[RE-wrenches] Trouble Shooting Bad Diode

Wed Nov 12 20:03:00 PST 2008

Chris......Less than 2 years.......

On Wed, Nov 12, 2008 at 7:59 PM, Chris @ the Oasis <info at oasismontana.com>wrote:

>  Hello Kirpal:
>
> What is the relative age of these modules?  For starters....  Just curious.
>
>  Chris Daum
> Oasis Montana Inc.
> 406-777-4309 local line
> 406-777-0830 fax
>
>
>  ------------------------------
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Kirpal Khalsa
> *Sent:* Wednesday, November 12, 2008 8:33 PM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] Trouble Shooting Bad Diode
>
> 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,
>>
>>
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>
>
> --
> Sunny Regards,
> Kirpal Khalsa
> NABCEP Certified Solar PV Installer
> Renewable Energy Systems
>
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-- 
Sunny Regards,
Kirpal Khalsa
NABCEP Certified Solar PV Installer
Renewable Energy Systems
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