[RE-wrenches] unbalanced string voltages

[David Katz]

Hi Larry,
A PV module is not exactly a constant current source. It has a power curve. At 0 volts in any given insolation, it's current will be the highest. As the voltage rises, the current drops, until the current is 0.
If you put two strings in parallel that have different voltages, they will both have the same voltage. That is physics.
That voltage will be determined by the mppt algorithm of the inverter, which should pick the point that supplies the most watts.  In the lower voltage array, the modules will operate at a current that is slightly closer to open circuit voltage.
David Katz

Sent from my iPhone

On Apr 29, 2013, at 8:44 AM, "Larry Crutcher, Starlight Solar Power Systems" <larry at starlightsolar.com<mailto:larry at starlightsolar.com>> 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
NABCEPTM Certified Inaugural Certificant
NYSERDA-eligible Installer
VT RE Incentive Program Partner
802.863.1202

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