干式直流电容器聚丙烯薄膜绝缘性能及其改进方法研究进展

doi:10.19595/j.cnki.1000-6753.tces.211556

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Abstract In converter valve halls of ultra-high voltage direct current (UHVDC) projects, dry-type metallized film capacitors (MFCs) play important roles, such as damping buffer, voltage support and auxiliary commutation, etc. Dry-type MFCs are mainly composed of polypropylene (PP) film overlapped and wound with evaporation electrode, whose performances depend on the insulation characteristics of the dielectric films. With the improvement of the system voltage level and the development trend of the equipment miniaturization, more stringent requirements have been put forward on the dielectric properties of the insulation for capacitors. This paper focuses on the existing problems of PP films for dry-type capacitors. The evolution law of dielectric properties and insulation failure under complex operating conditions are not completely clear, leading to the lack of the guidance to the cooperative regulation of dielectric loss and breakdown strength of PP films. Based on the evolution process of the micro and macro performances of PP films under DC superimposed harmonics or pulse conditions, this paper summarizes the influence of the microstructure, thermal field, complex electric field and mechanical stress on the dielectric properties of the films. In terms of both internal structure and external environment, the mechanism of the insulation aging and even breakdown failure is thoroughly analyzed. It is found that molecular chain configuration, crystallinity and crystal type have important effects on the crystal growth mode, micro morphology and thermal properties of PP films. The polar functional groups in the ash of the material can intensify the ionization of impurities in the film and increase the conduction loss, which is an important reason for the shortening of the capacitor life. Under DC superimposed harmonic voltage, the increase of harmonic amplitude or frequency will lead to the formation of low density area of thin film medium and accelerate the insulation failure. Pulse overvoltages may cause more damage to insulation than DC or harmonic voltages, which needs more concern in future researches. The high temperature environment promotes the injected charges from the electrode, which will increase the dielectric loss and further aggravate the temperature rise inside the capacitor. The localized electric field near the microscopic defects distorts significantly, which ultimately reduces the breakdown strength of the film. Under the combined field of electric field and mechanical stress, multiple partial discharges further decompose the molecular chain and develop into local cracks, which aggravate the partial discharge and insulation aging. The main methods to improve the insulation performance of dielectric films include ultra-fine production of the granular material, optimization of film stretching process, modification of the film material and whole structure design of capacitors. From the aspects of the physical blending, organic addition, ash suppression, crystallization design, chemical grafting and interface modification, the current research status of film insulation modification at home and abroad is summarized. Several methods of regulating the dielectric properties of PP substrates and films for dry-type capacitors are introduced. The effectiveness of different modified films are analyzed based on simulation and experimental data. Traditional inorganic nano addition, physical blending and chemical copolymerization are comparatively not suitable for micron-thickness films. Compared with inorganic nanoparticles, the dispersion of organic/semi-organic nanomaterials in the matrix are better. The breakdown performance of polyphenyl-ring compounds with capture ability of thermoelectrons is improved by introducing chemical traps. Modified films based on ash suppression and crystal morphology design have also proved to be excellent. The interface processing method needs to be further studied for its industrialization feasibility. Finally, the insulation failure and suppression methods of PP films for DC capacitors are summarized and prospected. This paper can provide a reference for the development of dry-type MFCs based on PP insulation suitable for operating conditions in UHVDC projects.

Key words： Dry-type capacitor polypropylene (PP) films insulation breakdown microstructures operating conditions property improvement

Received: 30 September 2021

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TM852

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	Du Boxue
	Ran Zhaoyu
	Liu Haoliang
	Xiao Meng

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Du Boxue,Ran Zhaoyu,Liu Haoliang等. Research Progress of Dielectric Properties and Improvement Methods of Polypropylene Film for Dry-Type Capacitor[J]. Transactions of China Electrotechnical Society, 2023, 38(5): 1363-1374.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.211556 OR https://dgjsxb.ces-transaction.com/EN/Y2023/V38/I5/1363

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