Well first you need to find the cause of the PD failure and then apply the necessary maintenance based on specific components,

In Core and Coils Assembly, oil-barrier-paper structure and oil, Electrostatic Shields, Leads typical faults may occur like:

Oil/surface contamination with particles, bubbles, static electrification, bad impregnation, high moisture, Partial breakdown in oil; surface discharge; creeping discharge  or Sparking and arcing between bad connection “Conductor under floating potential” discharges,Tracking in wooden blocks..

or in Bushing:

Localized defect within the core: bad impregnation, high moisture, short-circuits between layers, sparking across the core surface Breakdown in oil; surface discharge across the porcelain

or LTC:

Partial breakdown in the selector and in the diverter switch compartment

Poor or worn out contacts.

and etc.

Dear Mehmet,

First, you have to determine the location of partial discharge and then take measures to eliminate them when possible and economically more convenient.