Transformer winding movement monitoring in service - key factors affecting FRA measurements

This paper presents data from a study conducted to identify the key factors that contribute to frequency response analysis (FRA) measurement results. The results show that the lower the shunt impedance, the higher the sensitivity of the FRA measurement for the detection of winding movement. On transformers with large bushings, the bushings can mask the detection of winding movement. To detect minor winding movements, the FRA comparison has to be made using the same connections each time. With a typical test circuit, the FRA measurement sensitivity range of the standard lead arrangement is up to approximately 2 MHz only. For optimum measurement, the lead lengths need to be as short as possible, and the test configuration must remain constant for repeated tests. The higher the frequency range used, the more critical this becomes. From the results, it is apparent that minor winding movement or winding looseness generally shows in the high frequency region of the FRA admittance. All of the factors investigated impact on the ability of FRA measurements to detect winding movement. To optimize the sensitivity of FRA measurement method, these factors need to be taken into account.