【作者】 鲍明晖；

【导师】 何俊佳；

【作者基本信息】 华中科技大学 ， 电气工程， 2011， 博士

【摘要】 随着交联聚乙烯(XLPE)电缆越来越广泛的应用于输电系统,XLPE电缆的绝缘击穿特性严重影响了电力输电系统的稳定运行。电树枝是高电场作用下固体绝缘失效的主要原因之一,因此研究XLPE电缆绝缘中电树枝的生长特性和生长机理对提高电力输电系统的安全可靠性有重要意义。本文结合国内外研究的现状,对XLPE电缆绝缘中电树枝的生长特性进行了实验研究和理论分析。本文采用了显微摄影系统,对不同电压、频率和电极间距条件下电树枝的生长过程进行了实时观测实验,并从实验图像中计算得到了电树枝径向最大长度、空间分形维数、生长速度时间序列分形维数等实验数据。根据实验结果,分别从电树枝生长实验现象、电树枝生长空间特性、电树枝生长时间特性三个方向出发,对电树枝的生长特性进行了研究。针对电树枝的发展过程的三个阶段：起始、发展、击穿,分别分析了电树枝的实验现象。并分析了不同条件和不同形态下的电树枝的生长速度、空间分形维数和时间序列分形维数变化规律。分析结果表明,电压、频率和电极间距对电树枝的生长特性的影响有明显的规律性。不同形态电树枝有着相同的起始结构。随着电压的增大和电极间距的减小,起始时间值增大,分散性降低。而频率对起始时间影响较小。实验中主要观测到两类击穿,电树枝发展到接近地电极时发生的击穿和加压后的立即击穿。后者发生在电压较大时,且击穿前都有相似的电树枝横向生长结构。频率、电极间距和电压对电树枝发展过程有明显的影响规律。当保持频率4kHz不变,随着电压的升高,电树枝形态从稀疏枝状转为稠密枝状再转为丛林状,对应的分形维数和生长速度不断增大。当保持电压不变,随着频率的升高,电树枝形态从稀疏枝状转为较稠密枝状再转为藤枝状,分形维数逐渐变大,生长速度逐渐变快。利用电场驱动模型分别对高频电压下交联聚乙烯中电树枝的形态特性和空间生长速度特性进行了研究。对电树枝形态特性的的分析结果表明,局部电场强度和频率对电树枝形态的影响有明显的规律性。丛林状电树枝只在较高电压下形成,而且针尖附近小区域内的局部电场强度是形成丛林状电树枝的重要影响因素。纯藤枝状电树枝只在较高频率下出现,而且这个频率随着电压的升高而增大。双结构电树枝的结构转换位置的电场强度在不同的电压和电极间距下基本保持不变,但是随着频率的升高而明显增大。对电压、频率和电极间距对电树枝空间生长速度特性的影响进行了分析。根据电树枝生长速度值的大小,把电树枝生长速度曲线分为了三个区域,并求出了区域分界线对应的局部电场强度值,近似认为该值即为电场强度临界值Ec。结果表明,生长速度区域分界线的位置的变化趋势与局部电场强度的变化趋势相一致。随着电压的增大、频率的增大和电极间距的减小,电树枝生长速度的缓慢生长区域明显变小。电场强度临界值Ec随着电压的变大有小幅度的增长,而频率和电极间距对Ec值影响较小。基于电树枝生长动力学模型对电树枝生长时间特性进行了研究。在现有的动力学模型基础上,基于电树枝的生长机理,考虑了频率的影响作用,对模型进行了改进,建立了电树枝生长时间和外加电应力、频率之间的联系。基于改进的电树枝生长时间计算模型,引入了一个新的参数：能量阈值,表征了微孔形成的难易程度,该参数可以通过实验结果计算得到。计算结果显示不同形态下和不同外施条件下的能量阈值变化有明显的规律性。不同频率下的能量阈值随频率的变化关系的计算结果与理论值相近,说明了改进的电树枝生长时间计算模型中对频率影响因素的考虑是合理的。根据所有电树枝每个时间点的实验结果,采用随电树枝生长动态变化的局部电场强度计算公式,根据能量阈值计算公式计算了能量阈值和对应的电树枝尖端局部电场强度,并拟合得到了能量阈值定义式中的未知参数,从而得到了电树枝生长时间计算模型中的关键参数。根据该模型,可以由频率、电压、电极间距、电树枝长度和分形维数计算得到电树枝的生长时间。更多还原

