[RE-wrenches] Connecting Two Grid-Battery Backup Inverters to 120/208

[Philip]

We had exactly this scenario at Dankoff Solar, to back up all the computer loads in the business. The building was 120/208, and the two SW inverters would sync to their respective phases, 120 degrees apart.  The fun part was watching them on a scope when you shut off grid power -- they would drift to 180 degrees over a ten count.  Reconnect AC input, and they'd readjust back to 120 degrees and latch to grid.  This was with or without the stacking cable, BTW.  The SW didn't really stack together, as much as it stacked apart.  The stacking cable primarily sent a timing signal to tell the other inverter to be in opposition. There was also some communication on charging, etc, but it was pretty rudimentary as far as "coordination" went. 

The trick is that you can only feed 120v loads, and each phase needs its own neutral (or neutral MUST be sized for 2x inverter capacity).

Since the original system didn't and shouldn't have multi-phase loads, this can also work with the OutBack GT3048 inverters. However, you will need to unplug one from the Hub, program both as Masters, and should also disable charging on that single inverter. The inverter on the hub with the charge controllers will be the "master" of the system, the other will just help sell power to the grid in normal circumstances, and power loads on its output when the grid is down. 

Phil 


On Nov 28, 2012, at 10:06 AM, Christopher Warfel <cwarfel at entech-engineering.com> wrote:

> We were asked to replace two failed Trace SW4048 inverters with two Outback GT3048 inverters. Upon start up the slave inverter would not connect.  We did not realize that the building service is 208, 3 phase. The bypass is 120/240 which the SW4048s could connect to without a problem. Outback says their inverters will not connect to this system because of the phase angle of 120 and their software.
> 
> The phases are always going to be at this rotation, so I don't see a transformer helping solve the problem, but this is something I am really unfamiliar with.
> 
> So the input at the inverter panel is two 120 volt phase at 120 degrees. The output to the EP is 120/240.
> 
> I am asking if anyone has an idea of how to fix this problem? The goal is to have a grid interconnected system with battery backup.
> 
> Chris
> 
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