植被火条件下多相体放电与空气间隙绝缘强度下降的耦合机制

The vegetation fire in the overhead transmission line corridor can reduce the insulation strength of air gap significantly and may cause transmission line tripping faults and out of operation. However, the air-liquid-solid multi-phase mixture discharge in the air gap under vegetation fire condition is complicated and has lots of influencing factors, and the current air gap discharge theory can not explain its discharge process and breakdown phenomenon satisfactorily. In this project, in order to reveal the coupling mechanism of air gap insulation strength reduction caused by the multi-phase mixture discharge under vegetation fire condition, the AC breakdown characteristics tests of a short air gap with different typical vegetation types are carried out based on the former constructed simulated vegetation fire test platform. The difficult technical problems of partial electric conductivity measurement, large scale particles shape and distribution, and discharge arc path observation in the air gap discharge channels are solved during the tests and the vegetation combustion features' macroscopic physical parameters are obtained. The single and multiple influence rules of vegetation combustion features to the air gap insulation strength is deeply studied, and the multidimensional nonlinear relation between the vegetation combustion features and air gap discharge voltage is established by using the multivariate statistical analysis method. The influence rule of large scale particles on the discharge arc development path is analyzed with the measurements and simulation results of electric field and fluid field along the flame channel. The particle size effect of electric field distortion and triggering discharge of large scale particles in the flame is obtained, and the multi-phase mixture discharge theory of air gap is supplemented. The multi-factor discharge model of air gap in the vegetation fire is preliminarily built. The research results can give theoretical support to the air gap insulation design of overhead transmission line under the condition of vegetation fire, as well as the insulation failure assessment and protection.

架空输电线路通道内植被火易使空气间隙绝缘失效而导致线路跳闸停运。而植被火条件下空气间隙中气液固多相体放电复杂，影响因素多，现有空气间隙放电理论无法圆满解释其放电过程与击穿现象。为揭示植被火条件下多相体放电降低空气间隙绝缘强度的内在原因，本项目基于现有试验平台，开展典型植被火、短间隙的交流击穿试验，解决试验中间隙放电通道局部电导率测量、大尺度颗粒形状和分布、电弧发展路径观测的技术难题，获得植被燃烧特征宏观物理参量，深入研究植被燃烧特征参量对间隙绝缘强度的单一和综合影响规律，基于多元统计分析方法建立植被燃烧特征与间隙放电电压的多维非线性关系；结合放电通道电场、流体场仿真数据分析大尺度颗粒形状和分布对电弧发展路径的影响规律，得到大尺度颗粒畸变电场和触发放电的粒径效应，补充空气间隙多相体放电理论；初步构建植被火条件下空气间隙的多因素放电模型，为线路间隙绝缘设计、绝缘失效风险评估和防护提供理论支撑。

本项目针对植被火条件下多相体放电与空气间隙绝缘强度下降的耦合机制开展研究工作，搭建了模拟植被火条件下空气间隙击穿特性试验平台、半封闭试验空间，研制了火焰通道温度、电导率测量装置，解决了试验重复性、重要物理参量的观测等技术问题。主要研究工作和成果如下：1）系统研究了典型木本植被（杉树、松树、桉树）和草本植被（灌木、茅草、秸秆）导线—板短间隙交流击穿特性。获得了植被燃烧特征（火焰高度、温度、电导率、灰烬颗粒等参量）、击穿电压、泄漏电流等数据；确定了典型高风险植被：杉木（木本、燃烧火焰，比纯空气间隙降低约79.1%）、秸秆（草本、燃烧火焰，降低约87.5%）、茅草（草本、灰烬颗粒，降低约85.3%）。2）开展了杉木垛和秸秆火焰不同间隙距离下的击穿放电试验，获得了火焰完全桥接间隙时间隙放电曲线、电导率和泄漏电流特性。间隙平均击穿电压梯度U（kV）与间隙距离L（cm）的拟合关系：杉木垛U=1.248L-22.3，秸秆U=0.838L-17.28；火焰通道局部电导率随火焰强度、间隙中灰烬颗粒位置发生变化，靠近底部电导率高，杉木垛火焰20cm处平均电导率约为30μs/cm，高于40cm处的平均电导率；12kV交流电压下杉木垛泄漏电流峰值约为3.5mA，秸秆泄漏电流峰值约为13mA。3）植被燃烧灰烬颗粒多为长条形，茅草平均10cm（最长15cm）、秸秆平均7cm；仿真表明颗粒显著畸变间隙背景电场（最大16.3倍），间隙中多个颗粒在电场作用下运动形成颗粒链，当颗粒链桥接间隙程度最大时，间隙中的放电电弧沿着颗粒链发展，电弧路径与颗粒链分布完全重合。4）开展了杉木垛火焰下，模拟220kV架空输电线路导线—地空气间隙的1.2m、1.7m、2.2m导线—板间隙的击穿放电试验，火焰全桥接时间隙平均击穿电压梯度分别为64.8、82.9、94.2kV/m。5）初步揭示了植被火条件下电场、温度、电导率、大尺寸颗粒参数等放电通道物理参量降低空气间隙绝缘强度的耦合机制，初步建立并修正了植被火条件下空气间隙击穿放电的多因素放电模型。研究成果可为架空输电线路空气间隙绝缘设计、绝缘失效风险评估和防护提供理论支撑。