From degradation diagnosis to corrective actions

30 MVA transformer, voltage 130/15, in an hydropower plant.
13 years of service, no failure;
Fluid is affected by a heavy degradation, which cannot be corrected by normal oil maintenances.
DDF is hugely out of normal values, and acidity increases quickly.

Oil properties

A plan of characterization of oil degradation (3 controls in 12 months) clarifies the problem.Evidences:
Acidity: rate of increase 0,069 mg/g/y; Tg delta: rate of increase 0,224 unit/y; Copper: rate of increase 5,35 ppm/y
Diagnosis:
Phenomenon of progressive copper corrosion, due to the oil tendency to dissolve copper (probable presence in the oil of weak acids or complexing agents).
Increase of DDF related to copper accumulation in the oil.
Action:
Oil depolarization to remove copper and other polar compounds influencing DDF.Removal of acid compounds from the oil.
Stability in time:
After depolarization the equipment had the normal release of contaminants from the absorbed oil (10-12%).
Within 2,5 years the degradation stabilized to values matching IEC requirements for mineral oils in service.
Oxidation stability, controlled on 2006, is conform to the requirements for new oils.After depolarization