Sudden asymmetry in parallel installation

[Janvi]

My 3 device MP2-5k parallel installation now worked about 3 month like expected with grid feed in power over 30kVA. This evening, a 3x50Amp circuit breaker between ESS-In and Grid triggered while feeding only 9kVA into the grid. It required several attempts to startup the system again. Always when feed in power ramped up, the MP2 contactors stopped again and start new trial after some minutes. No notifications under/overvoltage or sync failures were indicated for the reason.

Trying small grid feed in powers showed suspicious asymmetry with lower power for L2. Of coarse all parallel cable length are matched for the devices. L2:L3 is almost ratio 2:3. Switching the ESS multiphase setup from total to individual cannot not change anything.

For comparison, the Ziehl EFR shows similar values as a independend instance.

VRM also shows approximately same voltages why I assume, it is not a N problem.

A current clamp instrument shows, that both parallel devices of L2 have any problem. The load for the L3 parallel devices is significantly higher to compensate the L2 drop:

The only idea for the moment is to check the AC voltage at the MP2 terminals directly. Maybe there is a problem with the N-Wiring? Any other ideas to check?

[MikeD (Mike)]

Given that “Par 1 & 2” of phase 3 also have higher currents while all the masters are matched would indicate that the problem is common to those 4 inverters: “par 1&2” of both L2 &3.
Could be either neutral wiring or some problem in the negative battery terminal wiring of those inverters.

[Janvi]

AC and DC Voltages between all L2 devices are very same. In the meantime, the AC currents of L2 slaves went up to about 3 Amp and power ration L2:L3 improved to 2,4 to 2,9kW without changing anything.

The deviations seems to be normal tolerance between the multis. Attached are some of my records up 25kW. Sorry for the rain and not enough sun. Only parallel unit 1 of L2 seems to have a little less power and L3 is catching this up. For higher powers, my digital multimeter is increasing the unstable values to about plusminus 0,2 Amp. Not all signals and not always. Did not watch with the scope but have no idea why the 50 Amps circuit breaker triggered yesterday