Research On The Moving Mechanisms Of Charged Metal Particles In DC GIL And Suppressing Methods

Some urgent issues are becoming prominent in nowadays China,such as shortage of corridors for UHV transmission lines,local power delivery of distributed energy resources,special demand in striding over rivers and la kes during transnmission,and the inevitable requirement for planning future DC power grids,for which cases gas insulated transmission lines(GIL)are gaining more and more attention from academia and industrial sectors.In the insulation system of DC GIL,due tomechanical collision,equipment vibration and thermal expansion friction during the interim process of manufacture,transportation,assembly and operation,GIL will inevitably produce metal-particle contamination.The metal particles will get charged and move,leading to air-gap breakdown within the cavity or even flashover along the insulator surface,which may result in dramatic deterioration of the GIL insulation and seriously affect the reliable operation of the DC transmission system.This paper mainly studies the charging mechanism and moving dynamics of the metal particles in DC GIL as well as corresponding suppressing methods.The key issues and the achieved results are given as follows.Fluid mechanics is used to analyze the impact of the mixed flow resistance in the motion process of the metal particles as toexplore the inelastic random collision phenomenon occurring between the metal particles and the conductor and/or the shell based on elastic collision theory.With the above aspects being incorporated,a motion model for spherical metal particles is developed.Accordign to simulations and experiments it is indicated that,in contrast to AC GIL,the particles in the DC GIL normally move through the gap between the electrodes,rendering a la rger distribution range and a higher degree of danger.In spite of t he fact that the particles move in a random style,it also shows some specific mode of distribution within the sealed cavity.Also,the degree of activity sees positive correlation with th e random angel of reflection and the voltage amplitude,while there encounters a maximum point as the particle radius varies accordingly.Exploration and elucidation of the motion characteristics of particles present s a theorical basis and technological reference as to evaluate particle criticality as well as develop suppressing measures.The motion of free metal particles may trigger partial discharge inside the internal cavity of the DC GIL,and a severe discharge will probabaly lead to breakdown of the whole air gap.Extensive studies are conducted to account for the influencial factors on partial discharge such as voltage polarity and amplitude,SF 6 air pressure,particle length,radius and number,based on an experimental platform.In the meanwhile,the impact of the above factors exerting on the motion dynamics and discharge features is explored,and the criticality of the metal particles under various factors is studied in details.Further,a high-speed camera is utilized to observe the process of air gap breakdown caused by free metal particles.By analyzing the electric field distortion after the spherical particle lifting,a micro-discharge space between the moving particle and the plate is characterized based on the micro-discharge criterion of the streamer theory.It is found there exists 3 modes of breakdowns caused by the metal particles in the DC uniform field,namely the stationary-direct breakdown,the micro-discharge breakdown and the lifting-voltage breakdown.For the jumping free linear particles,the micro-discharge breakdown distance increases with increased particle length,while the breakdown voltage decreases with the increase of the particle length.In addition to triggering air gap breakdown,the attached metal particles may also pose a dramatic effect on the charge accumulation of the insulator surface.Taken into account of the contamination of metal particles as well as the effects of ions’ formation,recombination,migration and diffusion in gas space,a microscopic model denoting accumulation of the insulator surface charge is established through COMSOL,a multi-physics simulation software,and a rotary experimental platform is thereby built for surface charge detection.It can be drawn from both the simulations and the experimental results that,a correlative consistency exists between the accumulation of insulator surface charge and the normal component of the electrical field,and the particles adhering to the insulator surface may cause a surge of surface charge accumulation.In addition,the charge amount caused by the particles attached to the middle part is more remarkable.Also,the polarity of the charge accumulation at both ends of the particle is opposite,which is also opposite t o that of the electrode facing the tip of the particle.As regards the suspended particles,little effect is seen on the accumulation of surface charge,and this effect can be neglected if the vertical distance between a suspended particle and the insulator surface is more than 4 times of the particle diameter.Development of effective suppressing technology is one of the key issues to be addressed.On the one hand,the suppressing mechanism of the electrode surface coating on the metal-particle lifting is studied.Based on the gas ionization theory and the interface charge accumulation,a model for the metal particle’s charging and lifting against the coatings impact under DC stress is proposed and it is found that,the static adsorption generated by the coatings under the DC stress can improve the lifting voltage significantly,and both the dielectric constant and bulk conductivity have effect on the charging time which presents a clarified guideline as to choose the insulating medium as coating material with high dielectric constant and low bulk conductivity.On the other hand,based on electric field analysis and particle capture experiment,a specific width-thickness ratio is proposed to describe the girdlattice particle trap in the DC GIL,and the motion behavior of the metal particles nearby the insulator is also studied to optimally arrange the traps in the concentration area of falling metal particles.The above studies present engineering significance in practical design.