Isc at STC close to MPPT max ok?

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mickenorlen asked Sep 22, '23

Isc at STC close to MPPT max ok?

Hi,

I have the Inverter RS 48/6000 230V Smart Solar, link, and I want to over size my panels for better winter production in an off-grid installation. I know similar questions have been discussed before, but I couldn't find any clarity on this particular case, with Isc at STC close to max Isc.

MPPT spec summary
Maximum DC solar charging power = 4000 W
Max operational PV input current = 18 A
Max PV Short circuit current reverse polarity protection = 20 A
Max Isc = 30A*

*(from manual) This outside of specification potential allows for system designers to connect larger arrays, and can be useful
to understand in case a certain panel configuration results in a short circuit current just slightly above the maximum of the reverse
polarity protection circuit.

The manual states "slightly above reverse current", which is only 20A.

With this in mind I'm wondering if it is really safe for me to put an array with Isc at STC just below 30, eg: 27-28 A? Does this max Isc refer to Isc at STC and so includes some internal margins, taking into account potential rare irradiance events, that might increase Isc above 30 after all?

Should I be worried or am I within spec? Also, are there any other negative long term effects of working the Amps close to max?

Thank you!

MPPT SmartSolar

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Answer 1 · 2023-09-23T04:45:41Z

kevgermany answered · Sep 23, 2023 at 04:45 AM

Should be ok as long as you fuse the panels. The bigger concern is voltage increase with low temps.

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Answer 2 · 2023-09-23T05:47:00Z

klim8skeptic answered · Sep 23, 2023 at 05:47 AM

@mickenorlen Isc increases with panel temperature rise and must be calculated at expected panel temps, esp if you are near the limits.

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Answer 3 · 2023-09-23T09:43:54Z

mickenorlen answered · Sep 23, 2023 at 09:43 AM

Thank you for the information. I think we would also have to take into account extreme irradiance events of about 1500W occuring even in Europe, https://www.quora.com/If-the-irradiance-of-the-sun-increased-somehow-over-1000w-m-2-does-the-power-of-the-solar-panels-increase-too-or-it-is-constant

Looking at this 400W panel, https://www.solarenergypoint.it/immagini/pdf/download/fotovoltaico/moduli-fotovoltaici/luxor-solar/eco-line-half-cell-full-black/luxor-solar-eco-line-half-cell-glass-glass-full-black-m108-395-415-w-scheda-tecnica-moduli-fotovoltaici-monocristallini.pdf

Isc_stc = 13.6
T_stc = 25
T_max = [Central Italy] = 75
Tk_i = 0.049
Ir_stc = 1000
Ir_max = 1500
The electrical characteristics graph show a linear relationship between Ir and I

Isc_max = Isc_stc * (1 + Tk_i/100 * (T_max - T_stc)) * Ir_max / Ir_stc
= 13.6 * (1 + 0.049/100 * (75 - 25)) * 1500 / 1000 = 20.9 A => 2P = 41.8 A => 11.8 A over 30 A limit => Not safe

So to stay within the limit for a 50% overirradiance event, I would need a panel with an Isc rating of no more than:

Isc_safe = Isc_lim / (1 + Tk_i/100 * (T_max - T_stc)) / Ir_max * Ir_stc
= 30 / (1 + 0.049/100 * (75 - 25)) / 1500 * 1000 = 19.52 A
(or 9.76 A for 2P array)

Does all this seem correct? Other factors to take into account?

Otherwise I could use fuses on the panels? Could you please explain to me quickly how that would work, and if there any potential downsides. I'm new to this field.

Thank you!

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klim8skeptic ♦ commented · Sep 23, 2023 at 12:20 PM

@mickenorlen

Isc_stc = 13.6
T_stc = 25
T_max = [Central Italy] = 75
Tk_i = 0.049
Ir_stc = 1000
Ir_max = 1500

The maths is pretty simple.

Isc at 50c above ambient gives 50 x 0.049 = 2.45% above Isc STC figure. Ie Isc @ 75c is 13.94a.

Dunno where irradiance above 100% comes into calculations.

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Answer 4 · 2023-09-23T16:34:17Z

mickenorlen answered · Sep 23, 2023 at 04:34 PM

Oh thank you I see that I made a mistake in my slightly overcomplicated math, I updated my previous post. I like using the formulas for excel sheet comparisons. But so with two series in parallel its 2 * 13.94 = 27.88

Then so far I'm still under 30 A for these panels. But I do believe overirradiance events have to be taken into account as well. Look at this article for example https://pv-magazine-usa.com/2023/08/17/new-study-claims-pv-industry-is-neglecting-overirradiance-issues/

What do you think?

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gazza commented · Sep 24, 2023 at 08:08 AM

Interesting article! It is relevant in the sense that we only really have access to STC data. Clearly there are variations in “the real world” numbers. Despite taking into account all the “known” coefficients to attempt to stay within limitations it does make a strong case for always allowing headroom even above that….. The question is how much more?

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Answer 5 · 2023-09-28T08:10:56Z

After having contacted a couple of vendors, as well as Victron themselves. This is the somewhat confusing information I got.

Below is reply from Victron's sales dept. I couldn't find a way to get in direct contact with the tech dept.

Can we interpret this that the warranty is still valid for PV arrays with short circuit currents up to 30A at STC?

Yes correct , but you need to stay will below this level of course

In case the current in rare circumstances would increase above 30A with such an array, e.g. during an over irradiance event, is there an overcurrent protection fuse to safeguard the integrity of the device or would it risk direct damage?

No , as this protection is rated up to 30A , so having a higher current here will result in severe risk for all

From a Vendor, even after I showed him the above reply from Victron:

I will stand my ground with the information already written in my last mail:

“Please do just connect not more than 20A in STC Conditions (as written in the datasheet of the RS48/6000)” !!!

Victron will not answer this “what-if” questions, as all information are written in the manual you already read!

(additional Info you might not have thought about: about 30A are also the maximum current what MC4-connectors can bear (especially if it is a random MC4 and not a very high rated/quality like Evo2), so maybe also think about physical damages due to high current on the connectors and not the RS48/600 itself. In both cases Victron will refuse warranty, as those defects clearly state a “not intended use”)

In the end my interpretation of this is that the warranty and device is designed to hold up for currents up to 30A, but YOU have to be the one to make sure that these currents are not exceeded, which would otherwise be considered a mistake in system design and probably void the warranty.

Regarding the MC4 connectors 30 A limit, there will be 2 separate strings connecting to the 2 MC4 inputs of the tracker, so max current at each connector should only be around 15 A.

The most important takeaway for me is that there is no protection beyond 30A. So I will consider adding some fuses to protect me from any over irradiance events.

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