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alaskannoob asked

Float, VOC, and MPPT RS - Is Temp Correction Required for the 8*Float Limit?

Continuing my habit of learning something new after I make expensive decisions on what I think I know...

I have seven arrays of 7 x LG440N2T-E6 bifacial panels in series, with each array connected into a tracker of two MPPT RS 450/200 units.

When I sized the arrays, then designed ground mounts & had them engineered & sent to a metal fabricator, I had sized the arrays based solely on this calculation:

VOC * 7 * 1.23 (my temp correction) = 423V = less than 450V = good to go

Now I see that my PV string cannot be more than 8 x the minimum battery voltage when at float.

My batteries are Pylontech US5000. The float value I see in the Compatibility Guide is 51V. Does this mean my arrays cannot exceed a VOC of 408V?

The VOC of my panels is, I think, 48.9V. 48.9 x 7 = 342.3V.

342.3 is less than 408V so I think I am good here and can use strings of 7 panels?

Or did I do my string VOC calculation incorrectly by not including a temp correction? Because if I use my temp correction, then my string VOC is 421V which is more than 408V, so I may have an issue. Many thanks for any help!

mppt.png


mppt rs
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JohnC answered ·

Hi @AlaskanNoob

I question the '1.23' Temp Comp factor you've used. I'm sure it gets cold in Alaska, so let's use the -40 degC rating of your panels (65C below STC, pretty cold).

From the specs -0.26 %/degC x 65 degC = 16.9%. So 1.17 rather than 1.23.

For the string, 48.9Voc x 7 panels x 1.17 = 400V. All sweet.

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alaskannoob avatar image alaskannoob commented ·

For our location the lowest temp recorded historically is -24F.


That puts us at a 1.23 correction factor according to this chart.

chart.png


I like your math and temp corrections better though. I hope yours are better, you got that math from the panels specs themselves? That seems like a much better source of information.

If the temp correction I used is correct, I still have a tiny issue, I think. IF the part of the manual that references "the maximum open circuit voltage of the PV array" means the VOC corrected for temperature (which I suspect it does, because what it cares about is the max voltage the MPPTs will see) then at -5F, and a 51V float as the Victron Pylontech compatibility guide lists, we will be about 3 volts over the float voltage * 8 number.

We have a few days each year where we see -5 to -7F. In the five years I've been here I've seen -7F once.

I suspect that a -5F sunny day that produces 3V more will likely not be a huge issue. I suspect that isn't enough of an "over voltage" to damage the unit especially given that 411V may be slightly over the 8*float number, but it's still less than the 450V rating of the unit. So I suspect if it does shut the PV down and show "over voltage" that it will just be annoying and not actually be a big issue.

I might also be able to increase the float voltage to 52V but I don't know what effect that would have and given the batteries will be commanding the MPPTs, it probably wouldn't matter anyway.


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JohnC avatar image JohnC ♦ alaskannoob commented ·
@AlaskanNoob

I'm sure LG know better than a 6yo National Code publication, generalised for all makers and panel type and age. And you don't get down to the temps I allowed for anyway.

In practice you'll never see Voc over that limit in the RS, which indeed will be looking at the real Voc from the panels.

I wouldn't hesitate to install it. Your planning is fine.

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alaskannoob avatar image alaskannoob JohnC ♦ commented ·
Thanks so much for the reassurance and showing me how to calculate the VOC more accurately. Cheers!
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alaskannoob avatar image alaskannoob alaskannoob commented ·

I'm watching this video now to try to learn about what I think you did.

https://youtu.be/53CAit1sHLU

Okay, got it, thank you!

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