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Feeding solar power back into municipal grid: Issues and finger-pointing - Page 25

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Posted by Bruce Richmond on April 19, 2011, 11:38 am

On pg 76 it mentions a DSP


It can take DC voltage and build a sin wave using PWM (Pulse Width


As shown on pg 74 that is smoothed and fed into a transformer.  A
feedback circuit measures the current flow and adjusts the voltage of
the constructed wave as needed.  Not sure what you want here.  If you
want the exact details about how each part works google is your
friend.  30 years ago I was working with drive panels for large
industrial DC motors that did all this to feed current back into the
lines when doing regenerative braking.  When they want to slow down
they don't worry about sharing a load.  They know there are loads out
there and they just pump the power into the line.  At that time it was
all controlled using analog circuits.

Posted by Mho on April 18, 2011, 4:16 pm
Absolutely correct.

Two voltage sources in parallel are at the same voltage.
Two exact voltage in parallel can supply the same load and split the load
between them based on the impedance from source (including it's own internal
impedance) to the load in the loop formed.

Yes the trolls are only trying to wreck another group. Sad from some
mentally damaged types but it happens.

"David Nebenzahl"  wrote in message
It *sounds*--and I'm sure you'll correct me if I'm wrong--as if you're
agreeing with me, and with Smitty, and others when we say that it is
*not* required that the photovoltaic inverter supply a higher voltage in
order to transfer current to the grid. (I take this from the last
sentence in the next-to-last paragraph, where you say " ... will adapt
itself to the line voltage, whatever it may be".)

The arguments against this, with all the pseudo-science being thrown
around (most of it by the ones who are also slinging insults) are
getting quite tiresome here.


Posted by Bruce Richmond on April 18, 2011, 3:15 am
Picture two water tanks connected with a loop of pipe underneath.  Put
a ball in the connecting pipe.  The two tanks are connected in
parallel.  The ball in the pipe doesn't go anywhere because it has
equal pressure on both sides.  Now pour some water into one of the
tanks to raise its level (voltage).  There will now be more pressure
applied to the ball by that tank and the water will flow into the
other tank, pushing the ball in that direction.

Attach a single drain pipe to the middle of the connector pipe above.
If the tanks start at the same level and the resistance to flow in the
pipes is the same then water will flow out of both pipes at the same
rate.  If one tank is higher than the other it is pushing harder and
more water will flow out of it than the lower tank.

The power company has many stations and wants them to all contribute
their share.  We OTOH have spent big $$$ on our PV system and if it
produces 5KW we want that whole 5KW to go to some load so that we get
paid for it.  We are not interested in playing nice and sharing a load
if it means we don't get to contribute the full 5KW.  So we raise the
voltage just enough to flow the current we need to.  That will result
in less current flowing from the grid because the load is only going
to accept so much flow and the PV system is taking more than its

Yes there will be flow at 119, 120 or 121, but from what source?  Like
I said, we aren't interested in playing nice.  We are entitled to pump
the power we produced into the line and get paid for it.  If the power
company has to reduce their production a bit to keep the voltage from
climbing too high so be it.

Posted by Daniel who wants to know on April 18, 2011, 3:32 am
But... but... but... EVERY car insurance company is cheaper than its
competitors and every alkaline cell is the longest lasting... LOL

Seriously, the power stations are not higher than each other, but every one
is higher than the grid, imagine three power stations, or 3 generators in
the same station all putting out exactly 48KV, the substation/transformer
they are connected to is getting slightly less, say 47,990 volts.  If they
then need to bring a fourth generator online they use a syncroscope to
adjust the prime mover to sync up the frequency and phase, adjust the field
to match the voltage IE 47,990, close the breakers, and slowly increase the
field and prime mover so that the load is shared and now all 4 are at 48KV.

Posted by Mark on April 18, 2011, 12:53 pm
 To Smitty

You can't have two voltage sources EXACTLY in parallel, its like
dividing by 0.  There is always some small resistance between voltage
sources or the sources themselves have resistance.

The sources have to be higher then the grid, not higher then eaach
other.  Lets say the grid is some point in the middle of a square and
it is at 120.0.  Lets say there are 4 sources feeding that center
point from the 4 corners.  Each feed has a small resistance between it
and the "grid point".  Each one can be at 120.1 for example and power
will flow from each to the grid point.  If feed point one is a big
nuclear plant it might be at 120.3 and your small solar plant at point
2 might be at 120.01.  But each will feed power TO the grid point
load. Each plant adjusts itself to the right slight increse in voltage
to feed the amount of power it has avaialbe.

(This is an oversimplification of what really happens.  What really
happens also has a lot to do with frequency and phase.)

If the load is at the grid point and is at 120.0, then any source
under 120.0 will pull power and not source it., but the load as a
reistor will pull current no matter what voltage is on it.  A resistor
can never geneate power.  But a motor connected to a grid can pull
power if you load the shaft or it can push power if you hook an engine
to the shaft and drive it.


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