Development and Verification of an Advanced Method for Diagnosing Measuring Transformers

Abstract

The aim of the work is to develop a method and means for diagnostics of instrument transformers, allowing high accuracy to identify damage to the winding or core of a transformer. It implies definition of a clear diagnostic criterion that allows assessing the transformer under test conditions. The suggested method for detecting damage is based on mathematical processing of transformer magnetizing current, which involves analyzing its harmonic composition and determining amplitudes and initial phases ratio of magnetizing current fundamental and higher harmonics. The applied digital signal processing includes digital filtering and Fourier series decomposition of the signal. The magnetizing current signals are used as initial data for analysis by the proposed method and can be obtained during routine work on taking current-voltage characteristic of the transformer using the RETOM-51 (61) test device. Based on analysis results of experimental data obtained from the various types tests of instrument transformers, recommendations were formed for application of the proposed method: it is recommended to record the instantaneous values of current simultaneously with taking the current-versus-voltage characteristic for its linear and nonlinear parts. In this case, to confirm presence of damage, it is sufficient to fix phase or amplitude deviation of calculated phasor, determined by harmonic components ratio of magnetizing current, at least by 10% at one or more points. Application of described method will make it possible to reliably identify internal faults of current transformers without significantly complicating the diagnostic and test equipment.