Dielectric Behavior Evolution Mechanism And Breakdown Strength Enhancement Of Polymer Insulation In High Magnetic Field

The magnet coil insulation plays the role of isolating and supporting conductors,and its reliability under high voltage and high magnetic field affects the safe and stable operation of the tokamak device.In this thesis,the epoxy resin and silicone rubber commonly used as insulation in magnet coils are chosen as the research objects.In high magnetic field,the polarization and carrier migration laws of polymer materials are studied based on electrodynamic analysis,and the insulation failure mechanism is studied based on the evolution law of partial discharge.Based on the addition of ferromagnetic nanoparticles and surface fluorination modification,the method of inhibiting degradation and breakdown of polymer insulation in high magnetic field is studied.The research work and main conclusions are as follows:(1)The dielectric parameters of polymer materials were tested by using the platform for testing dielectric properties of materials in high magnetic field.The conductivity of the polymer materials is increased,and the apparent permittivity is decreased in high magnetic field.Based on quantum chemical and electrodynamic analysis,it is found that the energy level splitting of polymer molecular orbitals leads to narrowed forbidden band width,lower interface barrier and increased density of shallower carrier traps.Based on the energy conservation of electro-magnetic-thermal conversion,it is found that the high magnetic field perpendicular to the direction of the electric field has an inhibitory effect on the polarization of the polymer materials;(2)Based on the research on the characteristics of partial discharge and polarization behavior of polymer insulation in high magnetic field,it is found that high magnetic field can inhibit the polarization of polymer materials,resulting in decreased polarization strength and faster depolarization speed.Partial discharge amplitude and frequency is decreased and increased respectively under AC voltage.High magnetic field promotes the migration of carriers in polymer materials,which leads to the accelerated dissipation of the charges at the insulation interface and the increase of the duration of partial discharge under DC voltage;(3)Based on the research on partial discharge and electrical tree degradation of polymer insulation in high magnetic fields,it is found that the higher discharge frequency in the magnetic field parallel to the electric field causes the electrical tree to rapidly extend to the ground electrode.More partial discharges with lower amplitudes in the magnetic field perpendicular to the electric field generate more side branches.Gradient magnetic field makes the apparent permittivity distribution in the insulation uneven,and the electric field near the needle electrode is enhanced,so that the electrical tree branches are more easily induced and rapidly extended.The polymer insulation interface barrier decreases and the conductivity increases in high magnetic field,which leads to an increase in the accumulation of space charges and reduces the DC breakdown strength of the insulation;(4)The method of adding ferromagnetic nanoparticles to improve the magnetic permeability of the material is used to weaken the field strength of the ma gnetic field concentration area.The insulation polarization behavior is regulated to reduce the partial discharge frequency,and quantum dots are formed to absorb the partial discharge energy,thus suppressing the insulation degradation and breakdown under AC voltage.Based on surface fluorination modification to regulate the distribution of insulation surface traps,the interface barrier is improved and space charge accumulation is reduced,suppressing insulation breakdown under DC voltage;Ferromagnetic nanoparticles addition and surface fluorination modification are combined to improve the insulation breakdown strength under AC and DC voltage simultaneously in high magnetic fields.Through the research of this thesis,the evolution law of the dielectric behavior of polymer materials in high magnetic fields is obtained,the mechanism of high magnetic fields on partial discharge and degradation as well as breakdown of insulation is revealed,and a method for improving the insulation performance of polymer materials in high magnetic fields is proposed.The research results provide theoretical and experimental basis for the insulation design of magnet coils.