Article

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

IEEE Transactions on Applied Superconductivity (Impact Factor: 1.32). 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.

0 Bookmarks
·
36 Views
• Source
##### Article: Study of the dielectric breakdown properties of hot SF6–CF4 mixtures at 0.01–1.6 MPa
[Hide abstract]
ABSTRACT: The dielectric breakdown properties of SF6–CF4 mixtures were investigated at different ratios of SF6, 0.01–1.6 MPa, and gas temperatures up to 3000 K. Initially, the equilibrium compositions of SF6–CF4 mixtures were calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. Then the electron energy distribution function was obtained based on those data by solving the Boltzmann equation under the zero-dimensional two-term spherical harmonic approximation. Finally, the critical reduced electric field strength (E/N)cr of SF6–CF4 mixtures, which is defined as the value for which total ionization reaction is equal to total attachment reaction, were determined and analyzed. The results confirm the superior breakdown properties of pure SF6 at relatively low gas temperatures. However, for higher gas temperatures (i.e., T > 2200 K at 0.4 MPa), the (E/N)cr in SF6–CF4 mixtures are obviously higher than that in pure SF6 and the values of (E/N)cr increase with the reduction of the ratio of SF6.
Journal of Applied Physics 08/2013; 114(5). · 2.19 Impact Factor
• ##### Article: Prediction of the dielectric strength for c-C4F8 mixtures with CF4, CO2, N2, O2 and air by Boltzmann equation analysis
[Hide abstract]
ABSTRACT: The dielectric strength of c-C4F8, and mixtures of c-C4F8 with CF4, CO2, N2, O2 and air, is studied through solution of the Boltzmann equation. The reduced ionization coefficient α/N and reduced attachment coefficient η/N are calculated, allowing the reduced effective ionization coefficient (α–η)/N and the critical reduced electric field strength (E/N)cr (the reduced electric field for which (α–η)/N = 0), to be determined. A high value of (E/N)cr for an electronegative gas, such as those considered here, indicates good insulating properties. It is found that c-C4F8–N2 and c-C4F8–air have very similar (E/N)cr values, higher than those of the other three mixtures, and superior even to that of pure SF6 for c-C4F8 concentrations above 80%. Comparison of the results obtained for c-C4F8 and c-C4F8–N2 with experimental values from the literature supports the validity of the approach taken here and the parameters used.
Journal of Physics D Applied Physics 09/2014; 47(42):425204. · 2.52 Impact Factor
• ##### Article: Design of 154 kV Extra-High-Voltage Prototype SF6 Bushing for Superconducting Electric Power Applications
[Hide abstract]
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.
Japanese Journal of Applied Physics 09/2012; 51(9). · 1.06 Impact Factor