Sweep Frequency Response Analysis (SFRA)

The technique of SFRA is a major advance in transformer condition monitoring analysis.

The acronym "SFRA" is meaning Sweep Frequency Response Analysis
The role of SFRA is growing in the transformer field and the results interpretation is becoming more and more important day by day; this is done by performing a measurement looking at how well a transformer winding transmits a low voltage signal that varies in frequency (the results  are in the form of frequency response traces).

Just how well a transformer does this related to its impedance, the capacitive and inductive elements of which are intimately related to the physical construction of the transformer.
Changes in frequency response as measured by techniques may indicate a physical change inside the transformer, the cause of which then needs to be identified and investigated.

It is important to highlight that SFRA test is a "non-destructive test".
SFRA is in adding an Off-Line test and it can be carried out for any voltage rating of Power Transformer.

The SFRA Analyzer identifies the following abnormalities in the transformer before they lead to failure:

- Core movement;
- Winding deformation and displacement;
- Faulty core ground;
- Partial winding collapse;
- Hoop Buckling;
- Broken clamping structures;
- Shorted circuited turns and open winding.

This is a proven technique for making accurate and repeatable measurements:
(a) First to obtain initial signature of the transformer sweep frequency response as a record for the future reference / comparison;
(b) Periodical measurement as a maintenance check;
(c) Immediately after a short circuit;
(d) Transportation or re-location of transformer;
(e) Earthquakes;
(f) Pre-commissioning checks.

These tests show, in trace form, the winding transfer function of the transformer and are valuable to determine if any damage has occurred during shipping or during a through fault.
These tests should be conducted before and after the transformer has been moved or after experiencing a through fault.
Results should be compared to baseline tests performed at the factory or as soon as possible after receiving the transformer.
If the SFRA tests cannot be performed at the factory, they should be conducted as an acceptance test before energizing it (or a new or rebuilt transformer) in order to establish a baseline.

All these topics have been discussed by Mr. Renato Franco, SEA SpA Testing Manager, during a Conference hold in Padua on 19 September, highlighting some Case Studies performed by SEA R&D Department.

An Abstract of the Presentation is available by following link: "SFRA Analysis for power transformers diagnosis".

For more information about SFRA test please contact SEA SpA Sales Department: info@seatrasformatori.it