AC-coupling XW6848+ and SMA SB-5.0

aj164aj164 Solar Expert Posts: 116 ✭✭✭
I would really like to get the AC-coupling working reliably for my own system and for customers.  When the grid is down, we want the PV to power loads and charge batteries by AC-coupling via the XW's AC output (loads).  

I have "AC Coupling" enabled on the XW+.  Firmware version is 2.07 BN004.

I set the SMA to:
grid type:  off-grid 60Hz
operating mode active power red.:  "linear frequency gradient"
diff. between starting frequency and grid freq.: 0.2Hz 
 active power gradient: of 60%. 
 diff. between reset freq. and grid: 0.2Hz   (not sure what this means)
 active power change rate after fault ends: 200%/min  (default is 10000%)
 lower freq. deviation limit prior to active power release: -5.00Hz  (default)
 upper freq. deviation limit prior to active power release: 1.5Hz
 recovery 200%

It works some of the time, but not all the time.  I've tried various different gradient settings.  When the load changes or the sun comes out from the clouds (and the batteries are already charged), the battery voltage goes up and up until the XW shuts down.  The SMA doesn't release until the XW actually cuts power.  Watching my frequency meter, I notice the line frequency rarely seems to go above 60.45Hz.  I did see 60.58 or so but never anything close to the 61.5Hz described in the XW AC-Coupling technical documentation.  

Anyone else get this to work?  



  • Dave AngeliniDave Angelini Solar Expert Posts: 5,802 ✭✭✭✭✭
    Yes but with SMA TL series. You are north America?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 31,427 admin
    Dave, Adam is probably somewhere around Austin/Houston Texas.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,802 ✭✭✭✭✭
    edited February 22 #4
    Thanks Bill !

    I would ask SMA what is required to do this with their battery inverter the sunny island? It might give you a clue. The TL series required nothing I recall. I assume this is for back-up only, no selling.

    XW pro, the replacement has many enhancements including the ability to just power loads and only charge when Soc gets into the area you program for. Soc did not work really well with XW+ in AC coupling. The + model is really triggered by voltage set-points. 

    The other way around all of this is to use the Schneider HV Mppt-100. You would use a 600v rated A or B switch and then just avoid the AC coupling when the power is out.
    The bulk of my experience is offgrid and AC coupling really is for really large systems or microgrids where someone is watching the system. Even then there is a combination of AC and DC coupling.

    So how was it for you last week?
    Good Luck!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • aj164aj164 Solar Expert Posts: 116 ✭✭✭
    Yep - Central TX.  I did fine with power, despite losing the grid for 7-8 hours at a time.  The batteries held up just fine.  I have eight C&D 225AH AGM "nano carbon" batteries.  The system powers everything but the heaviest loads (such as HVAC).  I heat the house using a wood-burning stove even during normal times.  Hot water is solar-thermal with electric backup. 

    Thanks for the reply.  There is a technical paper on Schneider's renewable energy site regarding AC coupling with the XW.  It describes how to set the parameters on their grid-tie inverters for power curtailment vs. frequency.  It appears the same parameters are available in the SMA SB-x.0-US series inverters.  These are transformerless despite having dropped the "TL" from the model numbers.  One thing I'll try next is to shorten the frequency window to .5Hz instead of 1.5Hz.  The AC-Coupling tech paper curiously shows 50.5Hz as the point at which the grid-tied inverter should curtail 100% (i.e. 0W output) for 50Hz systems, but for 60Hz nominal, it shows 61.5Hz.  Maybe that's a typo, or maybe the firmware in my XW+ has a bug.  Either way, it looks like my inverter isn't going much past 60.5Hz.   I'll post what happens!  

  • Dave AngeliniDave Angelini Solar Expert Posts: 5,802 ✭✭✭✭✭
    Good to hear about you! I know Texans are not as the news portrays them. I try not to listen to the drivel but it can sneak thru my filters....

    There was a list of known inverters to use with XW+. Probably a typo as xw+ is worldwide and 50hz also.
     Read the release notes for your firmware, sometimes they will list known bugs or issues for the future. There will not be anymore fixes as the + model is now legacy. There also are special revisions of firmware that are hidden and saved for special cases. Usually microgrids where there is alot of Schneider equipment and Engineering is involved. Good Luck !
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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