Tigo RSS not activating subset of circuits

[hazardandy]

New install. 54x 370W panels, rooftop. 30x panels on house, 24x on garage. Tigo TS4-A-2F per 2 panels. House is 3x strings of 10 panels (5x Tigos per string), Garage is 2x strings of 12 panels (6x Tigos per string). Growatt MIN10000TL-XH inverter for the house, MIN7600TL-XH for the garage. All 5 circuits run through a single metal conduit from the roof to the garage.

I connected all the negatives of the PV circuits to the inverters, left the positives floating, and measured each (I wanted to be 100% certain I didn't swap polarity anywhere along the chain). The Growatts have a Tigo RSS signal gen on the negative side, and when I see it blinking away with the estop pulled. All 3 house circuits measured in the 200V range, and when the positives are connected to the inverter, they go to 360V and the inverter makes power.

On the garage inverter I get something different:
-circuit 4 measures 3.6V regardless of RSS TX or not, regardless of both pos & neg connected to the inverter or just the neg. It appears the Tigos are not turning on
-circuit 5 measures 3.6V disconnected, and when RSS TX is enabled (through just neg or with pos and neg connected to the inverter), I see the meter jump to 100+V and then stay in the mid 70V.
-I removed C1-C3 from the house inverter, wired C4 & C5 into the house inverter to check if one of the RSS TX was faulty, exact same result
-each Tigo RSS is suppose to output 0.6V in the "off" state, so 3.6V leads me to believe at least one valid panel is plugged into each of the 6 Tigos per circuit, that they are contiguously connected, and that the polarity is correct. With polarized MC4s, it would've been nearly impossible to swap the polarity of anything anywhere. There's a slim chance one connector is not mated somewhere, but I'd love to narrow it down before ripping 24 panels off the roof to look for it.

Is there anything else I can do to debug? It's gonna really suck to have to go back up on the roof. Seems like the negative-most Tigo RSS on C4 is not closing, and maybe the 2nd most negative on C5 is also not closing? I'm happy to stick another box between the inverters and the PV to either run a single Growatt RSS TX or even buy the real-deal Tigo RSS TX, but my testing thus far tells me it won't change a thing.

 

[zanydroid]

(commenting to help boost your visibility in the forum)

Can you paste the datasheet of your panel? So we can try to guess what the voltages ought to be. And link the datasheet/instructions of the RSS.

Ouch, this sounds kind of painful to debug with no panel level monitoring.

I haven't installed DC panels yet, but I have gamed out similar scenarios theoretically.

It may be you are on the bottom edge of the documented string length, but you looked at those data sheets more than I have. (5 / 6 are pretty low for sunspec RSDs; I'm not sure if the dual RSDs have double the per-RSS voltage). And your 5-long strings are working while your 6-long strings are not working. And I doubt your Growatt cares about the dark start voltage, wrt starting up.

Are your panels set up for easy servicing where you can do one at a time? You might have to do binary search to isolate the problem (and this would likely require making an appropriate length set of jumpers to bypass intervening panels.

What is the off voltage of your 5-long string? 3V?

All 3 house circuits measured in the 200V range, and when the positives are connected to the inverter, they go to 360V and the inverter makes power.

This is very confusing to me. Open circuit voltage should be higher than operating voltage. What are the two reference points of your multimeter (they should be connected to DC+ and DC- since this is an ungrounded PV circuit)? What does the Growatt display or app show as the string voltage?

Seems like the negative-most Tigo RSS on C4 is not closing, and maybe the 2nd most negative on C5 is also not closing?

How are you concluding this? IE, it sounds like you are aware of a asymmetric behavior depending on which Tigo is missing. I think it depends on a model of how partial failure modes may cause the signal to be propagated partly through the string.

• Obviously Circuit 5 is in a better shape, from guesstimating your voltage. The 100V/70V is weird if disconnected, but if connected it's possible that is just from the MPPT's circuit behavior outside its voltage range.
• Circuit 4 behavior is as if something is nuking the RSS signal at the beginning. I guess that's what you mean by "negative most is not closing".
• The fact that you see 0.6V implies that the DC DC step-down in off-state is working, and there is continuity, and there is power coming from the panels.

For circuit 4, the one I would check / bypass is the first on the string. On circuit 5, I would check either the one corresponding to the number of panels needed to achieve that Voc you measure, or the one after that. The reason for this is that it's probably possible for (A) Tigo to turn on but eat the signal, or (B) stay off and eat the signal. For case (A) that would be the last one in the string corresponding to the Voc you measure. For case (B) that would be the next one (IE, the one with the last on panel is OK, and it's the one after that eating the signal).

Ideas to try

• Can you put a small load on it, like a 3.6V DC fan or LED, and guesstimate if it has the right power output as spec'd by the data sheet? This guards against a false voltage signal. Or, put the DMM in low impedance measurement to give it a "real"-er load.
• Buy or make some MC4 jumpers as appropriate to test the various scenarios.
• Is the RSS setup on the growatt capable of sending it down the + side? (I'm not sure if the way Tigo "forwards" the signal is compatible with this, I'm not the right kind of engineer to be able to make an educated guess. IE if it is intended to receive on - side and repeat on + side because there's no sane way to pass the PLC through the DC DC converter, then this won't work)
• Do you have extra spare panels or TIGOs available on the ground to bench test with/test your understanding/test operating the system in non-standard ways as may help with debugging? (you could consider ordering another TIGO or some 100W RV panels).
• Post on more solar forums, and hopefully someone with more expertise sees it.

Are there other faults throwing/being logged? Are you sure you know ground truth of transmitter state? (IE, how do you know that the transmitter is NOT turning off due to detecting a fault). I guess you are going by the blinking. By comparison a standalone TX would not be capable of this intelligence.

Have you found resources on detailed RSS theory of operation? My belief (which is pieced together rather than from one authoritative description + deep EE knowledge) is that when they are off, they behave like a 0.6V floating voltage source with unspecified current passing capability. And I would presume that the current passing is just enough for dark start. So if you measure 100V (or whatever) open circuit, that would be from the off-state behavior + the on-state Voc of the strings that turned on.

Another question is how PLC transmitter/receiver will behave in various failure modes / disconnect modes on the string.

 

[zanydroid]

• Obviously Circuit 5 is in a better shape, from guesstimating your voltage. The 100V/70V is weird if disconnected, but if connected it's possible that is just from the MPPT's circuit behavior outside its voltage range.

Oh wait, this has a very flexible MPPT. So 100V -> 70V would be from the MPPT actually activating. I'm somewhat curious how much it can pull through the busted TIGOs, but that doesn't really help you.

Outside context idea -- could there be interference from outside busting the signal?

Loading…

signaturesolar.com

 

[hazardandy]

I bit the bullet and just connected up C4 and C5 to the inverters as intended, measured voltage. Sometimes they didn't even show 3.6V, but after a few seconds, I did measure 450V (which is about right for 12 panels).

Notes for anyone who may stumble across this:
-it appears you do not need a complete circuit for the RSS TX to trigger some of the Tigos, but it's super unreliable (I did find the docs that said it needs to be a full loop and into an MPPT, and it worked when I did that)
-if you want to be sure you got the polarity right, I'd look for positive 0.6V per Tigo on the string output. Sometimes it would go away, and I'm not sure why, but I was able to measure {qty of Tigos}*0.6V on all of them at one point or another
-the DC NEG terminal of each of the Growatt MPPT inputs appears to be isolated from each other, so if you want to measure a given string, poke the meter probes either in the release hole of the spring-loaded terminal block or in the terminal block directly above (each input has two ports stacked vertically)