[RE-wrenches] Trouble Shooting Bad Diode

[Kirpal Khalsa]

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 <billbrooks7 at yahoo.com> 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:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *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 at 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20081112/c175c014/attachment-0003.html>