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

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2 Author(s)
Forrest, J.A.C. ; Teshmont Consultants Inc., Winnipeg, Man., Canada ; Allard, B.

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 159°C. From examination of the insulation in the hot spot area, this estimate of hot spot temperature was considered reasonable.

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Power Delivery, IEEE Transactions on  (Volume:19 ,  Issue: 1 )