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Transformer windings and leads


Transformer windings consist of paper-insulated, current carrying conductors wound around sections of the core; a winding lead is an insulated conductor that connects a winding to another winding, to a tap changer, or to an exit terminal (bushing).  To withstand operational conditions, windings and leads must be properly insulated, supported and cooled.

Transformer winding failures have statistically been attributed as the most frequent cause for transformer faults (ANSI/IEEE, 1985), followed by load tap changers, and bushings.  A major reason for winding failure is insulation deterioration.  Factors that contribute to deterioration include moisture, voltage surges, overheating, vibration and mechanical stress created during transformer through faults.

The following electrical field tests provide information about the integrity of the transformer windings.


Winding diagnosis


  • Transformer turns ratio: checks the fundamental operational characteristic of a transformer, e.g., that it is transforming voltage as expected, and may help to identify short-circuited turns, open-circuited conditions, and incorrect winding connections.
  • Exciting current: particularly adept in the detection of partial turn-to-turn short circuits in a winding (sensitive to the deterioration of the turn-to-turn insulation that is not tested in a power factor measurement).
  • DC winding resistance: assesses the integrity of the current-carrying path between transformer bushing terminals; detects problems such as loose or defective connections, broken strands, open-circuit conditions, or high contact resistance in tap changers.
  • Leakage reactance/short circuit impedance: the premier test to identify or confirm winding deformation
  • Frequency response of stray losses (FRSL): alone in its ability to detect strand-to-strand short-circuits in a conductor bundle; also sensitive to problems that have resulted in increased losses of conductive structural components such as the transformer tank, clamping structure and tie plates.
  • Sweep frequency response analysis (SFRA): sensitive to short-circuited turn(s), open-circuited winding conditions and winding deformation
  • Power factor/Tan delta/dissipation factor/capacitance (@ line frequency): assesses the ability of the insulation that isolates HV or LV windings from ground potentials, or HV windings from LV windings, to efficiently store the energy placed across it; detects moisture and other contaminants, and increase in voids in the insulation.  Capacitance is sensitive to extreme winding deformation.
  • Variable frequency power factor/dissipation factor (VFPF); provides confidence in the interpretation of the power factor/dissipation factor results above and more discerning information about elevated power factor/dissipation results
  • Dielectric frequency response (DFR)


Winding leads diagnosis


  • DC winding resistance: very effective in the detection of problems such as loose or defective connections, broken strands, and full or partial open-circuit conditions.
  • Sweep frequency response analysis (SFRA): assesses mechanical health; sensitive to open or partially-open connections of a lead and even a situation whereby a lead is incorrectly and inconsistently routed to a bushing terminal as compared to the lead routing associated with the other two phases of the transformer.