SFRA: A Non-Invasive (Non-Destructive) Test for Verifying Mechanical and Geometrical Integrity of Transformers

SFRA is the acronym of Sweep Frequency Response Analysis. This particular test was invented by the Chinese Engineers/Scientists in the year 2004 and a Standard was issued to this effect by the People's Republic of China as DL/T 911–2004.

The Transformers, inherently have resistance, inductance and capacitances, therefore, they constitute a unique RLC circuit of each one of them. Since, the Inductance and the Capacitance are frequency dependent parameter, therefore the total impedance of the transformer shall vary if it is subjected to an AC supply with variable frequencies. SFRA has emerged as a powerful and sensitive tool for evaluating the mechanical and geometrical integrity of the core, windings and clamping structure of the Power transformers by measuring their electrical magnitude in dBs over the frequency range from 20 Hz to 2 M Hz. The health-status can be assessed without opening or dismantling the transformer. Thus, this test can be designated as “non-invasive test (non-destructive test)”. This test is invariably performed at the OEM's works and also at site to verify its health-status of the Transformers. Further, this test is also performed on the transformer after major-trippings and severe seismic conditions for verifying the health-status of the Transformers. The maiden frequency response curves are called as the signature curves or thumb-impression curves for each individual Transformer.

The Authors have narrated the basic theory of the SFRA test and have narrated” Where to look-into, over the frequency range for each type of suspected faults?