Can Partial Discharge occour in DC circuits?
Of course it can. DC erodes conductors much quicker. Isolators are normaly not eroded but damaged by chemical processes, manufacturing faults or just bad design. Partial discharge should be in the top 3 causes for capacitor faults... at least that is what why they  kick the bucket... Its also happens in coils like relais or transformers.
PS: e.g. an already damaged cable isolation will not &quot;erode&quot; but a Partial discharge can like burn away even more isolation until it fails completly.