[RE-wrenches] unbalanced string voltages

[Ray Walters]

I'd go for using that extra module with the damaged frame.  What is the 
frame damage going to do to the system?
Is it bad enough that the module could quit working later (bad edge seal)?
The voltage mismatch isn't the end of the world either, but your over 
all year round performance will be better with that extra module than not.
The thing to remember is that MPP is going to vary all the time with 
insolation and temperature, so predicting actual performance impact from 
the mismatch is not just a simple calculation.  I have software that can 
model one way or another, but it can't model the strings together, and 
what the inverter is going to actually do in various situations.
If the module damage is just cosmetic, as they say "if you can't see it 
from the ground.....", while the loss in performance will be visible.  
To make the customer feel good, sell him the damaged module at salvage 
cost or just throw it in, explaining you just want the system to work 
well, but you don't like selling damaged goods......
We sell used stuff quite a bit, and folks actually like saving a buck 
when they can.  I'd let the customer have the final say, then you're off 
the hook either way.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 4/28/2013 10:02 AM, Kirk Herander wrote:
>
> Hello everyone,
>
> I am at present adding to a -20- panel array of old Sanyo HIT "lipped" 
> 200 watt panels. They are configured in -4- series strings of -5- 
> panels, with a string MPP voltage of 279 vdc (55.8 v, 3.6 a each). The 
> customer wants to add another nominal 2 kw using the (now) Panasonic 
> 235 watt HIT's which have an MPP of 43.0 vdc, 5.5 a. I am locked in to 
> an existing SMA 6000US inverter. Obviously I cannot get all the 
> strings to the same voltage. But I can get relatively close by rewiring:
>
> String 1: -7- 200 watt @ 55.8 ea. = 390.6 v MPP
>
> String 2: same as string 1
>
> String 3: -6- 200 watt @ 55.8 plus -1- 235 watt @ 43.0 = 377.8 v MPP
>
> String 4: -9- 235 watt @ 43.0 = 387 v MPP
>
> Under this scenario there is a delta of about 12 vdc between high and 
> low MPP. I assume the entire array will operate at the 377.8 V MPP. I 
> know that the 235 watt panel will be current limited also in string 3. 
> Am I missing anything obvious in doing it like this? How will the 
> inverter arrive at an operational MPP voltage?
>
> PS. In a pinch I could install a leftover 200 watt HIT I have in the 
> shop. That way I could have -3- strings of -7- 200's @ 390.6 v MPP, 
> and one string of 235's @ 387v MPP. But I would prefer not to use this 
> panel since the frame is damaged.
>
> Kirk Herander
>
> VT Solar, LLC
>
> dba Vermont Solar Engineering
>
> NABCEP^TM Certified Inaugural Certificant
>
> NYSERDA-eligible Installer
>
> VT RE Incentive Program Partner
>
> 802.863.1202
>
>
>
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