SMA SB7000TL-US-22 Inverter - Accessing individual MPPT performance information?

[sensij]

sensij,

From the voltage/power graph you provided it looks like you are able to monitor individual MPPTs too. What software application are you using?

Also, I see you are in San Diego too, just like me. What area are you in? I'm in Clairemont in the Mt. area on Mt. Laudo Dr. You didn't happen to have Rancho Solar do your installation did you? Maybe we can exchange horror stories

I have a SolarEdge system, which offers individual panel monitoring. It reports the input and output of each panel's optimizer, as well as what the inverter is doing. Navigating the SolarEdge portal to get at the useful information takes some work, unfortunately none of the panel stuff is available through their API.

I'm off of Mt. Acadia, near the Tecolote Canyon golf course... just a couple mi away from you. My installer is actually active on the forum, member ButchDeal.

With your application building abilities, sharing your data on PVOutput.org should be a breeze if you are interested. My system is in my signature, the data is accumulated as part of Team San Diego. We have quite a community of PV owners!

[sensij]

The SMA link I provided above was thin on detail, here is a better paper from Fronius:

http://www.fronius.com/cps/rde/xbcr/SID-AF30EAED-B3F2642E/fronius_international/SE_TEA_Efficient_East_West_orientated_PV_systems_with_MPP_tracker_EN_357684_snapshot.pdf

DaveDE2... you've posted your thoughts on this topic in a couple of threads, but seem unable to reconcile your understanding with the data generated by actual studies and experiments. You might want to model numerically the actual irradiance differences and effects on impedance and power output to see if you can explain why that gap exists.

[MinorDeity]

sensij,

From the voltage/power graph you provided it looks like you are able to monitor individual MPPTs too. What software application are you using?

Also, I see you are in San Diego too, just like me. What area are you in? I'm in Clairemont in the Mt. area on Mt. Laudo Dr. You didn't happen to have Rancho Solar do your installation did you? Maybe we can exchange horror stories

[DaveDE2]

...actually, I take what I said above about current flowing from one string into the other string back. The two strings don't act like batteries, but rather like current sources. No current should flow from one string to the other with different irradiances.

[DaveDE2]

My thinking is that the way the system is wired now neither of the two combined strings are operating at their MPP except at around high noon when they are equally illuminated. At mid morning and afternoon there may be a severe mismatch. For example if one string sees 600w/m^2 while the other is seeing 300w/m^2 then the first string will be putting out roughly twice the current as the second whilst both have about the same voltage. This means the MMP (or input impedance of the inverter, R=V/I) for each array will differ by a factor of two. Since the inverter can only operate at one input impedance, it won't be optimal for either array, but something in between. On top of that, since both strings want to put out different voltages, there will be current flow from one string into the other which is wasted power. It's like connecting two batteries of slightly different voltages in parallel. Granted, leaving the configuration as it is now will work one of the MMPTs less hard. Whether that has a significant impact on inverter life is as you say difficult to know. It depends on design specifics of the inverter.

[sensij]

The SMA data sheet for your inverter indicates each MPPT can be driven up to 18A. So it appears that a large imbalance between the MMPTs is ok. You'd expect that, particularly on systems with arrays pointing in different directions. If it were me, I'd put both southeast strings on the same MPPT.

Imbalance isn't the only factor... by putting both SE strings on the same MPPT, that tracker will see higher peak power than if the SE and SW strings are combined. All else being equal, I think the inverter will be happier with the somewhat lower peak sustained somewhat longer that the current configuration provides.

Ultimately, either configuration is "ok", and the discussion drifts towards what may be more efficient, and what will prolong the inverter's life. Reliable data on both of those are hard to come by, and not super easy to model given the dynamic nature of the system's operation.

[DaveDE2]

The SMA data sheet for your inverter indicates each MPPT can be driven up to 18A. So it appears that a large imbalance between the MMPTs is ok. You'd expect that, particularly on systems with arrays pointing in different directions. If it were me, I'd put both southeast strings on the same MPPT.

