[RE-wrenches] unbalanced string voltages

Mon Apr 29 12:21:04 PDT 2013

So the string with the highest current AND voltage dictates where the inverter MPP locks. Other strings are "pulled" up past their MPP which means lower current from them. Makes sense.

I was only considering constant current under a load equal to the potential. In that case a module is constant current. The chart was helpful to rethink what happens with mis-matched strings. Thanks!

Larry Crutcher

PS: I believe you!

On Apr 29, 2013, at 10:40 AM, Ray Walters <ray at solarray.com> wrote:

Modules aren't exactly constant current devices. <I-V_curve.gif>
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> 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> 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
>> NABCEPTM Certified Inaugural Certificant
>> NYSERDA-eligible Installer
>> VT RE Incentive Program Partner
>> 802.863.1202
>>  
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