Influential factor analysis and computation on electric field of converter transformer barrier

Bushing and barrier system are the critical components in ultra high voltage (UHV) converter transformer for safe operation. Electric field distributions within bushing and barrier system are complicated in UHV converter transformer under combined AC and DC voltages, which, as a consequence, results in some technical difficulties in insulation structure design and manufacture. The paper presents the FEM electric field simulation of busing barrier structure under AC, DC and polarity reversal operation voltages. Firstly, the research results show that transformer oil with lower relative dielectric constant undertakes higher field strength under the AC voltage, whilst the insulating pressboard with higher resistivity bears higher field strength under the DC voltage, and the maximum field strength occurs in the insulating coating on the shielding ring surface. Moreover, extremely high field strength appears in transformer oil due to the space charge accumulation during voltage polarity reversal. Secondly, the research indicates that temperature has a big influence on electric field distribution of barrier system under DC voltage, specifically, with reduction of temperature the electric field distribution among oil and pressboard becomes increasingly uneven. Finally, the feasibility of applying nonlinear insulating pressboard to replace tradition pressboard in barrier system insulation structure is discussed and concludes that the nonlinear pressboard could improve electric field distribution dramatically.