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# Breakdown Characteristics of Liquefied ${\rm SF}_{6}$ and ${\rm CF}_{4}$ Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

• ##### B. W. Lee
IEEE Transactions on Applied Superconductivity (Impact Factor: 1.2). 01/2011; 21(3):1430-1433. DOI: 10.1109/TASC.2010.2090638

ABSTRACT 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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##### Article: 50 years in the development of insulating liquids
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ABSTRACT: The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.
IEEE Electrical Insulation Magazine 10/2013; 29(5):13-25. · 1.32 Impact Factor