Jonggi Hong

Design of Current Leads for a High Voltage Superconducting Apparatus

In the Republic of Korea, it is widely thought that the subcooled liquid nitrogen cooling system is the most promising method to develop a high-voltage superconducting apparatus. This method involves injecting a gaseous insulation medium, such as He, Ne, or N2, into the superconducting apparatus to control the system pressure [1]. Therefore, a study on the dielectric characteristics of gaseous and solid insulation materials must be conducted to design the current lead parts for a high-voltage superconducting apparatus, such as a superconducting fault current limiter, superconducting cable, and superconducting transformer with a subcooled liquid nitrogen cooling system. In this paper, a study on the electrical breakdown characteristics of gaseous insulation materials is conducted to estimate the thickness of the gaseous layer between the current lead and the metallic enclosure. An analysis on the dielectric characteristics of the GFRP (glass fiber reinforced plastic) is performed to determine the thickness of the GFRP sheath for a current lead. It is found that the electric field intensity of the gaseous insulation media at flashover can be represented as the function of field utilization factors at various pressures: 0.1, 0.2, 0.3, and 0.4 MPa. Also, it is revealed that the electric field intensity at sparkover with an electrical breakdown of the GFRP can be represented as a constant value regardless of system pressure, and a field utilization factor under the conditions of quasi-uniform electric fields. All dielectric experiments are conducted under liquid nitrogen at a temperature of 77 K. The conceptual insulation design of condenser-type current leads for a 22.9 kV and a 154 kV superconducting apparatus is undertaken.

Analysis on the Dielectric Characteristics of Solid Insulation Materials in

Several types of solid materials are used in the development of high voltage superconducting magnet systems as the former and insulation barrier. The solid insulation materials used in cryogenic applications require excellent dielectric characteristics as well as robust mechanical characteristics. In this study, the dielectric experiments for three types of solid insulation materials, including glass fiber reinforced plastic (GFRP), epoxy (Stycast 2850FTJ), and Bakelite in liquid nitrogen (LN2) at 77 K temperature are performed. Each experiment on the solid insulation materials is performed according to the type of applied voltage (lightning impulse and ac voltage) and the system pressure (0.1 ~ 0.4 MPa). Also, experiments considering the stacking effect of insulation sheets are performed. All electrical breakdown voltages are observed when the solid insulation materials are fully penetrated. Electrical breakdown experiments on GFRP and Bakelite sheets according to the number of stacking are performed and the results are compared. The relationship between the dielectric characteristics of solid insulation materials and the distribution of electric field intensity is calculated and analyzed by the finite elements method (FEM). The results revealed that the dielectric characteristics of solid insulation materials in LN2 are directly affected by the distribution of electric field intensity and the number of stacking sheets.

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A study on the dielectric characteristics of various cryogenic materials should be conducted to design an electrically reliable high-voltage superconducting apparatus. Especially, the dielectric characteristics of gaseous and solid insulation materials are important for designing current lead parts, and those of gaseous and solid insulation materials are indispensable for designing superconducting coil parts. A subcooled liquid nitrogen (LN2) cooling system is the most promising, with respect to insulation, thermal stability, and current capacity, in the development of a high-voltage superconducting apparatus. In this paper, dielectric experiments on gaseous nitrogen ( GN2), LN2, and glass fiber reinforced plastic are conducted under ac and lightning impulse voltage for various pressures. Sphere-to-plane electrode systems are used to examine the dielectric characteristics of insulation materials according to field utilization factor (ξ). The experimental results are analyzed by a finite element method. The empirical formulae for calculating the electrical breakdown voltages of various cryogenic insulation materials at sparkover are presented.

