Gao Guoqiang

Microscopic degradation mechanism of polyimide film caused by surface discharge under bipolar continuous square impulse voltage

Polyimide (PI) film is an important type of insulating material used in inverter-fed motors. Partial discharge (PD) under a sequence of high-frequency square impulses is one of the key factors that lead to premature failures in insulation systems of inverter-fed motors. In order to explore the damage mechanism of PI film caused by discharge, an aging system of surface discharge under bipolar continuous square impulse voltage (BCSIV) is designed based on the ASTM 2275 01 standard and the electrical aging tests of PI film samples are performed above the partial discharge inception voltage (PDIV). The chemical bonds of PI polymer chains are analyzed through Fourier transform infrared spectroscopy (FTIR) and the dielectric properties of unaged and aged PI samples are investigated by LCR testers HIOKI 3532-50. Finally, the micro-morphology and micro-structure changes of PI film samples are observed through scanning electron microscopy (SEM). The results show that the physical and chemical effects of discharge cut off the chemical bonds of PI polymer chains. The fractures of ether bond (C—O—C) and imide ring (C—N—C) on the backbone of a PI polymer chain leads to the decrease of molecular weight, which results in the degradation of PI polymers and the generation of new chemical groups and materials, like carboxylic acid, ketone, aldehydes, etc. The variation of microscopic structure of PI polymers can change the orientation ability of polarizable units when the samples are under an AC electric field, which would cause the dielectric constant e to increase and dielectric loss tan δ to decrease. The SEM images show that the degradation path of PI film is initiated from the surface and then gradually extends to the interior with continuous aging. The injection charge could result in the PI macromolecular chain degradation and increase the trap density in the PI polymer bulk.

Influence of grounding mode on surge voltage in the process of pantograph rising for high speed train

Surge voltage on the carriage which transports along high speed train occurs frequently in the process of pantograph rising, the surge voltage is likely to cause electromagnetic interference or logical confusion to vehicle control and communication system, and even damage the vehicle-mounted components. That maybe a potential threaten for normal operation of high speed train. And the amplitude of surge voltage varies with the change of the grounding mode. In order to study the surge voltage systematically, an equivalent circuit model was built in the process of pantograph rising based on the grounding mode of a certain-type high speed train. The mechanism and propagating characteristics of surge voltage were studied and the influence of different grounding modes on the surge voltage was analyzed in the process of pantograph rising. The results show that the grounding mode affects the carriage surge voltage significantly. The maximum amplitude of surge voltage reaches 4.85kV and rapidly decays in 15 μs in the original grounding mode, while it decreases to 2.4kV in direct grounding mode. However, the transient impulse voltage that the weak current system can withstand should not exceed 2kV. The surge voltage value can be effectively decreased by parallel connecting a capacitor with a grounding resistor. The amplitude of surge voltage relates to the value of the parallel capacitance, and the value of surge voltage will be reduced below 2kV when the value of capacitance is 10μF. The conclusions provide theoretical basis for studying the influence of grounding mode on surge voltage in the process of pantograph rising for high speed train.