| High-speed railway is built rapidly in recent years. Inverter-fed traction motors are employed for electric-multiple-units (EMUs) and are one of nine key techniques for EMUs, whose insulation is subjected to Pulse Width Modulated (PWM) impulse voltage. Insulation system is weak part of motor, and it is the key of secure running for EMUs. Higher power, smaller volume and lighter, as well as more reliable is direction of future traction motor. So insulation system of motor should be improved to meet the need of future motor. The thesis is to promote insulation aging and breakdown mechanism under high frequency impulse. Insulation breakdown and its life law, partial discharge (PD) activity and its effect on life under high frequency impulse, insulation aging and its characterization were investigated. It has important academic value and project significance.For magnet wires used in the stator coils of traction motor, electrical breakdown and electrical tree breakdown, meanwhile combining with scanning electron microscopy (SEM) and energy spectrum were analyzed, and the corresponding breakdown mechanisms were studied. Insulation failure time of magnet wires was tested under different voltage amplitude, impulse frequency and impulse rise time. The results show that during the process of electrical breakdown, when energy of electron received from electric field is higher than that transferred to the crystal lattice of polymer, conductance will become unstable. As a result, insulation was breakdown very quickly. During the process of electrical tree breakdown, tree branch resistance of polymer was enhanced because of nano particles. Life models of voltage, frequency, and rise time are all accord with power function.PD test system under high impulse voltage and high frequency was adopted. PD characteristics of twisted pair, magnet wire and polyimide film specimens were tested. PD activities, including partial discharge inception voltage (PDIV), PD parameters (mean discharge quantity, discharge number and phase resolved PD pattern) were analyzed. Relation between space charge and PD was discussed, and effect of PD on insulation life was investigated. The results show that PD activities would be more serious with increase of frequency and shortness of rise time, and move to the zero of impulse voltage. When polarity of impulse is reversed, electric field of cavity is enhanced because of space charge. Memory effect of space charge has great impact on PD activities. Diffusion ability of polymer is improved because of nano particles, which reduces the distortion of local electric field. So PD activity is weakened.Testing of electrical aging was adopted, and insulation aging characteristic was investigated. The destroy instance of films’ surface, residual insulation strength as well as space charge distribution of polyimide films were tested. The effect of space charge on insulation aging was discussed, and relation between PD parameters and aging time was researched. The result shows that reduce of residual insulation strength for nanocomposite film is lower, because accession of nano particles weakens the effect of PD on aging action for polymer. There is relationship between skewness of mean discharge quantity and aging time, and insulation condition could be judged by skewness of mean discharge quantity. More charges were injected into the film, with increase of frequency, shortness of rise time and increase of aging time. Free radical chain reaction of polymer will be induced by hot electron, which would accelerate the insulation aging process. |
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