String inverter - one array vs multiple arrays (pros vs cons)

[arf88]

It doesnt matter if it ever ever ever ever reaches that wattage. It was installed incorrectly with a string that has too much wattage ( potential, stc, solar, whatever) on it.

The installer responded that they would schedule to rewire it, but "both options 2 strings or one string are acceptable temperature effects on voltage and with fewer panels sometimes you have less voltage"

I think it was mentioned earlier in this thread about panel voltage and minimum number of panels.

[ButchDeal]

What will happen to the system if a string with too much wattage is installed, as in this case? Will it limit to the ratings of that string of the inverter, or will there be damage?

primarily voided warranty, and potential damage.

[ButchDeal]

Great question, wouldn't the fail safe inside the inverter prevent damage? I assume clipping here would not apply b/c it is post input from the PV system.

The fail safe in the inverter controls the optimizers but if the string is out of range then they may likely be unable to limit the power properly.

[ButchDeal]

The installer responded that they would schedule to rewire it, but "both options 2 strings or one string are acceptable temperature effects on voltage and with fewer panels sometimes you have less voltage"

This makes no since in an optimized system, this statement is talking about non-optimized strings.
The minimum number of optimizers is 8 per string. Earlier we thought that we were talking about a total of 16 which would bring it to minimal string lengths.

[jflorey2]

primarily voided warranty, and potential damage.

Doesn't the inverter communicate with the optimizers to set appropriate voltages? Does a long string prevent this communication from working properly? I thought the optimizers defaulted to low or zero output if communications is not seen (to comply with rapid shutdown.)

[arf88]

The fail safe in the inverter controls the optimizers but if the string is out of range then they may likely be unable to limit the power properly.


This makes no since in an optimized system, this statement is talking about non-optimized strings.
The minimum number of optimizers is 8 per string. Earlier we thought that we were talking about a total of 16 which would bring it to minimal string lengths.

hmm, one would think the optimizer would have the ability to throttle the output based on feedback from the inverter before it reached that max limit. guess not.

Agree, just passing the message, they agreed to rewire it which is good enough for me for now. But would still like to understand why the damage could occur if there is a constant two way handshake between the optimizer and inverter which is supposed to do limiting.

[ButchDeal]

hmm, one would think the optimizer would have the ability to throttle the output based on feedback from the inverter before it reached that max limit. guess not.

Agree, just passing the message, they agreed to rewire it which is good enough for me for now. But would still like to understand why the damage could occur if there is a constant two way handshake between the optimizer and inverter which is supposed to do limiting.

The optimizers absolutely can except when installed out of specifications. Just like the engine on my bike can move it easily and accelerate it nicely to speed, but if installed in a dump truck will not be able to move it. There are specifications for the installation of a product and equipment and when you exceed the amperage ratting, things go badly.

[ButchDeal]

Doesn't the inverter communicate with the optimizers to set appropriate voltages? Does a long string prevent this communication from working properly? I thought the optimizers defaulted to low or zero output if communications is not seen (to comply with rapid shutdown.)

The problem is that the optimizers are limiting devices like a fuse. If you put too many amps on them then you have a problem.
The problem is not the communications but that the rating of the optimizers is exceeded. Why would you expect it to work when installed improperly? would you think you could connect 1 million watts? 1 billion watts? there is a limit that the manufacturer specifies and it has been exceeded by more than a little.

[arf88]

So damage can occur, but how can it be measured?

[ButchDeal]

So damage can occur, but how can it be measured?

how can what be measured?

[jflorey2]

The optimizers absolutely can except when installed out of specifications. Just like the engine on my bike can move it easily and accelerate it nicely to speed, but if installed in a dump truck will not be able to move it.

Right. But if you install a small electric motor with a good inverter on the dump truck it won't burn out or anything - it will limit the current to the motor, and just not move the truck. It sounded like you were suggesting it could be damaged.

The problem is that the optimizers are limiting devices like a fuse. If you put too many amps on them then you have a problem.

Right - but from your descriptions in the past, they are fully functional and protected DC/DC converters that can create whatever voltage/current is needed by the system. And almost all DC/DC converters incorporate current limiting to prevent damage due to shorts and overloads. (Which I think is what you meant by "limiting device like a fuse." If they do that, then the inverter is protected.

The problem is not the communications but that the rating of the optimizers is exceeded. Why would you expect it to work when installed improperly? would you think you could connect 1 million watts? 1 billion watts? there is a limit that the manufacturer specifies and it has been exceeded by more than a little.

I wouldn't expect it to work and convert 1 billion watts. But if I connected 1 billion watts of solar to an SMA inverter (for example) it would just produce up to whatever it was rated to produce, and then limit. (As long as you keep DC voltages within its operating limits of course.)

In the case of the Solaredge inverter I could see it shutting down in that case, if the optimizers couldn't throttle back enough or something. But where would the damage come in? The optimizers have to be instructed to turn on and produce X volts. Why would the inverter instruct them to provide a damaging voltage or current?

[arf88]

Right. But if you install a small electric motor with a good inverter on the dump truck it won't burn out or anything - it will limit the current to the motor, and just not move the truck. It sounded like you were suggesting it could be damaged.

Right - but from your descriptions in the past, they are fully functional and protected DC/DC converters that can create whatever voltage/current is needed by the system. And almost all DC/DC converters incorporate current limiting to prevent damage due to shorts and overloads. (Which I think is what you meant by "limiting device like a fuse." If they do that, then the inverter is protected.

