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<font size=3>Jeff:<br><br>
If I read this correctly, it sounds like you have an inverter with an
unusually narrow voltage window -- 12 modules is too few and 13 is too
many. This complication is above and beyond the physical
limitations imposed by the structure. There is too little
information in your e-mail to allow one to double check the
numbers.<br><br>
I assume that this project is in the bid phase because these details
would be worked out at that point. You need to change inverters or
modules and CAD a design that satisfies the electrical and physical
requirements. We recommend that these projects be designed to the
nut and bolt level prior to bidding. It hurts when you put that
much effort into a job you don't get, but it hurts worse wen you win a
bid and find out there are unpleasant surprises.<br><br>
Good luck.,<br><br>
William Miller<br><br>
At 09:28 PM 12/7/2009, you wrote:<br>
<blockquote type=cite class=cite cite>We are completing an institutional
solar project about 120 kW going on a large parking deck. The large spans
require some really large beams to support the multiple rows of modules
since support columns are as much as 60 feet apart. In other words,
adding more modules to each multiple row is a really big structural issue
and they are at the limit now.<br><br>
However, no matter how we design the strings, the number of modules in
each row can only be divided by 12 and multiples of 12 which is the only
combination of modules per string that also works with the number of
modules per row, the number of rows per system, and the number of modules
stacked N-S related to shading issues between rows. However,the inverter
manufacturer wants 13 modules per string to avoid low voltage cutoff on
hot summer days. if I go to 13 modules per string, I am really hitting
voltage max during average cold days and believe me, this site will see
more cold and snow days (reflections) with temperatures far below average
winter temperatures, then it will have hot summer days. The summer
average temperature is 83 degrees and I calculate the minimum array
voltage is still over 316 volts, on a 480 VAC 3 phase inverter.
<br><br>
Since by both structurally and module count, there is no way we can
increase row lengths to provide 13 module strings, and we cannot reduce
modules per row to achieve 13 module strings as this would make the
system far below what the client expects in system capacity.
<br><br>
Weather data for this location indicates it exceeds our summer design
temperature by onlyv60 hours total per year, and no doubt most of these
hours occur near the late afternoon at the end of a solar day.<br><br>
Whats the best way to deal with this less than ideal combination of
string sizes since we are getting nailed at both ends - too few modules
per string can shut down inverters when hot summer days drop module
voltages, and too many modules per string can do some real high voltage
damage on a very cold sunny day with a foot of snow on the ground.
<br><br>
What say yea?<br><br>
Jeff Yago</font></blockquote></body>
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