What are the acceptable acidity levels in transformer oil?

The acceptable acidity levels in transformer oil, measured as Total Acid Number (TAN), should ideally be below 0.1 mg KOH/g for new oil. For in-service oil, levels up to 0.3 mg KOH/g may be tolerated, but values exceeding this indicate potential degradation and increased risk of insulation failure. Regular monitoring of acidity is crucial for maintaining transformer performance and longevity.