Acid number (acidity) is the amount of potassium hydroxide (KOH) in milligrams (mg) that it takes to neutralize the acid in 1 gram (gm) of transformer oil. The higher the acid number, the more acid is in the oil. New transformer oils contain practically no acid. Oxidation of the insulation and oils forms acids as the transformer ages. The oxidation products form sludge and precipitate out inside the transformer. The acids attack metals inside the tank and form soaps (more sludge).
Acid also attacks cellulose and accelerates insulation degradation. Sludging begins when the acid number reaches 0.40; it is obvious that the oil should be reclaimed before it reaches 0.40. It is recommended that the oil be reclaimed when it reaches 0.20 mg potassium hydroxide per gram (KOH/gm) . As with all others, this decision must not be based on one DGA test, but watch for a rising trend in the acid number each year. Plan ahead and begin budget planning before the acid – number reaches 0.20.