Seokho Nam

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.

\hbox{LN}_{2}

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

This paper deals with the current decay characteristics of a high-Tc superconducting (HTS) coil operated in persistent current mode (PCM) under alternating magnetic field. In superconducting synchronous machine applications, such as linear synchronous motor (LSM), the superconducting coils are designed to operate in PCM to obtain steady-magnetic field with DC transport current. This superconducting coil operates on a direct current, but it can be exposed to alternating magnetic field due to the armature winding. When the field coils are subjected to an external time-varying magnetic field, it is possible to result in a current decay for those coils in PCM operation due to electrical AC loss. In this research, a current decay measurement system, which consists of field coil and armature coil, was fabricated to verify current decay characteristics of field coil in PCM operation against external alternating magnetic field. The current decay rate for the field coil was measured by using a hall sensor as a function of amplitude and frequency of armature coil.

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

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.

Experimental Analysis on Initial Current Decay Characteristics of Persistent-Mode HTS Coil by External Alternating Magnetic Field

The accidents caused by dielectric instability have been increasing in power grid. It is important to enhance the dielectric reliability of a high voltage apparatus to reduce the damage from electrical hazards. To develop an electrically reliable high voltage apparatus, the experimental study on the electrical breakdown field strength is indispensable, as well as theoretical approach. In this paper, the lightning impulse breakdown characteristics considering utilization factors are studied for the establishment of insulation design criteria of an high voltage apparatus. The utilization factors are represented as the ratio of mean electric field to maximum electric field. Dielectric experiments are performed by using several kinds of sphere-plane electrode systems made of stainless steel. As a result, it is found that dielectric characteristics are affected by not only maximum electric field intensity but also utilization factors of electrode systems. The results are expected to be applicable to designing the air-insulated high voltage apparatuses.

Dielectric Characteristics of Solid Insulation Materials With Respect to Surface Roughness

A significant number of recent studies have explored the impact of many kinds of winding insulation conditions on high-temperature superconducting (HTS) coils for the application of electric devices. HTS coil co-wound with turn-to-turn inserted materials could be an appropriate alternative to the no-insulation coil because of the poor charge-discharge delay and unexpected quench behavior of the no-insulation coil. In addition, the co-wound HTS coil has good thermal stability and mechanical integrity, making it useful for superconducting applications such as superconducting magnetic energy storage (SMES) and superconducting fault current limiter (SFCL). However, studies of the co-wound HTS coil have explored dc electrical characteristics, but not ac electrical characteristics. Research into ac electrical characteristics is essential because of the operation sequence of the SMES and fault occurrence for the SFCL. In this paper, the ac electrical characteristics of HTS coils co-wound with various inserted materials were experimentally analyzed. Tested coils were co-wound with either Kapton, stainless steel, or copper tape at every turn of the winding, and one no-insulation coil served as a reference. AC loss, fault current, and recovery time were measured at 77 K, self-field.

A Study on the Lightning Impulse Dielectric Characteristics of Air for the Development of Air-Insulated High Voltage Apparatuses

The environmental pollution caused by green-house gases such as and has been becoming the main issue of industrial society. Many developed countries are making efforts to minimize the amount of emission come from the operation of high voltage electrical apparatuses. As a part of these efforts, 180 countries signed the Kyoto Protocol in 1997 to cut back on their green-house gas emissions. Nowadays, the study on the development of dry air insulated switchgear (DAIS) which is known as an eco-friendly electrical apparatus is in progress. A DAIS is advantageous for minimizing the effect of impurities and enhancing the dielectric characteristics over an air insulated switchgear (AIS) by applying constant pressure to an enclosed cubicle. Therefore, a study on the electrical insulation performance of dry air as a gaseous insulation medium is conducted in this paper to substitute a gas insulated switchgear (GIS) for DAIS. As results, it is verified that the AC dielectric characteristics of dry air are similar to those of air and the lightning impulse dielectric characteristics of dry air are superior to those of air at 1bar pressure condition. However, dry air is of inferior dielectric characteristics to at 4bar pressure condition. Finally, it is suggested that the internal pressure of DAIS should be over 4bar pressure to develop a high voltage switchgear which has similar electrical performance to a conventional GIS. Also, the empirical formulae on calculating the maximum electric field intensity at sparkover of dry air are deduced by experiments according to pressures. It is expected that these results are helpful to design and develop a high voltage electrical apparatus.

Experimental Analysis on AC Loss and Fault Current Test of HTS Coils Co-Wound With Various Inserted Materials

This paper proposes a millisecond-scale electromagnetic levitator using high-temperature superconducting (HTS) coils. The proposed apparatus consists of an AC power supply, short-circuit switches, an aluminum plate, and two HTS coils. The HTS coils are wound in opposite directions and electrically connected in parallel. Thus, in a symmetric current distribution, the magnetic fluxes generated by the two HTS coils ideally cancel each other. However, in a sudden asymmetric current distribution created by the short-circuit switches, the magnetic fluxes are not cancelled, and the effective magnetic flux induces an eddy current in the aluminum plate placed above the HTS coils. The magnetic flux generated by the eddy current in the aluminum plate and the effective magnetic flux generated by the HTS coils together generate the repulsive force to levitate the aluminum plate. In this paper, numerical calculations and experimental verification of the repulsive force are performed. We show that this apparatus is able to provide a levitation force that we expect will be helpful for devices that need fast switching behavior of the levitation force.

Analysis on the Dielectric Characteristics of Dry Air According to Pressures for Developing an Eco-friendly High Voltage Switchgear

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.