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

[bcroe]

You don't need to wire up N, just L1, L2 & Gnd. You should be able to find 8/2 for about 20% cheaper than 8/3. It used to be hard finding 8/2 local electrical wholesalers (they had 8/3), but now most of them carry it for the solar installers.

My string inverters connect to neutral, for the line voltage monitors. Your
results may vary. Much of my wire came from the internet, lots of choices.
Bruce Roe

[pclausen]

Ah good to know. I had N wires to all the Micro Inverter strings and just left the THHN #10 wires there. I'll go ahead and pull them out for a much cleaner look. Fortunately I already had stock on hand, so I didn't buy anything additional for this project, other than the 300 MCM and #10 PV wire for the exposed runs.

[nerdralph]

So with that finally out of the way, I completed wiring the #6 L1/L2 to each inverter, along with #10 Neutral and GND. I also did Ethernet homeruns to each inverter. Yes I know you can daisy chain them all together, but I always homerun each wired device back to the nearest switch.

You don't need to wire up N, just L1, L2 & Gnd. You should be able to find 8/2 for about 20% cheaper than 8/3. It used to be hard finding 8/2 local electrical wholesalers (they had 8/3), but now most of them carry it for the solar installers.

[pclausen]

Man it sure is hot and humid here in VA today under mostly sunny skies. Played around with the layout a little:



Still trying to figure out how I want to group and label the individual DC strings.

I do have some basic graphing in place. Here's one of the 15 panel strings:



Not sure what is causing the spikes, but maybe that's when the sun pops out from behind a cloud?

I wonder what the graph will look like on cold clear days?

[pclausen]

Turns out there is an integration already developed for SMA in Home Assistant. I added integrations to the 4 SBs I have online so far and made a very crude dashboard:



Since all the sensors are now there and data is updated every few seconds, I can do something like what I have for my Davis weather station, which looks like this:



Or I can use Grafana to do something like this:



Or setup a table like view similar to what I have for my mining rigs: (that was the catalyst for going solar in the first place)



But first I want to get the other 4 inverters online, then tinker with collecting stats and monitoring.

[pclausen]

Got the 60 panels on the pole barn re-wired and spread across a pair of 7.7s. As mentioned before, for now I'm doing 4 strings with 15 panels in each.

I'd like to build a nice dashboard that graphs AC and DC voltages, current and power. The Sunny Portal leaves a lot to be desired. Right now with just 4 of 8 inverters running, I keep 4 windows open like this:



Full rez link since the forum really scales down the image:

https://i.imgur.com/FNxseFP.jpg

But that takes up way too much screen real-estate, even on a 49" 4K monitor, and doesn't look pretty at all. I run Home Assistant, so maybe I can log all this stuff there and build a nice dashboard? Will have to look into that.

My SMA 7.7-40 is on firmware 2.4.98.R which is the latest for the 40 series. The new 41 series I got last week are on 3.2.15.R, which newer than the latest on the SMA website, which is 3.2.13.R. Go figure. 3.2.13.R supposedly added DC string stats to Sunny Portal, but I'm not seeing it anywhere.

[pclausen]

Yep all good now. Wish I had kept my old analog multimeter. I don't recall throwing it away, but I haven't seen it in years.

I had the sun peek out briefly between showers and maxed out the 6.0 inverter. Those old (circa 2016) 255 W Ubiquiti panels still got it. 24 x 255 = 6,120 W. I was not expecting them to be able to max out a 6.0, but was pleasantly surprised.



So with that finally out of the way, I completed wiring the #6 L1/L2 to each inverter, along with #10 Neutral and GND. I also did Ethernet homeruns to each inverter. Yes I know you can daisy chain them all together, but I always homerun each wired device back to the nearest switch.

[bcroe]

So system verified and working now? 1 Meg ohm is probably too high
for that equipment to measure with any accuracy. My own stuff with 60
panels running at 400VDC, plus a 250 foot feed, in conduit, measured
more in the area of 100 meg ohm. Either is high enough not to present
a hazard to man or machine.

