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Tesla switched inverter to Delta (no optimizers) from promised solaredge (with optimizers)

nwdiver

Well-Known Member
Feb 17, 2013
7,945
10,306
United States
Shading is problematic for all string inverters but based on various forum posts it looks like Delta has particularly poor/limited MPPT that can't handle even moderate shading.

That doesn't make sense from a functional perspective. I don't 100% understand how the MPPT 'sweeps the band' but I can see the results and I know what it's looking for. Current through a series string needs to remain the same and the current a solar cell passes is proportional to the sunlight it receives. So a shaded cell may only pass 1A while its neighbors in full sun are pushing 8A. The MPPT will 'see' that at (1A)(350v) it gets 350w OR (8A)(270v) it gets 2160w. It's gonna go with option 2.

The only area this gets complicated is with parallel strings. But Deltas have 3 or 4 MPPT trackers so having independent strings is possible. If they're paralleling strings then that's a problem with the installation not the inverter.
 

holeydonut

Supporting Member
Jun 27, 2020
1,495
873
East Bay NorCal
It's gonna go with option 2 ... which I believe you mean to be 350w OR (8A)(270v) it gets 2160w

If this is the case, then how do you explain why the OP's production decays so rapidly compared to his neighbor's production? It seems like the Delta has some logic to avoid "seeing" those "1A" cells as you identified.
 

nwdiver

Well-Known Member
Feb 17, 2013
7,945
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If this is the case, then how do you explain why the OP's production decays so rapidly compared to his neighbor's production? It seems like the Delta has some logic to avoid "seeing" those "1A" cells as you identified.

Either he is mis-reading the production data or they paralleled the strings on the roof. For Example; If you have parallel strings of 10 and 2 or 3 panels on one string are shaded then the remaining 7 - 8 on that string will have significantly reduced production. I have an older string inverter with 1 MPPT so my rooftop array is 3 strings of 14 in parallel. In the evening one string loses 3 panels to shade and my production drops by ~30%. With a newer 3 channel inverter I would only lose those 3 panels for a ~7% drop.

It's not physically possible to shut down an independent string by shading a few panels unless you drop below the minimum voltage for the input but that should require shading ~70% of the string.
 
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SaveOurPlanet

Member
Jan 6, 2018
237
140
San Francisco
Either he is mis-reading the production data or they paralleled the strings on the roof. For Example; If you have parallel strings of 10 and 2 or 3 panels on one string are shaded then the remaining 7 - 8 on that string will have significantly reduced production.

It's not physically possible to shut down an independent string by shading a few panels unless you drop below the minimum voltage for the input but that should require shading ~70% of the string.

Not meaning to hijack the thread, apologies in advance. I have 2 strings each wired in series, then the 2 series are wired in parallel before the junction box. Is there a reason for that, I just want to learn something.

Actually I have 4 strings on the roof, 2 strings in series with 5.32A down to the inverter, 2 other strings in series then parallel with 10.64A down to the inverter.
 
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nwdiver

Well-Known Member
Feb 17, 2013
7,945
10,306
United States
Not meaning to hijack the thread, apologies in advance. I have 2 strings each wired in series, then the 2 series are wired in parallel before the junction box. Is there a reason for that, I just want to learn something.

As @taphil mentioned. Laziness or ignorance. Frustrating. If you have an inverter with multiple MPPT channels and they're only using one that's just dumb.

~99% of the time it won't be a problem but the small expense of running them separately is 100% worth the cost for the 1% it is an issue. It's also a great diagnostic tool. You can compare the strings to ensure everything is fine. A failed panel can be masked when the strings are wired in parallel.
 

SaveOurPlanet

Member
Jan 6, 2018
237
140
San Francisco
As @taphil mentioned. Laziness or ignorance. Frustrating. If you have an inverter with multiple MPPT channels and they're only using one that's just dumb.

I edited my my post, string 1 and string 2 in series down to the inverter. String 3 in series, string 4 in series then parallel down to the inverter. On the inverter string 3 and 4 are jumped. My production is fine with shading and all, just curious.
 

cali8484

Member
Jul 8, 2018
186
107
California
That doesn't make sense from a functional perspective. I don't 100% understand how the MPPT 'sweeps the band' but I can see the results and I know what it's looking for. Current through a series string needs to remain the same and the current a solar cell passes is proportional to the sunlight it receives. So a shaded cell may only pass 1A while its neighbors in full sun are pushing 8A. The MPPT will 'see' that at (1A)(350v) it gets 350w OR (8A)(270v) it gets 2160w. It's gonna go with option 2.

