Over-paneling Victron MPPT controllers.

[FilterGuy]

Is Voc of an off-angle or shaded panel greater than Vmp of a panel receiving direct sun?
If so, no need for the diode.

In actual operation, not standard tests, the one in full sun would be hotter, reducing its Vmp further.

I am not following. I think I can model the situation like this:


We want the panel in the sun to be operating at Vmp. (V2=Vmp)
The question becomes "what is the voltage of the panel in the shade. In the extreme case, there is no light on the 'shaded' panel and full sun on the other panel. In that case V1= 0 < V2 and that will cause current from the sunny panel to back-feed through the shaded panel.


With blocking diodes, the current cant back-feed.



Please let me know if I have this wrong!!

 

[Hedges]

Yes, but with a small amount of indirect light, like panel oriented away from sun, what is its Voc then?

 

[MrM1]

So building an array that will see no shade all facing same direction would be more stable not seeing current extremes?

 

[FilterGuy]

Yes, but with a small amount of indirect light, like panel oriented away from sun, what is its Voc then?

I think I see what you are saying..... if the current is zero (or negative?), it will operate at Voc for the conditions it is experiencing. It is a good question and it depends on whether a panel will reach Voc when off-angle.

I had been 'taught' early on that in the case we are describing, the less lit panel can experience back feed.... but you are suggesting that is not the case. I never had a reason to question it, but I am willing the explore the possibility it behaves differently than I thought. It is a reasonable assumption that when one of the panels is totally blacked out, it will experience back feed. It is also a reasonable assumption that when the two panels are equally illuminated there will be no back-feed. So, someplace between blacked out and fully illuminated, the back feed will stop. Is it a linear progression from full back feed to no back feed, or is it an upside-down hockey stick curve as the light increases and if so, where is the knee of the curve?

It would be an interesting test to set up and try. Just hook two in parallel (no controller) and measure currents with one in direct sun and vary the angle of the other. A more elaborate test would be to also set up an MPPT controller on a near-empty battery and measure the power produced as the 2nd panel's angle is changed.

 

[Hedges]

So building an array that will see no shade all facing same direction would be more stable not seeing current extremes?

All one orientation on a given MPPT is simple, slightly more efficient.
Avoiding partial shade can make things much more efficient if multiple strings in parallel.
Multiple orientations could fit production to consumption better so you don't draw batteries as low.
Multiple orientation can support 140% of PV on SCC or GT inverter, or run them at 70% the wattage, 50% the power dissipation.

 

[Hedges]

It would be an interesting test to set up and try. Just hook two in parallel (no controller) and measure currents with one in direct sun and vary the angle of the other. A more elaborate test would be to also set up an MPPT controller on a near-empty battery and measure the power produced as the 2nd panel's angle is changed.

Yes, and I was close to that setup when I tried 2s panels in sun feeding 1 panel in darkness, testing for electro-emission of IR.
I suppose with a Y cable, I could parallel one dark pane with one panel of my series PV string running on MPPT, and measure current with clamp meter.

But to KISS, just read string voltage from my operating array, and measure Voc of a single panel tilted various orientations.

I've seen warnings that very little light is needed for PV to produce enough voltage to cause shock. Obviously more of a concern for high-voltage string than single panel.

 

[MrM1]

Multiple orientation can support 140% of PV on SCC or GT inverter, or run them at 70% the wattage, 50% the power dissipation.

So you're saying there's no universe where I could run 32 370w panels in two 4s4p configurations going into the 2 PV inputs of the Growatt 12kw 250? VOC would be around 192 vdc

 

[FilterGuy]

But to KISS, just read string voltage from my operating array, and measure Voc of a single panel tilted various orientations.

That would probably give a good indication.... but since the panel voltage is related to current, I could imagine getting some surprises when current is actually flowing..... Then add the MPPT controller in the mix and I could imagine even more surprises as it tries to optimize the power-point. (I guess I am admitting that I don't have an intuitive feel for how all the different variables might interact)

I've seen warnings that very little light is needed for PV to produce enough voltage to cause shock.

So have I..... and that is inconsistent with the warnings of back feed. That was the chain of thought that made me willing to question my beliefs when you pointed it out.

 

[Hedges]

So you're saying there's no universe where I could run 32 370w panels in two 4s4p configurations going into the 2 PV inputs of the Growatt 12kw 250? VOC would be around 192 vdc

I can't seem to get the full manual, just data sheet.
That indicates 7000W array and 120A charging current. It lists 250 V max, doesn't give details of watts or amps limit for each of two MPPT tracker.
It is obviously intended to get at least half its charging power from each MPPT, don't know how much more than half.
Don't know if it has a hard limit on Isc, or available watts.

