Re: [RE-wrenches] unbalanced string voltages

[Allan Sindelar]

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 al...@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 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 <r...@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 <dk...@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" <k...@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   _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org

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