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Jeff,<br>
<br>
The maximum voltage of the PV modules will probably be a little less
than the spec'd value and it will decrease slightly as the modules
age. But going over the inverter's voltage rating can (probably will)
be catastrophic. It looks like your leaning towards avoiding that
possibility by using 12 modules and I think that is the right
approach. While inverters do have a minimum operating voltage, the
more likely circumstance is that the array voltage drops outside the
MPPT window. The inverter will still operate and the output power will
drop off but by less than the percentage by which the maximum array
voltage drops outside the MPPT window. So if you are loosing 5% of
your potential output for 60 hours a year, that's not a serious
problem. But you still need to make this decision carefully. In 10 to
20 years the system voltage will drop 5 or maybe 10%. How far are you
going to be outside the MPPT window then? Is the inverter still going
to operate? <br>
<br>
Another potential gotcha working against you is that some inverter
manufacturers string calculators assume that the peak power voltage
coefficient is essentially the same as the open circuit voltage
coefficient; that is to say the percentage change applies to both
numbers. It doesn't! In fact the peak power voltage changes almost as
many volts as the open circuit voltage. Take a look at the
voltage/temperature coefficients for the Evergreen modules; Evergreen
publishes both coefficients. While most manufacturers only publish one
value, the numbers are similar for all poly or mono crystalline cells. <br>
<br>
Sounds like this module/inverter combination may not be a good match.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
Jeff Yago wrote:
<blockquote cite="mid:20091207212852.47D3C505@resin15.mta.everyone.net"
type="cite">
<pre wrap="">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.
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.
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.
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.
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.
What say yea?
Jeff Yago
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