The SFRA test is one of the few field tests that can determine if there is a problem with the geometry of the windings. The test set applies a 10v p-p sine wave to a selected winding, then measures the signal input/output from the other end of the winding as a function of frequency and using a network analyzer displays the response in graphical form on a laptop. The types of problems detected are:
The test can be done with the unit full of oil or empty. Any before and after comparisons have to be made with the same conditions—Results from an empty tank SFRA should be compared to another empty tank result. This would be typical of a test run prior to transport and then run again upon arrival at its destination. The waveforms should be identical unless there was movement during shipping and handling. We've had experience with this test on furnace transformers and large power units sister transformers) with good repeatable results.