Study On Damage Characteristics Of Polyimide Film Caused By PD And Material Modification

High-speed railway has been favoured by the world because of its advantages of speediness, high efficiency and environmental protection and so on. Inverter-fed traction motor is one of the key techniques for electric-multiple-units (EMUs). The stability and reliability of traction motor can safeguard the secure running for EMUs. The insulation of inverter-fed traction motor is subjected to pulse width modulated (PWM) impulse voltage with steep rise front and high repetition frequency, which would lead to premature failure for insulation of traction motor. Therefore, it is very important to carry out comprehensive research for insulation of inverter-fed motor. The insulating material will be aged gradually and change its chemical composition and physical morphology, which have been performed thruogh very little related work.In this thesis, the microstructure and micromorphology of PI film were studied during the aging processes. Then, the degradation mechanism of PI film was explored. At last, the PI/Al2O3 nanocomposite was prepared by using in-situ polymerization technique. The main research content is as follows.(1) A partial discharge detection system under square impulse voltage was designed based on the ASTM 2275 01 electrode standard. The PD parameters of pure PI film and PI nanocomposite film in different temperature environment were analyzed. The effects of thermal exposure on dielectric strength of nano PI film and pure PI film were investigated. The comparison of the breakdown field strength of single layer film, double layer films and three layer films were also studied. The results show that the breakdown field strength of PI films is greatly decreased with the tested temperature increasing, which is due to the motion and pyrolysis degradation of PI macromolecules caused by overheating. Higher thermal conductivity of PI nanocomposite film has the better breakdown stability in high temperature environment. Then, the molecular simulation technology was used to analyze the thermal degradation mechanism of PI film. It is found that the ether bond and imide ringare cut down easily and lead to the PI film degradation.(2) The damage mechanism of pure PI film and PI nanocomposite film was discussed. The micro-morphology and micro-structure observed form SEM images reveal that the degradation path of PI film is initiated from the surface and then gradually extends to the interior with continuous aged. Physical and chemical effects of discharge cut off the chemical bonds of PI polymer chain. The FTIR show that the fracture of ether bond(C-O-C) and imide ring(C-N-C) on the backbone of PI polymer chain leads to the decrease of molecular weight, which results in degradation for PI polymer. The results reveal that discharge phase presents widening trend with the increase of aging time. The appearance of the discharge in higher phases is the sign of that some faults are expanding rapidly, which means the sample reaches its terminal stage. The PD resistance of PI film is improved because of nano particles.(3) The in-situ polymerization technique was adopted to prepare the PI/Al2O3 nanocompostie. SEM images reveal that Al2O3 nano-particles disperse homogeneously into the PI matrix. Influences of frequency, temperature and Al2O3 nanoparticles doped concentration on permittivity and tanδ of PI/Al2O3 nanocomposite film were studied. The results shows that permittivity and tanδ of nanocomposite film increase with the nano-Al2O3 particles doped concentration increasing. The dielectric behaviors of pure PI and PI/Al2O3 samples show the frequency dependence and temperature dependence. The electrical breakdown strength declines obviously with the wt.% content of Al2O3 nanoparticles increasing, but improved the corona-resistant property. The elevated temperature would reduce the breakdown voltage and corona-resistant property.