Sweep frequency response analysis

Sweep frequency response analysis (SFRA) is a 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. 
Methods.
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:
Process.
Transformers generate a unique signature when tested at discrete 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.
Problem detection.
SFRA analysis can detect problems in transformers such as: 
Uses.
SFRA can be used in the following contexts: