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

[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]

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

[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.

[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.

[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]

I definitely agree that the SunMax inverters are not representative of micro-inverters in general. Like I said, there was a flaw in the manufacturing process that cause a lot of failures. So I have nothing against microinverters in general, but in my use case, with no shade to speak off for 98% of my panels, they don't offer an advantage over string inverters.

I do have a couple of strings that will be slightly impacted by shade in the am, and one of them only in the winter months. I know SMA has this thing they call ShadeFix, that supposedly compensates for a few panels seeing shade. So what does the MPPT actually do when a couple of panels in a string see shade?

peakbagger Great suggestions! I think I'll incorporate both. Really like the idea of a wireway. Much cleaner than the "spaghetti of conduit I already have.

So one question I have that might have been overlooked, is if I can use only 2 of the 3 MPPT inputs and still reach the full potential of the SMA SB inverters. Looking at the specs for the 3 larges models, I see this:



So it looks to me like I can easily reach the 7,700 watt rating of the 7.7 model by using inputs A and B only with like 15 panels in each string and prove about 4,000 watts per input (470 volts @ 8.63A) which is within the specs.

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.

[solarix]

I rest my case.

[peakbagger]

My comments for what they are worth on the proposed Inverter Installs. Good that you are on the north side of the building for cooling. I cannot see how deep the roof overhang is but by the looks of the dirt on the siding, it may not be deep enough to keep the inverters out of the rain. If I was going through the hassle to install inverters I would consider installing an overhang below the existing overhang that sticks out past the front of the inverters with some extra. Anytime I can keep electronics out of drip line I do so. The other thing is considering installing one may be two horizontal runs of electrical gutter (wireway) with a internal divider to keep the DC separate from the AC. Sure conduit will work but that is lot of "spaghetti", gutter will look a lot neater. If you just put a gutter under the bottom set you can run verticals to the top row or just run an upper and lower gutter.

Makes me glad I only have 4 microinverters and 3 string inverters in my collection.

[solardreamer]

Sounds like Ubiquiti was in over their heads and sunMax was/is hardly representative of microinverters in the market. There are much more reliable microinverters available.

[Mike90250]

On an occupied building ( HOUSE ) you may have to use a Rapid Shut down System to meet local code, using optimizers at each panel as a shutdown device, to make the array "fire department" safe.

[slinthicum]

Quite a story that to some degree makes me thankful my system does not have microinverters. Yes, the old-style string inverters fail (mine did before the completion of its first operational year), but it is a single device and not multiple devices providing the opportunity to have to deal as you've had with the "labor and return shipping costs until you reach the `10 year warranty expiration."

[pclausen]

The original 80 were purchased 10/2016 and deployed 12/2016 so about 5.5 years. I raised enough stink with them in 2017 that they sent me 56 replacements around 6/2017. I deployed 48 of them on the hill side arrays in fall 2017. Since then I loose about 10-20 per year, both originals and replacements. Its a known manufacturing process flaw that causes some capacitors to be installed too tightly during the wave soldering process which cause premature failure.

The other issue I have with these inverters is that during the middle of the day on sunny days, I get these over-frequency errors. I worked extensively with their developers in the spring of 2017 on this issue and they went so far as to ship me some very expensive data logging equipment that I connected to my mains and we setup VPN to allow them access to said equipment as well as my gateways to tail logs on the inverters. We went through several different firmware versions and while the issue got better, it was never completely resolved.

They stopped development and abandoned the whole SunMax line in 2019 and halting software development sometime in 2018. Unfortunately Ubiquiti has a bad habit of dropping product lines at the drop of a hat, and SunMax was no exception.

I'll definitely give the guys I worked with an A for effort in trying to sort out my issues and sending me 56 free inverters and continuing to honor warranty replacement for all the 130 or so inverters I have deployed. But I just don't want to put up with the labor and return shipping cost until I reach the 10 year warranty expiration. Hence my decision to cut my losses and move to string inverters. And I think I should be able to recoup most of that cost from selling all those 130 inverters and y-cables and such.

[oregon_phil]

Mounting system/hole pattern is slightly different between -40 and -41. -41 has integrated sunspec transmitter for communication with rapid shutdown devices via DC lines. Firmware is not interchangeable.

[solarix]

What is the age of the microinverters?

[pclausen]

At the time I was looking (summer/fall 2016), these seems to be very competitively priced ($203 for a 260 watt panel and $92 per inverter). I also really liked their roof mounting system and the whole concept of individual panel monitoring. But probably the most determining factor what that I was already a heavy user of Ubiquiti network gear and cameras and like the concept of a "unified" experience of all my gear and had visions of sunMax integrating into that stack for a true single pane view of all my stuff.

Had it not been for the extremally high failure rate of the microinverters, which are otherwise very stout with a cast aluminum enclosure and low power Bluetooth communication with Gateways connected via Ethernet directly to my local network.

Here's how many failed inverters I have in my system right now: (red - I see the inverter but power output is 1 watt or so. orange - no comms)



I get about 10-20 failed inverters every 12 months. So I decided to end it this year and go 100% string inverters and the 2 SMA 7.7s I have are rock solid.

I did have to register and get a grid guard code so that I could bump the allowable AC output to be 272 volts, in order to overcome the fact that my local utility really likes to delivery your power "hot". In my case that is at least 250 volts as observed in the morning and evening when I know the solar plant isn't pulling up the numbers.

Anyway, after upping the max AC voltage, they have been solid as a rock going on 2 years now. The first one was a 40 model and the second one is a 41. Does anyone know if there is any difference between them? My guess is that maybe the only thing is some additional mandated safety features on the 41?