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<DIV><SPAN class=218495416-15112010><FONT size=2 face=Arial>Hola
Wrenches,</FONT></SPAN></DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>The topic of string
level monitoring keeps coming up in my circles. Personally, i think it's dumb in
most non-R&D applications. Dumb is actually a little weak for how i
really feel about it, but that's the word i'll use today. </SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010>For the purpose of keeping it
straight, i'm talking about real strings. Not to be confused with re-combiner
inputs. i advocate mapping and monitoring all re-combiner inputs in
central-inverter applications.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Part of my
perspective comes from the fact that monitoring string data inherently assumes
that you are monitoring the system-level outputs anyway, and know how to
evaluate that data. </SPAN></FONT><FONT size=2 face=Arial><SPAN
class=218495416-15112010>Any "benefit" of string-level monitoring must be over
and above the benefits of monitoring without it. </SPAN></FONT><FONT size=2
face=Arial><SPAN class=218495416-15112010>System outputs are what matter, right?
So why focus on strings? What are you waiting for? Expecting something wierd to
happen? </SPAN></FONT><FONT size=2 face=Arial><SPAN class=218495416-15112010>Are
you looking for binary information (On/Off) or comparative data from one string
to the next? How do you account for instrumentation accuracy tolerances compared
to module tolerances? Are you looking for trends, such as seasonal shading from
one string to another? Or trends such as decreasing current in strings over
time? Are you really gonna make a point of going back and normalizing
point-in-time current measurements to point-in-time environmental conditions
overlayed with point-in-time soiling data and comparing them? Really? And then
what? </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010><EM>"Gee, there's a
7% difference in normalized String 5 from the same 15-minute
interval last year, George. Ya think we should go out and see if there's
something wrong?"</EM> </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>The real answer is,
no. You and yours are not gonna go out there for that unless you are gluttons
for punishment and don't have anything better to do. Or simply don't know
better. Here' why: Unless there is some other sign of system output being off,
it's not worth chasing the wild goose. </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>If the system output
is down from what it should be, you gotta go out and find the problem anyway.
When you get there, you will follow these steps:</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>A) Visual
observation </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>B) Data
gathering from meters and displays</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>C) Compare
field observations and data to monitored data</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>D) Determine
whether or not there is actually a performance problem and begin troubleshooting
if necessary</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>E)
Troubleshooting Step 1: Clamp individual string inputs at the combiner and
compare to others.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>You gotta go thru
these steps anyway. Whether or not you have string monitoring.
</SPAN></FONT><FONT size=2 face=Arial><SPAN
class=218495416-15112010>So string monitoring doesn't save you a
danged thing here.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Now, i know you
youngsters trust that monitoring thing, and are ready to jump right in the
middle of String 5 and skip all that troubleshooting nonsense... Hey, your
i-phoidberry said it was String 5, right? </SPAN></FONT><FONT size=2
face=Arial><SPAN class=218495416-15112010>You go right ahead. Let me know how
that works out for ya... Over time. No, silly... Not "overtime"... Two words.
Over. Time. </SPAN></FONT><FONT size=2 face=Arial><SPAN
class=218495416-15112010>Actually, when it comes right down to it, i don't
really care how it works out for you as much as i care how it works out for the
customer and your boss. You're getting paid whether it works out or not. At
least for awhile. The customer and your boss are most likely NOT getting paid if
it doesn't work out... Keep that in mind.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010><EM>"Are you sure
that's String 5? I thought String 5 was this one... Dang! We didn't actually
number the strings on the modules, did we?. I guess we gotta open up that
combiner anyway...."</EM></SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>And what are you
gonna do to String 5 anyway? Pull it all apart and... What? Without
checking it side-by-side with the others in the combiner first? Really?
</SPAN></FONT><FONT size=2 face=Arial><SPAN class=218495416-15112010>What's that
gonna get you? A pile of modules about one string high and no answers. That's
what.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Ever had to have a
test instrument calibrated? Ever heard of a thing called "drift"?
Measurement accuracy tolerance? Improper use of a calculator? How about just
being too uptight and data-centric to see the big picture? Ever heard of any of
these? I've already seen thousands of dollars wasted in truck-rolls to "fix a
string", only to find out that a channel on string level monitoring
board failed and there wasn't really a performance problem after all. Unless you
count the monitoring board's lack of performance as a
problem.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>i can certainly see
where having string level monitoring could be handy as a reference. Assuming
that it's working correctly and all. But only in a very limited number of
circumstances. For example, just before going out to do maintenance
on a system, you take a look across the graphs and see if there are any
problem-children out there so you can take an extra good look at some part of
the array when you are out there. Current, in this application, is real-time and
not cumulative. It's not like you're doing amp-hour calculations... Which might
have value in a pocket-protector-basement-of-the-science-building sort of way...
IF you were doing high-resolution sampling and data parsing and actually still
have a pocket protector and reside in the basement of the science
building. </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT size=2
face=Arial><SPAN class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>i advocate
doing a clamp-compare-test, on each string in string inverters and each
re-combiner input for central inverters, as part of the periodic
maintenance and any diagnostic testing. Look, it's only gonna take a couple
minutes to do that anyway. </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010></SPAN></FONT><FONT
size=2 face=Arial><SPAN class=218495416-15112010>From a performance monitoring
standpoint, i think string-level monitoring is a waste of time and money. It
introduces additional layers of reliability and data management
problems that, in my estimation, will be net losses over the life of most
PV systems. </SPAN></FONT><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>For me, the cons
outweigh the pros. </SPAN></FONT><FONT size=2 face=Arial><SPAN
class=218495416-15112010>Am i all wet on this? What do you
think?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>i would like to get
feedback from other Wrenches on this topic. Some questions to get
your contemplative juices going... </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010></SPAN></FONT> </DIV>
<DIV><SPAN class=218495416-15112010></SPAN><FONT face=Arial><FONT size=2>H<SPAN
class=218495416-15112010>ave you </SPAN><SPAN class=218495416-15112010>installed
one or more systems with string level monitoring?</SPAN></FONT></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Have you
specified one or more systems with string level monitoring?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Are
you currently specifying and/or installing systems with string level
monitoring?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Have you had any
seriously negative experiences with equipment of this type?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Have you had any
seriously positive experiences with equipment of this type?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>Please
describe.</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>What do you see as
the best feature associated with string level monitoring?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>What do you see as
the worst feature associated with string level monitoring?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>When that current
sensor board fails at year 12, are you really gonna replace
it?</SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN class=218495416-15112010>If string level
monitoring is so spiffy, why not? </SPAN></FONT></DIV>
<DIV><FONT size=2 face=Arial><SPAN
class=218495416-15112010> </SPAN></FONT><SPAN
class=218495416-15112010><FONT size=2 face=Arial></FONT></SPAN></DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2 face=Arial>Really looking
forward to others' opinions on this topic.</FONT></SPAN></DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2 face=Arial>Thanks in
advance,</FONT></SPAN></DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2
face=Arial></FONT></SPAN> </DIV>
<DIV><SPAN class=218495416-15112010><FONT size=2 face=Arial>Solar
Janitor</FONT></SPAN></DIV></BODY></HTML>