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Find out about SFRA the practical way!

Tony Wills - Application engineer

Sweep frequency response analysis (SFRA) testing on transformers provides invaluable information about the mechanical integrity of the transformer’s components – information that can’t be obtained in any other way without dismantling the transformer or at the very least, by performing a thorough internal inspection. Mechanical movement or deformations within the transformer produce changes in the transformer’s inductance and capacitance distribution. It is these changes that SFRA testing measures.

Some engineers and technicians however, find SFRA test results daunting to analyse, particularly if they have not had much experience with using this technique. Having comparative results, ideally from tests performed when the transformer was new or in known good condition, is a big help, because SFRA test results should not change throughout the life of a transformer. But when changes in the test results are seen, what do they signify? International standards organisations have developed guides and brochures to help answer this question but for those who prefer a more practical hands-on approach, Megger’s FDB101 demonstration tool is a very attractive alternative.

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Fig. 1: Frax demo tool FDB101This demonstration tool, pictured in Figure 1, has proved exceptionally popular among those who have seen it. At its heart is a real – but, of course very much scaled down – transformer. Tests can be performed on this transformer in its as-new condition, and then switches on the box allow various fault conditions to be introduced so that users can observe the differences these faults make to the SFRA results. There are switches to introduce shortcircuited turns, loss of turns and a bad connection. In addition, users can squeeze the HV winding with a clamp and move the core clamp to simulate magnetic problems.

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Fig. 2: Influence of a damaged core

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Fig.3: Radial deformation of HV windings

Fig. 4: Influence of shorted turn in LV winding

Just a few examples of results generated using the demonstration tool are provided in Figures 2 – 4, but it is incredibly versatile and can be used to show many other types of results such as those relating to axial displacement of windings, loss of turns in the LV winding, bad connections and the influence of shorted turns on interwinding TTR measurements.

SFRA is an important test technique for transformers. It saves time and money, because when test results overlay perfectly with previous test results, certainty is provided that there are no electromechanical faults present in the transformer. And, with a little experience, SFRA test results become a bit easier to interpret. Now, thanks to the Megger FDB101 SFRA demonstration tool, engineers and technicians can gain this experience quickly and easily, right at their desks.

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