Well the DC link voltage on this inverter was rated at over 700V, and the IGBTs I was using were quite quick so it would have been running at about 1400V/us. The key parameter (as has been mentioned elsewhere I think) is the inter boundary capacitance between the hot and cold side. The faster the dV/dt, the greater the capacitive current between the two sides. If this becomes excessive then this can lead to failure of the insulation, which obviously is a disaster. This was just a large AC drive.