Thermal problems caused by harmonic frequency leakage fluxes in 3-phase, 3-winding converter transformers

Harmonic frequency leakage flux can be a limiting factor in 3-phase, 3-winding HVDC converter transformers. Investigation of a 3-phase, 3-winding 240 MVA converter transformer failure indicated the failure was caused by harmonic fluxes. Calculations indicated that the magnitudes of these harmonic fluxes to be approximately 45% of the power frequency leakage flux for the transformer, and are little affected by the transformer impedance or the converter firing angle. A study of the failed transformer loading during its life was made, and a calculation made of the hot spot temperature considering various insulation half-life factors. Based on published information on insulation half-life factors it was estimated that at full load the hot spot temperature of the transformer was about 159°C. From examination of the insulation in the hot spot area, this estimate of hot spot temperature was considered reasonable.