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Open circuit string voltage of 17 volts...is this normal?

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sorka

Well-Known Member
Feb 28, 2015
11,597
9,570
Merced, CA
A new system installed yesterday with the Tesla inverter. Tech could not get it working yesterday. Claimed it was software and they'd fix it on the back end.

Two strings. 5 panels on one and 7 panels on the other.
Strings measure 0.588 and 0.590 closed circuit. Removing the positive lead, of each, strings measure a little over 17 volts with mid morning sun. This doesn't seem even remotely right but what are the chances that both strings would be messed up in the same way?
 
These aren't micro inverters and there are no optimizers. Should a simple string of PVs have an open circuit voltage equivalent to their unloaded output?
I don’t know how smart the panels are standalone. But they might need to see load to fully fire up. If you already know that and asking what unloaded should be(safety mode) I don’t know. Probably can find online. But a very low voltage when unloaded might be perfectly normal.
 
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Since the inverter is in "wait for solar" state, then it seems likely that if its a software issue and not hardware, then the inverter is not sending the proper signal to activate the MCIs. But since it's in that "wait for solar" state, the inverter probably thinks it has done what its supposed to.
 
Tesla uses mid-circuit interrupters (MCIs) after every third panel or so that will open the circuit if the string is not hooked up to the inverter and the inverter has AC power.
and just to be minorly pedantic, they clearly don't fully open the circuit as clearly demonstrated here and that would make communication and troubleshooting slightly difficult. I believe with Solar Edge each optimizer puts out 1 V when it is waiting for a command to operate so you can count the number of modules in a string with just a multimeter. I don't know if the MCIs do something similar.
 
and just to be minorly pedantic, they clearly don't fully open the circuit as clearly demonstrated here and that would make communication and troubleshooting slightly difficult. I believe with Solar Edge each optimizer puts out 1 V when it is waiting for a command to operate so you can count the number of modules in a string with just a multimeter. I don't know if the MCIs do something similar.
Thanks probably better to say high impedance state, although the spec sheet for the Tesla MCI does call its passive state as "normally open." I would be interested to learn what the actual circuitry is in these devices and how the signalling work between the inverter and the MCI but haven't been able to find any information online.
 
That 1V per panel sounds familiar for Solar Edge.

I know with my Solar Edge with two strings wired in parallel. The Panels come on 2 at a time. So there is some sort of handshake going on. The panel/micro is not dumb and just sees a load or inverter. It is signaled to come on.

Another thing I recall when they installed it that it had to be capable of shutting down (below some voltage) in some amount of seconds. I forget the number. I think it was in single digits.
 
They swapped out the inverter but it appears one string is almost dead. String 1 should be a bout 7 amps, not 0.6. Total production should be 3.8KW right now with slight clipping. i.e. if the inverter had higher capacity, it would be just about 4KW right now at this time.

No shading on either string. All panels facing exactly the same direction.
Screenshot_20220518-124217_Chrome.jpg
 
Sounds like either bad panel or bad intermittent MCI. I had the intermittent MCI and took a while for. Roof crew to come out and replace 3 of the 4 MCI (they didn't bring enough) on the string. Luckily that seem to have fixed it