Solar installers giving me different info

[PVAndy]

Different manufacturers have different limitations to how unbalanced a dual MPPT inverter can be. I don't know off the top of my head for SMA. They do have an online string sizer, so not knowing all of your details, I entered a hypothetical system in NYC, with 12 250W panels S and 13 E (I couldn't remember your exact specs). I tried two 3000W TL-22, and one 5000TL-22, one 6000TL-12, and one 7000TL-22. It's projected annual output below:

two 3000W TL-22, 7145kwh
one 5000TL-22, 6936kwh
one 6000TL-22, 7283kwh
one 7000TL-22. 7223kwh

http://www.sunnydesignweb.com/sdweb/#/Home

Warning: It is not at all intuitive to use.

The -22 does not have a limitation on panel inbalance. You can hang all your panels on one MPPT if you choose and just leave the other one open. (Unlike the ABB's where you have to jumper) The -12 was just discontinues and I would not use it. It has no DC-DC conversion and has a very high start voltage as well as a narrow MPPT range. The -22 starts at 150VDC and tracks down to 125 VDC. If you enable Optitrac it will also find the peak MPPT even under partially shaded conditions. The -12 was a European inverter that was designed ot run with a max string voltage of 800VDC. I have 2 -22 6K on my house with 60 PhonoSolar PS250P-20/U. We have installed thousands of them with good results.

[rdo]

With the strings both getting sun, measure the voltage on the + lead of both strings (back at the array or anywhere before the wires enter the inverter.
If the two voltages are equal, then both strings are connected to the same MPPT input.
If you want to be completely sure, you can cover one or more panels in one string and confirm whether the voltages are still equal.
If this SMA uses ungrounded array input, you may need to measure from - to + on each string to compare.

Okay, is there any way of finding that out from Sunny Explorer?

[PVAndy]

Okay, is there any way of finding that out from Sunny Explorer?

No you can't find it out with SunnyExplorer since it only can measure what is at the inverter. Regarding measuring the voltages, there is no difference (other than a minor voltage drop) measuring the strings at the panels or where they connect into the DC Disconnect.

[subdriver97]

Did you look into SolarEdge? With a SolarEdge inverter system each panel will have it's own MPPT (Solar Edge calls it a Power Optimizer) and they all run to a Power Inverter (which is essentially a string inverter...)

advantages:
+ multiple MPPTs as stated above (solves you unbalanced problem and will offer optimized production over SMA String)
+ SolarEdge also offers individual panel monitoring for no additional cost

disadvantages:
- SMA, as a company, has a proven reliability track record (SolarEdge is relatively new to the market, however the technology is still based on reliable MPPT/string inverter technology. SolarEdge inverters have a 12 year warranty and they offer an extension to 20 or 25 years for additional cost.)

[macholini]

No you can't find it out with SunnyExplorer since it only can measure what is at the inverter. Regarding measuring the voltages, there is no difference (other than a minor voltage drop) measuring the strings at the panels or where they connect into the DC Disconnect.

Actually, I was wondering about this as well. SunnyExplorer can provide spot values for power, current, and voltage of each string, which can also be monitored and downloaded with software like SBFspot. Isn't this enough to figure out if the string are connected to the same MPPT or to separate ones? I thought that if they are connected to the same MPPT, then they must have identical voltages but varying current (and therefore power). Is that not the case?

E.g., here is my PVOutput for a SB7000TL-US-22 inverter with 2 strings of 13 panels each: http://pvoutput.org/intraday.jsp?id=...57&dt=20141223
The extended data chart shows all the spot values sampled every 5 mins, and while they are normally very close to each other, yesterday (12/23) happened to be a day when they varied quite a bit in the morning and afternoon, and had very different voltages. Isn't this possible only if they are connected to separate MPPTs on the inverter?

[inetdog]

Isn't this possible only if they are connected to separate MPPTs on the inverter?

Yes.

The -22 models have two separate MPPT inputs. Now if you happen to have a blocking diode on each of the strings (not needed or wanted, but....) then you could measure a lower voltage on a shaded string and it would not supply any current to the inverter at all.

The only reason (other than defect) for the panels strings to differ in Vmp is partial shading. And that is why you need two MPPT inputs if your strings are subject to different shading during the day.

[rdo]

Actually, I've checked SE today to find out that the voltage and currents in both strings are different (although not much) and it didn't look they're connected in any way.
Later in the afternoon the difference was even more obvious, as one string went into shade:
Unshaded string operated at 325V and around 2A, generating ca 800W.
Partially shaded string operated at 125V, around 0.2mA generating ca 100W (OptiTrac was certainly doing its work).

Each string is 6xE20.

[macholini]

Yes.

The -22 models have two separate MPPT inputs. Now if you happen to have a blocking diode on each of the strings (not needed or wanted, but....) then you could measure a lower voltage on a shaded string and it would not supply any current to the inverter at all.

The only reason (other than defect) for the panels strings to differ in Vmp is partial shading. And that is why you need two MPPT inputs if your strings are subject to different shading during the day.

No, I was asking if the difference in voltages can be used as a proof that the 2 strings are connected to separate MPPTs. In other words, can we detected if the two strings are properly connected using Sunny Explorer by looking at whether the the voltages of DC1 and DC2 are different? Or is it possible to see varying voltages even if both strings are connected to a single MPPT in parallel?