【Abstract】 Since the XLPE cables are widely used in power system, the dielectric breakdown characteristics of XLPE cables are of great concern for the security and stability of power system. Electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses, so it’s of great importance to study the electrical treeing characteristics and mechanisms in XLPE cable insulation for the improvement of safety and reliability of power system. In this paper, based on the current research works, the electrical treeing characteristics in XLPE cable insulation are experimentally and theoretically studied.Treeing process in XLPE samples subjected to various voltages, frequencies and pin-plane spacings are uninterrupted recorded by an online microphotograph system. The radial extent length and spatial fractal dimension of electrical tree structures, and the time series fractal dimension of tree growth rate are computed from the experimental results. The investigations of treeing characteristics are divided into three parts:electrical treeing phenomenon, spatial characteristics of electrical treeing and time characteristics of electrical treeing.The phenomenon in the three steps of electrical treeing:initiation, propagation and breaking down are separately described and analyzed. Tree growth rate and fractal dimension under different voltages, frequencies and distances between electroces are found out. It can be concluded that the influences of voltage, frequency and pin-plane spacing on treeing characteristics are full of regularity. Trees in different structures have the similar initiation structure. With the larger voltage and shorter pin-plane spacing, the time for initiation was longer and less dispersed. The influence of frequency on initiation is relatively small. Two types of breakdown were observed in experiments:the breakdown happened when the tree was closely to the plane and the breakdown happened right after the voltage was applied. The latter type of breakdown was observed when the voltage was higher, and the trees have the similar lateral propagation structure before breakdown. The influences of voltage, frequency and pin-plane spacing on tree propagation are full of regularity. Under the same frequency, with the increasing voltage, the tree structures changed from sparse branch-type to dense branch-type and then to bush-type, and the corresponding fractal dimension and growth rate increased. Under the same voltage, with the increasing frequency, the tree structures changed from sparse branch-type to dense branch-type and then to vine-type, and the corresponding fractal dimension and growth rate increased..Based on FDTG (Field Driven Tree Growth) model, the structure characteristics and spatial growth rate characteristics of electrical trees are investigated. It can be found in the structure analysis that the local electric field and frequency regularly affect the tree structure. Bush-like trees only formed under higher voltages, and the electric field in a very small zone near the needle tip is an important influencing factor for the formation of bush-like trees. Pure vine-like trees formed only under high frequencies, and the lowest frequencies that pure-vine-like trees appear increased with voltage. For double-structure trees, the local electric field at the transition location almost stayed constant under different voltages and pin-plane spacings, but obviously increased with frequency. During the spatial growth rate analysis, the growth rate curves were divided into three regions according to the values, and the local electric field at the dividing line were computed, which can be seen as the critical electric field EC.It can be concluded that the change tendency of dividing lines is coincident with the change tendency of local electric field. Higher voltage, higher frequency and smaller pin-plane spacing all could result in the narrower region of slowly growth. The critical electric fields Ec increased slightly with voltage, and stay almost the same with frequency and pin-plane spacing.Based on electrical treeing kinetic model, the time characteristics of electrical trees are investigated. According to the influencing mechanics of frequency on electrical treeing process and the existing electrical treeing kinetic model, an improved electrical tree growing time model has been built, in which tree growth time depends on electric field and frequency. A new parameter:energy threshold, which is derived from the improved electrical tree growing time model, was introduced and computed from the experimental results. Calculating results of energy threshold regularly change with different conditions and structures. The change trend of energy threshold under different frequencies according to the model are similar to the calculating results from the experiments, which means the consideration of frequency in the improved model is correct. According to the experimental results of all time points in all of the electrical trees, using the dynamic electric field calculating equation, the energy threshold and the corresponding local electrical field are calculated, and two important parameter of the growth time calculating model are calculated by curve fitting. Finally according to the improved growth time calculating model, electrical tree growth time can be calculated from frequency, voltage, pin-plane spacing, tree growth length and fractal dimension.更多还原