Partial dischage is a claer cut function of Voltage Stress in insulation .
The harmonics have a single point major effect , that it makes the waveshape PEAKY which can cause additional voltage stress in the insulation causing increase in Partiual Discharge .
This pheniomena is not so relevent to power transformer . The phenomena could be more pertinent in Dry type trasformer because the insulation system ( epoxy casting or resin impregnation )of the Dry type trfr are normally not free from voids . The breakdown voltage across the voids can be easily crossed with minor increase in volatge stress causing more PD .