[PVAndy]

No, I was asking if the difference in voltages can be used as a proof that the 2 strings are connected to separate MPPTs. In other words, can we detected if the two strings are properly connected using Sunny Explorer by looking at whether the the voltages of DC1 and DC2 are different? Or is it possible to see varying voltages even if both strings are connected to a single MPPT in parallel?

Yes if you look at the MPPT input voltages there will probably be a difference. If bother strings were connected to one MPPT then the othere MPPT would show zero voltage and current. If both strings are in parallel (highly unlikely) you would not be able to see any voltage difference. Current in each string would differ.

Andy

[inetdog]

Yes if you look at the MPPT input voltages there will probably be a difference. If bother strings were connected to one MPPT then the othere MPPT would show zero voltage and current. If both strings are in parallel (highly unlikely) you would not be able to see any voltage difference. Current in each string would differ.

Andy

And connecting both strings together in parallel and then connecting to both MPPT inputs is almost certain to cause problems.

[macholini]

Yes if you look at the MPPT input voltages there will probably be a difference. If bother strings were connected to one MPPT then the othere MPPT would show zero voltage and current. If both strings are in parallel (highly unlikely) you would not be able to see any voltage difference. Current in each string would differ.

Andy

Well, I don't know for sure the MPPT input voltages, I haven't measured anything manually. Sunny Explorer shows 2 sets of spot values (called [A] and [B]), but I am not sure if [A] and [B] refers to the 2 MPPT inputs or to the 2 strings I have (which theoretically may be connected in parallel to a single tracker). But given that I have observed differences in voltage [A] and voltage [B] at the same time, I just wanted to make sure this means definitely that the two strings are connected to two separate MPPTs. Sounds like that's the case, which is great.

[rdo]

Well, I don't know for sure the MPPT input voltages, I haven't measured anything manually. Sunny Explorer shows 2 sets of spot values (called [A] and [B]), but I am not sure if [A] and [B] refers to the 2 MPPT inputs or to the 2 strings I have (which theoretically may be connected in parallel to a single tracker). But given that I have observed differences in voltage [A] and voltage [B] at the same time, I just wanted to make sure this means definitely that the two strings are connected to two separate MPPTs. Sounds like that's the case, which is great.

I came to similar conclusions, as the situation displayed in my Sunny Explorer looks exactly the same. I do remember when the electrician was commissioning the system, I noticed nil values displayed in one ''string' on the inverter's display. I pointed that out what made the electrician to double check. He confirmed there is voltage on both pairs of cables, but now I'm thinking he must have initially connected them to the same MPPT. He reconnected for me, so both 'strings' started show some figures now.
What I reckon may be confusing is the inverter refers to each MPPT as to 'string'.

[PVAndy]

I came to similar conclusions, as the situation displayed in my Sunny Explorer looks exactly the same. I do remember when the electrician was commissioning the system, I noticed nil values displayed in one ''string' on the inverter's display. I pointed that out what made the electrician to double check. He confirmed there is voltage on both pairs of cables, but now I'm thinking he must have initially connected them to the same MPPT. He reconnected for me, so both 'strings' started show some figures now.
What I reckon may be confusing is the inverter refers to each MPPT as to 'string'.

You don't even need to use SunnyExplorer to tell if the strings are connected to separate MPPT's You can read the string voltages off the display on the inverter.

Also from SunnyExplorer, if you have 2 voltage readings you have 2 separate MPPT's in use. There is no way that the inverter can tell if it has one string or 2 connected to an MPPT. You can tell because the current with be roughly double.

The -22 (and most inverters) are ok with 2 strings of equal length connected to one MPPT in parallel even if they have different orientation since the Vmp is primarily set by cell temperature and if the panels are producing different currents the currents sum. I routinely do this on systems I design. My personal system is running 3 strings of 10 PS250P-20/U 250 W per SunnyBoy 6000TL-US-22.

Andy

[macholini]

The -22 (and most inverters) are ok with 2 strings of equal length connected to one MPPT in parallel even if they have different orientation since the Vmp is primarily set by cell temperature and if the panels are producing different currents the currents sum. I routinely do this on systems I design. My personal system is running 3 strings of 10 PS250P-20/U 250 W per SunnyBoy 6000TL-US-22.

Doesn't this depend on the panel? E.g., the SB7000TL-US-22 has a 18A max current per MPPT (30A across both MPPTs) and my panels (LG300N1C-B3) have an Impp current of 9.4A so 2 strings connected in parallel could potentially exceed the single MPPT current limit. At least, that's what the Sunny Design tool said when I was playing with different possible design configurations.

[bcroe]

Doesn't this depend on the panel? E.g., the SB7000TL-US-22 has a 18A max current per MPPT (30A across both MPPTs) and my panels (LG300N1C-B3) have an Impp current of 9.4A so 2 strings connected in parallel could potentially exceed the single MPPT current limit. At least, that's what the Sunny Design tool said when I was playing with different possible design configurations.

i think your system is fine, this is just theoretical? Sure if the strings connected to an inverter
exceed the inverter spec, it will limit current to a safe value and excess available power will be
lost. Sometimes multiple strings may have different orientations and never reach their
theoretical Impp at the same time. This may allow parallel connection with no clipping loss. I
had this issue with some strings and had to rearrange them between 2 inverters to avoid
clipping on one during under utilization on the other. Bruce Roe