Over paneling MPPT, short circuit current too close?

[mickenorlen]

Hi,

I'm planning an off-grid solar setup and would like to over size my panels to maintain decent production in the winter. For the MPPT at 4kW I would like at least 5kW PV (25% over size). With 2 strings in parallel I'm worried that the short circuit current is too close to the max PV Isc of the MPPT.

These components are already purchased. The only thing missing is to decide the panel setup.

- Battery: 48V, 16 x EVE LiFePO4 3.2V => V_float = 54.4, according to this post
- MPPT: Victron Inverter RS 48/6000 230V Smart Solar, spec
Maximum DC solar charging power = 4000 W
Max Vsc = 8 x 54.4 = 435.2
Max operational PV input current = 18 A
Max PV Short circuit current reverse polarity protection = 20 A
Max Isc = 30A

Relevant clarifications from MPPT manual, section 3.4, link

The Inverter RS Solar single tracker model contains multiple PV input connectors. However these are internally connected to one single Maximum Power Point Tracker.

...

The maximum operational input current for each tracker is 18 A.

MPPT PV inputs are protected against reverse polarity, to a maximum short circuit current of 20 A for each tracker.

Connecting PV arrays with a higher short circuit current is possible, up to an absolute maximum of 30A, as long as connected
with correct polarity. 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 maximum open circuit voltage of the PV array must be less than 8 times the minimum battery voltage when at float.
For example, where a battery has a float voltage of 54.0 volts, the maximum open circuit voltage of the connected array cannot exceed 432 volts.

Panels
So now I'm looking at these panels below and find these values at STC.
- Luxor, 400W, Isc: 13.60, Vsc: 37.01
- Jinko, 435W, Isc: 13,72, Vsc: 39,36


Questions
Could it cause any problems to connect them in two parallell strings, eg: 7S2P?

14 x Luxor, 5600W, Isc: 27.20, Voc: 259.07
14 x Jinko, 6090W, Isc: 27.44, Voc: 275.52 (it might be enough with 12 panels here)

The excerpt from manual above sais they allow for Isc "slightly above" the max polarity protection circuit of 20A. So I'm worried that Isc at 27.5A could stress and reduce the life expectancy of the MPPT. Given that panels can sometimes produce more than their STC specification, I'm also worried that the current could even rise beyond the absolute max Isc at 30A and cause direct damage.

Let me know if you think there are other types of panels that would be more appropriate for my use case. I'm also curious which panel you would chose of the above. Would you say the double glass design from luxor makes them superior in terms of longevity?


Thank you for your help

 

[sunshine_eggo]

Light gives voltage.
Intensity of light gives amperage.

In order to exceed Isc output on your panels, you will need to be in conditions where the panels are exposed to over 1100W/m^2, AND they are operating well under Vmp... which like a very rare possibility.

 

[ghostwriter66]

Not exactly a "correct" answer BUT having done a few billion of these ground systems -- I have personally found that you CAN push the AMP limit BUT in no way shape or form EVER NEVER push the voltage limit ...

again not technically correct but higher AMPS will make the system run hotter as it hits its max ... higher voltage simply releases the factory smoke ...

been there -- done that ...

several times ...

 

[sunshine_eggo]

when a manufacturer is this specific:


4) Normal operation is regulated to 18 A, with maximum reverse polarity protection 20 A

I'm not pushing past 30A on this particular controller.

 

[mickenorlen]

EDIT, saw my question was already answered above.

Thank you!

 

[zanydroid]

when a manufacturer is this specific:

View attachment 168837
4) Normal operation is regulated to 18 A, with maximum reverse polarity protection 20 A

I'm not pushing past 30A on this particular controller.

Amps rating expressed as Isc should always be interpreted as already factoring in 1.56 factor from continuous load and lensing, right?

Ie you can do straight adding and comparing of all amps ratings listed as Isc

But amps ratings not labeled as Isc (wiring, breakers, fuses) need derating.

How do you account for the 20A reverse current protection in how you handle or install this? A straw man might be you stick to 30A for permanent install and 20A for temporary install with more potential for reverse polarity.

 

[mickenorlen]

But do you guys think I can safely go ahead and use panels with 27.5 A at STC then? Or do I have to consider this possibility of rare irradiance events as mentioned by @sunshine_eggo? I don't really understand the thing you said about "operating well under Vmp".

 

[zanydroid]

But do you guys think I can safely go ahead and use panels with 27.5 A at STC then? Or do I have to consider this possibility of rare irradiance events as mentioned by @sunshine_eggo? I don't really understand the thing you said about "operating well under Vmp".

