Bill and all.......So we were at the site today and these were our findings:<br>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. <br>
String 1 had a Voc of 368, String 2 had Voc of 312. <br>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.....<br>
The system was 2 strings of 9 BP SX170's connected to a PVP2800XV. <br>Module nameplate Voc was 44V. Cell temp of the modules at time of measurement was roughly 85-90 degrees F. <br>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. <br>
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.).<br>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.........<br>
<br>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?<br>We will be filing a warranty claim on the single panel that is obviously defective.......<br>
Thanks for your advice and input.....<br>Cheers,<br><br>Kirpal Khalsa<br>
Renewable Energy Systems<br><br><br><div class="gmail_quote">On Fri, Oct 31, 2008 at 10:33 AM, Bill Brooks <span dir="ltr"><<a href="mailto:billbrooks7@yahoo.com">billbrooks7@yahoo.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div link="blue" vlink="purple" lang="EN-US">
<div>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">Kirpal,</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">When you parallel, your voltages have to be the same, so you are
correct in that assumption.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">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.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">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. </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">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.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">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.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);">Bill.</span></p>
<p><span style="font-size: 11pt; color: rgb(31, 73, 125);"> </span></p>
<div style="border-style: solid none none; border-color: rgb(181, 196, 223) -moz-use-text-color -moz-use-text-color; border-width: 1pt medium medium; padding: 3pt 0in 0in;">
<p><b><span style="font-size: 10pt;">From:</span></b><span style="font-size: 10pt;">
<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" target="_blank">re-wrenches-bounces@lists.re-wrenches.org</a>
[mailto:<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" target="_blank">re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Kirpal
Khalsa<br>
<b>Sent:</b> Friday, October 31, 2008 10:43 AM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> [RE-wrenches] Trouble Shooting Bad Diode</span></p>
</div><div><div></div><div class="Wj3C7c">
<p> </p>
<p>Greetings all......I was hoping i could have someone help me
confirm a troubled series string on a system we have installed.....<br>
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.....<br>
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?<br>
Thanks for the help and advice.....<br>
Cheers,<br><br>
</p>
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<br></blockquote></div><br><br clear="all"><br>-- <br>Sunny Regards,<br>Kirpal Khalsa<br>NABCEP Certified Solar PV Installer<br>Renewable Energy Systems<br>