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

Harmonic frequency leakage flux can be a limiting factor in three-phase, three-winding HVDC converter transformers. Investigation of a three-phase, three-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 159degreesC. From examination of the insulation in the hot spot area, this estimate of hot spot temperature was considered reasonable.