Interpretation of Power Transformer Frequency Responses Using the Matrix of Winding Admittances

Abstract

Frequency response analysis (FRA) is widely used to assess the mechanical and electrical conditions of the power transformer and shunt reactor windings. The requirements for the FRA and for frequency response measurement equipment have been standardized in the IEC 60076-18 standard. However, there still remain many questions concerned with interpreting the frequency response measurement results. As is well known, electrical faults and serious mechanical damages of certain transformer windings entail significant changes in the winding frequency responses. In the general case, the measured transformer winding is coupled with the other windings, a circumstance that has an effect on the frequency responses obtained for an individual winding and adds difficulty to assessing the condition of the transformer windings. It is shown—based on representing the transformer as a multiterminal circuit with the matrix of winding admittances—that the frequency responses measured according to the standard schemes are coupled with it. It is shown, taking as an example the transformer with a delta-connected winding, that the mutual influence of windings can lead in practice to an incorrectly assessed condition of individual winding phases. Matters concerned with the use of non-standard measurement schemes, including those for measuring the winding admittance matrix entries, are considered with the aim to obtain additional diagnostic information and interpreting the frequency response measurement results.