Your also going to have to consider what's going to happen to your high temp
voltages in 5-10-15 years as the PVs derate with age plus add in dirt/dust
etc. You'll be losing more and more of those summer hours as time goes on.
Seems like an extremely narrow voltage window on the inverter and/or very
high Voc per PV. Are you locked with the PVs you've spec'ed...and the
inverter? You say you're completing this system, but still trying to
configure the strings??
How about a temperature controlled disconnect with 13 PVs.
Bill
Feather River Solar Electric
4291 Nekson St.
Taylorsville, CA 95983
----- Original Message -----
From: "Jeff Yago" <[email protected]>
To: "RE-wrenches" <[email protected]>
Sent: Monday, December 07, 2009 9:28 PM
Subject: Re: [RE-wrenches] large array string sizing
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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