Study On Low Temperature Inner Insulation Problems For Superconducting Fault Current Limiter

With the discovery and rapid development of HTS materials,superconducting technology is being more and more important in powerapplication. Compared with traditional power apparatus, superconductingpower apparatus, such as superconducting fault current limiter (SFCL),superconducting transformer and superconducting cable, has greatadvantages. For commercial use of SFCL, higher and higher voltage use isbeing taken into consideration. However, as liquid nitrogen (LN2) is used ascooling material, cryogenic technology and insulation problems limit thedevelopment of superconducting apparatus. Insulation property is crucialand limits the development at high voltage level. So it is important andmeaningful to study the insulation characteristics in the environment ofliquid nitrogen, and it is helpful for the design of SFCL and to ensure thereliability and safety of the apparatus.There are mainly two kind of low temperature insulation problems inSFCL, that is, inner insulation and outer insulation. Inner insulation refers toinsulation problems for liquid nitrogen inside the apparatus, while outerinsulation concerns with the problems with bushing. This paper studies theinner insulation problems in SFCL and mainly completes the followingwork:(1) The experiment setup producing bubbles is designed to imitatequench condition, and the AC breakdown voltage is measured. This paperpresents the bubble behavior and the influence on insulation properties of liquid nitrogen. Spherical brass electrodes are used to producequasi-uniform electric field. The relationship between breakdown strengthand electrodes’ gap length and bubble amounts is investigated.(2) Flashover experiment setup in liquid nitrogen is built to measurethe AC surface flashover breakdown voltage of different electrodes.Simulations are conducted to study the theoretical electric field distribution,which can help explain the experiment results.(3) Based on the fundamental design of SFCL module in Shanghai JiaoTong University, U-type electrode and different structures of HTS electrodeare designed to imitate the practical structure. Partial discharge test andsurface flashover test is done for U-type electrodes to help design suitabledistance in a module. Besides, simulations are conducted to verify theresult. And in this case, bubble influence on insulation property of practicalstructure can be investigated.The results show that bubbles have great influence on degradingliquid nitrogen’s insulation property and G10has excellent insulationcharacteristics. The experiment data and result will provide usefulinformation for220kV SFCL’s insulation design. It is important to ensurethe reliability and safety of the apparatus and do not make the device toolarge.