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

Abstract

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 ε 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.

Comparison between AlN and Al 2 O 3 ceramics applied to barrier dielectric of plasma actuator

“…[19,20] Although this approach works, it presents issues relating to discrete thickness, handling difficulties, partial discharge between layers [21] and most significantly, material degradation. [22] Therefore, after long-time plasma process, the top layer of the polyimide film usually degrades then the underlying adhesive appears. [20] Compared with the widely used polymer dielectrics, ceramic offers the advantages of corrosion resistance, high and low temperature resistance, excellent dielectric properties and heat conduction, which make it a good dielectric barrier candidate for the future.…”

Surface Degradation of Oil-Immersed Nomex Paper Caused by Partial Discharge of High-Frequency Voltage

“…Another reason is that the higher charge injection volume and the frequent injection and drawing of the charge under HF stress [17][18][19] cause an increase in the number of charges detrapped on the surface of the oil-impregnated Nomex paper. As a result, the area of collision ionization is more concentrated.…”

“…However, the above-mentioned literature mainly studied PD damage characteristics and the mechanism of oil-paper insulation under AC voltage. Under HF voltages, some researchers [16][17][18][19][20] have been exploring PD damage characteristics and mechanisms of solid insulation, while the related theories of oil paper are scanty and still unclear. In addition, it has been found through experiments that the PD damage mechanisms of oil-paper insulation between HF and AC voltages, and of HF voltages between oil-paper insulation and solid insulation, are different.…”

Functional nano‐fillers in waterborne polyurethane/acrylic composites and the thermal, mechanical, and dielectrical properties

“…PD phenomenon can induce degradation of material, because the activity generates high energy electrons and accompany with chemical oxidation, heat release and irradiation, which cause the chain and bond rupture, further destroy the structure of materials and ultimately lead to insulation breakdown. [72][73][74][75] 4 | CONCLUSION In this work, three different kinds of nanofillers including CNTs, CNC, and Al 2 O 3 with organic and inorganic attributions were employed to modify PUA dispersions. The functions of the nanofillers in the polymer matrix were investigated, as well as the micro-structure and applied performance, especially the mechanical and dielectric properties.…”