① Oil sample testing – Performance indicators such as withstand voltage and impurities should be tested every three years. This interval can be shortened for transformers operating at full or overload for extended periods.
② High and low voltage insulation resistance should not be less than 70% (10MΩ) of the original factory value. The difference in the three-phase average DC resistance of the windings at the same temperature should not exceed 2%, and should not exceed 2% compared to the previous measurement.
③ Transformer working grounding resistance should be measured every two years.
④ The cycle for power outage cleaning and inspection should be determined based on the surrounding environment and load conditions, generally every six months to one year.