Question about expanding grid tie system

Hi, I've done some searching and am struggling to find relevant answers. I have an existing 6kW grid tie system using 22 LG panels and a 6kW Fronius Primo inverter. I recently acquired a whole additional 3kW system (8 Astroenergy 405W panels, 1 3.6kW Solis inverter), that someone was selling at discount due to aborting their project. I want to add this new system to my house.

From what I've read of the Solis inverter, it's compliance with "Rule21" and UL1741SA gives a few different options for how the inverter controls the power it outputs such as "Volt-Watt", "Volt-Var". It seems to me that either of these options would allow the two inverters to be connected to the same 240V split phase service panel. Correct? I have not found similar documentation from the Fronius inverter; only that Fronius:

"inverters can be combined in parallel by using the Fronius Solar Net input and output connections. This establishes a communications ring for the paralleled inverters which allows them to work together for increased total output capacity. This means that you can add additional Primo inverters at a later date."

Can anyone offer advice?

Check with your AHJ and utility.

Two grid tie inverters of the same or different brand should have no problem working together when connected to a common main panel and there is no need for communication between them. The reason is because grid tie inverters have the built in function which enables them to monitor the grid voltage and frequency then adjust accordingly their own output voltage and phase angle as necessary.
I have an 11 panel, 3kW Enphase M250 system and a 12 panel, 2.8kW on a Fronius IG Plus. They both feed into the main panel on different breakers. Been working well together for over 4 years and the output is as expected. Could the output be increased if instead it were 2 Fronius inverters working together? Maybe but not by much.
Off grid inverters are different, generally if you wish to operate them in parallel (stacking) they need a communication cable to keep the frequency and voltage synchronized.

Thank you @BentleyJ, that was what I expecting/hoping. Encouraging to hear from someone with experience. I'd like to understand how the Anti-Islanding function works better in case of a grid failure and two inverters suppling the same panel. I guess in the scenario when the grid is not there to accept the output power, the voltage would quickly rise above tolerance and one inverters would shut down and the other would soon follow? Hopefully this happens in milliseconds? I guess this is exactly the scenario off-grid systems are trying to avoid?

gdowie said:

I'd like to understand how the Anti-Islanding function works better in case of a grid failure and two inverters suppling the same panel.

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That's actually a good question, I'm not sure how grid-tie inverters "sample" the presence of grid AC or lack thereof vs. power from another source like the second grid tie inverter. Perhaps one of the electronics expert on this forum could answer?
After adding the second grid tie solar system I can verify both grid tie inverters do indeed shut down if the main breaker is turned off or grid power is lost.
More recently I have also added a Schneider XW+ 6848 in a whole house back-up configuration. It is AC coupled to both grid tie systems. When grid power is lost the 6848 begins inverting (8ms is the published specification). Its interesting that the Fronius will shut down then restart but the Enphase system almost always stays on and can ride through the brief power interruption.

That is interesting about the Enphase system staying up. From what I've read, there's a push to make newer grid-tie solar inverters add robustness to the grid in cases of significant distributed generation (lots of residential solar) by being able to ride-through power fluctuations so that there is not a cascading event of a small power ripple shutting down all the grid tied inverters and therefore amplifying the fluctuation. I guess this is the objective of UL 1741 SA. My Fronius was installed in 2015, but it sounds like all these changes for grid stabilization came circa 2017. Still searching to find if my Fronius Primo 6.0-1 will perform these grid support functions.

I found this presentation informative:

BentleyJ said:

I have an 11 panel, 3kW Enphase M250 system and a 12 panel, 2.8kW on a Fronius IG Plus. They both feed into the main panel on different breakers.

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How is your export to the grid metered by your utility? We have 2 meters, one is for the service entrance (billing) and one is for the solar system only. We get charged for the non-negative kWh's on the billing meter and paid for the metered "usage" on the solar meter. I think I want both my solar systems to supply that single solar meter but then they would enter the main panel on a single breaker. It does seem like independent devices (inverters) should have independent breakers. Maybe a subpanel with two breakers to combine the two solar inverters before supplying the main panel through the single solar meter?

gdowie said:

It seems to me that either of these options would allow the two inverters to be connected to the same 240V split phase service panel. Correct?

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They are both grid tie inverters and can be paralleled without any special communications. @BentleyJ explained that earlier. My Enphase micros coexist with the output of my Skybox with no issues. The issues that are important are the conditions of your existing NEM agreement any restrictions in that agreement that may limit any increase in capactity.

gdowie said:

Still searching to find if my Fronius Primo 6.0-1 will perform these grid support functions.

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The question turns on whether Fronius did a firmware upgrade that made it compliant with UL1741SA.

Ampster said:

They are both grid tie inverters and can be paralleled without any special communications. @BentleyJ explained that earlier. My Enphase micros coexist with the output of my Skybox with no issues. The issues that are important are the conditions of your existing NEM agreement any restrictions in that agreement that may limit any increase in capactity.

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Thanks. I guess NEM stands for Net Energy Metering? My original agreement states I'm allowed up to 15kW system. I have spoken to my utility, and they tell me they will honor my original tariff under that agreement from 2015, and still allow me to produce up to 15kW.

gdowie said:

How is your export to the grid metered by your utility?

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Like Bentley I have a single meter. I do have a limit in my NEM agreement about only being able to add one kW so when I added the Skybox it had the capability to limit export and I set that to the limit in my NEM agreement. The micros flow through the Skybox but when the grid is the Skybox curtails its own output to keep export within the setting. . As far as I can tell this has not happened becauuse much of my power is self condumed but the benefit is that I am getting a lot more kWhs of genereration than I would have not had otherwise. It sounds like you have a lot more flexibility to add production,

gdowie said:

Thanks. I guess NEM stands for Net Energy Metering? My original agreement states I'm allowed up to 15kW system. I have spoken to my utility, and they tell me they will honor my original tariff under that agreement from 2015, and still allow me to produce up to 15kW.

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We only have one digital meter. The outdated term "bi-directional" meter is still being used but most of the analog meters with the spinning disk have been replaced with digital, 2 channel meters. Channel 001 totals up the incoming power and channel 071 totals up the outgoing power.

As far as adding additional solar and the battery back up to our original permitted system. I didn't ask, I just did it. So far SCE or the building dept. has not complained.

I always wondered if the poco ever "catch" folks with more solar than they are supposed to have. I know in VA where I live, residential systems supposedly max out of 20kW. I started out with 20kW and was inspected for 20kW when my net metering agreement was established back in 2016. I have since grown my system to 60kW and do at times push 50kW+ into the grid.

As long as you have a big enough transformer on the pole (I got a 50 KVA one) and 400A service, AND you don't end up with an excessive bank balance of kWh on your account, they will probably leave you alone. At least that's what I keep telling myself, lol.