Experimental and numerical study of the polyimide film insulation characteristics in the liquid nitrogen of superconducting transformers

With the increasing voltage level of superconducting equipment, the insulation strength is critical to ensure the safe operation of the equipment. The polyimide film is usually selected as the main insulation material because of its good insulation performance. In this paper, the insulation properties of polyimide films in liquid nitrogen environment were studied experimentally. The extremely uneven electric field was established by the rod-rod electrodes in the pure liquid nitrogen environment inside the cryostat container. Between the rod-rod electrodes, the power frequency voltage withstanding experiment and the impulse voltage withstanding experiment were both carried out on polyimide films with different thickness. Partial discharge experiments were carried out on films of different thickness. In order to analyze the influence of immersing the polyimide film in liquid nitrogen for different time, 10 polyimide film samples of each thicknesses were immersed in the liquid nitrogen environment. Use a scanning electron microscope to observe whether nitrogen atoms penetrate the surface of the film and analyze the influence of intruding nitrogen atoms on the insulation strength of the film. The research results of this paper will provide the design specification of insulation for high voltage level, large capacity and compact superconducting transformers.