空间电荷对直流接地电树枝影响的实验及仿真研究

Effect of Space Charge on Grounded DC Electric Tree:Experiment and Simulation

为研究聚乙烯中直流接地电树枝的生长特性与材料中空间电荷分布的联系,该文以某商用级XLPE高压直流电缆绝缘材料为对象,分别进行了20℃下不同单次预压时间和短路次数的直流接地电树枝实验。实验结果表明:正负极性下电树枝长度都随着短路次数的增加而增加,正极性下电树枝长度随着单次预压时间的增加先增加再减小,负极性下单次预压时间对电树枝的影响不大;负极性下电树枝的扩散系数整体大于正极性下的电树枝。同时,基于双载流子模型建立了针板电极的三维仿真模型,在综合考虑了电荷的注入、迁移、扩散、入陷和脱陷等行为的基础上,计算出XLPE中随时间变化的空间电荷分布,并据此提出了空间电荷有效注入深度这一新的的概念,以此表征能够引发电树枝的那部分空间电荷在材料中的分布范围,从空间电荷的角度对实验所发现的电树枝特性进行了合理的解释,证明了受陷电荷是直流接地电树枝的成因。

In order to study the relationship between space charge and the growth characteristics of grounded DC tree in cross-linked polyethylene（XLPE）, periodic-grounded-DCtreeing experiments under 20℃ with positive and negative polarities were carried out on a commercial grade high voltage DC cable insulation. The results show that the length of electrical tree grows with the increase of short circuit times under both polarities. At the same time, the length of electrical tree rises and then decreases under positive polarity with the increase of single DC-prepressing time, while it shows little effect under the negative polarity; the expanding coefficient of electric tree under negative polarity is greater than that under positive polarity. A modified 3 D model of needle-plate electrode based on bipolar charge transport mechanism, is established to simulate the space charge in XLPE, and the model takes charge injection, migration, diffusion, trapping, as well as detrapping into account. With the model, the time-variant distribution of space charge in XLPE could be figured out. Moreover, a new concept of effective charge injection depth has been presented to characterize space charge distribution range, at which the density of space charge is large enough to initiate electrical tree. With the modified model the feature of electrical treeing is explained reasonably based on the concept, proving trapped charge is the cause of grounded DC tree.