[sensij]

Some violence in the voltage in the morning and evening seems to be a normal side effect of MPPT... there isn't much power there, to the optimization routine has a harder time figure out where to operate. Here is an example from a single panel on my system, showing voltage and power on an otherwise clear day. I have some structural shade in the afternoon, but the morning is about as clear as it can be in a residential area without a perfectly clear view to the horizon. V on 2-15.JPG



The mpp voltage on your SE and SW arrays is probably not as different as you are imagining, and the mismatch won't amount to much in the way of losses... voltage is only weakly affected by the irradiance, and is much more strongly affected by temperature. Current, on the other hand, is much more sensitive to irradiance than to temperature.

Before SMA got into the multiple MPPT game, they aggressively defended the idea of multiple orientations in parallel on a single MPPT (as long as the strings were of equal length). One study showed a mismatch power loss of only 0.25%. In your case, that could easily be wiped out by the difference in efficiency from the MPPT loading.

http://www.smainverted.com/files/201...-TEN122510.pdf

With respect to your daughter's system, there isn't a problem, really. A lightly loaded inverter will have a different efficiency than one more heavily loaded, and from curves I've seen, once you drop below 20% loading the efficiency starts to fall off quickly. In other words, especially in the morning and evening when there isn't much sun, the inverter efficiency is probably a bigger deal than voltage mismatch in the panels. I'm not sure if there is any benefit to jumping the two MPPT's together in her case, but that is an option available in installations that require only a single mppt.

Efficiency test data on the SB6000TL-US-22 is available, and most other inverters you might be interested in can be found here:

http://www.gosolarcalifornia.org/equ...ests/summaries

[MinorDeity]

this is probably a better design than putting the SE strings in parallel and the SW string by itself, since that configuration would lead to less balanced power between the MPPT's.

My main concern was that the voltages might be significantly different on the SE and SW arrays when the sun first comes up and goes down. Although I assume a higher voltage from one array wouldn't cause reverse current flow in the lower voltage array due to protection diodes, couldn't it potentially exceed their reverse breakdown voltage? As you can see from my voltage plot for the 2-string MPPT, there is a significant downward spike near both sides, but there is also a similar spike on the AM side of the 1-string MPPT. I don't know the cause or significance of these, however.

Also, you seem to indicate that an MPPT power balance is good. My daughter has a 2-SB6000TL-US-22 inverter system with 3ea 13-panel strings. Thus, only one MPPT is being used on one of the inverters. Is there a problem with doing this?

[sensij]

Actually, the 10-panel inverter 2 single-string is facing approximately Southwest and the double 10-panel inverter 1 paralleled strings are facing approximately Southeast. It seems a little strange to me that the Southwest array peaks slightly earlier than the Southeast array, but the whole system has been in operation for over a year and has exceeded its estimated production. I suppose it's possible the installers may have erroneously paralleled the SW string with one of the SE strings. Do my graphs give you any indication that this might be the case and if so, would there be anything to gain by correcting the issue? I suppose an empirical way to find out would be to through a blanket over part of the SW array and see if both power curves dip.

If you have a total of 3 strings on this inverter, 10 panels each, the data are clearly showing that a SE and SW string are in parallel on MPPT 1, and a SE string is by itself on MPPT 2. With no shading (it doesn't look like you have any), this is probably a better design than putting the SE strings in parallel and the SW string by itself, since that configuration would lead to less balanced power between the MPPT's.

[MinorDeity]

Looks like you are running 3 strings total, 10 modules per string, 2 in parallel into "Inverter 1" and the other into "Inverter 2". The panels on inverter 2 appear to be facing a bit more east than the other two strings?

Actually, the 10-panel inverter 2 single-string is facing approximately Southwest and the double 10-panel inverter 1 paralleled strings are facing approximately Southeast. It seems a little strange to me that the Southwest array peaks slightly earlier than the Southeast array, but the whole system has been in operation for over a year and has exceeded its estimated production. I suppose it's possible the installers may have erroneously paralleled the SW string with one of the SE strings. Do my graphs give you any indication that this might be the case and if so, would there be anything to gain by correcting the issue? I suppose an empirical way to find out would be to through a blanket over part of the SW array and see if both power curves dip.

