Analysis of Electric Field Redistribution in Oil-Pressboard Insulation of a ±525 kV HVDC Converter Transformer under AC/DC Superimposed Voltage

HVDC converter transformers are subjected to complex electrical stresses because AC and DC voltage components coexist on the valve winding. This study investigates time-dependent electric-field redistribution in the oil-pressboard insulation of a ±525 kV VSC-HVDC converter transformer under an earth-referenced AC/DC superimposed valve-winding voltage obtained from system-level steady-state analysis. A time-dependent finite-element electric-field analysis was performed in COMSOL Multiphysics using fixed electrical properties of insulating oil and pressboard. The electric field was initially concentrated in the oil channels but gradually shifted toward the pressboard barriers due to Maxwell-Wagner field redistribution. The Maxwell-Wagner relaxation times for the different barrier configurations ranged from approximately 3.49 to 5.79 s, and the 50 s analysis period covered a period in which the field variation became gradual following the major redistribution. With the inter-barrier oil-gap thickness fixed at 2 mm and the total insulation distance maintained at 110 mm, the pressboard barrier thickness and number were varied, thereby changing the overall pressboard fraction. At 50 s, the maximum pressboard electric field decreased from 16.9 to 10.8 kV/mm, corresponding to a reduction of approximately 36%, while the oil-channel electric field remained within a relatively similar range despite changes in the overall insulation configuration. These results highlight the need to consider time-dependent electric-field redistribution caused by the sustained DC component of AC/DC superimposed voltage in HVDC converter-transformer insulation design.