for Development of High Voltage Magnet Applications

Three kinds of solid insulation materials, such as glass-fiber-reinforced plastic (GFRP), Bakelite, and MC Nylon, are often used in high-voltage superconducting apparatuses as formers and insulation barriers to improve dielectric characteristics. GFRP, Bakelite, and MC Nylon are considered promising candidates as cryogenic solid insulation materials for developing a high-voltage superconducting apparatus due to their excellent dielectric characteristics and robust mechanical strength under cryogenic conditions. The surface roughness of solid insulation materials could differently vary depending on the machining processes. These effects may result in reduced dielectric characteristics. Currently, the details of any relationship between surface roughness and dielectric characteristics under cryogenic conditions have not been analyzed, and there are only a few reports. Thus, a study of the dielectric characteristics of solid insulation materials with respect to surface roughness in cryogenic conditions should be conducted when designing high-voltage superconducting apparatuses with high reliability. In this paper, we present a study on the dielectric characteristics of several materials, such as GFRP, Bakelite, and MC Nylon, used as formers and insulation barriers as a function of surface roughness to assess the effects of changes in surface roughness. Experiments with respect to surface roughness were performed with alternating-current voltages in gaseous nitrogen (GN2) and liquid nitrogen (LN2). A barrier-type solid insulation material was installed between a rod-to-plane electrode system. Penetrating electrical breakdown, creep discharge, and sparkover voltage with a barrier system were observed in the experiments. It was found that the penetrating electrical breakdown, the creep discharge, and the sparkover voltage with a barrier were influenced by the surface roughness of the solid insulation barriers.

Study on the Dielectric Characteristics of Gaseous, Liquid, and Solid Insulation Materials for a High Voltage Superconducting Apparatus

The current lead for a high-voltage superconducting fault-current limiter is made of copper or brass, and the cryostat chamber is often composed of a nonmagnetic material, such as stainless steel. Therefore, sparkover breakdown may occur at the current lead or the superconducting coil of such a superconducting coil system. The measurement of sparkover and partial discharge inception voltages is considered an important technique for diagnosing the dielectric conditions of a high-voltage superconducting instrument. In this paper, the sparkover voltage and the partial discharge inception voltage with respect to the electrode material were examined under an alternating-current voltage. Sphere-to-plane electrode systems made of stainless steel, copper, brass, and iron were tested in dielectric experiments to verify the sparkover and partial discharge inception voltages. It is observed that the sparkover and partial discharge inception voltages in gaseous nitrogen (GN2) were strongly dependent on the electrode material. In addition, it is found that the sparkover and partial discharge inception characteristics of GN2 conform to the work function of the electrode material.

Dielectric Characteristics of Solid Insulation Materials With Respect to Surface Roughness

The electrical insulation design for a superconducting system is important when developing a high-voltage superconducting apparatus as a substitute for a conventional one. In this paper, the degradation characteristics of 2G high temperature superconducting (HTS) wires, with respect to electrical breakdown tests, were studied. It was found that the superconducting materials in 2G HTS wires can be damaged in electrical breakdown, and the damaged structure of 2G HTS wires results in the degradation of the Ic and index number. As a result, it was found that the degradation characteristics of the 2G HTS wires were affected by the stabilizer material and applied breakdown voltage. Thus, the hardness and electrical conductivity of a stabilizer material can be considered as design parameters in developing a high-voltage superconducting coil. Finally, the cross-sectional views of 2G HTS wires were presented using a scanning electron microscope.

A Study of the Dielectric Characteristics of Gaseous Nitrogen With Respect to the Electrode Material for Developing a High Voltage Superconducting Fault Current Limiter

Since 2011, the Institute for Basic Science has been developing a linear accelerator, called RAON, for a rare isotope science project. The linear accelerator utilizes an electron cyclotron resonance (ECR) ion source for generating intense highly charged ion beams. A superconducting magnet system for a 28-GHz ECR ion source consists of six hexapole coils nested inside four solenoid coils made with low-Tc superconducting wires of NbTi. The superconducting magnets provide an axial magnetic field from four solenoid coils and a radial magnetic field from six hexapole coils to extract an ECR plasma stream that controls the flow current. The electromagnetic force acts on the superconducting magnets due to the magnetic field and flowing current. Therefore, electromagnetic characteristics of the superconducting magnets should be analyzed to minimize the electromagnetic force that causes coil motions and coil strains during excitation. In this paper, the design of superconducting magnets for a 28-GHz ECR ion source is deduced by analyzing the magnetic field to extract intense highly charged ion beams and an electromagnetic force to minimize coil motions and coil strains. The arrangement of the superconducting magnets is inferred by analyzing the magnetic field and the electromagnetic force. Also, the application of an iron pole is considered to improve the stability with a lower operating current. Analysis is conducted by using finite element method simulation.

