Converting 50kW system from micro inverters to 8x SMA inverters

[nerdralph]

Ignoring shade concerns, are there any downsides to only using 2 inputs on a 3 MPPT capable inverter? Seems to me I can reach higher voltages that way by having more panels in series across 2 inputs vs. fever panels per string in series when using all 3 inputs, and thus reduce loss, not to mention simplified wiring as I need to run fewer wires back to the inverter location, which for some arrays, is almost 200' away.

Peak efficiency is usually between 350V and 400V DC input, so you'll get better efficiency too. Even with inverters that have a low startup voltage, I try to put at least 6 panels per string for that reason.

[pclausen]

You are spot on with the peak efficiency. On the SMA inverters, 375V seems to be the sweet spot:



Here's my revised design:



Ground Array #1 has 8 panels get gets shade in the AM during the winter months, so I put those on their own string.

Also, Hill Side Array #1 has 4 panels that gets shade in the AM. Furthermore, between the 2 Hill Side arrays, I have 4 panels with cracked glass. I'm going to consolidate all those into the 4 shady positions. Here you can see the 4 panels that gets AM shade:




When I installed the 2 hill side arrays, I lined up all the poles in a straight line for both arrays, not thinking that of course the array on the right, which was higher up than the one on the left, would cast a shadow on it in the AM. I certainly won't make that mistake again!

I also decided that 45 panels on the Pole Barn array was not a good match for 2x SMA 7.7s (already have 1 wired to 36 panels with the remaining 9 on micro-inverters, so I went ahead and extended the pole barn on the South side, to accommodate another 15 panels, for a total of 60 panels arranged in 4x15 strings. The bonus is that the pole barn will be better able to accommodate my equipment as I can back low implements all the way back against the new South wall.



Inverters should ship next week. I think I did pretty good on price:



The only part I dread about all this, it undoing the shop roof array and pulling all the inverters. It won't be fun, and I'm still trying to decide how to do it, given the way the SunMax fastener system works.

[pclausen]

After thinking long and hard about where the place the inverters, I decided a better option is to mount them inside the shop on the wall near where the meter base is on the outside. This will also allow me to place the solar subpanel right in between the inverters and keep everything symmetrical.



I'll need to move all this stuff that is currently up against that wall and also build out a frame that comes out past the metal bracing.

So something like this:



I'll be able to run all the wiring behind the super strut frame to keep things looking tidy.

[bcroe]

Are you going to put AC and DC disconnect switches outside? Bruce Roe

[pclausen]

There is a 400A AC disconnect located on the exterior of the main building here:



When in the off position, it kills everything to/from the shop building where all the solar arrays and inverters are located.

As for DC disconnect, I just have the ones build-in to each SMA inverter on the inputs (rotary switch). Since the shop building is not occupied, I don't believe I need actual DC disconnects or fuses between each string and the MPPT inputs on the inverters. Both of my current SMA 7.7 inverters just have each string wired directly to the MPPT inputs with nothing in between. My current plan is to continue that tradition when converting the other arrays over the string inverters, unless that's a bad idea.

[bcroe]

An outside AC disconnect can work, that is what I have used to shut
things down (an extremely rare event). I hate to open an operating
DC switch, as the 400V 21A current draws such an arc. It will still
leave the DC disconnect inside, hope no inverter smokes.

String inverters KISS, some of the things going here could not be
done any other way. Bruce Roe

[pclausen]

Yeah, the SMA manual stresses to always disconnect the AC power before disconnecting the DC disconnect, as, like you say, that could draw a serious arc.

In keeping with KISS, perhaps I should think about using these where each string exit the array:

[bcroe]

If multiple strings are connected in parallel to an inverter, they need individual
fuses. Some inverters have fuse positions to do that. Or bring them together
in a combiner box with fuses, then make a heavy wire run to the inverter. In
line string fuses work, but could be a service problem to locate.

The new software will not allow me to show a combiner box. Bruce Roe

[pclausen]

I won't have any strings in parallel. All will be in series. Here's a summary of how many strings and the number of panels in each:



That last string with only 4 panels are the ones that are shaded in the AM and the glass is cracked in the panels. Maybe I would be better off keeping micro inverters on those 4 panels vs. turning them into a small string?

But yeah, I suppose having in line string fuses at the arrays would be a service issue, especially for the roof mounted arrays. They do make nice combiner boxes such as these:



But I have not seen something similar for individual string fuses being combined into a single box.

[sdold]

How did you upload those images, did you use the "Upload attachments" or "Upload URL" buttons, or something else?

[bcroe]

Here strings with 720 cells (12 panels of 60) run MPPT at 360V
in summer. But in cold Feb, MPPT can hit 420V. That means
before inverter turn on, Voc can be 525V. Inverter specs 600V
absolute max, I see that as a point where catastrophic inverter
failure could happen. Some run one more panel in the string,
but I would like to have some margain to failure. Things like
power lines and lightning strikes (yes has happened in Jan)
might add some more voltage.

Here a string is panels connected in series, connecting another
400V string in series causes the voltages to add up way beyond
the practical limits. The combiner box here handles 2 inverters
with 6 strings each, has a fuse for each string. You would need
more than that. The combiner box is certainly a convenient
starting point for any troubleshooting.

Any cracked panels I would consider probable failure points
before long, and keep them apart from the rest of the system.
Bruce Roe

[pclausen]

How did you upload those images, did you use the "Upload attachments" or "Upload URL" buttons, or something else?

I did the following:



Click the picture icon along the bottom, paste in my url and then uncheck the "Retrieve remote file and reference locally".

[pclausen]

bcroe Good point about open voltage total. I added that to my summary:



So I come in a little below the 600V maximum input voltage spec of the SMA SB7.7 on my array with 15 panels per string.

That said, when using the SMA designer, it does give me the following error when I got from 14 to 15 panels in each string:



Not sure why since I'm below the 600V rating.

As an alternative, I suppose I could do 6 strings of 10 panels each and use all 3 inputs on each SB 7.7 inverter instead of just 2.

[pclausen]

I was able to wring some more details out of the SMA designer and it shows the following for Voc @ Tmin:



So 642 V. Where does that figure come from? When I look at the label of those panels, I see this:



A bit of Googling reveals that the Temp. coefficient of Voc = -0.36%/C at STC.

Above panel has a Voc of 38.5V, so 38.5 x -0.36%/C = -0.2386V/C. Given the NOCT of 45C, at 0C that would in increase of 10.737 volts. So Voc @0C would be 49.237V.

But 49.237V x 15 panels would be a whopping 738.555 Volts where the SMA calculator is coming up with 642V, which is 42.8 Volts per panel.

What am I missing? It sounds like I should limit myself to 14 panels per string, but I'd like to better understand why.

[oregon_phil]

I found a spec sheet that shows your panel's temperature coefficient Voc =-0.31%/C.

My panel's reference air temperature is +25C. My guess is that you chose your lowest air temperature to be -11C (12.2F). The difference from between +25C to -11C = -36C.

Voc change would be = -36C * (-0.31%/C) = 11.16%

11.16% * 38.5 Volts = 4.3Volts

Voc at -11C = 38.5 + 4.3 = 42.8 Volts

15 string voltage = 42.8 * 15 = 642 Volts at -11C

Feel free to check my math. I haven't had my coffee yet.

There are quite a few Voc max calculators on line. All of them rely on your estimated lowest temperature among other variables.

Added: SMA 7.7 MPP voltage range is 100 to 550 Vdc.