32 x 370W (STC) = 11840W
Obviously it won't process all of that.
If each 16 panel sub array had 4s2p facing SE and 4s2p facing SW, a 90 degree angle, area presented to sun would be sqrt(2) as much.
11840 / sqrt(2) = 8372W (STC)
Typically PTC rating of panels is about 85% to 90% of STC
8372W (STC) x 0.85 = 7116W (PTC)
That's when tilt of panels matches sun; off-season, output is reduced.

Seems to me that having a setup where your 32 panels aren't all facing same orientation, rather have 90 degree angle between them, would be just about fully paneled but not over paneled. No available power wasted, would keep inverter MPPT running at 100% load longer.

But it does carry some risk, depending on how the inverter's MPPT behaves.
If sun is positioned so the two sub arrays are both at 45 degree angle, max output of array, and there are clouds off to the side not blocking direct sun but reflecting light onto it, available current is a bit higher. It is because this extra light can increase output that we are told to design wire/fuses for 25% extra current beyond Isc. If MPPT never allows itself to draw more current than it can handle, no problem. But if it keeps its transistors on longer, pulling array voltage down to maximum power point and taking in more current than MPPT can handle, could cause damage.

Some manufacturers explicitly say extra available watts is OK, but don't exceed voltage spec. They also give a max Isc spec.
You should try to find manual with full specs, because data sheet doesn't list Isc.

https://www.growatt-america.com/upload/file/contents/2021/07/60ed37db74ad1.pdf

 

[Hedges]

So have I..... and that is inconsistent with the warnings of back feed. That was the chain of thought that made me willing to question my beliefs when you pointed it out.

I just did a test, as the sun was about to set.
A Sharp 165W panel, happened to be oriented toward the sun but shaded by a bush except for a glimmer of light directly on a couple panels.
Voc read 30V. I put my hand in the way of the little bit of direct light, Voc read 28.5V
This was one of panels I had yanked for underperforming.

Spec is 43.1 Voc, 34.6 Vmp

https://www.apexpowerconcepts.com/sol_dow_165WSS.pdf

On a different day, PV string was 29.4 Vmp (per panel, I read string voltage at inverter)

Looks to me like measured Voc in the shade is almost exactly Vmp in direct sun.

So that backs my claim no need for anti-backfeed diodes.

Anybody else what to post measurements?

 

[MrM1]

@Hedges thanks for that info.

I can't really do the 90° angles due to design constraints, so I am considering dropping 8 panels (two 4s strings), one string from each sub array giving me two 4s3p arrays of 370w panels.

That should be 370 x 12 = 4440w x 0.85 = 3774w on each PV Input, just over the 3500w per pv input per inverter / scc spec.

Would that keep me in a safe enough range? We would then save the 8 panels until the pole barn is built and put them there and add a small 150vDC second SCC.

A second more detailed option is to take 1 4s string from each sub group and point one tilted E (4 panels) and one tilted west (4 panels). Keeping the 4s4p per sub array but having part of each sub array only taking morning and afternoon sun. But that might involve blocking diodes or introduce other issues and /or still get me up to hi in incoming watts and isc for the mppt scc.

So just kinda thinking it out

 

[Hedges]

All same orientation? Sounds relatively safe. But would be good to see more specs including any Isc limit, and/or ask manufacturer about going oversize on watts.

If you can tilt some, that's better. Mostly South? and then one string each East and West?
No need for diodes. you can parallel multiple identical strings of different orientation.

The only thing that significantly reduces output from what's available is multiple strings in parallel, some with large percentage shade.
I think 10% of a string shaded doesn't lose any but the shade panels. For you, one panel 25% of string might. 2 panels in one string would cause loss of the production of all four. One panel in each of 2 strings, with 3 strings in parallel, might lose production of 8 panels, produce like 4s. Or just 3 panels lost, produce like 3s3p. It depends on the power/voltage curve and the MPPT algorithm.

 

[MrM1]

Got a little more anecdotal info about the Growatt 12kw 250 and the idea of over paneling from Watts247.

"You can overpanel, however this can be a waste when an sc48120 (dual mppt charge controller - same as what's in this unit is just a few hundred bucks) port your extra panels into a new charge controller."

So this is saying that the Growatt's internal Mppt SCC is the same unit as the sc48120 mpv? Still not much info out there on it.

 

[Hedges]

If you get an OK on overpaneling from an authoritative source, then less concern about damaging it. This is from a vendor?
Try to get the installation manual.

Considering what an insolation website calculates for your location, you can know exactly how much available power would be wasted by over-paneling. These days, PV panels are fairly cheap, so it is a reasonable way to squeeze more power out of an SCC. Over-paneling can mean clipping excess production in the summer, having more production in winter (or morning/evening) than a smaller array.

Of course you may be able to add an external charge controller. Then it is more like a component system, rather than hybrid or all-in-one. Does the growatt keep track of battery state-of-charge? Does it communicate with BMS to adjust its charging? In the event you add an extra charge controller, is control/monitoring integrated with it?

