First - understand what partial discharge (PD) is ... and is not.
Partial discharge (PD) happens when a small area of insulation in a high voltage environment cannot cope with the electrical stress and breaks down. It doesn't span the whole distance between two insulated electrodes – that's why it's called 'partial'.
So - to ensure something stays "PD free" requires at least one of the following conditions: 1) an insulation with absolutely no imperfections that can well exceed the applied local voltage stress, or 2) a low enough voltage that the existing insulation (with or without imperfections) can withstand the stress.
Note that the severity of partial discharge can also be affected by environmental conditions such as altitude, humidity, presence of (external and/or internal) contaminants, proximity of conductors from different polarities/phases, etc.
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