Converter transformers are the important components of ultra-high voltage (UHV) DC transmission systems. With the increase of UHV transmission voltage levels, the main insulation of converter transformers is subjected to the combined effects of high-strength and extremely uneven fields, such as electrical, magnetic, thermal, and mechanical. The main insulation system of the valve side winding for the converter transformer is subjected to complex electric fields such as AC-DC composite, DC superimposed harmonics, and polarity reversal voltage for a long time, which causes the dielectric constant of the oil-pressboard insulation to be in a dynamic state and affects the insulation performance of the converter transformer. The vibration of the iron core and winding during transportation and operation inevitably causes deformation of the converter transformer winding, resulting in defects in the insulation system and affecting its electric field distribution during operation, thereby posing severe challenges to the insulation system. In this paper, a full-scale simulation model of a converter transformer is conducted and the changes in electric field under potential defects (burrs, dents, bubbles) in the valve side winding are analyzed. The simulation results can comprehensively improve the operational reliability and safety of the converter transformer.