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1、3.3 液体电介质的击穿 3.3 Breakdown in liquid dielectrics,关于液体电介质击穿的机理，到目前还很不完善。不少研究得到的实验结果差异大，无法进行分析比较。 The breakdown mechanism of liquid dielectrics is up to now still not perfect and many results of the experiment are so different that they can not be able to be analyzed.,近年来随着液体净化技术不断的提高，建立了一些比较粗糙的液体介质击穿
2、理论。 In recent years,with the improvement of liquid purification technology, some primary breakdown theory of liquid medium was established.,3.3.1 高度纯净去气液体电介质的电击穿理论 3.3.1 Electric breakdown theory of high purity deaerated liquid dielectric 3.3.2 含气纯净液体电介质的气泡击穿理论 3.3.2 Bubble dielectric breakdown theo
3、ry of pure liquid containing gas 3.3.3 工程纯液体电介质的杂质击穿 3.3.3 Impurities breakdown of engineering pure liquid dielectric,本节内容 Contents in this section,假设液体分子热振能量是量子化的，那么当液体分子基团的固有振动频率为时，在与电子的一次碰撞中，液体分子平均吸收的能量仅为一个振动能量子h（h是普朗克常数）。 Provided hot vibration energy of liquid molecules is quantized, when the i
4、nherent frequency of liquid molecular groups is , in a collision with electron, average energy absorbed by liquid molecular is only a vibration quantum of h (h is Plancks constant).,1. 碰撞电离开始作为击穿条件 Breakdown beginning from collisions ionization,3.3.1 高度纯净去气液体电介质的电击穿理论 3.3.1 Electric breakdown theory
5、 of high purity deaerated liquid dielectric,电子在相邻两次碰撞间从电场中得到的能量大于h时，电子就能在运动过程中逐渐积累能量，至电子能量达到一定值时，电子与液体相互作用时便导致碰撞电离。 In the two continuity of collisions, if electron obtains energy from the field higher than h, then electron can accumulate more energy gradually in motion until reaches a certain value
6、, interactions between electron and liquid will cause collision ionization.,击穿场强为 Breakdown field strength for,(3-17),各基团固有振动频率的平均值； The average value of groups natural vibration frequency;,图3-9 直链型碳氢化合物液体Eb与(m-1)(/M)的关系 Figure 3-9 Relationship of straight-chain hydrocarbon fluids Eb and (m-1) (/M),
7、直链型碳氢化合物液体的击穿场强与 成正比，与图3-9所示的实验结果一致。 Breakdown field strength in the straight-chain hydrocarbons liquid is proportional to , the same with the experimental results as shown in Figure 3-9.,2.电子崩发展至一定大小为击穿条件 A certain degree of the avalanche as breakdown conditions,为液体介质上一个电子沿电场方向行径单位距离平均发生的碰撞电离次数，则正比
8、于碰撞总数1/乘以电离几率e-Chv/(eE)，即 represents the average frequency of impact ionizations when an electron moves unit distance along the direction of electric field, then the collision is proportional to the total number of 1/ multiplied by the ionization probability of e-Chv / (eE), which,(3-18),则发生击穿时 Then
9、 the breakdown occurs when,(3-20),所以 So,(3-21),说明在其他参数一定时， 1/ln(d)，即液体介质层的厚度减薄时，击穿场强应增大。 When the other parameters are constant, Eb is far less than 1/ln(d).That is when the thickness of liquid medium layer decreases, the breakdown field strength should be increased.,气泡击穿理论： The bubble breakdown the
10、ory 不论由于何种原因使液体中存在气泡时，由于在交变电压下两串联介质中电场强度与介质介电常数成反比，气泡中的电场强度比液体介质高，而气体的击穿场强又比液体介质低得多。 No matter how there are bubbles existing in liquid, due to alternating voltage in two series medium electric field intensity inversely proportional to the permittivity. The electric field strength in bubbles is high
11、er than that of the liquid medium, moreover, the breakdown voltage of gas is much lower than that of liquid medium.,3.3.2 含气纯净液体电介质的气泡击穿理论 3.3.2 Bubble dielectric breakdown theory of pure liquid containing gas,所以总是气泡先发生电离，这又使气泡的温度升高，体积膨胀，电离将进一步发展；而气泡电离产生的高能电子又碰撞液体分子，使液体分子电离生成更多的气体，扩大气体通道，当气泡在两极间形成“气
12、桥”时，液体介质就能在此通道中发生击穿。 Therefore, the bubble will always be ionized previously and makes bubble temperature rise and volume expand, leading to further ionization. What is more, the high-energy electrons from bubble ionization have a collision with liquid molecules that will generate more gas, when bub
13、bles form “the gas bridge” between two electrodes, breakdown of the liquid medium happens in this passage.,热化气击穿 Thermalization gas breakdown,当液体中平均场强达到107108V/m时，阴极表面微尖端处的场强就可能达到108V/m以上。场致发射，大量电子由阴极表面的微尖端注入到液体中，估计电流密度可达105A/m2以上。 When the average field in the liquid up to 107108V/m, field at the c
14、utting-edge of cathode is likely to reach 108V/m above. Field-emission, a large number of electrons will be injected into liquid through cathode surface tiny tips, the current density estimation of 105A/m2 above. 单位体积、单位时间中的发热量（=电流密度电场强度），约为1013J/(s m3) Thermal power per unit volume and time equals
15、to the product of current density and field strength), about 1013J / (sm3),当液体得到的能量（转化为热量）等于电极附近液体气化所需的热量时，便产生气泡。 When energy obtained by the liquid (change into thermal power) equals to the heat for liquid vaporization near electrode, bubbles appear.,以产生气泡条件作为液体击穿条件，即 Take condition of producing bu
16、bbles as the liquid breakdown condition, namely,(3-22),(3-22),式中 Where,m代表空间电荷影响的常数 ，约在1.52之间； the constant represents the space charge effects, about 1.5 to 2; T液体在电极粗糙处强场区滞留的时间； the time that liquid staying at electrode rough region of strong field; A常数；constant; C液体比热；specific heat of liquid; B液体气化热；liquid vaporization heat; E为液体击穿场强。 liquid breakdown field strength.,式(3-22)可以粗略地估算液体介质的击穿场强。当液体温度升高时，击穿场强下降。 Eq(2-22) can roughly
