| Lightning strike is still one of the most important factors for the tripping fault of overhead line.Although the existing "blocking" and "dredging" lightning strike protection methods have alleviated the lightning damage to a certain extent,there are still many bottleneck problems such as uncontrollable,unreliable and unsafe.Therefore,in order to greatly reduce the lightning trip rate and solve the existing lightning protection bottleneck problems,a new lightning protection idea of "impact dredging-power frequency blocking" is derived,among which the method of "impact dredging-gas arcing-power frequency blocking" shows its unique advantages in arc extinguishing.Based on the lightning protection method of "impact dredging-gas arc extinguishing-power frequency blocking",this paper studies the arc extinguishing and dielectric strength recovery characteristics of multi-fracture compression self-extinguishing structure.When lightning strikes,the structure can give priority to breakdown discharge to form a protective channel to prevent the insulator from being ablated by arc.After the breakdown discharge,the arc is divided into several sections along the structure.Each section of the arc is compressed by the structure so that the air medium forms a jet stream,which reacts on the arc to make it elongate,accelerate the energy dissipation and finally achieve a self-arcing extinction effect.From theoretical analysis,simulation,scientific experiment and practical application,this paper studies the development of arc,extinction of arc and recovery characteristics of medium strength after arc extinction in multi-fracture compression self-extinguishing structure.The specific work is as follows:(1)The analysis methods of the three stages of dielectric breakdown,arc combustion and dielectric recovery in arc development are defined.In the phase of dielectric breakdown,a thermodynamic non-equilibrium analysis was performed,and nitrogen and oxygen mixture was used to replace air.In the phase of arc combustion and medium recovery,the physical properties and transport parameters of air arc plasma are calculated by local thermodynamic equilibrium analysis.Through the study of the physical characteristics of the arc,it is determined that the main way to extinguish the arc of multi-fracture compressed self-extinguishing structure is "forced extinguishing".(2)By studying the motion state of arc compression and the formation source of arc compression state,it is concluded that the compression effect is conducive to arc dissociation.The effects of horizontal and vertical blowing on the energy attenuation of arc are studied..(3)A set of equations of magnetohydrodynamic for this structure was established,including the conservation equations of mass,momentum and energy,Maxwell's equations,Ohm's law and gas state equation,and an appropriate simplified boundary layer integral method was proposed to solve the problem.A two-fluid model suitable for the breakdown phase of the structure is established.The influence of multi-fracture airflow on arc segmentation is studied.It is found that the more the number of fractures,the easier the arc is extinguished.(4)Particle dissociation,arc length and arc temperature are used as arc extinction criteria to study the critical breakdown criteria for the occurrence of re-breakdown and arc re-ignition.COMSOL Multiphysics software was used to simulate the arc-extinguishing and dielectric strength recovery characteristics of the structure.Two energy input modes of "first lightning,then power frequency" and "first power frequency,then lightning" were adopted.A small system and a multi-system structure were set up on the simulation object.Simulation conclusion: 1)The left and right structure of 2 ms can basically extinguish the arc;2)In the "lightning first,power frequency" simulation of small systems,after the lightning impact,the200 s processing frequency current will cause the internal breakdown of the structure and the occurrence of arc reignite.At this time,the structure will generate jet flow with a reduced velocity,but this phenomenon will not occur in many systems;3)The arc blowout time and dielectric strength recovery characteristics of multi-system are slightly better than those of small system.(5)Lightning impact discharge,lightning impact volt-second characteristics,power frequency tolerance voltage,large current impact tolerance and power frequency continuous current blocking tests were carried out on the 10 k V and 35 k V compression automatic arc extinguishing devices based on this structure.1)U50% of 10 k V and 35 k V units are 112.4k V and 325.1 k V,respectively;2)The volt-second characteristic curve of the device is lower than that of the protected insulator;3)The 10 k V and 35 k V devices withstood the power frequency voltage with amplitude of 29.3 k V and 91.5 k V for 1 min respectively,and no destructive discharge occurred;4)Withstand the impulse current above 65 k A with an interval of 50 s to 60 s for 2 times,the devices has no obvious damage;5)The 10 k V and 35 k V devices successfully broke the power frequency continuation flow around 0.5 k A and 1.3 k A,and the blocking time was around 2.5 ms and 3.0 ms respectively,which met the requirements of the continuation flow of the power frequency within 1/4 power frequency cycle,and the amplitude and frequency of the power frequency voltage restored to normal within half a cycle.(6)Select 10 k V and 35 k V overhead lines in Guangxi,Yunnan,Sichuan,Fujian and other areas with high incidence of lightning strikes for practical application of the device,and adopt the full-line three-phase installation method.Application effect: after installation of the line,the average annual lightning trip times decreased by more than 90% compared with that before installation.After installation,due to the decrease of arc construction rate,the calculated average number of lightning trips per year is about 0.3 times,which verifies the effectiveness of multi-fracture compressive self-arc extinguishing structure for lightning protection. |
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