Over-paneling Victron MPPT controllers.

[FilterGuy]

I reached out to Signature Solar whom, while not the manufacturer, I would consider one of the 2 or 3 US based support contacts for the units.

The Growwatt North America website gives this email for support contacts: usa@ginverter.com

However, whenever I send a technical question to that email, it gets forwarded to china by the USA rep. He forwards it to is a list:

marco.liu@growatt.com
owen.liu@growatt.com
zhiwei.yu@growatt.com

I have seen replies from Owen.liu@growatt.com but not the others.

 

[MrM1]

So do you think the 7000ws on the 12kw unit is a hard limit for the total input or for each PV input (7000 PV1 and 7000 PV2)

 

[FilterGuy]

So do you think the 7000ws on the 12kw unit is a hard limit for the total input or for each PV input (7000 PV1 and 7000 PV2)

I think it is a hard limit per MPPT..... but I am not sure if there is one or two MPPTs. I sent an email to them asking about this.

 

[MrM1]

Thx. Based on what Sig Solar wrote me there are 2 MPPTs in the large low frequency inverters. And each can handle its own 7000w array. Will be interesting what answer you get back

 

[PVorBust]

If you dig into them, a lot of the MPPT controllers have limits like this. They are typically 30-50% more than the rated charging power.
It turns out that Victron has a limit to....but the limit is hidden in the Max Isc spec.

the best conclusion I can draw is that it's an economic limit, not an electrical limit.
Once you pass 150~200% panel capacity, you should increase SCC capacity to make the most of it.

 

[FilterGuy]

the best conclusion I can draw is that it's an economic limit, not an electrical limit.
Once you pass 150~200% panel capacity, you should increase SCC capacity to make the most of it.

There definitely is a practical limit to over-paneling, but the manufacturers insist there are electrical limits.

 

[FilterGuy]

Thx. Based on what Sig Solar wrote me there are 2 MPPTs in the large low frequency inverters. And each can handle its own 7000w array. Will be interesting what answer you get back

Well....... Growatt said there are 2 MPPT controllers.... but a limit of 3500W each for a total of 7000W. That implies they don't support any over-paneling beyond the 7000W. It is frustrating that the Growatt documentation is so bad, but the mixed messaging between the documentation, Growatt support, and the distributers is infuriating.

My guess is they can be over-paneled at least 30%, but my guess does not help much if you try it and smoke the unit.

 

[MrM1]

Well....My guess is they can be over-paneled at least 30%, but my guess does not help much if you try it and smoke the unit.

Thx. Yes that is disappointing.

What would cause the unit to smoke if I never exceed 250voc? I'm trying to wrap my head around it. I was planning a 4s4p of 370w panels (5920w) at a VOC of 189.8vdc and an Isc of 44.28a Per PV input on the Growatt 12kw. But as the watts of the array start rising in full sun, is there something unique/odd that happens to the VOC /Isc if the SCC is not able to use the power?

Looks like some SCCs smoke and some don't? Trying to understand why that would be.

 

[Hedges]

An SCC could draw maximum current from an overpaneled PV array and hurt itself. Or it could monitor current and never exceed what it can handle, even if Isc exceeds that.

Heat is created in SCC from IR drop through components like inductor, diode, transistor. MOSFET have very low on resistance, so most of their power dissipation comes while transitioning between on and off. The voltage during steady-state operation would swing between Vmp and zero, with Imp (or whatever lower current it is limited to by SCC wattage rating), dissipating energy J = 1/2 x V x I x falltime (during each transition.) Higher frequency switching dissipates more power, but allows smaller inductor (lighter, less expensive) less ripple in output.

We often compare energy loss in PV wiring (less for higher voltage strings) with loss due to efficiency of SCC (higher for high voltage strings). Also, high voltage strings saves money on copper wire. However, copper doesn't wear out (unless so hot the insulation gets damaged.) Shifting inefficiency to SCC in order to save money stresses SCC more. Over-paneling exacerbates that.

When overpaneling, could be worth aiming for a more efficient operating voltage of SCC so it generates less heat. At least, if you're in a hot climate.

