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Posted by Scott Willing on January 30, 2004, 2:15 am
 
On Thu, 29 Jan 2004 14:23:27 -0500, R. H. Allen


I don't think it was the case 10 years ago either; it was more than
that many years ago that I worked on direct-coupled DC/DC
up-converters that managed something like 95% efficiency, as part of a
UPS project.

The lead engineer on that project was a bit naive, and the issue of
isolation and the implications for the output of the inverter that
followed the upconverter did not occur to him until we were well into
the project. It was a real "oh-oh" moment when we figured that one
out, lemme tell ya.


I'm afraid so.

I'd hazard a guess that 99% of inverters under 500W do not employ
transformers, which includes pretty much anything that plugs into a
cig lighter.

These units use exactly the same topology that we were working with: a
direct-coupled high-frequency switching up-converter feeds a
high-voltage H-bridge inverter stage. You can easily tell these units
by measuring the voltage between either output terminal and ground.
With a non-isolated unit, the voltage will be half of the line voltage
on each terminal.

If you attempt to connect both DC negative and what should be the
neutral (in North America) AC terminal to ground, the magic smoke
escapes, since this move shorts the high-voltage rail to ground
through one of the inverter power switches within a half-cycle of
firing up.

The same basic topology is used in many larger inverters, except with
a transformer-isolated up-converter. The high switching frequency
permits the use of smaller magnetics, and the magnetic coupling of the
transformer provides isolation. Voltage conversion is a bonus.

The approach taken by Trace, and I presume also by Outback, is to
dispense with a separate switching up-converter stage, run the
inverter at the primary voltage and target output frequency and drive
a Big Whacking Transformer that provides isolation and voltage
conversion. The transformer requires a lot more iron in this topology
since you're operating at line frequency instead of 100kHz or
whatever.


I suspect what they are really thinking about is that you can take
this topology:

low-voltage DC -> DC/DC up-converter -> high-voltage inverter

and swap it for:
 
high-voltage DC -> high-voltage inverter.

But as has been pointed out many times, this means no opportunity for
isolation. With this scheme you cannot connect both the DC negative
input and *either* of the AC output terminals together, as should be
the case (again, NA) for code-compliant installations where both DC
negative and AC neutral are ground-bonded.


Well, I was opening the discussion a little bit further to encompass
rabid skepticism concerning any of various technologies proposed,
which include non-Si approaches.

As far as durability is concerned, "it depends." If I can have a 10kW
array for a couple grand and I have to replace it in 10 years, I'd
still consider that a bargain. I would want to know the environmental
implications of that (embodied energy? recycling possibilities? toxic
waste issues?) but a lot of folks wouldn't give a flying hoot.

-=s


Posted by R. H. Allen on January 30, 2004, 4:54 pm
 
On Thu, 29 Jan 2004 19:15:17 -0700, Scott Willing


Thanks for that post -- very educational about a topic that has been
murky at best in my mind for some time now. What knowledge I *do* have
about inverter design is from a very academic point of view. Any reading
you can suggest that describes real-world topologies for inverters of
the sort that one might use for residential- and commercial-scale PV?
Just a general overview would do -- I'm familiar with the basic
circuits, just not how they're actually put together with real-world
considerations in mind.

Posted by Scott Willing on February 2, 2004, 7:05 pm
 On Fri, 30 Jan 2004 11:54:26 -0500, R. H. Allen


I'm afraid I learned most of what I know on the job. A really good
applications manual from a semiconductor manufacturer (e.g. a company
that makes switchmode power supply controllers) can be a wellspring of
information at much lower cost than the typical multi-100-dollar
textbooks.



Posted by R. H. Allen on January 30, 2004, 6:34 pm
 On Wed, 28 Jan 2004 21:23:51 -0500, "Solar Guppy"


Where do you read that? According to the pdf linked to by the OP, the
module tested at Sandia was "constructed with 500 silicon solar cells
fabricated with the Sliver cell process." Testing a standard Si panel
would have been pointless.

Posted by R. H. Allen on January 30, 2004, 6:40 pm
 On Wed, 28 Jan 2004 21:23:51 -0500, "Solar Guppy"


Sorry about the double-post, I forgot to include something in the other
one....

FWIW, in a subsequent publication they backed off the
transformer-elimination thing and state simply, "These high voltage
modules could allow for direct conversion from DC to AC without the
requirement for voltage up-conversion," which seems consistent with
Scott Willing's thoughts in his reply to me farther on down this thread.

Oh, and the pdf specifies that the open-circuit voltage on the
Sandia-tested panel approached 320 volts. Definitely not a standard Si
panel.

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