proper inverter [RE-wrenches]

Bill Brooks bill at brooksolar.com
Mon Mar 20 16:04:36 PST 2006


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Darryl,

It seems fairly obvious that the installer wired all 32 modules in series to
get 648 (20.25 Volts each). Since they are 120 Watt modules, they are likely
AstroPower or Kyocera modules--correct? Why is there a big conduit? Was it
originally wired for 48 volts and then rewired for all in series?

I think I am very glad that the installer has moved to an area of expertise
(it would be improper to say he moved to "another" area of expertise, since
he clearly had no clue what he was doing in PV). There are numerous ways
this array could be wired. As William suggests, just confirm which module
you have and you could use a string sizing chart to size the array for
several different inverters. If the wiring has been brought down in
4-modules circuits, then the Outback PSSB switch is an ideal option. You
would need two so you could put 16 modules on each PSSB.

Have fun cleaning up this guy's mess. If he screwed up this much on his
array design, it really makes you wonder what else is wrong. A complete
inspection of all the wiring would be worthwhile. 

Good riddance Mr. 32 modules in a series string installer-man. I hope you
never come back (unless you decide to learn about the trade).

Bill.



Darryl:


This sounds like a tough situation.  What make/model are the modules?  Did 
you try using on-line sizing tools provided by the inverter manufacturers 
to get some string quantity options?

This brings out some philosophical points that were touched on recently in 
this forum, at least peripherally:  Top down versus bottom up.  Top down, 
to me, means the modules are mounted first on non-liftable rack and then 
wired.  Bottom up is when the modules are mounted on a liftable rack and 
wired from underneath.

The trend, of course, is top down and is made possible by the advent of MC 
cables.  My unique position on MC cables is well known.

One additional disadvantage of top down is that it is difficult to 
troubleshoot or modify wiring.  You state that the client wants a cheap 
fix, but that may not be possible.  You did not perform the original 
installation so you are not obligated to perform miracles on the 
cheap.  Purusant to redesign, this array may need to be completely 
disassembled and rewired.

Good luck and let us know how it works out.

William Miller




At 06:01 AM 3/19/2006, you wrote:
>Hello
>looking for some ideas or solutions.   Perhaps an
>inverter recommendation.
>I have a system not of my creation, that is composed
>of 32  (12 volt) modules mounted tight to the standing
>seam roof.  I have reason to believe the modules are
>120 watt. Connections are under the modules arranged
>in rows of 4 across and 8 rows high, tight together.
>The connections on the end of each row are brought to
>junction boxes and then 100 feet to the inverter
>location.  The conduit is large.  The installer has
>changed his expertise and left the area and no owner's
>manual.
>
>My problem is they are connected in series and I
>measured the open circuit voltage at over 648 volts on
>a heavy overcast and cool day. .  My guess is on a
>cold day this would exceed 720 Voc.  The owner wants
>it fixed at low cost, just a new inverter and change
>the wiring.  The roof is difficult to reach, being
>high and at a 45 degree angle.  I can reach a splice
>every 4 modules connected in series. Such that I can
>create 84, 168, 336, 672 Voc by reconnecting into sub
>arrays.  If my guess is correct the Vmp is 16.9 and my
>sub arrays would be 68, 135, 270, 541 Vmp.     I have
>worked mostly with Outbacks and Sunny Boys.  The 3840
>watts seems to high for the Outback.  The voltage
>range of 270 Vmp seems to low for the SB voltage
>window when the modules get hot.  What
>recommendations?
>I know you that work with lots of inverters this seems
>stupid.
>Thanks you
>Darryl
>

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