Re: [RE-wrenches] Matching modules into Xantrex GT

[Allan Sindelar]

Kent (continuing an older discussion thread), You addressed the particulars of the specific application below correctly; thanks. But the question I was asking was about he effect if the example had been the other way around. You write that "the combined array will be operating at the Vmp of the largest subarray" - right. In the case below the larger subarray operates at a lower Vmp, and thus simply pulls the VMP of the weaker subarray down to it, resulting in an insignificant performance loss. But what if the larger subarray had the higher Vmp? It would pull the Vmp of the weaker subarray up, rather than down. It seems to me that this would pull the weaker subarray past the knee of its I/V curve, resulting in a substantially reduced current output for that string. That's what I was asking about. Your thoughts? Allan Sindelar Allan@positiveenergysolar.com NABCEP Certified Photovoltaic Installer EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com On 1/16/2011 5:30 PM, Kent Osterberg wrote: Allan, As a first estimate, the mismatch losses can evaluated by assuming that the combined array will be operating at the Vmp of the largest subarray.  In this case that would mean the SW175s are operating at a voltage that 3% lower than their Vmp.  Since the power curve is nearly second order near Vmp, the power output of the SW175s will be about 1.5% off Pmax and the combined array will suffer from about 0.6% mismatch. In reality the maximum power point will occur at a point someplace in between the Vmp values and the mismatch loss will be a even less. Kent Osterberg Blue Mountain Solar, Inc. Allan Sindelar wrote: A little more musing here...and I'm looking for others' opinions on this theoretical piece. If you have two unmatched strings, won't the string with the larger Imp define the MPP of the inverter? If so, I would expect that the weaker string would be forced to operate at the Imp of the stronger string. If the stronger string operates at a higher Vmp, it will pull the weaker string's operating voltage above its own MPP, which could pull the current down substantially as voltage moved from Vmp toward Voc. Conversely, if the stronger string operates at a lower Vmp, it will pull the weaker string's operating voltage back toward its own MPP, which would maintain approximately the same current at a slightly lower voltage. This would reduce output slightly, but not nearly as much as in the other direction. Does this make sense? And does real-world experience bear this out? Allan Sindelar Allan@positiveenergysolar.com NABCEP Certified Photovoltaic Installer EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com On 1/15/2011 11:50 AM, William Miller wrote: Friends: I have a client with a Xantrex GT2.8 inverter.  Feeding it are 8 Solar World SW175 modules.  This is half the modules that this inverter can handle.  The client wants to realize the full potential of this inverter.  Adding 8 additional SW175 modules would be ideal, but they are no longer manufactured. I need to find a group of modules to match the 175s.  I am contemplating 9 SW240s.  The specifications for this match up are below.  I imagine the IV curves are similar for these two modules as they are the same chemistry and the same manufacturer.         8-SW175 9-SW240 Voc     403             382 Vmp     283             275 Imp     4.95            7.87 Pcsi    1252            1715 Ptotal  2.967 Might this match be close enough?  I know this over-powers the inverter slightly.  I imagine that the Xantrex will self regulate this very rare over wattage condition. Comments? William Miller Please note new e-mail address and domain: William Miller Miller Solar Voice :805-438-5600 email: will...@millersolar.com http://millersolar.com License No. C-10-773985

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