Surface Partial Discharge Characteristics Of Polyimide Films

With the advantage of excellent chemical property, mechanical property, thermostability and corrosion resistance, polyimide(PI) has been widely used in different fields, such as the aerial, nuclear, microelectronic industry,turn to turn insulation and turn to ground insulation of inverter-fed motors. However, its usage as an insulating material for inverter-fed motor is not successful up to now. Insulation early failures occur frequently. Due to the winding always works in the conditions of high-temperature, high voltage and partial discharge, it will accelerate the aging of insulation. Fluorination as change the chemical component in surface layer of polymers should give rise to the corresponding change in electrical properties of the surface layer thus influence the charge injection from electrodes when they are used as an insulator. It will weaken the intensity of partial discharge and inhibition the breakdown of insulation. Hence, study on the partial discharge mechanisms of polyimide is of great significance.1. PI samples were fluorinated in a reaction kettle for respectively 15, 30, 45, 60 and 90 min. Surface chemical compositions of the fluorinated films together with the original(untreated) films were characterized by attenuated total reflection infrared(ATR-IR) and EDS analysis. The relative permittivity and conductivity of the samples were also measured. It shows that direct fluorination of polymer results in the fracture of C-H bond subsequent by the formation of C-F, C-F2 and C-F3 groups.2. Effect of direct fluorination on surface partial discharge characteristics of polyimide films were investigated by using pulse stimulus at predefined voltage parameters. From the discharge distribution and image analysis, it can be seen that the fluorination has a significantly inhibitory effect on partial discharge intensity. Recurrent plot analysis of discharge current shows that the change of chemical component in surface layer of polyimide film by fluorination makes the partial discharge become more stable and regular. The mechanism for the influence on the surface partial discharge characteristics was analyzed.3. Effect of direct fluorination on surface partial discharge characteristics of magnet wires were investigated by using AC power. The differences of partial discharge inception voltage(PDIV), number of discharge pulses and magnitude of partial discharge for different fluorination time were measured. The effect of direct fluorination on surface partial discharge characteristics of magnet wires were analyzed by the relation between the discharge magnitude and the adjacent discharge magnitude.