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

Sun May 12 07:30:39 PDT 2013

Fellow Wrenches,

I realize that many of you are offering your time and comments in a genuine effort to help Ron, however, the problem he described is the EXACT behavior of all early 3024i controllers. This happened with overloaded controllers operating in 24 to 12 downconversion, just like in this case. When the 3024 would reach current limiting, the controller will lock itself to half the input voltage. Because high power is still available, the controller would still function and many did not even know they had a problem. By disconnecting PV and battery and reconnecting, the 3024 will reboot and the problem will not occur again until current limit is reached. Note that this only happens when using high voltage PV and down converting to 12Vdc.

Blue Sky Energy is our #1 selling controller and we have installed over 500 3024i's and 3024iL's. When I first discovered this issue and reported it, Rick at Blue Sky was able to duplicate it and made appropriate changes. Upgrading firmware always corrected the issue.  So, with respect to the many aged but wonderful minds on this list, I highly suggest the controller upgrade first. At the very least, reboot the controller in the morning and observe as it reaches the limit.

Larry Crutcher

On May 12, 2013, at 5:44 AM, Exeltech <exeltech at yahoo.com> wrote:

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 ...