AC Coupling Design - Would this work?

About 4-5 years ago, I picked up (5) SI-5048's at a bankruptcy auction.  They were $300/piece, which gave me grand plans of building out my backup's probably time that I either do it or sell them.

Along the way, I installed more solar.  Unfortunately, I wasn't able to find anyone in my area that still dealt with SMA so a solar edge system was installed; which left me with essentially two plants.  Here's what my current system looks like:

It's a massive system, but My life is 100% electric now.  Electric heat, hot water, well pump, cars.  While I'm grid tied, I haven't had to pay for electricity aside from a $2/month connection fee, a small fee to pay for essentially a large battery system.  Recently though, the outages have been more frequent ~1-2/month lasting 4-8 hours.  Some of those times it's a minor inconvenience, other times the loss of heat is no joke.  

So I want to expand my backup, especially since I already have the Sunny Islands.  If possible, I'd like to get my SolarEdge going in it.  That would keep me going indefinitely.  If not, I would need to end up sizing the battery bank up.

Here's my initial thoughts, I'd appreciate any feedback folks might have on it.

My idea is to isolate the two plans essentially to keep them from potentially interfering (or back feeding) into each other.  It also allows me to load shed should I anticipate a longer outage.  Shedding essentially the remaining Main Panel loads first followed by Priority Loads (ie, Hot Water, Fridge, Freezer, Electric Backup Heat, Stove, Car Charger) , and running on bare Critical Loads (ie, Heat Pump, Well Pump, Sump Pump, Internet, 2 lights)

The SMA system would power the critical loads, and continue to be a grid-feed configuration.  The SolarEdge system would operate the Sunny Islands in Off Grid mode.  It would be responsible for controlling the generator; turning it on only in a grid fault mode - where the ATS has kicked in - and the battery has drained to roughly 45%

Is all this feasible?  Or am I a mad man?  I'm open to suggestions to if this is just a whacky design.


  • rimcanyonrimcanyon Registered Users Posts: 5 ✭✭
    I’m a newbie but I am interested in your setup.  Can you explain how the SE system can be off-grid, but still be connected to the utility grid and capable of reacting to the ATS switching.
  • AmpsterAmpster Registered Users Posts: 172 ✭✭✭
    edited August 2020 #3
    rimcanyon said:
    I’m a newbie but I am interested in your setup.  Can you explain how the SE system can be off-grid, but still be connected to the utility grid and capable of reacting to the ATS switching.
    I can give you an overview of AC coupling and let the OP explain what he has learned from another forum about his system in particular.
    My System is a Solaredge GT inverter and an Outback Skybox. I originally had the Solaredge GT inverter installed by a contractor in late 2017. I knew I wanted battery back up as well and in late 2018 I installed the Skybox. I also moved most of my circuits from the main service panel to two panels inside the same wall in my garage. One of those panels was fed by a breaker on the service panel and the Skybox was fed by another breaker from the service panel. The Skybox has an internal transfer switch (ATS) which feeds the other panel in my garage which I call my essential loads panel. The Solaredge inverter was moved from my main panel to the essential loads panel. When the grid is up both panels see the grid and the Solaredge works normally. In the following example I will use the terms hybrid inverter to describe what the Skybox does  and GT inverter to describe how the Solaredge inverter reacts. 

    When the grid drops the hybid inverter  senses that the grid is down and disconnects from the non functioning grid connection and begins to power the essential loads panel. The GT inverter sees this AC frequency, tests it for several factors, and then begins powering up as if it were connected to the grid. This is known as AC coupling. The tricky part is that when the grid is up the GT inverter can send everything that the solar panels can deliver. However when the grid is down this power can only go to the hybrid inverter through the bidirectional connection from the essential loads panel to the hybrid inverter. The hybid uses that power to service the loads and to charge the batteries. If the GT inverter is producing more power than the hybrid inverter can use it needs to tell the GT inverter to modulate its output. The hybrid inverter does this by changing the frequency of the AC signal it is producing. The GT inverter sees that higher frequency and slows down the conversion of power from the panels. That is the magic of AC coupling and how a hybrid inverter can connect to and control a GT inverter. The GT inverter doesn't know or care about the position of the ATS. It is just looking for a signal that passes it's test for the grid.

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