If you overpanel by aiming half your array SE and half SW with a 90 degree angle between them, area presented toward sun is 0.7 times as much. That means you can over panel to 1.4x without exceeding electrical specs of SCC (most of the time; there is more area to capture extra light reflected off nearby clouds under certain conditions.)

With 90 degree angle, 5kW would be fully paneled for 3500W input. With an acute 60 degree angle, 7kW would be full paneled.
At 90 degrees each subarray has 45 degree angle to horizontal, isn't losing much from sun below horizon (but objects causing shading get in the way.)
At 60 degrees, more of panel's daily out put is lost to sun not above horizon.

 

[FilterGuy]

Looks like some SCCs smoke and some don't? Trying to understand why that would be.

I do not know why. In discussing this with Victron, they said there are times when the input is short-circuited....but would not elaborate on why or when this would happen. In addition, I have noticed the AIO units tend to be more restrictive than stand-alone chargers.

On the AIO units, it could be a heating problem. With the SCC, AC-DC charger and inverter all working at once, there could be a significant amount of heat generated. By limiting the PV wattage, it limits the time where the SCC is operating at it's max to a very short period.

 

[curiouscarbon]

time-offset overpaneling as @Hedges mentioned seems pretty clever to me.

if i lived in a cylindrical roof building like barracks then maybe i could overpanel by having a continuous curve and maximize overpaneling and minimize SCC wear and tear ?

then the strings would operate at the SCC max throughout the entire solar day but be overpaneled from the spec

 

[FilterGuy]

An SCC could draw maximum current from an overpaneled PV array and hurt itself. Or it could monitor current and never exceed what it can handle, even if Isc exceeds that.

Heat is created in SCC from IR drop through components like inductor, diode, transistor. MOSFET have very low on resistance, so most of their power dissipation comes while transitioning between on and off. The voltage during steady-state operation would swing between Vmp and zero, with Imp (or whatever lower current it is limited to by SCC wattage rating), dissipating energy J = 1/2 x V x I x falltime (during each transition.) Higher frequency switching dissipates more power, but allows smaller inductor (lighter, less expensive) less ripple in output.

We often compare energy loss in PV wiring (less for higher voltage strings) with loss due to efficiency of SCC (higher for high voltage strings). Also, high voltage strings saves money on copper wire. However, copper doesn't wear out (unless so hot the insulation gets damaged.) Shifting inefficiency to SCC in order to save money stresses SCC more. Over-paneling exacerbates that.

When overpaneling, could be worth aiming for a more efficient operating voltage of SCC so it generates less heat. At least, if you're in a hot climate.

If you overpanel by aiming half your array SE and half SW with a 90 degree angle between them, area presented toward sun is 0.7 times as much. That means you can over panel to 1.4x without exceeding electrical specs of SCC (most of the time; there is more area to capture extra light reflected off nearby clouds under certain conditions.)

With 90 degree angle, 5kW would be fully paneled for 3500W input. With an acute 60 degree angle, 7kW would be full paneled.
At 90 degrees each subarray has 45 degree angle to horizontal, isn't losing much from sun below horizon (but objects causing shading get in the way.)
At 60 degrees, more of panel's daily out put is lost to sun not above horizon.

If panels at different angles are put on the same controller, be sure to put blocking diodes between them.

 

[MrM1]

We are working on a re design now with different angles for the Growatt 12kw 250 with 2 mppt trackers
Pv1 input - 190v voc
- 4440w 4s3p facing south
- 1480w 4s1p facing east

Pv2 input - 190v voc
-4440 4s3p facing south
- 1480 4s1p facing west

For a total array of 4s8p on 2 4s4p sections each going into pv1 and pv2 respectively.

Still a bit hi for the 7000w total of the Growatt but

Hoping this might work?

And would we need a blocking diode for that? What are they? What do they do? (sorry for my ignorance)

If still too large we can either sell off some panels Bc we got a deal or get a second scc

 

[FilterGuy]

And would we need a blocking diode for that? What are they? What do they do? (sorry for my ignorance)

Will does a good job of describing it in this video

However, if the Growatt has two separate MPPT controllers, they are not needed.

 

[Hedges]

If panels at different angles are put on the same controller, be sure to put blocking diodes between them.

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.

 

[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?