直流电场下油纸绝缘楔形油隙局部放电仿真研究

Simulation Study on Partial Discharge of Oil-paper Insulated in Wedge-shaped Oil Gaps Under DC Electric Field

换流变压器是特高压直流设备的重要组成部分,在不均匀电场作用下,换流变压器中会发生局部放电现象,可用楔形电极模型进行模拟。为研究油纸绝缘在楔形电极的局部放电特性,基于双极性电荷输运和流体动力学漂移扩散理论建立二维轴对称仿真模型,计算获得纳秒尺度下的微观放电图像,得到空间电荷和电场强度分布的变化过程,并讨论电压幅值、电荷迁移率、油纸界面电荷等影响因素对局部放电特性的影响。仿真结果显示,楔形油隙的放电发展过程主要分为油中电晕、油纸界面水平沿面放电和绝缘纸纵向放电3个阶段。油纸界面闪络电压随纸中电荷迁移率的增大呈现出先增大再减小的变化规律,当纸板的电子迁移率和空穴迁移率为10^(-13) m^(2)×v^(-1)×s^(-1)时,横向放电发展受到抑制,闪络电压最高;变压器油的电荷迁移率对放电过程影响较小。放电滞留在纸板表面的正极性界面电荷形成电荷“屏蔽层”,对下一次的放电发展起到抑制作用,导致放电横向发展的速度和范围均随界面电荷量的增加而降低。该文的研究结果对阐明楔形电极的放电机理具有重要理论意义。

The converter transformer is an important part of ultra-high voltage direct current(UHVDC)equipment,and partial discharge(PD)will occur in converter transformer under the action of non-uniform electric field,which can be equivalent to the wedge-shaped electrode model.In order to study the PD characteristics of oil-paper insulation at wedged electrode,a model based on the bipolar charge transport model and the hydrodynamic drift-diffusion model was built.The micro discharge images at nanosecond scale were calculated to obtain the process of space charge and electric field distribution.Besides,the influences of parameters such as voltage amplitude,charge mobility,and oil-paper interface charge on PD characteristics were discussed.The simulation results show that the PD development process of the wedge-shaped oil gaps has mainly gone through three stages:the corona in oil,the horizontal surface discharge at the oil-paper interface,and the longitudinal discharge at the insulating paper.With the increase of charge mobility in paper,the flashover voltage first increases and then decreases.When the electron mobility and hole mobility in paper are 10^(-13) m^(2)×v^(-1)×s^(-1),the growth of horizontal discharge is suppressed and the flashover voltage is the highest.The charge mobility in transformer oil has less influence on the discharge process.The positive charge retained on the paperboard surface forms a“shielding layer”,which can inhibit the next discharge,resulting in the decrease of the horizontal speed and range with the increase of interface charge density.The results of this paper are of great theoretical significance to clarify the discharge mechanism of wedged electrode.