Sweep Frequency Response Analysis Test | SFRA Test of Transformer

What is SFRA Test?

Frequency response analysis (often referred to as FRA or SFRA) is a powerful and sensitive method for testing the mechanical integrity of transformer cores, windings, and press frames, in power transformers.

The winding of transformer may be subjected to mechanical stresses during transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc. These mechanical stresses may cause displacement of transformer windings from their position and may also cause deformation of these windings.

SFRA Test can detect,

• winding deformation – axial & radial, like hoop buckling, tilting, spiraling
• displacements between high and low voltage windings
• partial winding collapse
• shorted or open turns
• faulty grounding of core or screens
• core movement
• broken clamping structures
• problematic internal connections
• 
  

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 term 'theoretically' means in most cases the resistance, inductance and capacitance of an equipment have non zero values. Hence most of the electrical equipment can be considered as RLC circuit hence they response to the sweep frequencies and produce an unique signature.

As in a transformer each winding turn is insulated from other by paper insulation which acts as dielectric and winding themselves have inductance and resistance, hence a transformer became as a complicated distributed network of R,L and C or in other words a transformer is a complicated RLC circuit.


Now in SFRA sinusoidal voltage Vi is applied to one end of a winding and output voltage Vo is measured at the other end of the winding. Remaining windings are kept open. As the winding is itself an distributed RLC circuit it will behave like RLC filter and gives different output voltages at different frequencies.

Here if we make constant input voltage Vi and change different frequency than according to the complexity of each winding we can find different output voltage Vo with different frequency.Each winding have unique signature of frequency Vs Output voltage graph.


But after transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc, if we do same Sweep Frequency Response Analysis test and superimpose the present signature with the earlier patterns and observe some deviation between these tow graphs, we can asses that there is mechanical displacement and deformation occurred in the winding.

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