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Feeding solar power back into municipal grid: Issues and finger-pointing - Page 24

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Posted by Mho on April 18, 2011, 11:11 pm
The manufacturers are really the only ones that know. The rest of these here
are jus guessing from logic and experience with similar equipment. IOW: it
is a trade secret, mostly as to the exact details but..

The circuitry can adjust phase angle and open circuit voltage to the
grid-ties point and sense the current that results. From this can tell the
current magnitude and phase angle to maintain to deliver the quantity they
desire. Tiny changes in phase angle of current drawn can tell them if the
internal phasing to the grid phasing is getting "out-of-wack".

Voltage phasing is sensed when the grid-tie point (breaker) is open. Once in
parallel (breaker closed) this is not possible as they are all the same

I am sure different manufacturers have different techniques.

Now if you are asking about the waveform synthesis you need somebody else. I
know how to filter the crap out of a square wave but how the make a better
sinewave (less distortion), I have only have ideas that I will not bore you
with. I have repaired many of these things from 400kW 3 phase units down but
never a proper sinewave unit. Maybe just better filtering?

Talk about the "cloud" now?...LOL

"David Nebenzahl"  wrote in message
Y'know, that's the *second* time you've offered that document as a
supposed answer to a question, and it doesn't contain any more relevant
information to what I asked than it did the first time. It is chock-full
of other interesting details, but it does *not* answer my question at
all. The most they have to say is that a DSP is used to sense the
line-side voltage and relay it to the intertie; however, they don't
explain just how this all works in the detail I was asking for.

I invite you to point out specific sections that answer my question, as
I have a copy of the PDF handy, if you think I'm mistaken.

Posted by MarkK on April 19, 2011, 3:07 am


my suggestion to you is to google the term "current source".  Most sources
you are familiar with like batteries and generators are more like voltage
sources.  A current source is in a way the opposite concept and you need to
think about it for a while.)    A grid tie inverter emulates a current
source.  It puts out at its terminals (within limits) whatever voltage is
needed to cause the desired current to flow.

They all follow Ohms law I=E/R. With a voltage source, V is fixed and I
varies with R.  With a current source, I is fixed and V varies with R.

In a normal grid tie situation, the amount the V has to vary is very small
probably 1 or 2 volts  at the most.  That is the detail of how it controls
the current.  The voltage will rise or fall as required such that the
desired current flows.

You can Google the design of current sources for more detail.


Posted by trader4@optonline.net on April 19, 2011, 11:56 am
Google "Norton and Thevinin Equivalency" or "Norton Thevinin

You'll see that a resistive circuit can be modeled as either a current
source or a voltage source together with a resistor and that they are
 interchangeable and equivalent in how they behave.

Posted by krw@att.bizzzzzzzzzzzz on April 19, 2011, 10:49 pm
 On Tue, 19 Apr 2011 04:56:03 -0700 (PDT), "trader4@optonline.net"

Bingo!  It doesn't matter which you model the inverter as, the physics doesn't

Posted by trader4@optonline.net on April 19, 2011, 11:36 pm
 On Apr 19, 6:49pm, "k...@att.bizzzzzzzzzzzz"

Yes, we'e on the same page.  They are interchangeable, but I think
 Mark had a good idea in  bringing up the idea of the current source,
 as conceptually it's a bit easier to understand.  It appears to have
got Smitty at least partly convinced.

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