Unequal Parallels on the MPPT - Does it matter?

[nezek88]

Hey guys

So I've got this Growatt SPF12000T HVM and it has dual MPPT each running 15 panels but my configuration is bit weird and I wanted to double check i'm not making a mistake:

So two strings of 15 panels 3Panels x 5 and the wiring goes like this:



So in total I have 4 pairs of red and black cables, each pair going into a 32a double pole DC RCD and each two going into a 62a double pole DC RCD before being connected to each MPPT.

I guess the main question is if ok to wire it this way, and if would you do it in a better way?

Panel details:
ANP350W(72)

350W
VOC:46.68
SCC:9.45

MPPT MAX 7000W(3500x2)
MAX 150VOC
Max operating A120

Thanks

 

[littleharbor2]

Deleted

 

[DThames]

I am having problems understanding your planned setup. Normally one would create a series string of x panels and then sometimes parallel those strings. For 4 MPPTs, I understand you have an odd number of panels. I just can't understand what is in series.

 

[littleharbor2]

I am having problems understanding your planned setup. Normally one would create a series string of x panels and then sometimes parallel those strings. For 4 MPPTs, I understand you have an odd number of panels. I just can't understand what is in series.

That's why I deleted my original comment after I saw that the illustration is of 3 panel series blocks. It's not real clear being so small.

 

[nezek88]

That's why I deleted my original comment after I saw that the illustration is of 3 panel series blocks. It's not real clear being so small.

Sorry for this, i've edited the image - is that any better?

Thanks you both for taking the time to have a look

 

[littleharbor2]

Surprised your AIO has such a low Voc. limit. Your 3 panel strings look uncomfortably close to the 150 volt limit. Do you live in the Tropics? If you are in an area that gets cold you are likely to fry your unit with high voltage.

 

[nezek88]

Surprised your AIO has such a low Voc. limit. Your 3 panel strings look uncomfortably close to the 150 volt limit. Do you live in the Tropics? If you are in an area that gets cold you are likely to fry your unit with high voltage.

Yes I do live in the tropics, and with the cable distance it causes a small voltage drop and its fine..
When I tested the lines it was about 121-124 VOC

 

[nezek88]

I am having problems understanding your planned setup. Normally one would create a series string of x panels and then sometimes parallel those strings. For 4 MPPTs, I understand you have an odd number of panels. I just can't understand what is in series.

3 panels in a series 5 times wired 3+3 to an DC RCD and 3+3+3 to another DC RCD then joined to a single line going to the MPPT.
Take the above and X 2 (second MPPT to make a total of 30 panels)

 

[Rednecktek]

The picture shows 3s2p/3s3p/3s2p/3s3p, is that correct?!?! Having a 3s2p and a 3s3p paralleled in together is nerfing the larger set.
15 panels is a real pain in the ass to work with.

350W
VOC:46.68
SCC:9.45

MPPT MAX 7000W(3500x2)
MAX 150VOC
Max operating A120

That's pushing the raggedy edge of voltage with that setup, if it ever gets cool out it's going to fry your MPPT. If you kick that down to 2 panels you're going to need a LOT of ports on combiner boxes.

The safest way to deal with this setup would be either get 1 more panel and do 2s4p to each MPT input. That keeps you safely below the VOC limit and lets you use all the panels.

The only other option would be to do 2s3p to each MPPT input and then throw the last 3 panels on their own controller capable of taking 200v VoC and paralleling that right to the battery.

 

[littleharbor2]

Well I would get two fused combiner boxes Bring all 5 strings together in each box and then to your two MPPT inputs.

 

[DThames]

Electrically you have 3s5p on each MPPT. If your wiring runs are sound, having 6 panels on one wire and 9 panels on another wire would be no different than 15 panels on a single larger wire. I can't see anything that would be a glaring, "no".

 

[nezek88]

The picture shows 3s2p/3s3p/3s2p/3s3p, is that correct?!?! Having a 3s2p and a 3s3p paralleled in together is nerfing the larger set.
15 panels is a real pain in the ass to work with.

Yes that is correct, i suspected something would cause inefficiency - where can I find some reading material or an explanation for how and why the nurfing happen?

 

[nezek88]

Electrically you have 3s5p on each MPPT. If your wiring runs are sound, having 6 panels on one wire and 9 panels on another wire would be no different than 15 panels on a single larger wire. I can't see anything that would be a glaring, "no".

Essentially yes each MPPT has 3S5P yet 3S3P+3S2P are joined - Rednecktek mentioned it might cause nerfing to the smaller set?

 

[Solar Guppy]

From your image, you have 5 series triplets, and one feeder pair for 2 triplets and one feeder for 3 triplets . Functionality wise that would work but has two issues.

First and most important is if one has more than three panels ( or groups of panels in series as you do ) in parallel you are required to have overcurrent protection as if one of the series stings has a panel fault, all the current from the other 4 strings will feed the fault.

Second, since your have 2 + 3, more current will flow on that home run wire with three triplets than the two, which would have a slightly higher voltage loss, so in a mppt system, there will be some minor tracking losses but likely minor in daily harvest.