Ac coupling whit GTFX3048

Discussion about adding energy storage to grid-dependent inverters using OutBack Power technology
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yamill20
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Posts: 10
Joined: Wed Sep 05, 2018 7:44 pm

Ac coupling whit GTFX3048

Post by yamill20 » Thu Jun 18, 2020 9:26 pm

Hi have a question

I have two GTFX3048 2009edition
How to connect micro inverter for acin to sell and support inverter to charge batteries

Mike Curran
Forum Emperor
Posts: 1655
Joined: Sat Aug 28, 2004 6:28 pm
My RE system: Outback - Garage roof:
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied GVFX3524's classic stacked for 120/240VAC (2007)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS, OpticsRE. Tigo Energy ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 (2x7) Talesun 275W (DC array input to SB charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input (manual switch during grid outage only) from 14 Talesun 275W,
Enphase M215 microinverters, normally direct grid tied

All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: Ac coupling whit GTFX3048

Post by Mike Curran » Fri Jun 19, 2020 4:41 am

AC coupling connects your grid-tie microinverters to the output, or ACout, of your battery-based inverter (in your case those GTFX's), not the AC in.

Those GTFX's are not well suited for using them in an AC coupling application because they don't have any built-in way of limiting charge current to your battery when the grid is down and your loads aren't using all the PV power coming in from your microinverters. Without this mechanism you run the very real risk of overcharging your battery, and it'll happen quickly, too.

At a minimum you would need an FNDC to closely monitor your system battery voltage and use its built-in auxiliary relay contact to open a power relay between your microinverters and your GTFX's when battery voltage is sensed to be going high from overcharging. Outback used to offer a remote-operated circuit breaker ("ROCB") for this purpose but have discontinued it in their newer inverters (Radian, Skybox) in favor of a frequency shifting technique that is more reliable than an ROCB. This app note ( since removed from OB's website) describes how the ROCB works: https://documentcloud.adobe.com/link/re ... be8178511a

If that link doesn't work, try this:
app_note_acc_new.pdf
(1.93 MiB) Downloaded 20 times
http://www.tigoenergy.com/site.php?95b2dca2-ca6c
https://enlighten.enphaseenergy.com/public/systems/Hctc107221

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