That is what I was asking. I think Isc already factors in the lensing safety factor but I wanted to check some opinions.

 

[sunshine_eggo]

Amps rating expressed as Isc should always be interpreted as already factoring in 1.56 factor from continuous load and lensing, right?

good question. I don't think so. Isc is measurable in ideal conditions. It is theoretically possible for Isc to be above rated, BUT that would be a pretty rare case as I described above. +10%?

Ie you can do straight adding and comparing of all amps ratings listed as Isc

But amps ratings not labeled as Isc (wiring, breakers, fuses) need derating.

How do you account for the 20A reverse current protection in how you handle or install this? A straw man might be you stick to 30A for permanent install and 20A for temporary install with more potential for reverse polarity.

IMHO, it's just about understanding the implication. Reverse wire like a dummy, and > 20A likely to pop the controller.

But do you guys think I can safely go ahead and use panels with 27.5 A at STC then? Or do I have to consider this possibility of rare irradiance events as mentioned by @sunshine_eggo? I don't really understand the thing you said about "operating well under Vmp".

Vmp is about 80-83% of Voc where the panels will typically operate when producing maximum power. You'd need to be even lower than that voltage to get Isc. I'm really struggling with this. I've personally seen my array output > 3600W on a 2970W rating due to cloud edging and cold temps and some of that was elevated current for sure.

The wording about the 30A limit seems pretty strong, so I hesitate to say with certainty. Is that out of the manual?

 

[mickenorlen]

The wording about the 30A limit seems pretty strong, so I hesitate to say with certainty. Is that out of the manual?

Yes the quotes above are straight from the manual.

From what I've learned and understand so far Isc increases just slightly with temperature. For example for the luxor panel:

Isc_stc = 13.6 A
T_stc = 25 °C
T_max = [Central Italy] = 75 °C
Tk_i = 0.049 %/°C

Isc_max = Isc_stc * (1 + Tk_i/100 * (T_max - T_stc))
= 13.6 * (1 + 0.049/100 * (75 - 25)) = 13.93 A (or 27.87 A for 2P array)

So far we are still under 30A. But then as you say there is the issue of overirradiance. So far I haven't found any clear guidelines on this issue.
Here is an article saying it's in fact been neglected. https://pv-magazine-usa.com/2023/08...industry-is-neglecting-overirradiance-issues/

In your case there was ca 20% overirradiance. Here is a person noting 1400 W/m^2 (+40% from STC) in Germany and US. Then there are studies in Peru and Brazil reporting up to 1800W/m^2 etc.

As I've found that there is a linear relationship between Ir and I, it seems we would have to take all that overirradiance into account for the maximum Isc.

Isc_max = Isc_stc * (1 + Tk_i/100 * (T_max - T_stc)) * Ir_max / Ir_stc

Isc_lim = 30 A
Ir_stc = 1000 W/m^2
Ir_max = 1500 W/m^2 (assumed max overirradiance event in Italy)

=> 27.87 * 1.5 = 41.8 A => 11.8 A over limit => Broken charge controller?

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)

This looks awfully close to the 20 A they seem to have designed their system for with the implemented reverse polarity protection.

 

[zanydroid]

good question. I don't think so. Isc is measurable in ideal conditions. It is theoretically possible for Isc to be above rated, BUT that would be a pretty rare case as I described above. +10%?

I don’t really have a source other than my intuition, but for solar panel to solar panel Isc comparisons you don’t do any adjustments. So in this case it doesn’t make sense to add 10% since you would never be able to parallel any pair of solar panels together without fusing.

On a panel spec sheet I took a quick look at, the fuse rating in amps was around 1.56x the ISC rating in amps. So clearly fuse rating needs the derate.

 

[zanydroid]

Another thing to consider is looking at microinverter spec sheets. If you interpret the regularly encountered 15A max Isc as needing 1.56 downward correction then zero modern solar panels will work with their same generation microinverters.

 

[MisterSandals]

This looks awfully close to the 20 A they seem to have designed their system for with the implemented reverse polarity protection.

Your calculations look pretty thorough but i did not get thru very much of it.

I think if you spend 1/10th as much time with a volt meter testing polarity, you can stop worrying about reverse polarity.

 

[mickenorlen]

@MisterSandals Yes, i like to be thorough. No worries, but I didn't calculate this to ensure I'm not breaking reverse polarity. It was to explore what is an appropriate panel Isc to avoid breaking the limits of my charge controller under operation.

 

[zanydroid]

I feel the 30A ISC means 2p maximum.

