[RE-wrenches] not sure what's going on here

[Exeltech]

Hello Wrench Team,

Please forgive as this seasoned design engineer contributes
to your discussion:


The conditions and symptoms as originally posted are:

1) A PV array consisting of four Solarworld SW165 PV modules.
   [72-cell, mono PV, Voc=44.1V, Vmpp=35.3V, Isc=5.2A, Impp=4.7A,
   all values at STC.]  All four PV are parallel-connected.  (See
   item #2 for substantiation.)

2) The PV terminate into a Blue Sky Energy 3024i [30A max out,
   with automatic current limiting].  V input max for this unit
   is specified to be 57 volts.  This maximum voltage would be
   exceeded by just two of the above-mentioned PV connected in
   series.  Subsequently, my contention the PV are connected
   in parallel is supported.

3) One of the PV modules has permanently dropped from ~30-35v
    output to 13v, regardless of test or operating conditions.

4) When the failed panel is removed from the array, the three
   remaining panels come back to normal voltage, but after approx
   15 minutes drop down to around 13v on the Blue Sky display,
   even though the individual output from each panel when unhooked
   and tested in full sun show about 35v.

5) After turning the system off and on again a few minutes later
   the normal voltages return but drop again in about 15 min.
   The failed panel stays at 13v, does not recover.


Speculative long-distance diagnostics are always challenging.


First:

The PV module that shows 13 volts open-circuit voltage under all
test conditions (hot, cold, open-circuit, etc.) has experienced
permanent failure (short) of two of the three bypass diodes.

Explanation:
Bypass diodes typically shunt 1/3 or 1/4 of the cells in a module.
The specified PV have 72 cells, so there's one bypass diode across
each of 24 cells in this particular PV module.

When a diode fails, it shorts the portion of the module across which
it's connected, and the PV voltage drops accordingly.


Next:
Ron Young (original poster), stated "the voltage of the remaining
modules "drops" after 15 minutes use when the system is turned off,
then back on again".

Ron didn't specify the magnitude of this drop, so I can only guess
whether this is the normal Voc to Vmpp decrease .. or he means this
too is a decrease from nominal Voc to 13V or some similar voltage.

Presuming the latter, this would indicate the heating of the PV is
causing one or more bypass diodes in one or more of the remaining
modules to fail short intermittently.  To that, and to isolate the
affected module, the PV must be disconnected from each other, and
each module tested separately under the conditions under which the
failures were noted.

As a point of clarification, and since the four PV in this array
are parallel connected, there is no appreciable current being
forced through any one module by any of the other modules when
all are operating normally.  In the event of one or more shorted
bypass diodes in a module, then all current from the remaining PV
in the array will flow through the shorted diodes in the faulty PV.

Defects in the cell buss structure or connections could come into play
in a parallel array by causing hot-spot heating, along with possible
current being forced through the PV due to the excessive voltage drop
caused by that resistance.  This could also cause the bypass diodes
in the affected PV to be forced into the conductive mode.  If this
happened often enough, could in turn lead to eventual failure of
the diodes, as many bypass diodes relied on convective and radiant
cooling within the junction box - and this doesn't work very well.
I've got thermograph images of junction boxes with bypass diodes
conducting, and the measured temperatures are impressively high,
and very eye-opening.


That said...

The only means to accurately diagnose the PV would be to completely
disconnect them, face them into the sun, let them heat up, then at
an absolute minimum, take Voc and Isc measurements for each module.
It would also be highly recommended that the Isc measurement be taken
for a period of several minutes, because internal heating (and
subsequent failure of bad connections) may not show up until the
connection(s) gets hot enough to fail.


Also ...

To correct an earlier statement made within this thread, where a poster
said diodes usually fail "open" ... this is incorrect.

Diodes fail short.  The only time they fail "open" is when they are
mechanically faulty, which then causes the conductive path to open
(rare), or when they are totally destroyed -- and thus obviously open.

Can a diode fault be "intermittent"?  Yes.  Though not common, it
is always mechanical in nature caused by thermally-related
expansion/contraction.  Otherwise, "short" is the failure mode in
diodes.


Conclusions:
One of the four PV has suffered permanent failure in two of the
three bypass diodes.

Symptoms reported by Ron Young (the original poster) indicate
one or more of the remaining PV are experiencing intermittent
thermally-related short-circuit failures in the bypass diodes,
which then decreases the output voltage after ~15 minutes as
Ron reported.  When the PV cool, the thermally-induced short
in the diode(s) goes away, and the PV output voltage returns
to normal.


Remedy:
Depends on the accessibility of the bypass diodes.  If the
diodes and junction boxes are potted or otherwise rendered
inaccessible, then replacement of the affected PV is indicated.
If all four came from the same production batch, I'd recommend
changing out all of them, even if still functioning normally.
Solarworld would be responsible only for those PV that are
malfunctioning -- not for any normally operating product.

Option:
I'm not personally acquainted with this specific PV, but if its
vintage is such that the diodes themselves are accessible and
replaceable (some in older models of PV were), then replacement
of the diodes would be a consideration, rather than the entire PV.
If this is the case, ensure the diodes are more robust than the
parts used in the original product, and that all connections to
the new parts are solid.



Regards to all,



Dan Lepinski
41 years in solar energy ...


--- On Sun, 5/12/13, don at energysolarnow.com <don at energysolarnow.com> wrote:

From: don at energysolarnow.com <don at energysolarnow.com>
Subject: Re: [RE-wrenches] not sure what's going on here
To: re-wrenches at lists.re-wrenches.org
Date: Sunday, May 12, 2013, 12:22 AM

Ron et al-

The firmware in the controller sounds likely, but a faulty shorted
bypass diode does not. Isn't that what bypass diodes are supposed
to do -- turn on and short (bypass) a shaded row of cells?

We use to see this half-panel voltage as a failure mode with the
old Kyocera 120's when they were still blue. The solder mask fails
in a hot spot and opens, so no current can flow in that row of
cells. Then the bypass diode is just doing it's job by bypassing
current at lower module voltage, just as it does when half the
module is shaded. If there are other modules in parallel they
will push current into the reduced-voltage one until they are
also at reduced voltage, which is sensed by the controller.
Sounds like a lot of current is being pushed through the
damaged module, which will fry the hard-working bypass diode(s)
or the solder mask wire in that low module.  Best to disconnect
the low module or the others will kill it. I cannot think of
any physical way the rest of the solar modules can work at full
voltage for awhile without the damaged one, then have the
voltage drop suddenly under load, only to be seen at full
voltage again when disconnected. Do I have this sequence
right? -- This sounds like software (or firmware) attempting
to do MPPT in the controller.  If it is faulty firmware as
Larry C. wrote then it should be upgraded. So, could the
controller be complicit in damaging the low module?

Hmmm....

Don Barch
Energy Solar