A New Interpretation of FRA Results by Sensitivity Analysis Method of Two FRA Measurement Connection Ways

As a reliable and non-destructive technique, frequency response analysis (FRA) has been widely recognized for transformer winding deformation detection and assessment. In this paper, frequency response functions (FRFs) are obtained by two different FRA measurement connection ways, namely, end-to-end voltage ratio (EE) and transfer voltage (TF) ratio measurements. At the same time, according to the two ways, sensitivities of the FRFs to electrical parameters of the equivalent circuit model of the transformer winding are investigated. Based on Tellegen theorem, adjoint network method is derived to calculate the values of the sensitivities of FRFs to electrical parameters. Meanwhile, the sensitivity characteristics of circuit parameters are studied. In addition, by comparing the sensitivity results of the two ways, the variation laws of the sensitivities under the two different connection approaches are determined. Besides, this paper can provide a new and objective method to interpret the FRA results and give us a better understanding how much the parameter variations can influence the FRA measurement results.

I. Introduction

A FRA method is a popular and effective technique to detect and monitor transformer winding deformations [1]. As a promising diagnostic tool, it has been successfully proved to be a reliable and practical method. To obtain the response, a low-voltage sinusoidal sweep frequency signal is injected into one terminal of the winding, and the final response signal is received from another terminal. Hence, the voltage ratio of the amplitude of the response and source voltage is got. Then, it can easily obtain the FRA trace by plotting the voltage ratio as a function of frequency in dB scale. A change in the geometrical configuration of winding, which is caused by winding deformations, will have an impact on the winding’s electrical properties. Finally, the deformation will be reflected by the change of the FRA curve. Furthermore, FRA method is a comparative method. Analysis of the test results relies on the comparison between previous results and present results. If there is no initial FRA trace, it is also feasible to make a comparison among phases in this transformer or based on an identical transformer which comes from the same factory.