Trace 5548 in-series kicks out generator

Cutterzoo

I've been using two Trace 5548 inverter/chargers connected in series to provide 240 volts since 2001 for an off-grid system at a vacation house. They have been rock-solid... until now.

I've been using a portable Generac 7500 generator as a manual back-up since 2012.  As of last night while trying to top off a charge after a couple of cloudy days, one of the inverters would instantly kick out the gennie when transferred, which also kicked out the other inverter since they're wired in series.

I did a work-around by shutting off the inverter/charger breaker for the offending inverter, which at least allowed the other inverter to transfer and charge the batteries, but the offending inverter continued to kick out.

Stuff I've already tried:
Disconnecting all A/C loads at the breaker panel.
Checked voltage at the gennie - both phases at 122 volts with no load
Swapping legs/phases at the gennie - problem stayed with the original inverter, did not follow gennie phase.

The last thing I tried was disconnecting one of the gennie legs/phases.  As it turned out, I left the offending inverter connected to the gennie, and it transferred just fine, so the problem seems to be something with the series connection of the two inverters.

Anyone have any experience with this problem?

Thanks.

RCinFLA

Being that the system was working the first thing I would check is to see if the generator wandered off frequency over time.  One inverter might be a little more tolerant then other.  

Second would be any govenor speed warbling of generator.  Lock stability of inverters is much tighter then absolute gen speed frequency.  Again, one inverter may be a little more forgiving then other.

Other thing you can try is backing down the AC2 max current to lighten load on generator to see if that makes any difference..  

Check your serial stacking cable DB25 connectors are making good connections.  Did you check to make sure the two inverters are running 180 degs out of phase.  I don't remember but I think either the DB25 series stacking cable determines which inverter is master and which is slave or whichever inverter is turned on first becomes master.  Try reversing serial stacking cable between inverters.