As a “new infrastructure” investment support economic project, ultra-high voltage direct current transmission has ushered in a new round of development and acceleration period. The demands of today’s times, such as larger transmission capacity, higher voltage level, and better transmission quality, have put forward higher requirements for the insulation reliability of converter transformers. The oil-pressboard insulation system is the main insulation structure of its casing valve side winding, outlet device, and other core parts. Due to the complexity and variability of the withstand voltage and working environment, it is prone to produce bubble defects under the combined action of various electrical and thermal factors. The migration, deformation, accumulation or other behaviors of bubbles with the oil flow will further distort the extremely uneven field of the insulation gap, leading to partial discharge and insulation degradation. For the purpose of clarifying the dynamic behavioral characteristics of air bubbles in oil prior to air gap breakdown under AC and DC compound voltages, this paper establishes a multi-field coupled bubble dynamics simulation model, investigates the influence laws and mechanisms of different factors such as the effective value of applied voltage, voltage polarity, and transformer oil temperature on the migration and deformation of bubbles, to provide theoretical guidance for the design, manufacturing, and fault warning of converter transformers.

Converter transformers;Multi-field coupling;Gas bubbles;Dynamic characteristics