PV Inverters module strings [RE-wrenches]

Tue Mar 27 19:38:45 PST 2001

William and others installing Trace 10208 inverters in moderate climates.

Eric has some very nice information below, but I have just a couple of quick
things to add. The SR-100 has an open circuit voltage of 22.0 Volts. Since
you're in California your correction factor is 1.13 according to the 1999
code (takes you down to 14F--works unless you are high in the
mountains--then you go to the full 1.25 correction factor).

Take 22 x 1.13 = 24.86; divide this into 600V:  600V/24.86V = 24.1 modules
(24)

This means that you can install 24, SR100s in series and stay under 600
Volts.

Now here's a good question for Kent Sheldon--how low can the Trace 10208
really go. Since there is not a separate circuit boosting input as in the
higher power inverters, the minimum tracking voltage has to be much more
than 300 Volts. What is it?

I say that it has to be greater than the typical afternoon peak AC voltage
we get from the utility which is 124 Vac RMS. If we look at the peak-to-peak
voltage of this wave form it is +/- 177 Volts or 355 Volts. The 300 V number
in some of the Trace literature must be based on the minimum AC voltage of
106 Volts-- +/-152 Volts or 304 Volts peak to peak. Unless I slept through
electricity 101 or don't understand this wild PV inverter stuff (I am a
Mechanical Engineer after all), a better number for minimum tracking voltage
for the Trace 10208 is actually 360 Volts DC (a little above 355V due to
voltage drops in transistors, etc...). Please correct me if I'm wrong.

Now let's go back to Eric's nice set of calculations. The only mistake Eric
made was that his Vmp correction factor was a little light--the SR-100 has
a -0.079 voltage correction factor for Voc not Vmp. The Vmp number is
generally much higher--guess at -0.1. But it all comes out in the wash since
the Vmp at STC is 17.7 Volts--final number is 13.7 Volts per module.
Multiplying by 24 modules yields 328.8 Volts DC. This means that the Trace
10208 will go out of its tracking range on hot summer days with 24 modules
in series. Since it is about 1.3 Volts per module, that's not too bad for
worst case, but still gives me heartburn. I hope the array voltage doesn't
degrade too much over 20-years (yeah right).

So, William--you can install 24 SR-100 modules in series, or of course, you
can always choose the equally suitable 24-modules in series ;-). You have no
choice. Other modules with lower Voc may tolerate 25-modules in series, but
you have to do the math--no way of knowing otherwise. Good luck.

William, my friend, you have raised a very important issue--one that our
industry has not legitimately dealt with yet. By the way, don't even think
about trying to run anything but a crystalline array on a Trace
10208--anything else will not work.

For you poor Wrenches that live in cold climates (Illinois, Vermont, etc...)
it really sucks for you. All you can do is find a lower voltage module or
put 21.8 modules in series. The code does not allow us to round up. (22Voc x
1.25 = 27.5 V; 600V/27.5 = 21.8 (21). If we install 21 modules, then on hot
summer days the Vmp will be down to 287.70 and the Voc will be down to 396 V
meaning that the inverter will be operating the array at near open circuit
voltage (very low output). Maybe folks in those areas can tune their
isolation transformers down a few notches to be able to handle the lower
voltage of the array? Any other ideas?

Bill.

-----Original Message-----
From: Eric Smiley [mailto:Eric_Smiley at bcit.ca]
Sent: Tuesday, March 27, 2001 2:02 PM
To: RE-wrenches at topica.com
Subject: Re: PV Inverters module strings [RE-wrenches]

Use the MPP voltage and derate for the highest expected module
temperature.

Eg. A 36 cell module at 65 degrees C (40 degrees above STC)

0.002 V/cell/degree C  (typical temp. coef.) x  36 cells/module = .072
V/ module/degree C
.072 V/module/degree C x 40 degrees = 2.88 V/ module

If Vmp = 17 then 17 V - 2.88 volts = 14.12 Volts

300 V / 14.12 V = 21.3 modules in series minimum.

Now look at the Voc and "derate" for the lowest temperature according to
the NEC table.

Eg. Voc = 21 Volts x 1.25 (worst case) = 26.25 Volts

600/26.25 = 22.9 modules in series maximum..

I don't have the specs for the SR100, but as you can see there isn't
much room to play.

----------------------------------------------------------------------
Eric Smiley
Project Leader - BCIT Technology Centre
Photovoltaic Energy Applied Research Lab (PEARL) - BCIT
ph: (604)432-8657 www.bcit.ca/~tc/pearl/index.htm

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Hosted by Home Power magazine: 
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For info contact list moderator by email:
 michael.welch at homepower.com

____________________________________________________________
T O P I C A  -- Learn More. Surf Less. 
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01