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PV inverter technical characteristics and safety requirements

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Posted by Teo on August 29, 2007, 9:08 am
 
Dear All,
I'm interesting in understanding which differencies there are between
an inverter for PV and an inverter for drives or UPS (of course except
for speed control).

I mean voltage tolerancies, transformers requirements or not, voltage
ripples, efficiency, etc.

Unfortunately I cannot find anything in the web. Could you please
suggest me something? Technical contents are appreciated.

Thanks

Best regards

m


Posted by R.H. Allen on August 29, 2007, 1:55 pm
 
Teo wrote:

PV inverters typically have a DC input from the solar array and an AC
output to the load. Many PV inverters, particularly those used in
off-grid systems, can also use batteries and have an additional
input/output for that purpose. Additional circuitry that may or may not
be included in the inverter is maximum power point tracking (for
extracting maximum energy output from the solar array) and battery
charging (so you don't overcharge or over-deplete the batteries).
Grid-connected inverters also must synchronize to the grid (both
frequency and voltage), and in most places they're legally required to
automatically disconnect from the grid if the grid goes down (for the
safety of the line workers).

UPS inverters typically have an AC input from the grid and an AC output
to the load, with a DC input/output to the batteries. They need to
detect when the grid is up or down so they know when to draw power from
the batteries, and their output needs to be high-quality enough to run
whatever loads they're connected to, but there are no grid
synchronization issues.


The best place to look for that kind of information is the standards
that govern the type of inverter you're interested in. For example, in
the US, inverters used for distributed energy generation like PV are
usually required to comply with IEEE 1547 (interconnection) and IEEE 929
(anti-islanding measures). UL 1741 may also be of interest to you. To my
knowledge, there are no transformer requirements on PV inverters in the
US and Europe -- they're available both with and without transformers.
My understanding is that prior to about 10 years ago transformers were
near-universal, but that more recent designs have tended to eliminate
them. Here's an overview of PV inverter evolution in the '90s:

http://labs.hti.bfh.ch/fileadmin/user_upload/lab1/pv/wr01.pdf

As for UPSs, a Google search on "ups standards" turns up this document:

http://www.mgeups.com/techinfo/techpap/articles/0248-e.pdf

It refers to a number of standards for UPS design.

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