[RE-wrenches] unbalanced string voltages

Mon Apr 29 12:12:23 PDT 2013

Kirk:

As noted below the current will be shared by all modules in the string and the voltage will be the sum of the operating voltage at that current.
When mixed modules are used this results in differences between the operating voltages of each string in the system.
Having parallel strings with differing max power points will have an effect on the operation of the MPP tracker.
You should also consider the effect of the mixed modules on the MPPT of the inverter (or charge controller) and resultant reductions in energy yield.

Some inverters can "get stuck" on one of the a local MPP voltage as opposed to global MPP
This will reduce the overall energy yield - sometimes significantly.
A good max power point tracker will find the correct global MPP voltage.
The global MPP will be at a point that is between the upper and lower operating voltages of all the parallel strings and proportional to the current of each string.
Using your example Strings 1, 2 and 4 are close in voltage.
For simplicity I will consider them to be the same, i.e. approximately 389 Vdc with 2 strings at 3.6 Amps and 1 string at 5.5 Amps = 12.7 Amps
String 3 will be about 378 volts and 3.6 Amps.(the lower of the two module currents)
Note: You can calculate this voltage of string 3 more closely by looking at the IV curve of the higher current module when running at the lower current, i.e. this module will operate closer to Voc so the MPP voltage of this string will be slightly higher.

Using the numbers from your email, the voltage difference is 11 Vdc and the ratio of the currents is 0.779 = 12.7/(12.7+3.6) so the global MPP voltage should be about 8.6 volts (= 11*0.779) above that of string 3.
I would therefore expect global maxima to be around 386-387 Vdc (378 + 8.6 V) above the voltage of string 3.
Using 387 Vdc as an operating point you can estimate the mismatch power loss from the IV curves of the various modules.
String 3 will be operating above it's MPPT so the modules will be closer to Voc and the current will be reduced further and the overall power of that string will be lower.
Since the current changes as the modules move towards Voc you will need to iterate the calculation a couple of times to get close to the actual operating point and estimated reductions in energy yield.

Best Regards,

John Berdner
General Manager, North America

SolarEdge Technologies, Inc.
3347 Gateway Boulevard, Fremont CA 94538 USA  (*Please note of our new address.)
T: 510.498.3200, X 747
M: 530.277.4894

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Ray Walters
Sent: Monday, April 29, 2013 10:41 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] unbalanced string voltages

Modules aren't exactly constant current devices. [cid:image001.gif at 01CE44C8.11172300]
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
NABCEPTM Certified Inaugural Certificant
NYSERDA-eligible Installer
VT RE Incentive Program Partner
802.863.1202

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