[RE-wrenches] unbalanced string voltages

[Ray Walters]

Modules aren't exactly constant current devices. I-V curve
They are for much of their voltage range, up to a voltage a bit below 
the MPP, and then the current starts tapering down to 0 at Voc.  The 
entire string can only pass as much current as the lowest current 
module.  That means the higher current module will be forced to operate 
where it can, which is at a higher voltage than its MPP point, as David 
said.
If you don't believe us, hook up a couple of mismatched modules and 
measure voltage across each module, and the current. Current is always 
equal in a series connection, then the modules make up for it by moving 
up or down along their IV curve.

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

On 4/29/2013 9:44 AM, Larry Crutcher, Starlight Solar Power Systems wrote:
> David,
>
> Since a PV module is a constant current device, I don't believe this 
> will take place. For a given amount of light (plus all factors 
> affecting power), the current potential will be there. What force can 
> act upon the current to reduce it? Consequently, the lowest Vmp the 
> inverter can track will dictate the voltage for all strings. This 
> means less power from the highest Vmp modules. Do you agree?
>
> Larry Crutcher
>
>
>
>
>
>
> On Apr 28, 2013, at 9:33 AM, David Katz <dkatz at aeesolar.com 
> <mailto:dkatz at aeesolar.com>> wrote:
>
> Kirk
> The 43volt 5.5 amp panel will operate at 3.6 amps, so it's voltage 
> will move up toward open circuit voltage, raising the mppt voltage of 
> that string.  I would bet that the voltage of that entire string will 
> rise to match the higher voltage string by moving toward open circuit 
> voltage.  It should work fine.
> David Katz
>
> Sent from my iPhone
>
> On Apr 28, 2013, at 9:01 AM, "Kirk Herander" <kirk at vtsolar.com 
> <mailto:kirk at vtsolar.com>> 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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