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2s or 3s or a combination of both?


Kalliew
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Good day.  Newby questions.  I hope they make sense. 2s or 3s or a combination of both?

Recently purchased a house with an existing solar system.  I would like to increase the number of solar panels for a quicker battery charge during the day and increase available power capacity during the day (complete off grid in the day).  At night, the inverter switches to Eskom and we await load shedding.  I would like to not charge the battery from the grid (after use) and be off grid in the day.

Must 5kw inverter:

Solar Charging Current 80A

Max PV Array Open Circuit Voltage 145Vdc

PV Array MPPT Voltage Range 60-130Vdc

6 x Solar Panels: Connected 3s2p

295w

Maximum Power (Pmax) 295w

Maximum Power Voltage (Vmp) 32.4v

Maximum Power Current (Imp) 9.10A

Open Circuit Voltage (Voc) 38.6v

I understand the basics of string series or parallel panel connections, however could not find much information for calculating series strings connected in parallel.  Your advice, clarification and guidance regarding the best option volt / amp calculations will be appreciated.

I would like to add additional panels and preferable, large panels (+- 450w) to my 3s2p setup.  With 72 cell panels, I know I must use a 2s setup.

I am thinking of these different connection option, however not sure if the connections are possible and if so, calculated correctly?

1.)     My current 3s2p (series 3 x 38.6v & parallel 2 x 9.1A = 115.8v & 18.2a) connected with new 2s2p (+-series 2 x 50.1v & parallel 2 x 10.88A = 100.2v & 21.76A) resulting in 100.2v and 39.9A to the inverter?  The voltage will be reduced to match the lowest v string and the strings amps will be combined?  Correct?

2.)     Instead of 4 x 450w panels, add 6 x 295w panels to the current setup.  Result 115.8v and 27.3A?  Correct?

3.)     Change current setup from 3s2p to 2s3p (64.8v & 27.3A) and add new (+- 450w) panels 2s2p (100.2v & 21.76A) resulting in 64.8v & 49.06A to the inverter?  The 64.8v will be problematic as the MMPT minimum array is 60A? Correct?

4.)     Add 4 x 450w panels and connect 1 x 450w panel with 1 x 295w panel 2s4p (38.6v + 50.1v = 88.7v & 9.1A per series string).  Once in 4p, 88.7V and 36.4A to inverter? Correct?  Put the 2 x 295w in the garage and in the future, add another 2 x 450w panels resulting in a 2s6p system (88.7v and 54.6A)?

If my calculations are correct, point 1 appears to be the best option?  Could point 4 be a better option for the future?  Is the 88.7 voltage of point 4 a concern?

I will appreciate your assistance in verifying the calculations and assisting me in making the best choice for adding additional panels.

Thank you

Kallie

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I personally would go with option 2 because the panels will be perfectly matched plus you will get the highest voltage which will lessen the effect of voltage drop in your cables.

Also, if your current setup is 115.8V 18.2A then doubling up would give you 115.8V 36.4A, not 27.3A...

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6 hours ago, Kalliew said:

1.)     My current 3s2p (series 3 x 38.6v & parallel 2 x 9.1A = 115.8v & 18.2a) connected with new 2s2p (+-series 2 x 50.1v & parallel 2 x 10.88A = 100.2v & 21.76A) resulting in 100.2v and 39.9A to the inverter?  The voltage will be reduced to match the lowest v string and the strings amps will be combined?  Correct?

No, that's dangerous. The difference in panel voltage will result in current flowing from one set of panels to the other. It could cause a fire. When paralleling strings, they ideally should be identical in panel specifications, but if not, the total Vmp should match within 5%. You seem to be using Voc in (some of?) your calculations, but Vmp would be similar. You would have a mismatch of 115.8 vs 100.2 V, or over 15%. That's way too much mismatch.

6 hours ago, Kalliew said:

4.)     Add 4 x 450w panels and connect 1 x 450w panel with 1 x 295w panel 2s4p

That's at least not dangerous, but you would be reducing the 450 W panels' performance to effectively something like that of a 295 W. So that's impractical.

It may make more sense to use a separate MPPT for the new strings of 450 W panels, connected directly to the battery. It's a little tricky, but can be done. Just set the external MPPT voltage settings one or two tents of a volt lower than the inverter's settings.

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3 hours ago, Coulomb said:

No, that's dangerous. The difference in panel voltage will result in current flowing from one set of panels to the other. It could cause a fire. When paralleling strings, they ideally should be identical in panel specifications, but if not, the total Vmp should match within 5%. You seem to be using Voc in (some of?) your calculations, but Vmp would be similar. You would have a mismatch of 115.8 vs 100.2 V, or over 15%. That's way too much mismatch.

Thank you for the response and clarification regarding the different voltages per string.  Option 2 is the most straightforward at this time, so I will be adding additional panels of similar spec.  To clarify, if I do not find the exact same panels (the most important would be a similar Vmp in the new panels), find panels with similar specs and ensure that I connect the different but similar spec panels in 3s so that the total VMP per string is within 5% of each other? 

Another noob question, although I most properly know the answer.  I have been using the VOC of the panel (38.6v x 3) as my calculation to ensure I do not exceed the Max PV Array Open Circuit Voltage of the inverter (145Vdc).  If using the panels Vmp (32.4v) as a calculation, the max voltage for the current setup 3s is 97.2v.  Adding a 4th series panel, the Vmp (4 x 32.4=129.6) would be just below the max MPPT voltage range, but the 4 x VOC would exceed the inverters max PV Array open circuit voltage.  Am I correct that adding an additional panel for a 4s setup is not a option?  

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5 hours ago, Kalliew said:

To clarify, if I do not find the exact same panels (the most important would be a similar Vmp in the new panels), find panels with similar specs and ensure that I connect the different but similar spec panels in 3s so that the total VMP per string is within 5% of each other? 

Yes.

5 hours ago, Kalliew said:

Am I correct that adding an additional panel for a 4s setup is not a option?

Yes, definitely.

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  • 2 weeks later...

Have not managed to find stock of exact same panels.  Please assist with best option for additional panels:

Current:

Pmax 295w vmp 32.4v imp 9.10a voc 38.6v

Option1:

Pmax 285w vmp 32.8v imp 8.71a voc 39.42v

Option2: + R180 from option 1

Pmax 330w vmp 33.7v imp 9.8a voc 40.6v

Option3: + R310 from option 1

Pmax 330w vmp 34.24v imp 9.64a voc 41.08v

Option 3 is just over 5% of current vmp

Thanks

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1 hour ago, Kalliew said:

Option1:

Pmax 285w vmp 32.8v imp 8.71a voc 39.42v

That's the closest Vmp, so that's the best option. I know it hurts to "go backwards" in panel nominal power, but it will probably give better overall power than the other options anyway. Mismatched panels don't perform as well as matched ones.

No need to consider Imp, and Voc doesn't matter nearly as much, but still points to option 1 anyway.

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