Research on the Mechanism of Gap Breakdown Characteristics of Overhead Transmission Line Under the Condition of Forest Fire Flame

Abstract

This article mainly analyzes the effects of flame temperature, flame conductivity, and particles on the gap breakdown characteristics. Under high-temperature conditions, it can quickly increase the number of electrons and ions in the channel, increase the conductivity of the channel, and significantly enhance the thermal dissociation in the air near the electrode, resulting in a decrease in the breakdown voltage of the gap. Due to the effects of thermal dissociation and chemical ionization, there are a large number of charged particles in the flame, which makes the flame conductivity sufficiently high, provides sufficient current, promotes the development of discharge, and ultimately forms a stable discharge channel, leading to gap breakdown. After particles enter the gap, the electric field near the particles undergoes distortion. As the voltage increases, more and more particles are attracted, forming particle chains to bridge more gaps, ultimately leading to a significant decrease in the insulation strength of the gaps. Among them, the influence of flame conductivity is analyzed from four aspects: the source of charge, the characteristics of flame leakage current, the influence of conductivity on breakdown voltage, and the influence mechanism of conductivity on gap discharge. The discharge mechanism of the transmission lines under the condition of forest fire is described. Based on the similar characteristics of gas discharge and the on- site statistical analysis data of transmission lines under the conditions of forest fire, the breakdown mechanism of transmission lines under the conditions of forest fire is analyzed. First, the mechanism of gap discharge development under the conditions of forest fire is analyzed, and then, the breakdown model of transmission lines under the conditions of forest fire is analyzed by combining the line and forest fire parameters, According to the simulation test results (full-flame and semiflame test data), the average breakdown field strengths of fir branches, reeds, and straw in the flame zone are 24, 55, and 43 kV/m, while the average breakdown field strengths in the semiflame zone are 155, 160, and 150 kV/m, respectively. The mechanism of the influence of flame temperature, flame conductivity, and ash and smoke on the development of discharge in flames has been clarified. When a forest fire occurs, due to the high intensity of flame combustion, the thermal buoyancy and electric field will cause the ash particles generated by the combustion to float up and randomly suspend in the flame, triggering discharge near the high-voltage electrode, thereby reducing the insulation strength of the gap. The main impact of ash on the gap breakdown voltage is the “multiplication effect” of ash-triggered discharge. The strength of particles triggering discharge in flames is related to factors such as particle size and the proportion of bridging gaps. When the gap between the flames is not fully bridged, the safe distance between the 220-kV line and the flame body is calculated to be 1.83 m, which explains why combustible vegetation, such as straw and reeds, although their combustion height is not fully bridged, still causes the transmission line to trip. The breakdown voltage prediction formula proposed in this article can provide a certain reference for the management and logging of vegetation in transmission line corridors.