The only area this gets complicated is with parallel strings. But Deltas have 3 or 4 MPPT trackers so having independent strings is possible. If they're paralleling strings then that's a problem with the installation not the inverter.

The MPPT search algorithm is critical. Some string inverter MPPT algorithms do poorly because they don't have the horsepower to continuously search across the entire MPPT voltage range in granular steps so they make assumptions about the search voltage range near some previous reference point that can get stuck with a local maxima on the I-V curve. Such situation can happen when shading happens too quickly for primitive inverter MPPT. Using your example, if the inverter has been operating at 350V for a long time then quick shading happens and the inverter only searches +/-50V around the reference voltage point (i.e. 300V-400V) then it will not find the MPPT voltage at 270V. Delta inverters appear to have a wide MPPT voltage range (50-480V) so I wouldn't be surprised if they have this problem. Of course, there could be other problems like parallel strings as you mentioned.
 

nwdiver

Well-Known Member
Feb 17, 2013
7,945
10,306
United States
The MPPT search algorithm is critical. Some string inverter MPPT algorithms do poorly because they don't have the horsepower to continuously search across the entire MPPT voltage range in granular steps so they make assumptions about the search voltage range near some previous reference point that can get stuck with a local maxima on the I-V curve. Such situation can happen when shading happens too quickly for primitive inverter MPPT. Using your example, if the inverter has been operating at 350V for a long time then quick shading happens and the inverter only searches +/-50V around the reference voltage point (i.e. 300V-400V) then it will not find the MPPT voltage at 270V. Delta inverters appear to have a wide MPPT voltage range (50-480V) so I wouldn't be surprised if they have this problem. Of course, there could be other problems like parallel strings as you mentioned.

For the example given above the nominal voltage would have been ~300v so it's far more likely to settle on 270 than 350. I've seen MPPTs settle on the wrong input voltage but that's always with parallel strings that can have dozens of MPPs that might trick the algorithm. A single series string is significantly more straight forward.

The OP needs to post the string data. If he can show that one panel is in shade and the string is operating at a higher voltage than it normally does then.... yeah... his MPPT isn't working correctly.
 

dareed1

Member
Jan 15, 2021
77
71
Belmont, CA
There's a couple of technical issues here, at least. The Delta M series inverters have 3 MPPT channels for the M6 and M8 inverters, but just 2 MPPT channels for the M4, M5, and (surprisingly) M10. The OP has 36 panels, so 3 strings on the roof, two of which have to be connected in parallel, probably on the roof. The two strings connected in parallel will likely not do well in MPP tracking during shading.

Secondly, the OP posted a solar generation curve that appears to be clipped at 8.8 or 8.9 kW. The M10 max current per MPPT channel is 20A, so we expect clipping in good sun at 34V*12*20A = 8.16kW. The inverter likely clips at a little more than 20A, so it would appear that either 1) the string that isn't connected in parallel is not actually wired to the other inverter MPPT channel, or more likely 2) all three strings are connected in parallel and connected to just one MPPT channel. If 2) is the case, I suppose that the MPPT tracking could get easily confused.

If it were my house, I would get the M Tool app (password is the inverter date code seen on the side of the inverter; activation code is 6532), and query the inverter history to see the voltage and currents in full sun and with the roof partially shaded. I would also use the M Tool app to check the firmware level (I think 3.1.7 is reasonably current) and that the clock is set to PDT as seen on the graphs time axis. All of this has to be done in very close proximity to the inverter. I use phone screen shots to record the graphs.

I can agree with the OP that a SolarEdge system should have been installed in preference to a M10 inverter, or as an alternative, use two Delta inverters so that at least 3 MPPT channels are available. I imagine that showing Tesla repeatedly that system isn't performing correctly will be superior to hectoring them with emails. BTW, I had the best results by writing an email with graphs and pictures, and following that up with a phone call, waiting until I spoke to a live person. The first person you can talk to is not likely to have either the knowledge or agency to do anything, but I found that pleasant persistance would get me to a technical person.
 
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