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, Kirpal Khalsa Renewable Energy Sytems
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