Conext XWPro 6848 AC couple question

[mwk1000]

The Schneider ac couple doc says -
The XW Pro can be connected in various configurations from just a single unit to multiple units in parallel, or in a three-phase configuration. Regardless of the system configuration however, the maximum continuous allowable rating of AC-coupled PV inverter(s) is the lesser of:
125% of maximum allowable charge power of the battery (calculated as rated continuous charge current times rated charge voltage).
125% of continuous rating of XW Pro inverter(s).

I am not understanding what this limit is for ? AC1 grid connections would be well higher than this .... The unit will frequency shift to reduce ac coupled output when needed .... I must be missing something simple.

I was hoping to use 1 inverter that would provide adequate power overnight or in a grid failure by placing all the high draw non-essentials on a subpanel that the PV inverters feed and thru to the XWPro @ 50amp breaker ( or try a new lumin panel to be selective ) The idea was no sun = no "luxury" items. Sun charge the batts , run everything. I don't really see how/why I need 2 XW's to meet the above.

I have 560Ah 48V LiFePo4 planning to run at 60-70 DoD 1C max and 0.5c recommended in/out , my AC PV supply is 16.4K Best possible case ...

 

[mwk1000]

I forgot to add that the PV inverters have an external meter to set zero export and self limit their output ....

 

[400bird]

With any AC coupled PV you're limited on your ratio of solar inverter capacity to battery inverter capacity.

The issue comes in the transitions.
Say you're running a large load and sucking down 15kw
When that load instantly shuts off, that 15 kw needs to go somewhere, and when off grid, the only option is into the battery. Frequency shifting takes time, and in that 30+ seconds (might be minutes, I don't exactly know, I never graphed the frequency shift) you'll blow something up.
The speed of frequency shifting is set by the grid code you apply in the XW pro.

One option is to put a smaller portion (7kw) of PV in the back up panel with the rest grid side.

I forgot to add that the PV inverters have an external meter to set zero export and self limit their output ....

This is not something that any AC coupled inverter can control. When connected to the grid, the battery inverter can't frequency shift as the AC frequency is set by the grid.
It sounds like you might be saying this is something the PV inverters are doing with their own external current sensors, but I just want to be clear the battery inverter can't do it alone.

I was hoping to use 1 inverter that would provide adequate power overnight or in a grid failure by placing all the high draw non-essentials on a subpanel that the PV inverters feed and thru to the XWPro @ 50amp breaker ( or try a new lumin panel to be selective ) The idea was no sun = no "luxury" items. Sun charge the batts , run everything.

I think you've got the idea backwards, the sup panel is the backup "emergency loads" that get constantly powdered through the inverter and stay live when the grid power is removed.

The main panel or different sub panel (not through the inverter) would take all the luxury items. Any PV connected here would go dead when the grid goes down.

 

[RCinFLA]

The AC coupled hybrid inverter must absorb any excess PV generated power to battery when there is no grid to push to and consume excess.

The ability to absorb excess power is limited by power capability of low freq inverter to take the AC excess to DC to push to battery, and the ability of battery to absorb it without rising in terminal voltage to point where inverter shuts down for DC input voltage being too high.

There is no regulation on how much current is pushed to battery other than inverter shutting down for overload or inverter DC input voltage rising above inverter max limit also causing inverter to shut down. XP will limit how full it allows battery charging to raise SOC during normal operation, so it has some reserve to push current to battery during AC coupling back feed.

Common mistake is thinking you don't need much battery capacity because you have so much PV power available.

Frequency shifting is rather slow reacting. Inverter must first detect the back flow at 60 Hz, then slowly shift its frequency, and finally the GT inverter must recognize the frequency shift and reduce it output or shut down its output. This process can take up to a second or two. The process has to be relatively slow since sudden AC freq shift can cause current surges in AC motors and poor power factor loads actually generates a back feed current for a percentage of its AC cycle which inverter does not want to react to as back feed from PV.

Worse case situation is PV production is high with power consumed by house loads then a large load is switched off causing a sudden large back feed PV excess that must be consumed by inverter/batteries until GT inverter(s) power output reduced or shut down. The overload time can be greater for a GT inverter that has variable power control based on grid frequency versus a GT inverter that just shuts down when frequency goes out of range.