[Spam] RE: proper inverter [RE-wrenches]

Tue Mar 21 10:48:18 PST 2006

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Darryl and William,

As those of you know who have been to my courses, I strongly caution
installers when using the string sizing programs from inverter companies.
The biggest caveat when using an inverter manufacturer's string sizing
program is to stay away from the minimum voltage options given. Always try
to put as many modules in series as possible to stay under the Vmax of the
inverter. For the Fronius at 500Vmax, 18 modules in series would be the max,
but if they are wired in strings of 4, you would have to set up two parallel
strings of 16, which is fine.

As I said earlier, it is clear they are currently wired in series, but it is
equally likely that the strings of 4 modules are run down to the inverter
since there is a large conduit, and this joker would not have installed a
larger conduit than absolutely necessary. Getting on the roof and verifying
the wiring seems like a costly job, so if the groups of 4 can be tested at
ground level, you are good to go.

If these are KC120 modules, it is also possible that the modules have
problems, so that the strings will need to be tested prior to connecting
them into strings. It could be that the array needs to be replaced under
warranty, so it would sure be good to know that prior to spending a lot of
time.

Incidentally, Voc and Isc tests will not always detect the ribbon problems
in the KC120 module and other modules from other manufacturers that have
exhibited similar problems. It must be tested under load of the inverter, or
with a $20,000 IV curve tracer, or with a large power resistor sized to
operate the array near the maximum power point, or on a partially charged
48-volt battery (assuming the strings are available in groups of 4 modules).

If you are testing 48-Volt strings, you can size a power resistor to operate
the array near max power at 800 W/m^2. So how do you do this? What a great
problem for a PV exam. It requires knowing the main parameters of the
module. Let's use the KC-120 as an example.

KC-120
>From the module label
@STC (1000 W/m^2, 25C cell temperature)
Voc = 21.5 Volts
Vmp = 16.9 Volts
Isc = 7.45 Amps
Imp = 7.10 Amps
@NOCT (800 W/m^2, 47C cell temperature)
Vmp = 15.2 Volts
Imp = 5.74 Amps

So what to use from this? The NOCT data is the proper data set. Since Ohm's
law is V=IxR, then the resistor value should be R = V/I = 15.2V/5.74A = 2.65
Ohms. Finding an resistor of that value might be tough, but you can play
around with power resistors in series and parallel until you have a resistor
bank with the correct resistance, and the correct power dissapation (400
Watts minimum). Or you can try to rent an IV curve tracer. Or you can feed
each string into 48 volt battery at partial state of charge and test the
operating current and voltage of string separately (maybe the easiest).

Good luck,

Bill.

Darryl:

My take on this:  1. You have to determine the make and model of the 
modules and 2. You need to trust the manufacturer's sizing tools.  You can 
always run the calculations yourself to double check the string sizing 
information.  The Fronius configurator shows 2 strings of 16 KC120 modules 
as good down to -4 F.  You can verify this your self.

If it were me, I'd approach this as a T&M repair and if it takes a ladder 
or boom lift rental to safely asses the situation, that's what it takes, at 
the owners expense. This sounds like a killer roof and safety is your 
number one concern, not the owner's pocket book.  It may require on-roof 
scaffolding and certainly fall protection to perform the work once a design 
is established.  Please, don't take any risks to save a few dollars.

The Outback PSSB is useful only if all of the leads are accessible.  It 
sounded like the module leads all run down stairs through the "big conduit" 
but that is not entirely clear.

William Miller

At 09:46 PM 3/20/2006, you wrote:
>Thanks Bill and William
>The owner does not know the module brand, nor does any
>of the paper work he has state.  The roof is high and
>all I could reach was the bottom row. I could not see
>any labels.  With out a better look I would be
>guessing, (I think Kyocera).   I will rent either a 48
>or 52' extension ladder and hope when I do the job.  I
>think my best bet is to wire them into two sub arrays
>of 325 Volts OC.  When it gets cold in MN this is up
>north, I might see 400 Voc.  When it gets hot, ambient
>+ 20C or 20% loss in voltage so Vmp will be about 250
>volts in the summer.
>
>Bill mentiioned the Outback Combiner box, with just
>two arrays could I not use a 600 volt fused
>disconnect? then How would I find 10 or 12 amp fuses
>in standard form? ( guess I will stick with the
>Outback)
>
>I do not know how the collectors got up there, but
>there is almost no room underneath.
>
>For an inverter I am thinking the Fronius 4000.
>Anyone have experiance with this inverter?  I have a
>3840 nameplate watts (I think) array at STC, but my
>125% brightness and my 125% cold weather would I be
>pushing it?  (My amps seem to be in range) (BTW I do
>not feel confortable using manufacture's charts.
>Based on one experiance where the chart seems to be
>wrong.)
>
>Thanks Darryl
> > Darryl:

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