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Clarification on Assistants - When to use ESS or PV inverter assistant when grid is usually not available?

I am installing a AC / DC Coupled system with Quattro, Fronius Primo & AGM batteries, I fully understand the rules and how to go about sizing and installation, that has been completed. However, I am a bit un-cleared on the right assistant to use for the setup.

The installation is not off-grid but the utility supply is very erratic, may not be available at times in weeks, I fully understand that for a off grid system with AC coupled inverter, the PV assistant is recommended but since the utility power may be available at times (even though there may be very long outage) can this system be setup with ESS assistant even though the utility power may not be available most time.

Thanks for the guidance as I am thinking of setup as ESS, Is there any demerit in this system being setup as ESS?

@mvader @Johannes Boonstra @Guy Stewart @Matthias Lange - DE

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Alexandra answered ·

We are in a similar situation here. We are using ESS. When the grid goes down the Fronius is still frequency controlled as when there is grid. The disadvantage of the pv assistant is when you are connected to the grid the Fronius is idle. You do need to be careful with the generator input AC2 to prevent the Fronius from accidentally feeding back into it. We set the Generator to run at a higher frequency so the system throttles the Fronius. There are a few other tricks out there other assistants that can be addes so you don't necessarily need to use this solution

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@Alexandra Thanks for your insight, just wanted to have experienced / expert users opinion if the Quattro will still be able to frequency control the Fronius, knowing that the grid frequency is fixed and will not affect the system. What does your setup looks like?

Your question is sort of answered in this thread.

No frequency shift happens when there is grid as it is synchronized to the grid. But we use the zero feed in option and some settings on the Fronius to prevent that.

The frequency shifting does happen when there is no grid as the quattro then has control over that. ESS does not work when there is no grid so all it's usefulness is disabled.

I am unable to share pictures as it is a client site and they have not given permission to share information.

But the site has been running for more than a year though, through many black and brown outs no issues no overcharging of batteries either.

It is a 3 phase system with a fronius symo and about 64kwh of battery. 2 x 150/70 for system recovery.