Seungmin Bang

Dielectric Characteristics of Solid Insulation Materials With Respect to Surface Roughness

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.

Degradation Characteristics of Superconducting Wires With Respect to Electrical Breakdown Tests

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.

Surface Discharge Characteristics of GFRP in GN 2 and LN 2 for Designing a Superconducting Coil System

Solid insulation materials used in the application of a superconducting apparatus need to be robust in terms of both mechanical and dielectric performance. The surface discharge characteristics of solid insulation materials are inferior to their penetration breakdown characteristics. Therefore, dielectric accidents in a superconducting appliance can occur as a form of surface discharge rather than as a form of penetration breakdown. In this paper, dielectric experiments on the surface discharge characteristics of glass-fiber-reinforced plastic in gaseous nitrogen (GN2) at room temperature and liquid nitrogen (LN2) at 77 K are conducted to design a high-voltage superconducting coil system with high reliability. As a result, the electric field intensity at surface discharge voltage VSD under ac and dc voltages is deduced as an empirical formula.

Creepage Discharge Characteristics Along the Interface Between Two Solid Insulation Materials in GN 2

Glass-fiber-reinforced plastic (GFRP) and MC Nylon are widely used in superconducting apparatuses, such as in superconducting fault-current limiters, due to their excellent mechanical and dielectric performance. Many superconducting apparatuses are operated in a high-voltage condition. Thus, an unexpected dielectric accident can occur with damage to a superconducting magnet. Therefore, dielectric experiments on the creepage discharge characteristics of GFRP and MC Nylon were conducted for developing a high-voltage superconducting coil system. For this paper, the creepage discharges along the surfaces of solid insulators in gaseous nitrogen (GN2), and along the interface between two pieces of solid insulation in contact, were experimented on and analyzed, with respect to ac and dc voltages, at 300 K, under the absolute pressure of 0.1 MPa.

Discharge Voltage Effects on High-

Breakdown voltage can occur in a superconducting magnet system under high electric field conditions caused by inductive voltage, high-voltage operation, quench resistance, and keen edge of high-TC superconducting (HTS) tapes. Discharge voltage on a superconducting coil can degrade the critical current and the index number of HTS tapes. In this paper, HTS tape experiments are conducted to confirm the degradation characteristics of HTS tapes under dc breakdown voltage. In real systems, winding tension is applied in magnet systems. Therefore, tensile stresses were applied in the HTS tape when testing the degradation by breakdown voltage. The tension was controlled by using the bolt torque of a fabricated sample. In addition, degradation characteristics of BSCCO and YBCO tapes under dc breakdown voltage were compared. Our results revealed that HTS tape could affect breakdown voltage and tensile stress.

\text{T}_{C}

Abstract – Recently in accordance with the rapid development of the industrial society, the accidents caused by dielectric breakdown have been increasing in power grid. It is important to prevent the dielectric breakdown of a high voltage apparatus to reduce the damage from electrical hazards. To establish an electrically reliable database of insulation design criteria for high voltage apparatus, a study on dielectric characteristics test is indispensable. In this study, dielectric characteristics according to field utilization factors () which are represented as the ratio of mean electric field to maximum electric field are investigated. the dielectric breakdown experiments by using several kinds of electrode systems made with stainless steel are performed by AC breakdown voltage under air-insulation. Also, the experimental results are analyzed by the Weibull distribution. As a result, it is found that the dielectric characteristics of air-insulation are determined by  as well as arrangement of electrode systems. It is considered that the results of this study would be applicable to designing the air-insulated high voltage apparatuses. Key Words : AC breakdown voltage, Dielectric characteristics, Field utilization factor, High voltage apparatus †Corresponding Author : Dept. of Electrical Engineering, Korea National University of Transportation, Chungju, KoreaE-mail : kang@ut.ac.kr*Dept. of Electrical and Electronic Engineering, Korea National University of Transportation, Chungju, Korea**Guju Technology, Inc., Chungju, Korea접수일자 : 2014년 11월 18일수정일자 : 2014년 11월 25일최종완료 : 2014년 11월 26일

Superconducting Tapes With Respect to Tensile Stress

A cross linked polyethylene (XLPE) material has been widely used to develop a distribution power cable due to its excellent electrical characteristics and mechanical strength. However, several problems such as environmental disruption, electrical aging, thermosetting property, and impurities which cause degradation also arise. Therefore, a novel dielectric material should be developed to substitute for the XLPE. Several kinds of polyolefin materials to substitute for the conventional dielectric material, XLPE are developed and A cylindrical rod to cylindrical rod electrode system made with stainless steel is used to perform the experiments according to the ASTM D-149 protocol under an AC and Impulse input voltage condition. The experimental results are calculated by the Weibull distribution method and analyzed by an Finite Element Method(FEM). Finally, the dielectric characteristics of the conventional XLPE and novel polyolefin are experimented compared with each other in this study.