be conditioned in order to supply the grid?

[Chris Smith]

You use the out side ac lines to set the cycle on your invertor to get them in phase. <p>In other words, the out side line power, sets the cycle in your device,... and not your side. <p>As far as motor driven generators, Im not sure how to set the cycles? <p>I’ll ask the Cogen people in town one day how they tap their turbine driven generator into the main state grid for a match?

[josmith]

If you have a syncronous generator it will fall into sync by itself. You can drive an induction motor above it's syncronous speed to force power into the lines. A generator with it's own excitation may fall into sync on it's own, just make sure it's fused in case it doesn't.
Of course the bigger issue here is dealing with the power company.

[upsmaster]

Well i'm not real sure but the inverter must be in sync with the line hz...most of the time this is done electronicly by leting the line hz control the inverter hz within a narrow window of cycles and i think that the voltage must be controled also if the inverter ac voltage is lower than the line then the inverter becomes a load to the locale power company then bang no inverter.
joe

[ian]

Its interesting to note that this technology is very complicated for two reasons. <p>First of all, you're switching a low voltage at a very high current in an application that demands very high efficiency. At the high cost of power by solar panels a poor efficiency inverter just wouldn't be acceptable.<p>The second reason is the strict codes of the local utility. It wouldn't do to shove power onto the line at low voltages and not the high voltages. Something of a sine wave has to be maintained.<p>Another interesting thing about an inverter is that it should operate at the "peak power point" of the solar panel. Solar panels have a "peak power point", let me explain............
A solar panel will have its highest voltage but 0 watts when there is an open circuit, it will also have its highest current but 0 watts when there is a short circuit. Somewhere between drawing as much current as possible without bringing the voltage down too much is the highest wattage the panel can produce.
When solar panels are going straight into a battery through a charger the voltage is set by the battery and the "peak power point" is seldom exactly met. Although, most panels do have a peak power point around 12V, it's no coincidence.

[Chris Smith]

And yet it is done every day of the week. <p>The transformer on the pole, the one that steps down 8000 volts into to your house current, also works two ways, to step up the current and voltage back into the grid. <p>The fact that these systems already work is a testimony to the fact that it is not impractical, or impossible. <p>The miracles of physics have already been worked out in this field, a hundred years now and more. <p>Thank god we don’t take negative approaches in science, as nothing would ever get done?<p>[ May 07, 2005: Message edited by: Chris Smith ]</p>

[ian]

Chris, I'm not being negative at all. These inverters are not transformers, the technology is far more advanced, and quite impressive.

[josmith]

For D.C. you need a "syncronized inverter". It switches whatever voltage is available onto the line at a point from the zero crossing where the the line voltage is less than to D.C. level. The electronics are similar to a regenerative dc motor drive.

[ian]

josmith...........
Have you looked into this or
are you making this up?
I'm not familiar with regenerative DC motors but I do know the utilities would not certify devices that only boost part of the sine wave. Operating like that would cause unacceptable distortions on the grid.
My strong suspicion is you're pulling this out of the air and that you're wrong.

[josmith]

I saw this thing at a home show maybe 20 years ago.I don't know how good is is just that it exists.
Regenerative drives are common on machine tools. They synchronize the firing of scr's to control speed and direction of a dc motor. When the motor has an overhauling load the current is inverted back into ac and forced back into the line.<p>"My strong suspicion is you're pulling this out of the air and that you're wrong."
OUCH!!

[Chris Smith]

People don’t realize how easy it is. If your in phase, and you boost a signal current into a grid, electricity is a two way street. For starters it reduces the load or current coming into your house, and if the pressure is sufficient it flows back wards instead of into your house.<p> Electricity is exactly like water, and water pressure isnt that complicated. <p>The only major requirement is phase synchronization and this is done by using the line frequency for timing. <p>Alternators all over the country tie in together to make a single unit of power for the different US grids, and they operate 24/7 for the most part with out a hitch. <p>If you had a water tower, and you pumped that pressure back into your pipes, the water grid would have an excess of pressure and quantity that didn’t come from the main source, by the exact amount of your water tower, give or take some resistance and loss. <p>These devices are already installed in many houses and cities, and if they were “complicated” they wouldn’t be cutting down the consumption of H.C. fuels like they are doing as we speak.

[lkraemer]

josmith,
You are exactly right. Large DC drives, like the
MaxLine and Maxpak S6R made by Reliance are actually a controller for a Four Quadrant Motor.
(Three Phase Power with 12 SCR's)<p>1. Motor Forward (Under Load)
2. Motor Regen Forward (Deceleration Forward)
3. Motor Reverse (Under Load)
4. Motor Regen Reverse (Deceleration Reverse)<p>ie. Four Quadrant Motor<p>Under a stop condition as the Load decelerates
the Motor becomes a Generator and the Drive
Gates on the SCR's to let the energy be put
back into the grid instead of slapping it across
some Dynamic Braking Resistors or other load to
absorb the energy. <p>The phasing and gating circuits are very complex and must be to keep from blowing the drive.<p>LK

[MrAl]

Hello there,<p>
Many years back we developed solar power converters for Sandia Labs in
New Mexico. One model was basically a sine wave converter that took about
190vdc in and put 120vac out, and we used a phase locked loop to sync
to the line. The actual connection to the line wasnt made until sync
had occurred. Max power tracking was accomplished at first with a
rather complex digital circuit that tweeked the output current and made
power measurements to determine what the optimum output current should
be for the current sunlight level. Later, it was found that a single
solar cell made a good sampler for measuring light level and it's
output was used to adjust the converter to the max power point of the
array.<p>As ian was saying, these tasks arent exactly easy. First you need a
sine wave converter, a way to sync it to the line, then a way to adjust
the output according to how much sunlight is available.
Oh yeah, the sine wave converters had isolation output transformers,
which could be designed to be simple isolation transformers or step
up or step down, so you could change that to match your solar array
nominal output voltage...which of course means the output is always
120vac for whatever dc input.
The panel we used for testing put out about
190vdc and cost something near a million USD.<p>Take care,
Al

[Sterling Martin]

I understand the part of how the transformer acts as a step-up. The only question I had was how the inverter or whatever other device being used is made to "sync" with the grid. As for the million dollar panel, I think that maybe I'll need lots of solar panels to offset the panel cost ! I was actually thinking of some type of rotating generator device, because at this point in time, solar panels are not feasible. Unless of course you just want to be able to say "I did it" and ignore the financial aspect. Of course it might be easier to generate d.c. with my rotating device and the invert it so I don't have to worry about frequency. I'm kind of thinking about trying to make a windmill generator. I really don't have any intention to "upload" to the grid, but couldn't help but wonder how it is done. Thanks

[MrAl]

Hello again,<p>The converter has a base voltage controlled oscillator, which controls
the frequency and thus also the phase via varying the frequency very slightly.
The PLL (phase locked loop) detects the phase difference between
the line and the converter and generates a dc control signal that feeds
the base voltage controlled oscillator. With a large phase difference the
control voltage is plus or minus, depending on lead or lag. As the phase
comes in close to the line phase, this control voltage approaches zero
so that the phase doesnt change any more. In this way the PLL is able to
keep the phase of the converter in sync with the line at all times.<p>Yes, the solar panels are expensive, but if you use a dc generator and
a converter you'll be best off with a rather higher dc voltage for
efficiency reasons (say 200vdc).<p>
Take care,
Al