Contrast with the common 15A ISC rating which one would likely interpret as 1p max without further analysis if it were encountered on a spec sheet

Conventional current range of modules of course, not some weird outliners

Contacting victron might be a good idea

 

[zanydroid]

Getting back to the question about modules. Sounds like you are looking at 54/108 cell style panels. Are you able to find 72/144? You can also try punching those into a calculator.

54 cell has higher current and aren’t a good fit for older MPPT designed for 60/72/etc voltages and currents. When I was looking at them they would not achieve maximum power on microinverters not designed for them. In the case of your MPPT, 26A STC (a little less for NOCT) vs 18A max operating current seems more overpaneled than usual.

Data for my previous post above about microinverters

For instance here is the data sheet for IQ8. 15A ISC, 10.6A maximum continuous current.

 

[mickenorlen]

Thank you for the feedback. Interesting this thing that fuses have a 1.56x Isc rating. I've send the question to Victron, I'll let you know if I get a reply.

But for now I just realized how to use the Victron MPPT Calculator properly. It seems they will suggest my product as long as Impp < 20 A. Eg just adding 0.1A to Impp in the Burger Black MPPT calc (panel info below) will remove my product from the suggestion.

I have been looking through the panels from the main Italian online vendors that fit my criteria:
- All black design
- Made for longevity with warranty to back it
- Good hail resistance
- Max 0.4%/year efficiency degradation

Basically I looked at these keywords: N-type, HJT, All Black, Glass Glass (not sure if I like bifacial though)

Larger panels could have been interesting but I couldn't find all black and almost all of them still had the same Isc around 15 A anyways, just increasing voltage.

Smaller panels could potentially work for example Luxor M60 300W. But the ones I found seem to cost even more per panel in spite of the lower power rating and I don't really like the idea of so many small panels and connections anyways.

For mid size panels I did find some of the most cutting edge HJT models had a lower Isc rating. Of course they cost ca 70% more than other panels, but the general quality and 0.2-0.25% annual degradation is still attractive.
- REC pure alpha 420W, 8.88 A
- Meyer Burger Black 380W, 10.6 A
- Meyer Burger Glass 380W 10.4 A + bifacial gain, let's say max 25% => 10.4 + 10.4 * 0.25 * 0.907 = 12.76A => fails MPPT calc compatibility.

 

[zanydroid]

Did you enable overpaneling in the calculator? Or increase the allowed ratio? I wonder if you put something huge like 200% whether it will tell you a safety limit got hit. I ask because the 13A range panels are 144% overpanel.

The calculator on that site will sort of show you the expected output taking into account the clipping.

Is it worth it to you to take the 70% cost overhead on panels to get to lower overpaneling? Is your project goal to hit a KWh number per year, per day, levelized cost of energy across 30 years, …

This is the title of the thread, but you probably are aware that the only hard blocker here is whether 2P on the readily available panels will blow up the SCC.

Can you exchange the MPPT with a better fit one from Victron via your distributor? Are you allowed by local regulation to use any MPPT on the battery bus or does it have to be victron?

There are folks on this forum that go for $/W above all else. I figured if they appear to be bankable (IE above financial due diligence for large scale) and have the safety and utility commission approval it is fine.

70% more is probably more than enough to pay someone to replace one or two failed panels and stock extras on your own. Not sure if the panel maker will give you identical panel, just cash you out, or pay for labor.

 

[mickenorlen]

I linked the Victron MPPT calc for Meyer Burger Black 380W in my previous post, here again. It's the one that is just 0.1A Impp away from removing my SCC from the suggested compatible products. Feel free to tinker around with it Not sure what you mean by allowed ratio?

Thank you for your feedback on different design goals. My main concern is the longevity and stability of my system and to have enough kW in the winter. There is a somewhat limited budget, but right now I'm honestly very impressed with the quality of the Meyer Burger Glass panel. So I'm considering to get half of the number of panels for now, get some statistics on power generation this winter, earn some money and get more panels later. I'm still hoping there is some built protection for overirradiance events and that the 30A max Isc is actually refering to an STC rating, which would make most of these panels safe.

I can't change my MPPT or batteries as I bought them a while ago, and either way I'm still happy with these components. I just want to know what risks are involved with higher Isc and at what point it can start to compromise stability and longevity, so I can make an educated decision for my panels.

 

[zanydroid]

Not sure what you mean by allowed ratio?

Allow oversizing above 130%, is that already checked?

Wonder what folks here would think about a 30A fuse. With 1.56 scaling that is just above the 18A max operating current of the MPPT.