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'[EE]: Grid tie inverter schematics?'
2004\01\12@133029 by Gary Neal

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Guys,

Anyone have information on grid-tie inverter schematics?  Something that
will synchronize to the grid and allow someone to put power back on the
main grid?  Input= ~12V, Ouput =120V, 60Hz, up to ~1kw.

I can find stand alone inverter schematics, but not grid synched.

I know there's all kinds of safety issues, so please skip the flames.  Just
trying to get educated.

Thanks,

Gary

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2004\01\12@141410 by Richard.Prosser

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Gary,
The "simplistic" approach is to use the voltage /frequency droop method. As
the output current rises, the output voltage and frequency drops slightly.
You have to target a slightly higher voltage & frequency initially. (A
series resistor would do the trick wrt voltage but wastes power, electronic
voltage control is really required).
You still have to closely monitor output current and voltage to remain in
phase. If you have a voltage supply that is noisy and/or variable (eg  a
genset) then you can have real problems.

Even running a number of inverters in parallel can be difficult without
additional communication & sync lines.

If you look at how the phase difference between the voltage and current
then a leading current phase angle will indicate that you are running
slightly faster than mains, while a 0 phase shift with high current
indicated that the output voltage is high.

In any event you will probably need a disconnect/connect relay to alow
syncronisation before you start feeding power and an electronic current
limit to limit current quickly and safely in the event of mains failure/
phase reversals etc.

If you do find an easy solution I would be very interested in hearing about
it!

Richard P




Guys,

Anyone have information on grid-tie inverter schematics?  Something that
will synchronize to the grid and allow someone to put power back on the
main grid?  Input= ~12V, Ouput =120V, 60Hz, up to ~1kw.

I can find stand alone inverter schematics, but not grid synched.

I know there's all kinds of safety issues, so please skip the flames.  Just
trying to get educated.

Thanks,

Gary

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2004\01\12@180906 by Rick C.

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I would not recommend trying to do it yourself. It is not as easy as one
thinks. Your processor must watch and sense "islanding" among other things. You
must also be UL or CE listed. Other NEC, local, and state regulations must be
followed along with the proper permits. It's not worth reinventing the wheel. I
grid tie with a Trace/Xantrex and sell over 22 amps @ 117 volts back to the
grid when its sunny outside. I chuckle when the watt-hour meter spins backward.

Picture of my unit is at the bottom of this page:
http://www.pic101.com/genset

Rick

Gary Neal wrote:

{Quote hidden}

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2004\01\12@191604 by M. Adam Davis

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The theory is fairly simple, it's when the rubber hits the road that you
get burned.

A circuit will match the phase of the AC line (voltage) exactly when it
is neither consuming nor providing current to the mains.

The circuit, if it is consuming, will lag behind the AC line.  If
providing it will lead the phase.

The degree that it is out of phase determines how much current is being
consumed or provided.

Some old mechanical generators would simply speed up or slow down
according to how much energy they wanted to place online.  This method
of grid connection was specifically used so that the losses in
conversion were little, since there is no conversion.

You can still see the effects of this if you measure the line frequency
during a busy part of the day (high load, phase is slowly shifting
backwards) and again in the early morning (low load, phase shifts
forward.  They also try to shift it far enough forward so there are
exactly 24*60*60*60 cycles per day so mechanical clocks are on time)

I believe this to be an area where knowledge is interesting and useful,
but the cost savings of building your own really don't apply.  Don't
think of it as buying a grid-tie inverter.  Think of it as buying
liability insurance.

-Adam

Gary Neal wrote:

{Quote hidden}

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