1 引言
为探求不同电压等级下带电雾凇覆冰对分裂导线电场分布的影响规律,本文在人工气候室内完成了不同交流电场下雾凇覆冰后的形态对单、双及三分裂导线的表面电场影响试验,并测量相关参数,根据不同雾凇形态建立有限元模型进行仿真计算,得到导线表面场强最大值及其变化趋势。
2 试验装置、试品及试验方法
试验在内径为 2.0m、内长为3.8m的小型多功能人工气候室内进行,人工气候室内的温度可调,且室内安装有按国际电工委员会（IEC）推荐制作的标准喷头,用来喷雾并形成覆冰条件;气候室内的吹风装置即可使室内温度及雾粒分布均匀;试验电压从人工气候室一侧装设的穿墙瓷套管引入。
表1 分裂子导线基本参数
Tab.1
| 型号 | 2a/mm | 2b/mm | 2c/mm | n/股 |
|---|---|---|---|---|
| LGJ-70/40 | 2.72 | 2.72 | 13.60 | 12 |
注：2a为单股铝线直径;2b为单股钢芯直径;2c为绞线外径;n为绞线最外层股数。
图1
表2 雾凇覆冰形成条件
Tab.2
| 水滴直径 dα/μm | 液态水含量 W/(g/cm3) | 温度 Tα/℃ | 覆冰电导率 γ20/(μS/cm) |
|---|---|---|---|
| 20 | 2.5 | -15 | 30 400 800 1 200 |
3 试验结果及分析
表3 不同电场分裂导线雾凇带电覆冰系数
Tab.3
| 场强/ (kV/cm) | 单导线 | 双分裂 | 三分裂 | ||||
|---|---|---|---|---|---|---|---|
| 1# | 2# | 1# | 2# | 3# | |||
| 冰厚 /mm | 0 | 4.5 | 4.6 | 4.5 | 4.6 | 4.5 | 4.6 |
| 5 | 5.8 | 5.9 | 5.8 | 5.9 | 5.7 | 5.8 | |
| 10 | 6.5 | 6.4 | 6.2 | 6.5 | 6.2 | 6.4 | |
| 15 | 5.2 | 5.3 | 5.1 | 5.3 | 5 | 5.1 | |
| 20 | 4.6 | 4.7 | 4.6 | 4.7 | 4.6 | 4.7 | |
| 冰树枝 底径 /mm | 0 | 1.55 | 1.53 | 1.56 | 1.53 | 1.53 | 1.54 |
| 5 | 1.56 | 1.55 | 1.54 | 1.57 | 1.56 | 1.53 | |
| 10 | 1.54 | 1.52 | 1.56 | 1.54 | 1.54 | 1.54 | |
| 15 | 1.57 | 1.55 | 1.58 | 1.53 | 1.53 | 1.56 | |
| 20 | 1.54 | 1.57 | 1.55 | 1.54 | 1.55 | 1.55 | |
| 冰树枝 高度 /mm | 0 | 1.45 | 1.43 | 1.45 | 1.44 | 1.42 | 1.44 |
| 5 | 2.54 | 2.51 | 2.53 | 2.53 | 2.54 | 2.51 | |
| 10 | 5.15 | 5.17 | 5.13 | 5.17 | 5.19 | 5.18 | |
| 15 | 1.83 | 1.79 | 1.81 | 1.82 | 1.85 | 1.83 | |
| 20 | 1.01 | 1.1 | 1.07 | 1.07 | 1.05 | 1.03 | |
由图2可知,不同覆冰电场下雾凇覆冰后的冰树枝形态并不相同;雾凇覆冰使得导线表面变得极为不光滑,细小的冰树枝会增加导线的粗糙度。
图2
4 冰树枝尖端电场模型及有限元计算
4.1 冰树枝尖端电场模型
为研究冰树枝引起的电场畸变效应,通过对比雾凇覆冰形态数据及表面覆冰照片发现,雾凇冰树枝可以简化等效为圆锥体形状,如图3所示。
图3
4.2 雾凇有限元模型建立及计算
图4
图4
三分裂导线不同电场雾凇电场分布
Fig.4
Electric field distribution of rime icing under different electric field
不带电覆冰时,随着覆冰时间的增加,表面电场如图5所示。施加69kV（有效值）交流电时,未覆冰的三分裂导线表面电场为15kV/cm,而带电覆冰后的导线若继续在该电压下运行则表面电场分别为20.71kV/cm、19.56kV/cm、20.65kV/cm、20.2kV/cm和19.76kV/cm,呈波动趋势,如图5b~5f所示,这是由于覆冰电场为0~5kV/cm覆冰时,水滴受到电场吸引力的作用导致雾凇冰厚迅速增加和雾凇冰树枝长度增加,但较粗的直径会弱化冰树枝的电场畸变作用,故运行表面电场会出现第一次降低;当覆冰电场为10kV/cm覆冰时,冰厚较5kV/cm时增加并不多,但冰树枝却出现明显的变长变尖,故导线表面电场受冰树枝尖端畸变作用更大,导致表面电场增加到最大值;在场强15~20kV/cm下覆冰时,导线覆冰厚度随场强的增加而减小,理论上导线运行表面场强应该降低,但由于强场强下冰树枝电晕活动剧烈,大量的离子轰击和泄漏电流将使得冰树枝尖端出现退化现象,故对导线表面的畸变效应会逐渐减小,导致运行表面场强逐渐上升。
图5中,0kV/cm电场下0~60min覆冰后的三分裂导线若继续在67kV电压下运行,则表面电场分别为22.4kV/cm、20.6kV/cm、19.8 kV/cm和19.3kV/cm,呈逐渐减小趋势,但减小速度逐渐变慢;这是由于导线等效直径的增加会弱化冰树枝尖端电场畸变效应,这与覆冰所得的平均参数结果趋势分析相吻合。
图5
图5
三分裂导线雾凇覆冰电场分布
Fig.5
Field distribution of triple bundle conductor after rime icing
在导线表面电场均为15kV/cm情况下进行30min雾凇覆冰,覆冰之后均运行于69kV电压下,则三种导线表面电场如图6所示。
由图6可知,三种导线在表面电场为15kV/cm覆冰30min后,施加69kV交流电后发现覆冰导线最大电场分别为22.5kV/cm、21.3kV/cm和20.2kV/cm,呈逐渐减小趋势,这是因为相同覆冰时间内,冰树枝对分裂数多的导线畸变越小,故相同电压等级下分裂多的导线表面电场较小,且不容易发生起晕现象。
图6
图6
三种导线覆冰30min 表面电场分布
Fig.6
Field distribution of three kinds of wires after 30 minutes icing
5 结论
（1）雾凇覆冰使得导线表面变得极为不光滑,细小的冰树枝会增加导线的粗糙度,由于雾凇冰树枝尖端使得导线表面电场畸变严重,在很低的电压下冰面即会出现电晕放电,进而出现严重的电晕效应。
（2）随着覆冰电场的增加,形成的雾凇冰树枝形态各异,冰厚和冰树枝高度均出现先增大再减小的趋势。低场强时,水滴主要受到电场吸引力的作用导致雾凇冰厚迅速增加;较高场强时,冰树枝出现明显的变长变尖;强场强下冰树枝电晕活动剧烈,大量的离子轰击和泄漏电流将使得冰树枝尖端出现退化现象,覆冰厚度随场强的增加而减小。
（3）导线表面电场在覆冰电场增加过程中成波动趋势;相同电场下覆冰时,覆冰程度的增加会减小导线表面电场,但减小速度逐渐减慢;分裂数越多的导线覆冰后表面场强越低,故越不容易起晕。
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