Sweep Frequency Response Analysis (SFRA) is a powerful and sensitive method to evaluate the mechanical integrity of core, windings and clamping structures within power transformers by measuring their electrical transfer functions over a wide frequency range. SFRA is a proven method for frequency measurements.
Contents
Methods
The SFRA is a comparative method, meaning an evaluation of the transformer condition is done by comparing an actual set of SFRA results to reference results. Three methods are commonly used to assess the measured traces:
How SFRA works
Transformers consist of multiple complex networks of capacitances and resistors that can generate a unique signature when tested at discreet frequencies and plotted as a curve. The distance between conductors of the transformer forms a capacitance. Any movement of the conductors or windings will change this capacitance. This capacitance being a part of complex L (inductance), R (Resistance) and C (Capacitance) network, any change in this capacitance will be reflected in the curve or signature.
An initial SFRA test is carried out to obtain the signature of the transformer frequency response by injecting various discreet frequencies. This reference is then used for future comparisons. A change in winding position, degradation in the insulation, etc. will result in change in capacitance or inductance thereby affecting the measured curves.
Tests are carried out periodically or during major external events like short circuits and results compared against the initial signature to test for any problems. The basic functionality of SFRA supports following measuring modes. Voltage transfer function Uo/Ui (f) SFRA test reveals if the transformers mechanical or electrical integrity has been compromised.
Problems that can be detected
SFRA analysis can detect problems in transformers such as: