| DC gas insulated transmission line(GIL)is capable of meeting special needs in scenarios such as distributed energy in-situ transmission,urban power pipe gallery laying underground and power transmission line crossing rivers,which also tend to be the key equipment for the future development of DC power grid.Metal particles are inevitably generated during the production,assembly and operation of GIL,and the metal particle charge and movement can lead to equipment insulation failures.In AC GIL,the percentage of insulation failure caused by particles is up to 50%,while metal particles are more active due to unipolar electrical stress in DC GIL.Among them,the greater risk to operational safety is to present a special phenomenon of firefly,namely,particles suspended on the surface of conductors.Firefly particles move in the area of strong high electric field strength with long-term,easily triggering breakdown,meanwhile,the particles may move along the conductor and adsorb to the insulator surface thus inducing flashover.However,the charging process and force characteristics of the firefly particles are still unclear,and the mechanism of metal particle firefly movement remains to be explored,whereas the unclear mechanism also leads to the absence of effective suppression measures.Furthermore,the existing measures to suppress particle motion in the GIL,namely electrode coating and particle traps,still lack a specific design for the firefly motion,and the synergistic measures to suppress the particle firefly still demand further investigation.To solve the above problem,a systematic study on the motion and suppression of particle firefly is carried out in this paper,namely,the mechanism of metal particles firefly in GIL,the critical starting criterion of the particles firefly,the selection of the electrode coating,the optimal design of the grounded particle trap,and the synergistic suppression of the particles firefly,which provides a theoretical basis and an applied technical method for the management of particles firefly in the GIL.By establishing an experimental observation platform,we have obtained the behavioral characteristics of the particles firefly,and investigated the quantitative effects of particle size and gas pressure on the motion of particles,thus clarifying the essential factors affecting the behavior of particles firefly.In addition,particle charging characteristics and corona ion wind of the particles in the firefly state were tested,and combined with the force characteristics of the firefly particles,the occurrence mechanism of particle firefly movement was revealed.The polarity change of particle charge,resulting from the space charge generated by the particle corona discharge,is the main trigger for the formation of firefly motion,while the corona ion wind generated by the particle end corona promotes the formation of firefly motion.Based on the occurrence mechanism of particle firefly,the calculation model of the corona voltage of metal particles on the electrode surface was established.By taking the corona voltage of particles as the critical condition for the change of particle charge electrode property and the generation of corona ion wind,the charged particle motion model was developed,considering the ion wind lift and the charge concentration characteristics.As a result,we proposed the critical onset criterion for the occurrence of particle firefly motion,and realized the dynamic simulation of particle firefly in GIL.Furthermore,the critical onset electric field strengths of wire particles firefly were obtained in DC GIL with different voltage levels.For the 100 kV GIL prototype,the onset electric field strength of negative polarities was 2.78 MV/m for wire particles with 0.2 mm diameter and 5 mm length.The suppression mechanism of electrode coating on particles firefly motion was acquired,by testing the charge and corona ion wind of particles on the coated electrode surface.Then,taking the charge residual coefficient and corona ion wind as the evaluation parameters,the selection principle of coating material to suppress the particle firefly motion was presented.The film with small dielectric constant and large body conductivity is selected to be electrode coating,enabling the corona ion wind to be less than 1 m/s,and the charge residual coefficient to be less than 0.22.In addition,epoxy resin was selected as the electrode coating material,and the longterm service environment of electrode coating was simulated through accelerated thermal aging at high temperature.Then,the suppression effect of electrode coating on the particle firefly under long-term service was evaluated,from the perspectives of material cleavage,dielectric constant,bulk conductivity and firefly onset voltage.The trapping kinetics of metal particles near the particle trap was experimentally observed,and the critical factors affecting the trapping process were investigated.Whereby the key design parameters of the particle trap were proposed,and the wedge-shaped trap structure for DC GIL was designed.The simulation model of particle trapping motion was further established,considering the random characteristics of collision.Then,the visualization simulation calculation method of particle trap capture probability was proposed to obtain the effect of particle trap with different structure parameters on metal particles,as well as the optimal design parameters of particle trap.Finally,taking electrode coating and grounded particle traps as the main suppression measures for particles firefly,the auxiliary effects of repellent electrodes and step voltage procedures on particle entrapment were investigated in GIL,Consequently,a synergistic arrangement method incorporating multiple suppression measures was proposed,to improve the capture probability and reduce the capture duration of firefly particles in GIL,thus achieving effective capture of moving particles.The above studies provide a theoretical basis and technical support for the efficient management of particles firefly motion in GIL. |
