| With the development of renewable energy generation and HVDC transmission,higher requirements are put forward for the reliability of long-distance transmission in complex environment.As a special transmission medium,DC GIL is an ideal solution to this problem.Metal particle pollution is one of the main problems that restrict the development of DC GIL technology.Metal particles are more active under DC voltage,which affects the insulation safety of equipment.Therefore,it is of great value to study the characteristics of metal particle lifting,movement and air gap breakdown caused by moving metal particles under DC voltage,and to take appropriate measures to inhibit the activity of metal particles.In this paper,a parallel plate metal particle movement observation platform is built to simulate the slightly uneven electric field environment in GIL,and the movement of different metal particles can be observed with high-speed camera.It can be seen from the experimental results that the lifting voltage of metal particles increases with the increase of particle density.The lifting voltage of spherical metal particle increases with the increase of radius.The lifting voltage of linear metal particles increases with the increase of radius and it is independent of its length.The lifting voltage of sheet metal particles increases with the increase of thickness and it is independent of its length and width.When the number of linear and sheet metal particles increases,the voltage decreases when the particles start to lift compared with that of a single particle due to the effect of electric field distortion.The polarity of DC voltage does not affect the amplitude of metal particle lifting voltage.After lifting,the spherical metal particles move between the electrodes with random collision reflection angle and the process of air gap breakdown caused by it is analyzed.Linear and sheet metal particles exhibit different motion characteristics under different DC voltage polarity.They will rotate and jump on the surface of the lower electrode under the positive DC voltage,and move between the electrodes under the negative DC voltage.There will be so-called flying fireflies phenomenon under certain conditions.The decrease of linear metal particle radius or the increase of its length and the decrease of sheet metal particle thickness or the increase of its diagonal length make the air gap breakdown easier,which is analyzed quantitatively according to the experimental results.Linear and sheet metal particles are more likely to cause air gap breakdown under negative DC voltage and the reason is analyzed from the perspective of corona discharge development process.PI/TiO2 nano-composite film is coated on the surface of the electrode without glue in order to inhibit the activity of metal particles.PI/TiO2 nano-composite films are prepared in the laboratory and their properties are characterized,then a nano-composite film that is resistant to corona and has excellent overall performance is obtained.Finally,the effect of the coating is evaluated by the electrode coating experiment from the changes of the lifting voltage of the metal particles,the movement activity of the particles and the air gap breakdown caused by the particles.The results show that the electrode coating method proposed in this paper is better than the other two methods using binder and the lifting voltage of spherical,linear and sheet metal particles increased by 64%to 130%,32%to 100%,70%to 107%respectively when the electrode surface is coated with PI/TiO2 nano-composite film with a thickness of 25μm.The lifting voltage of the metal particles will increase when the thickness of the film increases.However,the coating thickness of 25μm is more reasonable in order to reduce the influence on the heat dissipation of the electrode.The lifting voltage of metal particles is basically the same when the lower electrode is covered with film or both electrodes are covered with film.The movement activity of the metal particles weakens and they gradually rest on the surface of the lower electrode after the electrode is coated with film and the air gap discharge process also stops.The electrode coating strategy is proposed according to the polarity effect of particle movement process:lower electrode coating for positive DC voltage and double electrode coating for negative DC voltage. |
