Breakdown Characteristics of Liquefied SF6 and CF4 Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

IEEE Transactions on Applied Superconductivity (Impact Factor: 1.24). 06/2011; 21(3):1430-1433. DOI: 10.1109/TASC.2010.2090638


Highvoltagecryogenicinsulationissuesneedtobead- dressed in order to promote the commercialization of high temper- ature superconducting (HTS) equipment. One of the critical com- ponents for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep tem- peraturegradient,commercialbushingswhichhavebeeninsulated with gas could not be directly applied to cryogenic equipment due to liquefaction of in the cryogenic environment; there- fore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the op- timum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as , and have been investigated. In particular, we noted the insulation characteristics of gas whose liquefication tempera- ture is much higher than gas. Thus, in order to investigate the possibility of substituting gas for gas for the bushings of HTS electrical equipment, impulse tests, AC withstanding voltage tests, and partial discharge (PD) tests have been performed. As a result of these tests, it was shown that mixtures of liquefied insu- lating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of gas should be evaluated due to freezing effects. On the other hand, gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insu- lation gas for high voltage bushings of HTS electrical equipment.

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    ABSTRACT: Since the discovery of high-temperature superconductor (HTS), liquid nitrogen(LN2) has not been only utilized as a coolant of superconducting electric equipment but also as an insulation material in cryogenic environment due to its dielectric performance. It also has a lot of advantages over other cryogenic liquid such as less expense and harmless substance, thus it has been widely used in the development of superconducting devices. Up to now, a lot of research works dealing with the breakdown characteristics of LN2 for distribution-class superconducting devices have been presented worldwide but, few research works about breakdown characteristics of liquid nitrogen in extra high voltage class have been reported due to the limitation of cryogenic test facilities in extra high voltage (EHV) class. In order to study the cryogenic EHV insulation technologies, we have built the cryogenic dielectric test facilities including a fiber reinforced plastic (FRP) big cryostat with cryogenic bushing, a 400 kV AC overvoltage and a 1.6 MV lightning impulse test systems. Using these facilities, we focused on the breakdown characteristics of liquid nitrogen in EHV level which is rather different comparing to the distribution level. With real scale big cryostat, AC overvoltage test and impulse tests have been performed. From the test results, the breakdown characteristics of liquid nitrogen in EHV were suggested. And these test results could be used as basic insulation design data to develop transmission-class superconducting electric equipment.
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    ABSTRACT: One of the critical matters yet to be solved for commercial applications of extra high voltage superconducting devices is the optimum design and development of high voltage cryogenic bushings which could withstand both severe insulation requirements and a steep temperature gradient due to the cryogenic environment. Neither conventional extra high voltage bushings insulated by ${\rm SF}_{6}$ gas nor composite materials are directly applicable to cryogenic bushings due to an extremely low temperature environment. In order to obtain suitable dielectric performance of bushings in the cryogenic environment, we focused on an alternative insulation gas instead of ${\rm SF}_{6}$ such as ${\rm CF}_{4}$, which shows excellent dielectric performance under extremely low temperatures, and also on the optimum design of cryogenic bushings, which have a longer creepage distance compared to conventional bushings. In this paper, design factors of cryogenic bushings were discussed, and test results of 60 and 100 kV extra high voltage prototype bushings were discussed in detail. Consequently, it was possible to obtain satisfactory results to verify the insulation level of newly designed extra high voltage cryogenic prototype bushings for superconducting electric power applications.
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    ABSTRACT: One of the critical components to be developed for high-voltage superconducting devices, such as superconducting transformers, cables, and fault current limiters, is a high-voltage bushing to supply a high current to devices without insulation difficulties in cryogenic environments. Unfortunately, suitable bushings for high-temperature-superconductivity (HTS) equipment have not been fully developed to address cryogenic insulation issues. As a fundamental step towards developing the optimum design of the 154 kV prototype SF6 bushing of HTS devices, the puncture and creepage breakdown voltages of glass-fiber-reinforced-plastic (GFRP) were analyzed with a variety of configurations of electrodes and gap distances in the insulation material. And design factors of high-voltage cryogenic bushings were obtained from the result of tests. Finally, the withstand voltage tests of manufacturing a 154 kV extra-high-voltage (EHV) prototype bushing has been performed. Consequently, we verified the insulation level of the newly designed 154 kV EHV cryogenic prototype bushings for superconducting electric power applications.
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