Degradation Characteristics of Superconducting Wires With Respect to Electrical Breakdown Tests

Electrical breakdown voltages of liquid nitrogen (LN2) are measured by dielectric experiments, according to the gap between electrodes, and the diameter of electrodes. Also, the distribution of electric field intensity at sparkover is calculated and analyzed by the finite element method. The dielectric characteristics of LN2 depend on the amount of bubbles, as well as the duration time of the bubbles. Therefore, it is important to observe and analyze the bubble suppression characteristics of LN2 according to the conditions of the applied voltage type, electrode material, and system pressure. The bubble suppression characteristics of LN2 according to these conditions are experimented by measuring the duration time. Also, the amount of bubbles generated after the electrical breakdown is monitored by a high-speed camera system. AC and lightning impulse voltages are applied to the sphere to plane electrode system made of stainless steel, copper, and iron to compare and analyze the bubble suppression characteristics of LN2. It is found that the electrical breakdown and bubble suppression characteristics of LN2 are determined by the electrode material. Also, the bubble suppression characteristics of LN2 with the system pressure (from 0.1 MPa to 0.4 MPa) are observed and analyzed.

Magnetic Analysis on the Design of Superconducting Magnet for Developing 28-GHz ECR Ion Source

This paper deals with the electromagnetic loss characteristics of enclosures for a 24kV high voltage switchgear by using a finite element method (FEM). A study on the electromagnetic characteristics of enclosures for a high voltage switchgear should be conducted to minimize the size and the temperature rising of a switchgear. Generally, the enclosures made by stainless steel are used to minimize the eddy current loss caused by the transporting current in Bus bars due to its non-magnetic characteristics although the price of stainless steel is expensive compared with other metal for enclosures. Therefore, a switchgear made by stainless steel enclosures could be fabricated as a small size and are applied to a switchgear in urban substations. On the contrary, the switchgear enclosures made by steel could be fabricated with relatively cheap manufacturing price. However, the temperature easily rises due to the transporting current in Bus bars because steel is a ferromagnetic material. Therefore, the size of a switchgear made by steel enclosures is relatively massive and installed in rural substations. In this paper, the electromagnetic losses in the enclosures of a switchgear according to various enclosure thicknesses are calculated and compared with each other. Especially, we proposed a hybrid type enclosures for a switchgear made by stainless steel (top and bottom enclosure) and steel (left and right enclosure). It is concluded that the cost electromagnetic performance of applying the hybrid type enclosure is favorable to develop a high voltage switchgear.

Visualization Study on the Bubble Suppression Characteristics of

The rated voltage has been rising in order to minimize the losses in power transmission. The high voltage electric machines should be minimized due to the constraints of space. Therefore, the temperature of high voltage electric apparatuses easily exceeds the temperature limits. In this paper, it is investigated that how to minimize the internal temperature rising of a high voltage switchgear by adjusting the arrangement of bus bars. High voltage switchgears consist of a circuit breaker, a CT, a PT, a earthing switches, bus bars, and so on. It is very difficult to estimate the electromagnetic properties of a high voltage switchgear due to these various environments and structures. In this paper, analyses are focused on the electromagnetic characteristics of bus bars according to the arrangement method and the enclosures to simplify the electromagnetic characteristics of a switchgear. It is found that the characteristics of electric field intensity and electromagnetic losses in bus bars are influenced by the arrangement method of bus bars. However, it is confirmed that the electromagnetic characteristics of enclosures are not affected by the arrangement of bus bars. In this paper, the arrangement methods of bus bars to minimize the electric field intensity and electromagnetic losses are suggested. It is expected that the research results are helpful to design and develop an electrically reliable high voltage switchgear.