Converter Transformer Load Losses


Locally the LV-winding had very high losses, causing conductors to overheat with consequent turn to turn failure. Because of the small amount of cellulose being degraded, the Dissolved Gas Analyses (DGA) may not alert the end user about the imminent failure.

The simulation provided an excellent match with the observed failure (burnt paper at middle axial height of LV-winding). It allowed the OEM to understand the nature of the failure and take proper corrective actions.
If the simulation had been used to validate the original design, the failure would have not occurred!


  1. Transformer data:
  2. 55 MVA, 124/22/1 kV
  3. HV: 2 Zig Zag windings in parallel
  4. Filter: 2 windings in series
  5. LV: Δ and Y windings, made up of double disks in parallel
  6. The 12 pulse (network side) rectifier injects in the LV windings:

-Harmonic 1, 11, 13, 23 etc. in phase
-Harmonic 5, 7, 17, 19 etc. in phase opposition

Technical Challange
In order to compute the correct current distribution in the windings it was necessary to take into account all the three phases and the filter circuit (coupling a complex circuit with FEM).

The loss calculation has taken into account (for each harmonic current):

  1. Uneven current sharing

among the double disks in parallel
among mechanical conductors in parallel inside each disk.

  1. Uneven current density distribution inside each conductor because of eddy currents.


1st, 11th, 13th harmonic leakage flux
with loss density of the affected double disks



5th, 7th harmonic leakage flux
with loss density of the affected double disks

Simulation allow OEMs to identify critical spots in the windings, to understand the nature of the problem and to validate corrective actions, for example chenge of:

    • type and number of conductors
    • number, shape and relative position of elementary windings making up the LV windings (to improve current sharing).