[RE-wrenches] Array mismatch question

[Dave Click]

Allan, I haven't tried this in the field but I believe you'd be pulling 
the string voltages to roughly that of a 10.67-module string. So your 
11-module string voltage is down by 3.3%, 10-module string voltage is up 
by 6.7%. Your 11-module string performance is down by about 1V/module 
(perhaps power reduced from ideal by 2%) and your 10-module performance 
is up by about 2V/module (perhaps power reduced from ideal by 4%) for a 
total power loss of about 3%. When you get on site and see consistent 
irradiance, you could use some jumpers to bypass modules and better 
quantify the loss by comparing the performance of a 32-module array with 
a 30-module array.

With this many modules in series, and given that most manufacturers 
don't specify voltage tolerance any tighter than 10%, it's probably not 
a big deal. If you are bored on a spare afternoon, you could sort the 
modules so your higher-voltage modules are on the 10-module strings... 
fun, right?

Dave

On 2012/6/26 11:38, Allan Sindelar wrote:
> Wrenches,
> We have been hired to serve as independent consultants on a large Sunny
> Boy/Sunny Island off grid project. The array and the battery bank (of
> unknown type and capacity at this time) have proven inadequate for the
> loads and the owners want outside review of the proposed upgrade, which
> we have agreed to provide.
>
> In reviewing project information, I have learned that each Sunny Boy
> inverter is serving 32 260W modules in three parallel strings. Two of
> the strings have eleven modules and one has ten. This is to us a
> fundamental violation of good design practice, such that we have never
> tried it to learn the results. That's my question: what will be the
> output of this array of mismatched strings?
>
> We figure that the inverter will find a MPPT "sweet spot" that is pulled
> down below the MPP of the two eleven-module strings, to where the
> ten-module string's contribution is close enough to its MPP to maximize
> the output of the three strings together. This would result in slightly
> more than 30 modules' worth of wattage output from the 32-module array.
>
> We have not yet been to the installation or seen photos of it. My verbal
> understanding is that the 96 modules are on eight pole-top racks. It
> would be fun to figure out the array wiring on this one.
>
> Has anyone encountered this mismatch situation? What did you learn?
>
> Thank you,
> Allan
> --
> *Allan Sindelar*
> _Allan at positiveenergysolar.com_ <mailto:Allan at positiveenergysolar.com>
> NABCEP Certified Photovoltaic Installer
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder and Chief Technology Officer
> *Positive Energy, Inc.*
> 3209 Richards Lane (note new address)
> Santa Fe, New Mexico 87507
> *505 424-1112*
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