Elimination of Residual Flux in Transformers by the Application of an Alternating Polarity DC Voltage Source

Francisco De Leon, Ashkan Farazmand, Saeed Jazebi, Digvijay Deswal, Raka Levi

Research output: Contribution to journalArticlepeer-review


The purpose of core demagnetization is twofold: 1) to reduce the inrush currents when transformers are energized; and 2) to make sure that the frequency-response analysis (FRA) tests are consistent to avoid false diagnoses of damage during transportation. The significance of demagnetizing is presented on field measurements of an 80 MVA unit with FRA measurements. A new demagnetizer device with an alternating polarity dc voltage source is prototyped. Experimental verification of this prototype is presented for the demagnetization of transformers. A nearly complete demagnetization was observed in the laboratory for a small single-phase isolation transformer. The method proposed in this paper is applied to three-phase transformers with different core configurations and connections. Topologically correct modeling and numerical simulations confirm the full demagnetization of all branches of three-phase (three-and five-limb) transformer cores. Inrush current measurements and FRA plots before and after demagnetization confirm the effectiveness of the process.

Original languageEnglish (US)
Article number6980140
Pages (from-to)1727-1734
Number of pages8
JournalIEEE Transactions on Power Delivery
Issue number4
StatePublished - Aug 1 2015


  • Demagnetization
  • Transformers
  • frequency-response analysis (FRA)
  • inrush currents
  • residual flux

ASJC Scopus subject areas

  • Energy Engineering and Power Technology
  • Electrical and Electronic Engineering


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