[RE-wrenches] unbalanced string voltages

Mon Apr 29 13:57:51 PDT 2013

Allan:

I posted an example using Kirk's original quesiton but for some reason it has not showed up yet.
You are mostly right.

>>IF<< the MPP tracker works correctly it will pick the global MPP point.
Some MPP trackers get lost and fins a local MPP and you end up losing a bunch of energy.

In reality the global MPP will be between the upper and lower MPP's for each string.
As a first order approximation you can estimate where between by taking the ratio of the currents for each string.
For example: If the voltage difference between the strings is 11 Vdc and the current from the lower voltage string is 23% of the total current, then the MPP of the inverter will be closer to the higher voltage strings by the ratio of the current, i.e. 77% of the voltage difference.
77% * 11 V = 8.6 V so the MPP point of the total array should be about 8.6 V above the optimal point for the lower voltage string.
This of course will move the lower voltage string towards Voc and reduce it's current and power
It will also move the higher voltage strings toward Isc and decrease their voltage and power.
Since the shape of the power curve is not symmetric around the MPP there is a bigger negative impact by moving towards Voc than there is by moving towards Isc.
The result is that the true global MPP will probably be slightly closer to Isc than the 8.6 V in the example above.

You can iterate this calculation and calculate the power for the entire array from the IV curves for each string.
A couple of iterations should get you really close to where the array will be and let you estimate the reduction in energy yield compared to an optimal MPP on each string or module.

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 Allan Sindelar
Sent: Monday, April 29, 2013 1:06 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] unbalanced string voltages

I question one part of this conclusion, but have not set up any tests. The only part I question is "the string with the highest current AND voltage dictates where the inverter MPP locks". Rather, it seems to me that the voltage at which the combined current of all strings is greatest dictates where the inverter locks.

A string whose string VMP is higher than the aggregate point will be pulled to a lower voltage (raising the current of that string slightly). A string whose string VMP is lower than the aggregate point will be pulled up to a higher voltage - but as current drops off sharply above MPP, very soon that string's current will drop off substantially. Whether the dropoff affects the combined VMP depends on the relationship of that string to the whole, especially to the number of parallel strings in the array. That is, the effects of a difference will have a different effect whether there are two strings or ten. If there are, say, two strings, the combined VMP will drop to just slightly above the VMP of the lower-voltage string, as that's where the combined current of both strings is greatest. If there are ten strings and one has a lower VMP than the others (such as a string with one failed module in a large array), the combined VMP of the other nine strings will determine the VMP of the array, and the string with the low VMP will be pulled up past its string VMP and its current will drop substantially - how much depends on how far it's pulled above its VMP.

Could someone please explain to me what I just wrote? ;^)  And could someone please tell me if I'm wrong, and why?

I think these principles would apply in Kirk's case. I suspect the combination would work, as Dave suggests. But certainly, the bastard-stepchild 5.5 Imp module in string three would put out at most about 4.0 A (3.9 plus a smidge), and the lower voltage of that string might pull it above Vmp and consequently lower the entire string's output. I second Ray's encouragement to add the frame-damaged Sanyo, as all four strings then match pretty closely.

My hunch is that it will work for an unmentioned reason - that degradation on the Sanyos will result in less voltage imbalance.
Back to work...
Allan
Allan Sindelar
Allan at positiveenergysolar.com<mailto:Allan at positiveenergysolar.com>
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com<http://www.positiveenergysolar.com/>

On 4/29/2013 1:21 PM, Larry Crutcher, Starlight Solar Power Systems wrote:
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<mailto: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<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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