Major insulation design consideration of converter transformer

Converter transformer is the key important equipment in HVDC transmission system, and its life span depends heavily on insulation design. Converter transformer is subjected to several types of voltage during factory testing and in operation, including AC, DC, combined AC-DC voltage and polarity reversal voltage, which makes the insulation systems of converter transformer significantly differs from AC transformer. Design of major insulation system is realized through the analysis of electric field distribution with FEA software at critical locations of a transformer. Three critical stresses should be analyzed in the insulation system design, including maximum local stresses in solid insulation, the stresses across the oil gaps and the creep stresses along the interface between solid and liquid. This paper will focus on the major insulation system design consideration for converter transformers, and the electric field distribution under all types of voltage are discussed to decide the consideration necessity during the design process. The result of this paper can provide guidance for the major insulation design of converter transformer.