Transformer diagnostics with combined measurement methods based on different physical phenomena

The paper presents the concept of transformer diagnostic based on complementary methods with two measurements related to different physical phenomena. Two separate approaches are described. The first one allow the assessment of moisture content, ageing level and sludge presence in paper-oil insulation with joint measurement employing Return Voltage Measurement (RVM) and Polarization Depolarization Currents (PDC) methods. The second approach uses complementary measurements with Frequency Response Analysis (FRA) and Vibroacoustic Method (VM) to assess mechanical condition of an active part. Such approaches allow to perform more reliable and exact diagnostics. The results obtained in each pair of methods analyzed together show additional processes, which are impossible to detect with a single method. The paper describes basics of all four methods and gives directions for combined assessment. There are also some examples given of measurements from real cases.