Looking for recommendation on reconfiguration of solar array setup

Q&A and troubleshooting
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Hello,

I have my solar system running for just over a year now, but at first it was temporarily installed with pair of MPPT 250/100 due to unavailability of MPPT RS 450/200 unit that I ordered.

Background:

My installation consist of 16 panels - Canadian Solar CS6W-555MS, with specs as follows:

Pmax: 555W

Vmp: 41.9V

Imp: 13.25A

Voc: 49.8V

Isc: 14.05V

Max System Voltage: 1500V

Max Series Fuse Rating: 25A

MPPT RS is connected to 3x Pylontech US3000C, providing c. 10.5kWh of storage.

The panels are currently split into 2 arrays, East and West facing. I assume due to the use of MPPT 250/100 at the start, each array was configured as 2 paralleled strings of 4 panels to keep the voltage under 250V. This setup worked well.

Problem:

When eventually the pair of MPPT 250/100 were replaced with my shiny MPPT RS 450/200 without modifying the arrays layout (simply using 2 of 4 trackers of MPPT), I noticed that on sunny day the MPPT trackers max out on 16A per tracker, which matches the limitation listed in the data sheet.

Search for solution:

I am looking for the solution as with the sun climbing higher every day, I see the system maxing out regularly, which obviously is a waste of solar power.

I am looking for the best solution, and as far as I can see there are only two solutions available (if not just one):

  1. Break the paralleling and use all 4 trackers to run 4 arrays of 4 panels in series. This sounds simple and logical at first but:

    • requires doubling up on the array protection and isolation equipment on the roof, which done right is costly
    • requires another two pairs of solar cable runs back to the MPPT. It’s only c. 25m run, but an awkward job requiring climbing into not very nice places
    • it limits any future capacity expansion to the existing area only as I’d be using all 4 trackers, so connecting further panels in series would be my only option

  2. Reconfigure each array as string of 8 panels in series, which would be much simpler, but after running the idea past Victron’s Click & Design I was told that this won’t work, it throws the following errors:

    • Increase the battery float voltage or reduce the voltage on tracker 1.
    • Increase the battery float voltage or reduce the voltage on tracker 2.

Configuration validates after removing 1 panel from each array (ie. 7 panels per tracker). I assume the error is related to this limitation of MPPT RS:

MPPT operating voltage range is constrained by battery voltage - PV VOC should not exceed 8 x battery float voltage. For example, a 52.8 V float voltage results in a maximum PV VOC of 422.4 V.

My battery float voltage is set to 51V, which as I understand is a recommended value for my batteries, but Click&Design only validates after float voltage is changed to 55V! With that in mind, something doesn’t stack up for me here as 8 x 51V (float V) must be more then 8 x 49.8V (Voc)… it doesn’t take a math genius to work that one out.

Questions:

  1. Am I missing something else that constrains my preferred reconfiguration?
  2. Am I completely nuts thinking about creating the setup so close to limits?
  3. What would happen if the array voltage was to exceed 8x float voltage?
  4. Is there anything else that I am missing that I should be aware of?

Thank you in advance for any help, recommendation or even pointing in the right direction.

Regards,
Sergiusz

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Summary

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From the manual … You will see this error

Where the array voltage exceeds this parameter the system will give a “Over-charge Protection” error and shut down.

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If you run the same spec on the online calculator. You get the same result.

The max wattage on each tracker is 4000watts.
It may be easier to just use 7 panels.in each string. Leaving 2 orphans sadly

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Thank you for your responses, @lxonline

I still don’t fully understand why the configuration fails to validate with 8 panels as according to my math the Voc doesn’t exceed 8x float.

But you pointed out yet another limitation, which is far from ideal - clipping the power output on 4kW per tracker, so that idea is out the window.

Using just 7 panels per string and orphaning 2 to me sounds incredibly inefficient, especially that my longer-term aim is to oversize the system to get more power from it during winter. I guess better solution would be to make 2 strings of 6 and string of 4 with aim to top it up to full 6 if I can get my hands on exactly the same panels. I would only need to get another protection set for now, and that still leaves me with 1 tracker for future expansion.

Thank you again.

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Maybe in cold temps? This adds quit a few volts to the upper limit.
Did you try the same config but with higher temps and 8 panels. I didn’t play much with the calculator just confirmed what you were experiencing.in both the click and deaign and checked if the online mppt calculator agreed.

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You’re right - 8 panels config gets validated with 51V float with minimum temp up at 16C. 52V float allows to lower temp to 9C. I’m in rather mild climate. 9C while sun is shining would be extremely rare, but I would not rule it out completely as a possibility, especially with the crazy weather this winter. Plus for Pylontech recommended float is 51V, and 16C minimum is definitely not a runner.

In any case, while that’s an interesting observation and learning, at this point your other argument of clipping on 4000W has already helped me to ditch the 8 panel string idea. Back to the drawing board for now, I think I’ll be hooking up 3rd tracker for now.

Thank you kindly.

Regards,
Sergiusz

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