Lowest DAYTIME temp records [RE-wrenches]

[Matt Lafferty]

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Hi All,

A couple of days ago Joel D referred to 240 systems that were installed in
SMUD territory which calculated at >600 VDC Open Circuit and didn't result
in any inverters failing due to Over Voltage.  

I'd like to add some clarification to that statement.

Joel D's experience is only part of the story related to this topic in SMUD
territory.  There are several SMUD-Owned Arrays, large and small, which
exceed 600V when you apply NEC 690.7. Two particular configurations
calculate out to >640V, in fact. To be clear... These are SMUD-Owned,
Utility-Scale, and sited in Sub-Stations, behind fences. SMUD is the one on
the hook for any repairs and they accept that.  Like Allan S said... 

Neither of these configurations have resulted in "over-voltage" failures to
my knowledge. Neither of them has ever met their "specs" to my knowledge,
either, but that's a whole different story... Be sure to buy the book one of
these days! (Michael: I haven't written it yet, so I don't think that
comment belongs on the Marketing List yet! Let me know if I'm off-base
here... )  

Back to the systems that Joel D was involved in...

First of all, these systems were installed using the old Omnion 2200 & 2400
series inverters. Many of these inverters required repair throughout the
years. Repairs were done by SMUD's Electrical Technicians and records were,
and still are, NOT kept regarding what type of failures and/or causes
ocurred. Repairs were made on an "as-required" basis to get the systems back
online. Control boards, capacitors, relays, cooling systems, whatever... I
do not personally know of any "over-voltage" failures, but that is only
based on my general conversations with the Techs over the years. With all
due respect, Joel, I don't see how anybody can make a statement that "None
of the 240 inverters failed due to high Voc."

For those of you unfamiliar with the Omnions, they are a Bi-Polar DC Input
inverter platform. What this means is that the input circuits are
"center-tapped" which results in the inverter "seeing" two different
voltages... A "+" and a "-" voltage as measured between each pole and the
center-tap. (The entire string is split into two source-circuits) The
inverters were rated at "+400 / -400" nominal.  In those days, the +/-400
referred to "Nominal" which is the operating voltage instead of the Open
Circuit voltage.  

The vast majority of the systems used the Solec SQ-80 modules, which have a
21.3VOC rating.  String configurations were as follows:  42 modules in
series with a center-tap between module #21 & #22.  This calculates out at a
total string VOC of 894.6 if measured between + & -.  The actual source
circuit voltages were +447.3 & -447.3 VOC, which results in a VMP of +359.1
& -359.1 respectively.  As you can see, this is well below the "Nominal"
rating of the inverters.

Some notes about this type of configuration:
The NEC used to consider each side of a Bi-Polar circuit as a separate
circuit for voltage ratings.  This is no longer the case. A Bi-Polar
cirucuit is now required to be rated at the total + to - voltage and all
conductors & components in that circuit are required to be rated at no less
than that figure.  In this case, the original configuration would not be
allowed on a residential application for two reasons... First of all it
would be >600V. Secondly, the wires are only rated at 600V, as are the
modules.

The Solec modules never reliably produced their rated voltages. Still don't.
We have been refurbishing these systems for customers who elect to purchase
them for the past two years. This process is called "converting" due to the
ownership being transferred from a SMUD-Owned system to a Homeowner-Owned
system.

During this process, the entire system is removed from the site, including
the inverters, conduit, etc. Roofs are repaired &/or replaced.  Inverters
and conduit are disposed of. Modules (Panels of 7 each) are taken to our
shop, dis-assembled, thoroughly cleaned & inspected, individully tested for
VOC & ISC and sorted according to the test and inspection results. We
experience an average of approximately 20% failure rate due to one of three
primary factors. (Physical failure of or in the J-Box; Low-VOC; Low-ISC.)
Modules which fail at this stage are transported to SMUD's Salvage facility
for sale at a later date in a Salvage Auction. (Caveat emptor! Remember,
they look nice and shiny by now!) Failed modules are replaced using "good"
modules from a supply of modules which come from "retired" systems. "Good"
Modules are then re-panelized on UniRac Solarmount rails. After roofing work
is completed, the system is re-installed with a new Sunny-Boy. Systems are
re-configured to meet current codes and go through the entire permitting
process. String configurations are 21 Modules in series (447.3VOC rated).
Complete commissioning tests are performed. Results are recorded. Under
favorable conditions (good sunlight & low temps) it is common to see VOC's
come in at 85-90% of the rating.  ISC's come in around the 70-80% range.
These values are "as compared to corrected for irradiance and temperature",
FYI.

For those of you concerened about the Customer... SMUD uses a very
aggressive formula for "de-rating" the entire system which comes out very
much in favor of the Customer. (The Customer ends up getting more system
than they pay for and they pay a very low $/Watt for the systems). SMUD also
provides a complete-system warranty of 5 years. In addition, before a
Customer is allowed to purchase their system, a complete site evaluation is
performed, including Orientation, Shading Analysis, and Structural
Evaluation. Due to historical siting standards, not all systems are deemed
to be suitable for conversion and get retired.  Which is where the "makeup
stock" of modules comes from...

Hopefully some of you find all this to be at least somewhat interesting.

>From the trenches of history, old and new...

Pray for Sun and please don't forget to donate to the Hammock Relief Fund!
(Yes, rum is considered an acceptable donation...)  :-)

Matt Lafferty 
mlafferty at universalenergies.com

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