Using off-grid inverter to provide grid forming reference for grid-tie inverter

[scotchonrocks]

I have 3x Fronius grid-tie inverters of total 10kW capacity currently. I would like to use a grid-forming (off-grid) inverter to provide a reference signal to my existing grid tie inverters and run the system with batteries as a microgrid, independent of the grid via an auto-transfer switch. Has anyone tried this and care to share your experience.

[JSchnee21]

In a word, don't. Why not just buy the correct type of hybrid/islanding inverter?

The problem, assuming the grid-tied inverters will even accept the power output from the grid forming inverter (they probably won't), you'll need to have a place to instantaneously store ALL of the variable output power coming from the Fronius inverters. Otherwise the "backpressure" will just drive up the AC line voltage, and the microgrid will go out of spec and the Fronii will shut down. You'll go through this cycle hundreds of times per day. Despite potentially having loads, and a charging circuit to charge your batteries, it's unlikely the consumption profile will "instantaneous" match your variable production profile. During times where production exceeds consumption, there's no where for the extra power to go.

[jflorey2]

I have 3x Fronius grid-tie inverters of total 10kW capacity currently. I would like to use a grid-forming (off-grid) inverter to provide a reference signal to my existing grid tie inverters and run the system with batteries as a microgrid, independent of the grid via an auto-transfer switch. Has anyone tried this and care to share your experience.

You can't do it the way you think you can.

This is possible - specifically, it's called an AC coupled system. But you would need a hybrid inverter that can sink/source 10kW. That's roughly $4000 worth of inverters, $2000 worth of batteries and a lot of protection/control/sensing gear.

[solardreamer]

I have 3x Fronius grid-tie inverters of total 10kW capacity currently. I would like to use a grid-forming (off-grid) inverter to provide a reference signal to my existing grid tie inverters and run the system with batteries as a microgrid, independent of the grid via an auto-transfer switch. Has anyone tried this and care to share your experience.

I've also looked at this and the most common solution seems to be using AC coupled hybrid inverters (e.g. Magnasine and Outback Radian) with batteries. From what I have seen the solution works but has shortcomings:

* It's an expensive and complex add-on relative to the GTI system.
* Requires batteries that need to be properly maintained continuously and replaced periodically.
* Still needs a generator for practical usage even for a few days due to low solar days and battery SOC maintenance.

For my goal of emergency usage, I chose to go with a generator at this point since it's necessary in any case and much cheaper. I am hoping there will be a simpler solution someday that can leverage the GTI system to supplement and lighten the load on the generator when there is adequate solar available at a fraction of the cost of the GTI system.

[jflorey2]

For my goal of emergency usage, I chose to go with a generator at this point since it's necessary in any case and much cheaper. I am hoping there will be a simpler solution someday that can leverage the GTI system to supplement and lighten the load on the generator when there is adequate solar available at a fraction of the cost of the GTI system.

It looks like the Enphase IQ8+ microinverters will support such microgrid operations. (supposedly available soon)

[JSchnee21]

Yes, but you'll need to purchase their storage solution as well if your loads consist of more than a few light bulbs. Unless you live somewhere that doesn't have any clouds.

[jflorey2]

Yes, but you'll need to purchase their storage solution as well if your loads consist of more than a few light bulbs. Unless you live somewhere that doesn't have any clouds.

Or a generator. Per Jim Jenal (Run on Sun CEO) the IQ8 will do generator support as well.

[peakbagger]

Do I detect the sweet alluring smell of vaporware?.

I believe a real solution is the Sunny Island inverter with storage AC coupled to the existing inverters since the owner already owns the inverters.

I do wonder if the new 1741SA inverters with the SA functions enabled are going to help with this effort? Unlike the prior 1741 inverters that were designed to drop off the grid when detecting any disturbance, the SAs are designed to keep running to try to support the grid. and even stabilize it. No doubt storage is still needed.