How to do SFRA Test?
The Sweep Frequency Response test method produces traces or “fingerprints” of the network impedance created by the physical geometry of the test specimen.
Interpretation of the results when compared to a baseline test can indicate core or winding movement in transformers.
If a baseline test is not available, interpretation can be conducted by comparing to a sister unit or phase-to-phase.
The frequency response method is the ratio of the transmitted waveform to the applied low voltage waveform, which varies with frequency.
The impedance attenuates the input voltage signal.
A series of measurements are made to obtain the response at various frequencies.
The method used is “Sweep” frequency, which is a clear and unambiguous method because the sweep systems make measurements at each point of interest.
The results are highly repeatable.
Various case studies involving problems detected using the SFRA test will be presented in this paper.
In addition, preliminary work will be discussed on applying this technique to rotating machines.
The SFRA theory and test method will also be briefly discussed.
As mentioned earlier, the frequency response plots are the ratio of the transmitted voltage waveform (Vout) to the applied voltage waveform (Vin) in decibels (dBs).
The response in dBs is calculated as 20Log10(Vout/Vin).
The basic circuit is described in Figure 1. A three lead system is used to measure both the input and output voltages simultaneously as well as apply the input signal.
Fifty ohm coaxial test leads are used to apply the input signal and measure Vin and Vout.
These impedances are matched to the measurement hardware to minimize waveform distortions caused by reflections.
This ensures good repeatability. The SFRA test method has no doubt proven to be extremely valuable in detecting problems associated with transformers.
It is another valuable tool in the maintenance engineer’s toolbox.
The SFRA test will provide ease of mind when shipping damage is suspected or if the unit received a fault.
The method will detect shorted turns, core ground problems winding deformation, core damage, structural damage, and so on.
It is considered a diagnostic test, but a benchmark test is highly recommended to ease in the analysis of future testing.
The question of whether this test method can be applied to rotating machines and provide valuable diagnostic information to the maintenance engineer, is a question that should be explored and answered.
The test has proven to be successful in detecting shorted turns and ground faults on salient pole rotors.
We have also received positive information that this test is much more sensitive than the pole drop test in detecting and locating resistive faults in generator rotors without having to disconnect the poles.
One problem area includes cylindrical rotors. In some cylindrical rotors, the turns may migrate and shift in the area under the retaining ring.
This is not detectable (unless a complete short occurs) without removing the retaining ring.
In theory, the SFRA test method should detect this movement and therefore, eliminate the need to remove the retaining ring to diagnose the problem.
Additionally, we would like to explore detection of turn-turn faults on stator winding coils, stator core damage, and open circuits on salient pole rotor amortiser windings.