MX60 Max Array Voltage? [RE-wrenches]

[Bill Brooks]

Ray,

I think in the near term you have a listing issue since it is not listed to
140 volts yet.

Also, the argument that sunlight and cold don't happen together is not
correct. The case in point is a winter cold front that moves through where
the array goes from low irradiance and very low temperatures, to very high
irradiance (even cloud edge effect) still at very low temperatures at 11:00
am (snow reflectance is another problem in some areas of the country).

Breakdown of components can happen in a seconds or minutes--about the time
it takes the array to heat up and lower its voltage. Ultimately it is
Outback's call on whether they feel comfortable with this situation if they
increase their listed maximum voltage, but using the NEC table 690.7 is the
safest bet for not applying too much voltage to your equipment. (42.8V x 3 x
1.25 = 160.5 Volts)

Bill.


-----Original Message-----
From: Ray Walters [mailto:remotech at taosnm.com]
Sent: Friday, June 27, 2003 5:24 PM
To: RE-wrenches at topica.com
Cc: Nick Stallard
Subject: MX60 Max Array Voltage? [RE-wrenches]


Hi All, but especially Chris F.;

What is the collective experience with running the MX 60 with a high
voltage array to a 48 vdc nominal battery bank. The manual shows a Voc max
of 140 which looks like I could go with a 72 V nom. array. I'm using 140
watt modules with a Voc of 42.8. Times three puts me at 128.4 Voc at 25C. I
know I need to add some Voc for cold weather, but my experience is that if
the sun is shinning on the modules enough to produce power, it will produce
heat too. I usually have measured about 30 to 40 degreesF higher cell temp
than ambient. I know air speed humidity etc play a part too. Basically, The
lowest temp I would anticipate in good sun would be 20F or -7C. Using
Outback's chart (1% increase inVoc for each 2.5C below 25C) and assuming a
20F warmer module (cell temp=40F = 5C) then I should add 8% to 128.4Voc
which would be a Voc max of  138.7F. Boy that's close.

What happens Chris, if the array does exceed 140Voc? Smoke and fire, or
temporary shut down? Did I mention the array will be over 300 ft away? Now
I know supposedly Voc is open circuit and that means no current therefore
no voltage drop, but wouldn't the meter still see some discernable volt
drop in real world conditions? ( meter has to pull at least a few micro
amps to measure voltage) Is the controller switching the array off
temporarily to check Voc every few seconds? Will the Voc measured at the
controller for this split second really be the same as at the array or
would it be a volt or 2 less?
Also, in the MX 60 Manual, they show a chart with various array voltages
labeled 40, 65, and 95 V Nominal. Is that a misprint and should that be
labeled V at Max Power instead?
We're even considering running two 24 v modules in series with a 12 v of
similar current rating, but the racking and look would be awkward.

Thanks for all input,

Ray
Taos Power

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