Those expensive digital meters are fine for precision lab measurements,
not so good for finding simple field faults. Bruce Roe

[pclausen]

Just had to be a little more patient:


And now the insulation resistance is above 1 MOhm. I didn't change a thing. Go figure...



So I pretty much chased my tail for the last 2 days for nothing it would seem. At least I learned something and got a cool new tool for testing against ground faults!

Also, to answer an earlier question, yeah I had to make 3.5 foot extensions to go between the panels as those Ubiquiti SunMax panel have really short leads and they were custom made for use with their inverters.



And I have started my pile of inverters and connection cables:

[pclausen]

I made some tweaks to the parameters:



Status went to ok but I'm still not producing any power:



And my insulation resistance actually increased all by itself:



Maybe I need to power cycle it before it starts making power?

[pclausen]

Thanks for that advice! I made up a tool like you suggested and the stray readings went away completely. Here's what I get when connected between B+ and B-:



And between GND and any of the positive or negative DC terminals:



I went ahead and brought the inverter online, but it is still not producing:



Note how the insulation resistance is 1,311 kOhm on the working inverter and 433 kOhm on the one with the fault. Maybe I just need to lower the sensitivity on the 6.0 inverter?

It is currently set for 500 kOhm:



So I can lower it to like 300 kOhm and the issue should go away I assume. Need to go dig up my grid guard code since that parameter appears to be locked.

[bcroe]

I am going to assume, you have a fault to ground in either an
array or the wiring. Here is how I would check that. A super
high impedance voltmeter tends to be mislead by static charges,
and normal, very small, insulation leakage. The tool I use is 4
75W 120V bulbs in series, which can respond to a fraction of an
amp at up to array voltage. The impedance is low enough to put
some load on the panels, while instantly draining static charges
and ignoring normal tiny insulation leakage. It operates at system
voltage, in case that is needed to reveal faults. I cannot show a
picture since the new software blocks my uploads.

With the arrays disconnected from the feed wires and some sun,
first use the bulb test from ground, to either a +, then a - array
output lead. Both should give no response, otherwise that array
has a ground. The voltage on either side should give an indication
of how far down the string, the fault is. Test connection to the
array leads should light the bulbs.

Next connect the feed wires to the array, but not connected to the
inverter. Do the same test from ground, if one wire lights bulbs,
the other wire has the ground fault.

I had a momentary ground fault, that blew a GFI fuse in the inverter.
Replaced that, and all was well.

Looks like your panels are strung horizontally, like mine. Unless your
panel leads are longer than mine, that requires an extention jumper
between panels going across. I turned half my panels around, to
eliminate half the jumpers. Another way is wire them top to bottom,
then back around to the top again, before a jumper might be needed.
I could not do that, because there is a snow drop through gap which
should have no wiring crossing it. good luck, Bruce Roe

[pclausen]

I'm currently testing just a single panel. Here it is by itself getting the expected reading:



Positive to the frame looks good:



And negative to the frame looks good as well:



I then connect the B MPPT input wires to the panel and the ground wire runs from the frame back into the shop building:



At the inverter, the single panel voltage looks good:



And B+ to the ground wire looks good as well:



But B- to ground is showing the full voltage of the panel again:



I just don't get it...

[pclausen]

nerdralph Good to now. In the 2 years those 4 panels have been cracked, I have not noticed any degradation in performance (this is with micro inverters). I do still plan to move those 4 panels to their own string, just as a precaution.

I was wondering about grounding the ground arrays and the pole barn. Currently they all have ground rods in addition to being bonded at the inverters, which in turn are grounded via a pair of grounding rods for the shop building. The only place ground and neutral connect, is 200' away in the 400A disconnect panel. I have ground and neutral wires running between the house and shop building.

I'll go ahead and disconnect the copper bonding between the rails from the rebar, just to see if it makes a difference.

I did a bunch of additional measurements this morning:



Still seeing the ~380V between B- and GND with the strings attached.

[nerdralph]

The copper wire is also connected to rebar as seen here:

I can't say that's your problem, but I suspect the NEC says you don't ground the array separately from your bonding connector that goes back to ground at the inverter. You could get ground loops if there is a difference in the potential at the 2 grounding points.