ET Online
December 2008
The ultimate tool for detecting transformer faults

The ultimate tool for detecting transformer faults

01 November 2008

Matz Öhlén M.Sc., E.E.  - Marketing and sales manager

Power transformers are invariably designed to withstand the mechanical forces they are likely to experience during transport and as a result of inservice faults. However, it is by no means unknown for the forces to exceed acceptable limits in the case of severe events, or when ageing has led to a reduction in strength of the insulation. Frequency response analysis (FRAX) is a powerful tool for determining whether a transformer has been damaged by problems of this type.

Electrically, a transformer is made up of multiple capacitances, inductances and resistances. It is, in effect, a very complex circuit that produces a unique “fingerprint” when test signals are injected over a range of frequencies and the results plotted as a curve. In particular, the capacitances in the transformer are affected by the distance between conductors. Movement in the windings will, therefore, alter the capacitances and change the shape of the curve.

The sweep frequency response analysis (SFRA) test technique for transformers is based on comparisons between measured curves, which allow variations to be detected. An SFRA test involves multiple sweeps and reveals whether the mechanical or electrical integrity of the transformer has been compromised.
Normally, SFRA tests are used to capture a “fingerprint” reference curve for each winding when the transformer is new or when it is known to be in good condition. These curves are subsequently used as the basis for comparisons during maintenance or when problems are suspected.

The best way to use SFRA testing is to take regular measurements on the same transformer over a period time, and to compare the results. However, it is also possible to use type-based comparisons between transformers with the same design. Finally, a construction-based comparison can be used in some circumstances, when comparing measurements between windings in the same transformer.

Comparative tests are typically performed before and after transportation, after severe through faults, before and after overhaul, and as a diagnostic aid if problems are suspected. A single SFRA test can detect winding problems that would otherwise require multiple tests with various kinds of test equipment, as well as problems that cannot be detected at all by tests of other kinds.

As a general guide, magnetisation and other problems relating to the core alter the shape of the SFRA curve at the lowest frequencies, up to around 10 kHz. Medium frequencies from 10 kHz to 100 kHz represent axial or radial movements in the windings, and high frequencies above 100 kHz correspond to problems involving the cables from the windings to bushings and tap changers. In modern SFRA test sets, built-in analytical tools simplify comparisons between curves.

The diagram shows an example of the type of results SFRA can provide. It represents a singlephase transformer where after an overhaul, the core ground was accidently left unconnected. In this condition, the transformer produced the SFRA curve shown in red. With the core ground connected, the green curve was produced. The problem was clearly revealed by changes at frequencies between 1 kHz and 10 kHz, and noticeable changes can also be seen in the 10 kHz to 200 kHz range.

SFRA testing offers a fast and cost-effective way of determining if a transformer has been damaged and whether it is safe to energise. In addition, if there is a problem the test results provide valuable information that can be used to help decide on appropriate further action. For mission critical transformers, an SFRA fingerprint reference measurement is therefore a sound investment as it will make analysis easier and more reliable should a problem occur.

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