proper inverter [RE-wrenches]

[John Berdner]

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Bob-O:
 
The Sunny Boys perform most efficiently at about 50% of the maximum dc
input voltage.
E.g. 300 Vdc for a SB 6000 Vdc and about 250 Vdc for the SB3800U
The difference in efficiency is about 1%  between the highest
efficiency (at lower dc input voltage) and the lower efficiency when
running at 500/400 Vdc respectively.
The lower voltage array would normally be the recommended array
configuration except in areas where shading could be an issue.
This is all covered in detail during our training seminars when we
discuss system optimization.
 
Best Regards,
 
John Berdner


Bill,
while I certainly agree with you to stay away from the minimum input 
voltage, I also try to say away from the Max input voltage as well. 
That's got little to do with giving the inverter an over voltage 
condition. All inverters have a "sweet" spot range where they 
operated most efficiently. For anyone who has taken the Fronius 
installer training, for example, they explain that in detail. Other 
manus, not so much. A competent, conscientious installer will press a 
manu for that data and try to hit it as a service for his/her clients.
Bob-O
On Mar 21, 2006, at 10:48 AM, Bill Brooks wrote:
>

> 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.

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