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The oil-pressboard insulation on the valve side of the converter transformer is subjected to the steep rising edge pulse voltage caused by commutation in actual operating conditions. The steep rising edge pulse voltage will aggravate insulation degradation and easily lead to insulation failure. However, the effect of voltage waveform steepness on insulation degradation is rarely considered directly in the current research. In this article, the actual voltage waveform of the main insulation on the converter transformer valve side is analyzed, and its harmonic content is determined. The experiments under steep rising edge pulse voltage and superimposed harmonics voltage are set up, and partial discharge pattern and space charge distribution are collected. The steeper the voltage waveform is, the narrower the phase distribution of the discharge pulse is, and the larger the maximum discharge quantity is. There is obvious charge accumulation inside the oil-impregnated pressboard under the steep rising edge electric field. The increase of waveform steepness makes the charge not easy to migrate under the externally applied electric field, and the retention effect is enhanced, resulting in an increase in charge accumulation. Moreover, the steepness of the voltage waveform becomes larger, and the charge injection is accelerated under the pulse voltage. The collision, trapping, and detrapping of high-energy electrons release more energy, which is more likely to cause insulation damage and form more traps. The increase of traps leads to charge accumulation, which results in electric field distortion and further promotes the generation of partial discharge. Based on the above analysis, the influence of voltage waveform steepness on oil-pressboard insulation failure is obtained, which provides a theoretical basis for insulation structure optimization and condition assessment of converter transformer.