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These IGBTs have the diodes inside from emitter to collector for each IGBT. There was not a continuous massive amount of current when B+ and B- were reversed, so that makes me think maybe they gracefully failed opened? There are several little thin wires (like 28 awg or something) inside the IGBTs that are floating in a gel, which connect to the collector and to the emitter tabs. I think they are designed to act as fuses. I would be interested in seeing the inside of the IGBTs. They have tiny screws in them, and then the little plastic top can pop right off, or you might need to bend the metal tabs first for the plastic top to be removed. My guess is that some of those little thin wires have failed opened, but I don't know for sure.
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