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Cold temperatures, optimum array angle, maybe snow on the ground. Just
add sun for lots of power, maybe even a lot more than rated power at
STC. It is easy to see why you are seeing clipping events. If the array
angle was on a 20 degree roof during the cold weather, the array output
may not go high enough for the inverter to max out. In the hot weather
when the sun angle is optimum for a 20 degree roof, the module output
will be down. How much solar is reasonable to put on the inverter
depends on the circumstances. <br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
Darryl Thayer wrote:
<blockquote cite="mid:439117.99611.qm@web39302.mail.mud.yahoo.com"
type="cite">
<pre wrap="">Location is 45 N latitude, central Minnesota, the array I have watched the most
is a seasonal adjustable, adjusted about once a month. the steepest is about 60
degree tilt. It is a ground mount and has a flat field in front and easy to
adjust. Weather data is near St. Cloud MN
----- Original Message ----
From: <a class="moz-txt-link-rfc2396E" href="mailto:drake.chamberlin@redwoodalliance.org">"drake.chamberlin@redwoodalliance.org"</a>
<a class="moz-txt-link-rfc2396E" href="mailto:drake.chamberlin@redwoodalliance.org"><drake.chamberlin@redwoodalliance.org></a>
To: RE-wrenches <a class="moz-txt-link-rfc2396E" href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a>
Sent: Fri, January 21, 2011 8:24:27 AM
Subject: Re: [RE-wrenches] Inverter oversizing
</pre>
<blockquote type="cite">
<pre wrap="">I have REC 215 on Enphase 190. I see a lot of clipping. Every sunny
day in the
winter and spring.
</pre>
</blockquote>
<pre wrap=""><!---->
What angle is your array? Where is it located?
What I am seeing may be regionally specific. I have seen some clipping,
but usually the inverters have some overhead left from the 199 W limit.
With the 30 degree angle, even on sunny days with snow on the ground the
inverters max out it the high 180s. On another system in the area (with a
string inverter)the roof pitch is 45 degrees. The array can peg the
inverter on those days. The inverter and the STC rating of the modules
are a perfect match.
</pre>
<blockquote type="cite">
<pre wrap="">
________________________________
From: Drake <a class="moz-txt-link-rfc2396E" href="mailto:drake.chamberlin@redwoodalliance.org"><drake.chamberlin@redwoodalliance.org></a>
To: RE-wrenches <a class="moz-txt-link-rfc2396E" href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a>
Sent: Thu, January 20, 2011 8:23:55 AM
Subject: Re: [RE-wrenches] Inverter oversizing
On a couple of Enphase systems I monitor regularly, the modules are
oversized
compared to the inverters. One system has Sharp 224 W modules and M-190s
(capable of producing 199 W). The array is a ground mount at a 30 degree
tilt
at 39+ degrees latitude.Â
When the system was new in the autumn, I noticed two days of some
clipping.Â
Since then, none. In the summer, when the sun angle is better, the heat
will
keep the wattage down so clipping will very seldom occur. The modules
will not
be close to STC, so the inverters should not be pushed to their maximum.
The same is true of string inverters. Since real power of the modules in
generally significantly below the nameplate, inverters will generally not
run at
maximum power.  When we take in all the factors affecting the actual
array
power, I think that 120% over sizing from nameplate wattage is generally
safe.
Drake
At 09:14 PM 1/19/2011, you wrote:
David,
</pre>
<blockquote type="cite">
<pre wrap="">Thanks for sharing that paper. The labeling the graphs for the
10-second and
1-minute data in Freiburg appears to be reversed - the one minute
averaging
seems to have more data in all of the bins above 1000 W/sq m.Â
Basically, these
graphs show that irradiance observations above 1100 watts per square
meter are
fleeting and disappear in hourly averages.  Such occurrences are also
masked to
a small extent by 1-minute averages.
Â
Kent Osterberg
Blue Mountain Solar, Inc.
David Brearley wrote:
This reminds me of a scholarly article I came across about a year ago
while
doing some research. Here’s a link to it if anyone is interested:
</pre>
<blockquote type="cite">
<pre wrap=""><a class="moz-txt-link-abbreviated" href="http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf">www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf</a>
David Brearley, Senior Technical Editor
SolarPro magazine
NABCEP Certified PV Installer â„¢
<a class="moz-txt-link-abbreviated" href="mailto:david.brearley@solarprofessional.com">david.brearley@solarprofessional.com</a>
Direct: 541.261.6545
On 1/19/11 12:29 PM, "Bill Brooks" <a class="moz-txt-link-rfc2396E" href="mailto:billbrooks7@yahoo.com"><billbrooks7@yahoo.com></a> wrote:
Kent,
Â
How often were your data records? To capture edge of cloud effects, you
need
one-second data. Not many people gather that fast or that much data on
inverters. I don’t think there is that much energy in these spikes,
but they are
real and make some difference. 15-minute average data will completely
wash out
this data.
Â
This is also a deficiency in modeling software since most models are
using
hourly data.
Â
Bill.
Â
From: <a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] On Behalf Of Kent
Osterberg
Sent: Tuesday, January 18, 2011 1:18 PM
To: Wrenches; Marco Mangelsdorf
Subject: [RE-wrenches] Inverter oversizing
Attached is a graph that I produced to document the effect of various
ratios
between the PV array size and the inverter size. I extracted output
power data
for a 1020-watt system located in NE Oregon that is on the Sunny Portal
<<a class="moz-txt-link-freetext" href="http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US">http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US</a>
</pre>
<blockquote type="cite">
<pre wrap="">. The data are publicly accessible so feel free to run your own
</pre>
</blockquote>
<pre wrap="">analysis.Â
Better yet, analyze the data for a system near you.Â
Using 2009 data, I looked at how much energy would have been lost if the
output
was clipped at 800W, 810W, .... 1020W. I used 2009 data because there
was a
period in 2010 when the Sunny Webbox didn't have internet access. At
800 watts,
power clipping would have happened on about 25% of the days. Yet the
energy
that would have been lost was only 0.38% of the annual total.Â
The results shown on this graph aren't universal, results would be a
little
different in 2010, it would be different in some other climate, it would
have
been different at another elevation, it would be different with a
different
array angle, ..., and the module tolerance and inverter efficiency also
effect
the results. Modules in this system are Suntech 170-watt +/-3%. The
inverter
is Sunnyboy 1800 that should be operating at close to 93% efficiency.
Kent Osterberg
Blue Mountain Solar. Inc.
</pre>
</blockquote>
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