AC & DC Coupling Simultaniously

[Positive Power]

Just a quick question that I'm sure I already know the answer to but would love a little confirmation.
One of my recent installations is a DC Coupled (FM100 AFCI) Dual Radian Grid Connected (Net Metering), system with 750Ah of SimpliPhi batteries. The System is working great and the client is happy. So happy in fact she wants to add more solar. Instead of using another FM100, I'd like to AC Couple the new solar array (using Enphase IQ7+) to the Radian's critical load panel. I have installed multiple AC Coupled systems, but just not one on my existing DC Coupled Systems. I see no reason why both cannot work harmoniously, assuming I stay below the 75% of the 16kW output of the dual radians (no more than 12.8kW of Grid Tied input) and the batteries can comfortably handle the added charge current (which they easily can). Does anyone else see any issues with this combining the two different strategies?

[Mike Curran]

The Radian has AC-coupling capability. There's no reason this won't work. My system does this with Skybox but the Radian is similarly capable. I'm sure there are others on the forum who are already doing this with their Radian(s).

[Positive Power]

That’s my thought too. Indeed I have seen others do it as well. Thanks for your feedback.

[oughtwin]

That’s my thought too. Indeed I have seen others do it as well. Thanks for your snake gamefeedback.

What are the best practices for monitoring and controlling an AC-coupled system?

[Mike Curran]

Make sure your battery based inverter responds properly when there's a grid outage and total PV input to your system exceeds your load requirements. In this situation your battery- based inverter should dither its AC output frequency to force the grid-tied inverter to disconnect. If this doesn't happen your battery could become overcharged.

[clarkef]

What are the best practices for monitoring and controlling an AC-coupled system?

As others said the main thing is to have the Radiian system larger than the grid tied it’s going to manage when grid forming. That way the Radian can freq shift the output as needed to modulate the grid-tie inverters. If the gridtie system is larger than the Radians, the radians lose the ability to form a grid and control the output of the grid tie by shifting the power frequency.

If you have a backup generator, when the generator is energized, the grid tie inverters must be disconnected from the backup panel. You cannot safely interconnect a generator and grid-tie inverters. Outback used to sell a relay for this but it was just a DPDT 240v AC relay powered by the generator AC in. Wiring is covered in the outback tech note for AC coupling.

[lyly19]

How will frequency shifting be coordinated when batteries approach full SOC will the Radian curtail the Enphase system reliably without interfering with DC PV production?
Retro Bowl College

[clarkef]

How will frequency shifting be coordinated when batteries approach full SOC will the Radian curtail the Enphase system reliably without interfering with DC PV production?

That’s exactly what it does. As batteries approach the float value configured in the radian charging section, the radian shifts the output frequency away from 60hz to slow the production of the grid tied inverters. The closer to full charge the higher the frequency shift until the frequency shift enough to shutdown the grid tied inverter(s).

[Mike Curran]

FWIW, the max GDI system output when ac-coupled to a Radian is 7.6kw, per this document:
https://outbackpower.com/downloads/docu ... radian.pdf

[clarkef]

FWIW, the max GDI system output when ac-coupled to a Radian is 7.6kw, per this document:

That is right. The Radian needs be able to muscle around the grid tie inverters to control their output. If the GTI are larger than the radian then they end up controlling the output frequency and the radian can’t shutoff or reduce the GTI output when loads are less than what’s being generated or used for battery charging.