特高压直流输电技术由于其优越性发展迅速，然而因空间电荷诱发的绝缘问题，作为关键设备之一的换流变压器故障频发，威胁电力系统的安全稳定运行。本文基于电声脉冲（Pulsed electro-acoustic, PEA）法空间电荷测量技术和双极性载流子输运仿真模型，对电热耦合下油纸绝缘中的空间电荷特性及其对击穿的影响开展研究，为换流变压器的设计、制造和运行提供指导。针对绝缘纸中的孔隙及其导致的表面粗糙度问题，通过声学建模仿真发现：粗糙度形成非线性接触面，导致测得的体电荷密度小于实际值；由于孔隙，油纸中PEA信号主要来自于快波，导致空间电荷测量值偏大。将它们的影响简化成一个串联系统，构造了PEA系统校正函数。基于该校正函数可提高油纸绝缘空间电荷测量准确性，拓宽PEA法测量对象的适用范围。油纸中以双极同极性电荷积聚为主；不同极性下的空间电荷在稳态时呈镜像分布；极性反转电压下的空间电荷特性具有方向效应，在实际操作中须予以关注。其主要原因之一是绝缘纸中的电负性基团造成能带弯曲。正、负电荷均可在油纸-油纸接触面处积聚；由于油纸-油纸界面处的能级结构呈U形结构，并且正电荷更难穿过该能级结构，因而接触界面两个表面上电荷符号相反，形成“双电层”结构，并且正电荷总量大于负电荷。该结构有效地解释了一直备受关注的油纸绝缘界面电荷的极性效应问题。理化特性研究表明孔洞是空间电荷动态过程导致油纸中形成缺陷的一个体现；因为正电荷更难穿过孔洞处油纸-油纸界面的U形能级结构，当老化条件中存在电场分量时，陷阱电荷以正电荷为主；孔洞的形成会加速绝缘老化，正电荷或孔洞总量可视为油纸绝缘诊断或失效的一个判据。在?20℃~120℃范围内油纸的击穿可划分为三个区域，即电、电热联合和热击穿；基于此以及油纸能带弯曲结构，联立双极性载流子输运模型和热平衡方程，构建电热耦合下的迁移率及击穿判据，建立了电热击穿模型，仿真结果与实验结果一致；基于孔洞的形成及其影响，简化和改进电荷输运模型，以入陷或脱陷电荷总量为判据，建立了电热耦合下油纸寿命公式。该公式表明反幂定律存在局限性，不能由高场强下的寿命推算低场强下的寿命。
Ultra high voltage direct current power transmission technology has been developed fast recently due to its excellent advantages, however, due to the insulation problem induced by space charge, failure happens to the converter transformer, one of the key equipment, frequently. It is a threat to the safe and stable operation of power system. Based on the pulsed electro-acoustic (PEA) space charge measurement technology and bipolar carrier transport simulation model, this paper studied space charge characteristics and its impact on breakdown in oil-paper insulation under the combined electro-thermal stress, which provided experimental and theoretical guidance to the design, manufacture and operation of converter transformer.As regards the pore distributing within insulation paper and the surface roughness resulting from it, an acoustic model was set up to analyze the problem. The simulation results showed that surface roughness formed a non-linear contacting interface, which made the measured volume charge density less than the actual value. Because of the pores, the PEA signal genaerated in oil-paper was mainly from the fast wave, the measured space charge density was consequently larger than its actual value. In addition, the experiment result verified that the impact of both of them meanwhile existed. Then the combined influence of them was simplified to a series system, and the calibration function of PEA system was established. On the base of the calibration function, the PEA signal can be corrected, space charge density within oil-paper measured by PEA method was more accurate and more materials were therefore suitable for space charge measurement by PEA method.Bipolar homogeneous charge accumulation took a predominant role in oil-paper. At steady state, “mirror image effect” charge distribution of different polarities occurred. And the voltage polarity reversal direction affected space charge characteristics, attention must be paid to which during pratical operation. Analysis showed that one of the main reasons was that the energy band of insulation paper was bended upward on account of electronegative group within oil-paper.Both positive and negative charge can accumulate at the contacting interface. The sign of charge on each surfaces of the contacting interface was opposite, forming an “electrical double layer”. And the amount of positive charge was larger than negative charge. This was due to the U-shaped energy level structure formed at the interface, which was more difficult for positive charge to pass through. The “electrical doubule layer” and U-shaped energy level structure can well explain the long concerned polarity effect of space charge at the interface.Physical-chemical property study showed that the cavity reflected the formation of defects in oil-paper caused by space charge dynamics. Once electrical stress was applied during aging, the trapped charge was mainly positive because it was harder for positive charge to pass through the U-shaped energy level structure at the interface, the formation of cavity can thereby accelerate the aging of oil-paper. So the amount of positive charge or cavity can be seen as a criterion for diagnosis or failure of oil-paper.It was found that in the range of -20℃~120℃, the breakdown voltage of oil-paper can be divided into three regions, namely electrical breakdown, electro-thermal breakdown and thermal breakdown. Based on this and the energy band bending, using bipolar carrier transport simulation model and the heat balance equation, considering mobility and breakdown criterion under combined electro-thermal stress, an electro-thermal breakdown model of oil-paper was established. The simulation result was consistent with experiment results. And then further based on the formation of cavity and its influence during aging, the bipolar charge model was simplified and improved, and the life of oil-paper under electro-thermal stress can be calculated, on the hypothesis that when the amount of trapped or detrapped charge reached a critical value, insulation failure took place. This life equation showed that there were limitations to the anti-power law of life, which meant that the life under low fields can not be directly estimated from the life under high fields.
