That is why when repairing such things, usually soldering in the new IGBTs would be one of the last things I do. I always verify for a switching waveform at each of the gate pads with all IGBTs removed.
Test every diode, resistor, etc in the gate drives even if it look physically fine.
All it takes is one faulty component in the gate drive circuitry of one IGBT and the whole thing goes bang again. If you test without the IGBTs installed, if there is still a fault, no magic smoke will escape because there is no current path.
Do also test to ensure no short circuit on the inverter output within the UPS itself (filter caps for instance).
I did go through all the resistors and caps, mostly looking for shorts since they are in the circuit... (including between all the IGBT connections on the pcb).
I'd like to try testing the gate pins with a scope.. (the IGBTs aren't install yet) I just need to research how to do it.
From the spec, the gate threshold voltage is 4 to 6.5v.. so I would expect to see pulse between the gate and the emitter?
I'll admit I've put most my focus on the area around the IGBTs (shown in the pic below - bottom right of the image).
https://redhat.systems/P1004680.JPGI can power it on with a 15A/120v source... even though it is a 120/30A power in, without any load or batteries connected, I doubt it would need much for amperage. One interesting thing about this UPS is the battery power supply is 10 x12VDC batteries in series, so at full charge, the total battery voltage in is around 136V. Therefore, not much of a requirement for a big heavy transformer to raise the battery voltage up to the 120AC line voltage for loads, like most other UPSs.