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.

https://iopscience.iop.org/article/10.1088/1674-1056/23/2/027703/pdf

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

Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

The charge carrier trap energy level is an important parameter that will affect the electrical and optical properties of insulating and semiconducting materials. Depending on the characterizing methods, it has a remarkable difference. The trap energy levels of two kinds of polyimide films, original (100HN) and corona-resistant (100CR) ones produced by Dupont, are estimated by depolarization current, space charge limited current and thermally stimulated current method. The corresponding values are 1.185eV–1.200eV and 1.168eV–1.220eV, 0.93eV and 0.76eV, 0.65–0.91eV and 0.60-0.90eV, respectively. These results show that the trap energy level of polymers comes significantly under the influence of heating procedures, and the shallower traps are constructed in corona-resistant polyimide film.

The charge carrier trap energy levels of two different polyimide films characterized by three methods

https://iopscience.iop.org/article/10.1088/0256-307X/34/4/048201/pdf

Research of Trap and Electron Density Distributions in the Interface of Polyimide/Al 2 O 3 Nanocomposite Films Based on IDC and SAXS

The field dependent conduction current of nano-Al2O3 and nano-SiO2 composite polyimide films, with Al2O3 content of 5%, 10% 12.5%, 15% and 20% and SiO2 content of 5%, 15% and 30% in weight ratio, were measured, respectively. The conduction current of 10% Al2O3 content film after corona aging was also measured. The results show that, with increasing the inorganic content, the conduction current of composite film increase and the electric degradation thresholds decrease, after corona aging, the conduction current and electric degradation threshold decrease. The breakdown strength of nano-Al2O3 composite polyimide film decreases and the conductivity increases with the increasing of the inorganic content.

Conduction Current Characteristics of Nano-inorganic Composite Polyimide Films

The electric conduction current characteristics of both Dupont original and corona-resistant polyimide films (100 HN and 100 CR) before and after corona-aging eight hours under electric field of 10 kV/mm and 20 kV/mm were measured. The results show that the conduction current of 100 CR is about one order of magnitude larger than that of 100 HN. Based on the space charge aging theory, the electric degradation threshold obtained from the space charge limit current plot is 40 kV/mm for 100 CR and 35 kV/mm for 100 HN, and decreases with the increasing corona-aged field. By measuring the depolarization current of the two films at 170°C, it is observed that the decay of depolarization current of 100 CR is faster than that of 100 HN. Based on the depolarization current curve, the apparent carrier mobility and trap depth of 1.2×10−13 m2/Vs to 3×10−14 m2/Vs and 1.052 eV to 1.105 eV for 100 CR, 1.12×10−13 m2/Vs to 8×10−14 m2/Vs and 1.054 eV to 1.069 eV for 100 HN were, respectively, calculated during depolarizing.

Conduction Current Characteristics and Carrier Mobility of Both Original and Corona-Resistant Polyimide Films

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Conduction Current Characteristics and Carrier Mobility of Both Original and Corona-Resistant Polyimide Films