Bill and all.......So we were at the site today and these were our findings: We covered each solar panel with cardboard 1 at a time.....we continued to see the same lower voltage on string #2. String #1 had an expected Voc. String 1 had a Voc of 368, String 2 had Voc of 312. We decided to disconnect the entire string. Luckily the interconnects were accessible without physically removing the modules.......I love array's that are only 2 rows deep with service aisles all around..... The system was 2 strings of 9 BP SX170's connected to a PVP2800XV. Module nameplate Voc was 44V. Cell temp of the modules at time of measurement was roughly 85-90 degrees F. all modules were in the 40.1-8 range....except module # 3 from the positive homerun side was 38.8V, module 6 was 36.6V and module 8 was 10V. We had no trouble determining that module # 8 was defective......In these modules I believe that the entire 72 cells are in a series.string...(am i right?roughly .5+V per cell.). Covering 1 module at a time did not give us any indication as to which panel was defective. The Voc with 1 module covered ranged from a low of 299VV to a high of 304V. This did not make sense to me as to reducing the string Voc from 312 to 304 by covering 1 module.........
Anyhow........the 2 modules that had reduced voltages......1 with ~38V and 1 with ~36V......would you consider those defective? Worthy of trying to get BP to warranty those? We will be filing a warranty claim on the single panel that is obviously defective....... Thanks for your advice and input..... Cheers, Kirpal Khalsa Renewable Energy Systems On Fri, Oct 31, 2008 at 10:33 AM, Bill Brooks <[EMAIL PROTECTED]> wrote: > Kirpal, > > > > When you parallel, your voltages have to be the same, so you are correct in > that assumption. > > > > Your assumption about the diodes is not correct. Diodes don't work like > that. You could have a shorted module (all three diodes shorted) which would > lower the Voc by one module. It looks like you have nearly two modules worth > of problems. It could be a total of 5 bad diodes, but that would be weird. > It could be you have a ground fault, and haven't tripped the GFP yet because > of lower sunlight levels—(it has to be very low sun to stay under 1 amp). > You also could have a wiring mistake, either because the factory reversed > wired a module (rare, but it happens)—that would be 2 modules low; or, you > could have skipped two modules somehow. > > > > It does seem like a quarter of the bad string is out. Your assumption about > bad diodes could be correct, but it only affects the groups of cells its > connected to. Each diode protects 24 cells—it looks like you have 5 diodes > worth of loss. It could be in 2 modules, or up to 5 modules. > > > > The shading test is the best way to find bad diodes. Disconnect the bad > string from the good one and take a piece of cardboard the dimension of the > module. With your meter testing Voc on the back string, completely cover > each module, one at a time. When the Voc is not affected by complete shading > of a module, that module is completely bad. If the shaded module only loses > a portion of one module's Voc, then you have one or two, of the three > diodes, bad. > > > > This is a great opportunity to learn how to troubleshoot a PV array. Take > the time, and the effort, to run through it carefully, learn the problems, > and report back with your findings. It's a beautiful thing to learn > something new. > > > > Bill. > > > > *From:* [EMAIL PROTECTED] [mailto: > [EMAIL PROTECTED] *On Behalf Of *Kirpal Khalsa > *Sent:* Friday, October 31, 2008 10:43 AM > *To:* RE-wrenches > *Subject:* [RE-wrenches] Trouble Shooting Bad Diode > > > > Greetings all......I was hoping i could have someone help me confirm a > troubled series string on a system we have installed..... > System consists of 16 BP 170 W 24V nominal modules......2 strings of 8 > modules.....Vmp of each module is roughly 34V and Voc of each module is > roughly 41V. One string is under-producing......String 1 Voc is 346V and > String 2 Voc is 272V. String 1 Vmp under load is 270V and String 2 Vmp > underload is 270V. I am hypothesizing that String 2 has a bad diode on the > 3rd panel in and is reducing the Voc of that string by th 2 panels ahead of > it in the string. I am hoping to figure this out without removing the > entire string and checking each panel if possible..... > I think the Vmp is equal due to the fact that both strings are paralleled > in the inverter and the 1st string is being pulled to the same level V as > the 2nd string under load......Is my logic good? > Thanks for the help and advice..... > Cheers, > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > -- Sunny Regards, Kirpal Khalsa NABCEP Certified Solar PV Installer Renewable Energy Systems
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