[J.P.M.]

The change in voltage as a function of irradiance is very small. I've penciled out some calculations in another thread using the voltage data from my SolarEdge system, and it appears to correlate to cell temperature well enough that I don't think the other factors are very significant. Clearly, at startup, shutdown, and during cloud/shade events the MPPT becomes dominant, but under clear sky operation, using voltage as a proxy for temperature has held up well enough to the limited testing I've put at it so far.

From the cell temps I've measured and voltages measured at my inverter display taken at approximately simultaneous and approximately "instantaneously" - 16 panel temps taken at 4 random locations/panel 2X - ~ 6 min. on either side of min. incid. angle time and under clear sky conditions, while also using displayed voltages at the inverter to est. array temp. seems to produce array temp. est. that are reasonably close to one another.

Simply put, the two methods I use seem to be good to an ESTIMATED +/- 1 to 2 degrees C.or so of a representative array average temp., and seem to head in the same direction, and agree with one another reasonably well.

As required my my attempts to find out how fast my array fouls and the nature of the fouling and how cleanin/rain affects the fouling rate, I did that side/side comparison about 35 times (Note to Sensij: still farting around w/ mean/std. dev./etc. on all the data - film at 11) on very clear days over the course of about 18 months or so, all data taken at the time of those days min. array incid. angle, and convinced myself that measuring voltages at the inverter was about as accurate as measuring temps. under the entire array - and yes - I can get under it. That was a design consideration. There's more to it than that but that's my story and I'm sticking to it.

[sensij]

At first I thought you could use voltage sag as a measure of average module temperature since for example CS6P-260P voltage output varies by -0.113V/C. On second thought though, that's not an accurate way of measuring module temp because the MPPT has an effect on the module voltage as irradiance varies throughout the day ie; the operating voltage ignoring temperature effects increases with irradiance due to the MPPT. Some of that voltage sag is due to DC line losses too. Perhaps MPPT effects and line losses could be calibrated out but sounds like a bit of a science project. Like you care about any of this. Ha!

The change in voltage as a function of irradiance is very small. I've penciled out some calculations in another thread using the voltage data from my SolarEdge system, and it appears to correlate to cell temperature well enough that I don't think the other factors are very significant. Clearly, at startup, shutdown, and during cloud/shade events the MPPT becomes dominant, but under clear sky operation, using voltage as a proxy for temperature has held up well enough to the limited testing I've put at it so far.

[DaveDE2]

Looks like you are running 3 strings total, 10 modules per string, 2 in parallel into "Inverter 1" and the other into "Inverter 2". The panels on inverter 2 appear to be facing a bit more east than the other two strings?

At first I thought you could use voltage sag as a measure of average module temperature since for example CS6P-260P voltage output varies by -0.113V/C. On second thought though, that's not an accurate way of measuring module temp because the MPPT has an effect on the module voltage as irradiance varies throughout the day ie; the operating voltage ignoring temperature effects increases with irradiance due to the MPPT. Some of that voltage sag is due to DC line losses too. Perhaps MPPT effects and line losses could be calibrated out but sounds like a bit of a science project. Like you care about any of this. Ha!

[MinorDeity]

Nice!
It looks like the big voltage sag is due to panel temperature. What kind of panels do you have? What software did you write the application with? Tnx.

I have a 7.8kW system containing 30ea Canadian Solar 260w panels. My custom monitoring setup consists of two separate applications written in C#. One runs 24/7 and merely collects data from the inverter every 5 minutes and stores it in daily history files, kind of like what Sunny Portal does even if you're not logged on. The second application can be run whenever you wish and it displays the graphs for whichever history file you select, like what Sunny Portal does when you log on. I've been thinking about something else of value to monitor but at this point it seems like the P/I/V from each MPPT kind of encapsulates it all.