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believe the answer you're looking for has to do with the pressure-altitude-voltage derating related to corona discharge, which increases (gets worse) with higher altitude and peaks at around 80-100,000' Typically this is considered to occur at votlages above 300 V peak, but be sure you are accounting for all transients and internal voltages.
The effects of such atmospheric parameters as pressure and humidity on the corona discharge performances of conductors are complicated. Especially as to the effect of air humidity on corona inception voltage, researchers have not reached an agreement.