I wouldn't expect it to work and convert 1 billion watts. But if I connected 1 billion watts of solar to an SMA inverter (for example) it would just produce up to whatever it was rated to produce, and then limit. (As long as you keep DC voltages within its operating limits of course.)

In the case of the Solaredge inverter I could see it shutting down in that case, if the optimizers couldn't throttle back enough or something. But where would the damage come in? The optimizers have to be instructed to turn on and produce X volts. Why would the inverter instruct them to provide a damaging voltage or current?

Well said, in addition isn't the point of clipping to prevent damage as well. This in theory should happen before the over voltage and beyond peak wattage is reached, all assumptions on my part?

ButchDeal by measurement I mean the damage to the inverter. There is an error page in the portal but beyond that how can one measure the damage and degradation caused. My assumption is there is no way unless its instrumented at the PCB level but that is not realistic.

[ButchDeal]

Right. But if you install a small electric motor with a good inverter on the dump truck it won't burn out or anything - it will limit the current to the motor, and just not move the truck. It sounded like you were suggesting it could be damaged.

ok let me give a better example. When I was in NJ, I took my bike to a shop with a mechanic and a painter (co-owners) the painter had a very extreme bike that started out as the same model as mine. He first added a turbo and nitros to the the engine which nearly doubled the power. Took it on his first ride and ripped the clutch apart. The clutch was fine for the 80ft/lbs of torque it originally had but nearly double that and it was not sufficient. He then put on a competition clutch and on the first drive he ripped the final drive belt apart. The belt was fine for the original specs but when he added the added power and the clutch that could handle it, the next weekest part took the abuse.

Right - but from your descriptions in the past, they are fully functional and protected DC/DC converters that can create whatever voltage/current is needed by the system. And almost all DC/DC converters incorporate current limiting to prevent damage due to shorts and overloads. (Which I think is what you meant by "limiting device like a fuse." If they do that, then the inverter is protected.

The optimizers can convert voltage but if you cut voltage then amps increase. Nothing can be done about that, lower voltage equals more amps, higher voltage lower amps.
The optimizers are very efficient and no place to put the power other than conversion of voltage.
for permitting and the way the devices work is they are either ON and producing or off (shorted) and spit out 1 volt almost no amps.
They are considered limiting devices by code and thus you do not need additional fuses between the strings and inverter, but (just like fuses) ONLY when installed correctly and to manufacturers specifications. The manufacturer has limited the wattage that can be on a string!

I wouldn't expect it to work and convert 1 billion watts. But if I connected 1 billion watts of solar to an SMA inverter (for example) it would just produce up to whatever it was rated to produce, and then limit. (As long as you keep DC voltages within its operating limits of course.)

NO the SMA inverter would go up in a ball of fire, because the AMPERAGE is way out of specifications (~2.5MA). There are MANY MANY specifications on the input. Wattage is one, amps and volts is another. String inverters are going to limit based on volts and amps. Optimizers because they can handle more are limited on wattage which is a combination of volts and amps. This makes the optimizers more flexible but you still have the limitation.

In the case of the Solaredge inverter I could see it shutting down in that case, if the optimizers couldn't throttle back enough or something. But where would the damage come in? The optimizers have to be instructed to turn on and produce X volts. Why would the inverter instruct them to provide a damaging voltage or current?

Optimizers are not "micro" managed like this. They work more as a collective and group think. Each puts out what it can best put out, and others come down to keep in range. The net result is quickly getting to optimal levels for all but not centrally controlled by the inverter. In any case ALL systems have limits and specifications. When anything is installed outside of its limits things break.

I really am having a hard time believing that you are not trolling this discussion.

[Mike90250]

You measure the damage by how often you replace failed units. They will self-protect to some extent, My MPPT controllers do it by allowing the array voltage to rise and that shifts things into less efficient ranges, and they can revert to PWM mode. But they can be overwhelmed. I'm not going to push the limits, I'm staying where the devices are specified to work.

[jflorey2]

The optimizers can convert voltage but if you cut voltage then amps increase. Nothing can be done about that . . .

Of course something can be done about that. MPPT charge controllers do it every day. If the charge current declines, the voltage doesn't have to go through the roof. The DC/DC converter within the MPPT controller limits the voltage and current at the same time. They do this by reducing the DC/DC converter drive pulse width until the current drops, while voltage remains the same.

NO the SMA inverter would go up in a ball of fire, because the AMPERAGE is way out of specifications (~2.5MA).

No, it doesn't - I've tried it. So do many other inverters. It's called "clipping" when you see it during operation. It stops drawing more current from the array, even though the current is available. The voltage can only rise to the Voc of the panels, at which point you are simply throwing away the unused current. And since you have to design for Voc anyway, you are safe from overvoltage too.

In the case of the optimizers, if the optimizers are really 'working together' to set a fixed voltage at the input of the inverter, then they can limit the current to some maximum by doing the same thing - operating at a point that "throws away" the excess available power, just as MPPT controllers and overpaneled string inverters do. Now, maybe the Solaredge converters can't do this, and there is a failure mode where they can't back off any further than X. But that's not because it's impossible, it's because someone decided not to cover that possibility.

The optimizers are very efficient and no place to put the power other than conversion of voltage. for permitting and the way the devices work is they are either ON and producing or off (shorted) and spit out 1 volt almost no amps.

OK, now compare this to your statement "The optimizers can convert voltage but if you cut voltage then amps increase. Nothing can be done about that."