[RE-wrenches] Trouble Shooting Bad Diode

[Bill Brooks]

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,
Kirpal Khalsa
Renewable Energy Sytems

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