Sweep Frequency Response Analysis Test | SFRA Test

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Key learnings:
  • SFRA Test Definition: SFRA test of transformer is a method used to assess the condition of transformer windings by analyzing their frequency response to electrical inputs.
  • Understanding RLC Circuits: Transformers are complex RLC circuits; their components create a unique frequency response pattern that SFRA tests measure.
  • Diagnostic Capability: SFRA tests identify potential problems in transformers, such as winding displacement, deformation, and other mechanical faults.
  • Test Methodology: By applying a sinusoidal voltage and measuring the output, the SFRA test captures how transformer windings react at different frequencies.
  • Importance of Consistency: Comparing historical and current frequency response patterns helps detect subtle changes in transformer windings, indicating possible damage or wear.

The SFRA Test is a highly sensitive and reliable tool for monitoring the condition of transformer windings. These windings may endure mechanical stresses from transportation, heavy short circuits, and lightning strikes. Such stresses can displace or deform the windings, and in extreme cases, lead to their collapse. This can ultimately cause insulation failure or dielectric faults. Sweep Frequency Response Analysis Test or in short SFRA Test can detect efficiently, displacement of transformer core, deformation and displacement of winding, faulty core grounds, collapse of partial winding, broken or loosen clamp connections, short circuited turns, open winding conditions etc.

Principle of SFRA Test

The principle of the SFRA test is straightforward. Since all electrical equipment has resistance, inductance, and capacitance values, they can be modeled as complex RLC circuits.
The term ‘theoretically’ means some equipment may have very low or zero resistance compared to their inductance and capacitance values again, some equipments may have very low or zero inductance compared to their resistance and capacitance and again some equipments may have very low or zero capacitance compared to their resistance and inductance but theoretically all of them can be considered as RLC circuit although may be R = 0, or L = 0 or C = 0. But in most cases the resistance, inductance and capacitance of an equipment have non zero values. Hence most of the electrical equipments can be considered as RLC circuit hence they response to the sweep frequencies and produce an unique signature.

In a transformer, each winding is separated by paper insulation acting as a dielectric, and the windings have their own resistance and inductance. This makes the transformer a complex network of resistance, inductance, and capacitance—a complicated RLC circuit.

rlc network of transformer
Because of that each winding of a transformer exhibits a particular frequency response. In Sweep Frequency Response Analysis a sinusoidal voltage Vi is applied to one end of a winding and output voltage Vo is measured at the other end of the winding. Other 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.

That means if we go on increasing the frequency of the input signal without changing its voltage level we will get different output voltages at different frequencies depending upon the RLC nature of the winding. If we plot these output voltages against the corresponding frequencies we will get a particular patter for a particular winding. 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.

sweep frequency response

In addition to that, SFRA test also helps us to compare between physical condition of the same winding of different phases at the same tap position. It also compares different transformers of the same design. Analysis Low frequency response

  1. Winding behaves as a simple RL circuit formed by series inductance and resistance of the winding (At low frequencies capacitance acts as almost open circuit).
  2. At low frequencies winding inductance is determined by the magnetic circuit of the transformer core. High frequency response.
  3. At high frequencies winding behaves as RLC circuits.
  4. Winding exhibits many resonant points.
  5. Frequency response is more sensitive to winding movement.

Different Connection During SFRA Test

Signal applied across transformer terminalsConditions
HV Red phase to NeutralLV Red Yellow Blue phases are open
HV Yellow phase to NeutralLV Red Yellow Blue phases are open
HV Blue phase to NeutralLV Red Yellow Blue phases are open
HV Red phase to NeutralLV Red Yellow Blue phases are shorted
HV Yellow phase to NeutralLV Red Yellow Blue phases are shorted
HV Blue phase to NeutralLV Red Yellow Blue phases are shorted
LV Red to Yellow phaseHV Red Yellow Blue phases and LV Blue phase are open
LV Yellow to Blue phaseHV Red Yellow Blue phases and LV Red phase are open
LV Blue to Red phaseHV Red Yellow Blue phases and LV Yellow phase are open

Example Data Sheet for SFRA Test Result

sfra result sheet
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