Effect Of Pulse Waveform On Ageing Of Inverter-Fed Traction Motor Insulation

With the development of high power semiconductor devices, frequency control drive systems are widely used in the field of locomotive traction. Pulse Width Modulated (PWM) technique which provides motor supply voltages is characterized by square waveforms, width modulated, with steep rise front slew rate up and high pulse repetition frequency. For this reason, electrical insulation of traction motor can be premature failures under such pulse waveforms. Because design and manufacture technology of AC motor is not completely suitable for traction motor supplied by such adjustable speed drives anymore, it is essential that comprehensive research for electrical insulation of electric motor supplied by PWM voltage. However, some past research achievements are generally based on lower power electric motor made by dispersed windings, and very little work has been carried out for high power electric motor made by module windings. The thesis is based on the latest research results at home and abroad, and partial discharge (PD) and space charge which are major factors leading to aging of insulations are used as main study objects. Activity and mechanism of PD and space charge of insulation system of traction motor supplied by adjustable speed drives is investigated .Furthermore, In order to investigate the influence of square wave pulse on aging and failuare of winding insulation, aging characteristics of winding insulation are also studied. These reasearch results will greatly promote advancement of theory of aging and breakdown under such pulse condition. As a result, frequency control technique can be widely used.A novel PD signal measurement system and statistic analysis software is presented under square wave pulse voltage. From comparison of response of sensors made by different ferrite core, the sensor made by NiZn ferrite core is more suitable for transferring PD signal at pulse voltage. Also, wavelet package is used to filter interferences generated by steep rising and trailing edge of square wave. According to PD statistical method under sinusoidal waveform, time window and PD statistics are performed. Depending on special statistic analysis software, phase resolved PD patter at square wave voltage is also defined to further investigation of PD activity and mechanism. Measurement method and influencing factors of space charge under pulse voltage is discussed, and effects of parameters of pulse waveform on activity of space charge are investigated. The device based on Pulse Electro-Acoustic method (PEA) is developed in order to directly measure distribution of the space charge, and thermally stimulated current (TSC) is also designed to measure energy level of space charge in polymer. Computing methods of trap parameters are compared through gauss fitting and kinetic model fitting. After different pulse frequency voltage aging of polyimide film, changes of microstructure of polyimide film is investigated. Moreover, lifetime of polyimide film is related to activation energy and charge quantity of high temperature peak in the TSC curve, whereas activation energy and charge quantity of lower temperature is nearly independent of lifetime. Under pulse voltage, it is possible to decrease of activation energy with aging. According to the measurement results of space charge, effects of different pulse frequency, voltage amplitude, polarity, rise time and insulation layers on activity of space charge are analyzed.Effects of different pulse waveform on PD activity of nano hybrid polyimide film, normal polyimide film and twisted pair specimens of winding are investigated under pulse voltage. Through measurement results of PD inception voltage (PDIV), PD parameters including mean discharge quantity and discharge number, phase resolved PD pattern of different materials specimens, increase of pulse frequency and decrease of rise time of pulse waveform both results in enhancement of inverse electric field when polarity of pulse waveform reverses. For this reason, different PD activity can be observed. Furthermore, through analysis of SEM (scanning electronic microscope) and PD parameters, three layers of nanophase hybrid polyimide film have been observed which lead to large surface conductivity and suppress PD activity. Due to protective effect of nanophase layers to PD damage, damage of PD hardly occurs in the interior of nanophase polyimide film.In order to investigate accelerated aging mechanism and characteristics of insulations under square wave pulses, aging experiments of twisted pair specimens of stator winding of traction electric motor are performed. Comparative tests are carried out under AC sinusoidal and square wave pulse conditions. Obvious clusters of discharge number occur with the increase of pulse voltage aging time, whereas no similar phenomenon can be observed after sinusoidal voltage aging. It indicates inverse electric field at pulse condition is more intensive than at AC sinusoidal condition. Furthermore, analysis of the moment parameters of PD statistics operatorsat different aging time is also performed in order to show relationship between PD statistics operators and aging time. The results show that there is a relation between skewness of mean discharge quantity and aging time.