Application of natural frequencies deviations patterns and high-frequency white-box transformer models for FRA interpretation

摘要

Frequency response analysis (FRA) is largely used to assess the mechanical and electrical condition of windings of power transformers and shunt reactors in service. The comparison of measured frequency responses in a wide frequency range using correlation analysis or integral indexes are widely used in practice for FRA interpretation. Despite its simplicity, these approaches, being based on the integral indicators, do not allow to take into account the specifics of the object under measurement and to explicitly interpret all the variety of possible types of defects. FRA interpretation can be made in a qualitatively different manner with an analysis of transformer design features and their influence on the winding natural frequencies. The shifts of these frequencies are usually associated with serious winding faults and have different patterns depending on types of faults and their location. The report develops an approach to the FRA interpretation based on the analysis of windings natural frequencies and patterns of their first natural frequencies deviations. The description of the approach for a determination of the presence and approximate location of internal short circuit faults in transformer windings using graphical representation of natural frequencies deviation patterns is given. The report illustrates the practical applications of high frequency modeling of transformer windings (white-box models) for a research and a qualitative analysis of the influence of internal faults to the winding frequency responses with the aim of FRA interpretation. The description of the white-box model used by the authors is presented. The problems of model parameters calculation and their adjustment to account for an influence of winding deformations and other damages in a transformer are discussed. The issues associated with an application of the models in the EMTP-type program for the simulation of FRA-measurements and for the calculation of frequency responses are considered. The report presents the results of computer modeling of frequency-response deviations caused by unearthing of electrostatic shields or magnetic core and by damages occurring in windings or coils connected in parallel.