[RE-wrenches] unbalanced string voltages

[dan at foxfire-energy.com]

Hey Kirk, I'm currently doing a project with Tigo Optimizers. Sounds like they might work for your project. db

-------- Original Message --------
Subject: [RE-wrenches] unbalanced string voltages
From: "Kirk Herander" <kirk at vtsolar.com>
Date: Sun, April 28, 2013 12:02 pm
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>

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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