Corona processing of insulating oil

It is well known that sustained corona discharge in insulating oil lowers its dielectric strength and simultaneously reduces its corona resistance. Therefore, for operating stresses in the corona regime, activity typically increases with time and, if allowed to continue, eventually leads to breakdown of the oil and failure of the component or system. It is, therefore, common practice to periodically replace oil in devices such as large power transformers and switchgear before breakdown occurs. Sealed components such as capacitors are typically replaced. Experiments have demonstrated that the dielectric properties of corona weakened oil can not only be restored, but actually improved by a simple regeneration process. These experiments were carried out on high voltage pulse transformer windings which were operated at high repetition rates until partial discharges formed. Reprocessing the oil after each operating cycle resulted in successively longer operational periods before partial discharges appeared. In a separate experiment, a process was developed to precondition transformer oil to raise its corona inception voltage before using it to insulate a high voltage component, thus giving it a longer initial service life for a given operating stress or permitting higher stress operation for limited operating times