Analysis, modeling, and simulation of the phase-hop condition in transformers: The largest inrush currents

Ashkan Farazmand, Francisco De León, Kuang Zhang, Saeed Jazebi

Research output: Contribution to journalArticlepeer-review


Inrush currents in transformers can have very disruptive effects, such as voltage sags, false tripping of the protective devices, and mechanical stresses in the transformer windings. This paper shows that there are operating situations that may cause a transformer to draw abnormally high inrush currents. Examples include the normal operation of offline uninterruptible power-supply (UPS) systems, interruptions, voltage sags, and notching. These conditions may produce inrush-like currents of more than twice the value of the 'normal' maximum inrush caused by energizing at voltage zero-crossing. For this condition, the term 'phase-hop' is used in this paper. Laboratory experiments were performed on four different transformers (1 kVA) with varied characteristics and show the impact of phase-hop in the magnitude of inrush currents. The experiments are also used to validate the Electromagnetic Transients Program model used for the analysis of multiple cases. In addition, the behavior of the magnetic flux in a transformer under phase-hop is investigated and compared with different operating conditions using finite elements. The results of this paper have implications in transformer design and in the operation and design of UPS systems to prevent the damaging effects of phase-hop.

Original languageEnglish (US)
Article number6742628
Pages (from-to)1918-1926
Number of pages9
JournalIEEE Transactions on Power Delivery
Issue number4
StatePublished - Aug 2014


  • Inrush currents
  • interruptions
  • phase-hop
  • transformer modeling
  • uninterruptible power-supply (UPS) systems
  • voltage sags

ASJC Scopus subject areas

  • Energy Engineering and Power Technology
  • Electrical and Electronic Engineering


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