Experience of Select Solutions with SFRA measurements demonstrates that the SFRA results can be adversely affected if many aspects of the measurements are neglected. Misleading diagnostic results can be obtained at the frequencies as low as around 400 kHz possibly leading to a wrong conclusion on the condition of the transformer.
To achieve high repeatability of SFRA measurements the following practices are recommended:
- Keep the ground extensions straight along the bushings and make them as short as possible.
- Perform the measurements using only flat type ground extensions with low impedance.
- Make sure connections are made to bare metal not to painted nor rusted surfaces.
- Before starting SFRA measurements, connect the source, reference and test clamps together clamped on the highest voltage HV bushing with the ground extensions along the bushing. The resulting graphs should be a straight line of zero magnitude across the whole range of measurement frequencies. On completion of all SFRA measurements, perform another calibration check in the same way it was done initially. This will determine the highest repeatable frequency by comparison of the initial and the final SFRA measurements.
- Perform the SFRA measurements in a consistent way to minimise external interferences.
High Voltage Test Services of Select Solutions is committed to perform SFRA measurements to the best abilities to achieve a high level of SFRA repeatability and quality of the measurements that allows users to obtain better interpretation of the test results.
 “Mechanical-condition assessment of transformer windings using frequency response analysis (FRA)”, CIGRE Brochure 342, WG A2.26, 2008.
 K. Feser, J. Christian, T. Leibfried, A. Kachler, C. Neumann, U. Sundermann and M. Loppacher, “The transfer function method for detection of winding displacements on power transformers after transport, short circuit or 30 years of service”, 12/33-04, CIGRE Paris, France, 2000.
 S.A. Ryder, ALSTOM Transformer Research Centre, Saint-Quen, France, “Diagnosing a wider range of transformer faults using frequency response analysis”, XIII International Symposium on High Voltage Engineering, Netherlands, 2003.
 J.A.S.B. Jayasinghe, Z.D. Wang, P.N. Jarman and A.W. Darwin, “Investigations on sensitivity of FRA technique in diagnostic of transformer winding deformations”, IEEE International Symposium on Electrical Insulation, 19 – 22 Sep 2004, Indiana USA, pp. 496 – 499.
 Zhongdong Wang, Jie Li, Dahlin and M. Sofian, “Interpretation of transformer FRA responses – Part 1: Influence of winding structure”, IEEE Transactions on Power Delivery, vol. 24, No.2, pp. 703-710, Apr 2009.
 Andrey A. Reykherdt and Valery Davydov, “Effects of Test Cable Ground Extensions on Repeatability of Frequency Response Analysis Measurements on Power Transformers”, IEEE Electrical Insulation Magazine, Vol. 28, No.3, pp. 26-31, May – June 2012.