With a hybrid containing both inverter and charge controller, it should be able to maintain a target battery charge current (e.g. 100A), and allow more current from the charge controller when inverter consumes that current. With a component system, that usually isn't available. (Victron can do it, and AC coupled systems like SMA can.)

Have to consider maximum charge rate of battery. With two charge controllers, you will deliver more current at times.

SC48120 MPV- 120A 48V, 24V and 12V battery MPPT 250V PV 120A Charge Controller only – Watts247 Wholesale

watts247.com

 

[MrM1]

If you get an OK on overpaneling from an authoritative source, then less concern about damaging it. This is from a vendor?
Try to get the installation manual.

Considering what an insolation website calculates for your location, you can know exactly how much available power would be wasted by over-paneling. These days, PV panels are fairly cheap, so it is a reasonable way to squeeze more power out of an SCC. Over-paneling can mean clipping excess production in the summer, having more production in winter (or morning/evening) than a smaller array.

Of course you may be able to add an external charge controller. Then it is more like a component system, rather than hybrid or all-in-one. Does the growatt keep track of battery state-of-charge? Does it communicate with BMS to adjust its charging? In the event you add an extra charge controller, is control/monitoring integrated with it?

With a hybrid containing both inverter and charge controller, it should be able to maintain a target battery charge current (e.g. 100A), and allow more current from the charge controller when inverter consumes that current. With a component system, that usually isn't available. (Victron can do it, and AC coupled systems like SMA can.)

Have to consider maximum charge rate of battery. With two charge controllers, you will deliver more current at times.

SC48120 MPV- 120A 48V, 24V and 12V battery MPPT 250V PV 120A Charge Controller only – Watts247 Wholesale

watts247.com

Yes the info I got is now from 2 vendors in the US. Both have said over panel is no problem for the 48V SPF 12kW DVM MPV 250VDC. We purchased the inverter from Sig Solar. But how much stock can we put in these responses?

Richard from Signature Solar Wrote:
As long as you do not exceed the VOC/VDC you can have more than 7000W of power generated from your panels, however it will not be inverted so it will be wasted.

Ian from Watts 247 Wrote:
You can overpanel, however this can be a waste when an sc48120 (dual mppt charge controller - same as what's in this unit is just a few hundred bucks) port your extra panels into a new charge controller.

But there is little to no info about the inverter and built in SCC from the web sites and manufacture web sites. I did find the very latest version of the manual, v1.1 that notes some info about warning beeps for PV over voltage, PV over amperage, etc. So it seems if it went over amp from the PV input it would sound a warning rather than send out white smoke signals?

Honestly you folks know way more than I, and I am just trying to get the best practices info before we do the install. We have done nothing yet. We have ordered almost everything. But the 370w BiFi panels, while we did get 34 (a few extra), were actually only $150 each, so IMHO they are very inexpensive compared to the rest of the project and worth having extra, over paneling if we can, or adding a second SCC if we cannnot.

The battery bank will be made up of 32 EVE LF280K cells in a 16s/2p config with Heltec 200amp BMS units on each 16s string. There will be No communication with the BMS from the inverter or SCC so I would assume adding a 2nd SCC would not be a problem, either in charge current or communication with the components.

 

[Hedges]

32, 280 Ah cells would be 28 kWh, so if 0.5C allowed that would accept 14 kW of PV (or 14 kW from charge controllers, if over-paneled)
That's at nominal 25 degrees C, at significantly lower temperatures, allowable current would be lower.

"beep" for over-amperage? of PV? That would have to mean the SCC was willing to draw more current than it wanted.
Sure that wasn't for AC overload, which could be sustained for a brief time until component temperatures got too high?

I like multiple angles, spreading power production over the day to better match consumption.
If your pole barn has a different orientation, you could run PV wires back from it. Either to same hybrid, or the separate SCC.
With multiple angles on the existing (or both) SCC, they operate for more hours at lower power level. Heating of transistors goes as current squared, which would be power squared. Stress from heating should be reduced this way, extending life.

 

[MrM1]

Yes the main solar array will point south with a 25° tilt and the pole barn would have slopes pointing due east and due west with even more slope.

So that would put me at 4s3p x2 south and 4s1p east 4s1p west with all 370w panels

 

[MrM1]

"beep" for over-amperage? of PV? That would have to mean the SCC was willing to draw more current than it wanted.
Sure that wasn't for AC overload, which could be sustained for a brief time until component temperatures got too high?

I have no idea exactly what it means, but it is in the newest manual... Error code 51?

 

[Hedges]

"Solar Charger Over Current"
Maybe it can hurt itself. If it knows to display a message, you'd think it would tell itself not to do that.

 

[MrM1]

"Solar Charger Over Current"
Maybe it can hurt itself. If it knows to display a message, you'd think it would tell itself not to do that.

LoL.

Like as in hurt you mean "White Smoke" pain?