Experience in Using Frequency Response Analysis to Assess the Condition of a High-Power Test Transformer

Frequency response analysis has become widely used in the world in recent years to assess the mechanical and electrical condition of the active part of power transformers. To date, the measurement technique has been developed and standardized in IEC, IEEE, and GOST standards. However, the issues of interpretation at the world level have not been fully resolved, and the interpretation of results is often a subjective expert opinion. A practical example of assessing the technical condition of a special single-phase high-power test transformer with an internal damage occurred during operation is presented. The results of measurements and analysis of the winding frequency responses, including those obtained by non-standard measurements, are discussed. It is shown that the application of the DL/T 911 methodology based on the pair correlation coefficients can lead to an erroneous assessment of the condition of the transformer. Calculated frequency dependences of the fault conductance, impedance, and inductance of the winding, as well as inter-winding capacitance are presented. The diagnostic parameters calculated from the frequency responses of the windings and from traditional diagnostic measurements are compared. The inspection of the active part of the transformers after opening the transformer tank confirmed the main conclusions on the condition of the transformer drawn after the comprehensive diagnostic examination.