Advice for replacing old string inverter

[RKCRLR]

I have a newer 6kW Enphase/Panasonic system and an older (2009) 4kW string system. The system is installed on an uninhabited metal building. I'm planning on getting 2 Powerwalls. My understanding is the Powerwalls will work with both inverters but I'm concerned about the old string inverter. It went out a few years after it was installed and was replaced under warranty with a new unit of the same model (Yes! ES 4000P). It is now out of warranty. I haven't yet decided whether to just replace it when I get the Powerwalls installed or wait until it goes out. Either way, I'd like to be ready so I know what to do.
The old inverter is not a smart inverter, there no optimizers (and I don't have a shading problem), and the system is not rapid shutdown compliant. My county is under NEC 2014 for new construction.
1) Will a smart inverter work better with the Powerwalls than a older type inverter?
2) Do I have to comply with the NEC 2014 rapid shutdown requirements if I replace the inverter or am I "grandfathered" in?
3) I was looking at the Sunny Boy 3.8-US-41 inverter, it seems like it would be a good fit. Are there other/better options I should be considering for my situation?
Thanks

[Ampster]

I can't respond to the code issues. Did your warranty inverter replacement require an electrical permit?
As to the benefits of a smart inverter, I do think it will AC couple better with your Powerwalls. The reason lies withing the requirements of CA 21 which I assume the Sunny Boy will comply. My understanding is that new protocol adds modulation to an inverters capability based on line frequency of the controlling inverter when off grid. In this example the Powerwall is the controlling inverter when off grid. It is not an issue when the grid is working.

On the TMC forum, there is a thread where a poster was complaining that his Powerwalls shut down his GT system when the Powerwalls were full. Then when the Powerwalls were discharged to 90% the GT inverters were powered up and the cycle repeated, creating unnecessary wear and tear on his Powerwalls. If his installer had used the next version of GT inverters they would have modulated their output to follow the house load and would not have shut down. I may not have a complete understanding.There may be phases to the CA 21 rules. It may be worthwhile finding out if the Sunny Boy can modulate in an AC coupled environment or if it just shuts down and later turns back on when frequency comes back into spec.

[RKCRLR]

^^^^
I don't know if the replacement inverter required a permit, it was replaced by the installer.
Thanks for your input, I'll try to find out if the Sunny Boy can modulate.

Edit: I found this document online:
https://ressupply.com/documents/sma/...rt_Details.pdf
It states:
"Inverters are also able to modulate their output power to support the utility grid interactively. Inverters react to changes in the utility grid by varying their power factor for example or by achieving an improved grid stability using other grid management services." Sounds like they can modulate if the Powerwalls use the same commands the grid uses.

[Ampster]

^^^^
I don't know if the replacement inverter required a permit, it was replaced by the installer.
Thanks for your input, I'll try to find out if the Sunny Boy can modulate.

Edit: I found this document online:
https://ressupply.com/documents/sma/...rt_Details.pdf
It states:
"Inverters are also able to modulate their output power to support the utility grid interactively. Inverters react to changes in the utility grid by varying their power factor for example or by achieving an improved grid stability using other grid management services." Sounds like they can modulate if the Powerwalls use the same commands the grid uses.

There are no commands. The Powerwall shifts the frequency to signal the AC coupled inverter. There is an algorithm in the AC coupled inverter that responds to different frequencies. Some turn off and some ramp down. The term curtail is sometimes also used. This is only an issue when the grid is down because the grid controls the frequency when connected and there is no need to modulate or curtail the GT inverter because the grid can take all the inverter can deliver.

The devil is in the details. What one person calls modulation another might loosely say binary on/off is similar. It seemed to matter to that other poster. In either case you may want to investigate if your old inverter is even capable of AC coupling. If it is, once your Powerwalls are installed your can mimic a power outage and see if you are happy with the result. The key is to find out if your old inverter will actually AC couple. Tesla should be able to tell from the make and model number.

[RKCRLR]

^^^^
Tesla has all the information on my inverter. As far as I can tell, Tesla's position is the Powerwalls can AC couple with any grid tied inverter. They make the old inverter go offline by increasing the frequency beyond the inverter's tolerance (60.5Hz according to my inverter specs) to make it think the grid is down. The Powerwalls then go back to 60Hz when they are ready for a charge and the old inverter will restart, at least that is my understanding. I believe the Powerwalls will curtail the Enphase inverter outputs but I don't understand the details of how the Powerwalls interact with the Enphase inverters and my old inverter at the same time.
One of my concerns is my old inverter (probably on it's last legs already) won't like the "grid" constantly going down and coming back online when the Powerwalls are calling the shots. The old inverter probably wasn't designed to run like that.

[Ampster]

^^^^
Tesla has all the information on my inverter. As far as I can tell, Tesla's position is the Powerwalls can AC couple with any grid tied inverter. They make the old inverter go offline by increasing the frequency beyond the inverter's tolerance (60.5Hz according to my inverter specs) to make it think the grid is down. The Powerwalls then go back to 60Hz when they are ready for a charge and the old inverter will restart, at least that is my understanding. I believe the Powerwalls will curtail the Enphase inverter outputs but I don't understand the details of how the Powerwalls interact with the Enphase inverters and my old inverter at the same time.
One of my concerns is my old inverter (probably on it's last legs already) won't like the "grid" constantly going down and coming back online when the Powerwalls are calling the shots. The old inverter probably wasn't designed to run like that.

The interaction of the Powerwall when off grid is the same frequency changing process you describe. The only difference with the AC coupled inverters is how do they respond to a higher frequency. Will they modulate down until the next increase in frequency by the Powerwall or will they shut down until the frequency comes back up. All your Enphase Inverters and your old Inverter care about it that they think the grid is up because they see frequency and load. It is up to the Powerwall to manage their output because the powerwall is presenting the load that those inverters need to see so they are fooled that the grid is up. The load that the Powerwall presents to those AC coupled inverters is a combination of your household loads and the charging of the Powerwall. It manages its charging load with the varying household load to give those inverters what they need to see or else it will raise the frequency. That is the essence of AC coupling.
If your inverter was designed for AC coupling it was designed to do that. The poster on the other forum that I mentioned, did not have a problem with that per se, his issue was that he was needlessly cycling his powerwall as his solar turned off and then on again. He also was concerned that when the sun was shining and his inverter was not powering his house and that was a waste in his mind. I agreed with his concept. If it were me I would upgrade my old inverter. It is too late to do it as part of your SGIP installation but I would do it around the same time and include it in my totals for the investment tax credit. That is not tax advice, just some guy on the internet that likes tax credits.

BTW, I learned most of the above from the TMC forum. If you want more than my opinion, pose your question over there and you will get answers from a variety of people, many of whom I have learned AC coupling principles from.

[RKCRLR]

I've done a little more investigation. Sunny Boy states their US-41 inverters are CA Rule 21 compliant. Rule 21 requires frequency-watt ability for interconnection to the grid. So the US-41 inverters should be able to curtail output based on frequency shift. But I'm not 100% sure of my interpretation.