[RE-wrenches] not sure what's going on here
Exeltech
exeltech at yahoo.com
Sun May 12 07:45:42 PDT 2013
Hello Larry,
A firmware upgrade may address some aspects of the issue, and
is certainly a recommendation, but it doesn't account for the
one PV that exhibits 13Vdc output when it's completely out of
the circuit.
That characteristic clearly exemplifies a failure within the PV
module itself.
It's entirely possible more than one failure mechanism is involved
here, such as something within the PV, and other that's related
to the firmware. Multiple-issue failures are some of the most
frustrating and challenging of all fault analyses. Then add
distance to the equation ....
Thanks too for the reference to "aged".
Dan
--- On Sun, 5/12/13, Larry Crutcher, Starlight Solar Power Systems <larry at starlightsolar.com> wrote:
> From: Larry Crutcher, Starlight Solar Power Systems <larry at starlightsolar.com>
> Subject: Re: [RE-wrenches] not sure what's going on here
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Sunday, May 12, 2013, 9:30 AM
> 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 ...
More information about the RE-wrenches
mailing list