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The termination part of a high temperature superconducting apparatus such as a superconducting fault current limiter or a superconducting cable is usually insulated by pressurized SF 6 gas for its excellent dielectric strength. However, SF 6 condenses into LN 2 because of its high boiling point (approximately 209 K) during operation. The condensed SF 6 into LN 2 may result in problems such as dielectric degradation and system pressure decline. In order to prevent the condensation of SF 6 into LN 2 , the termination of an HTS apparatus is divided into two parts by using a spacer: an upper part pressurized by SF 6 with room temperature and a lower part pressurized by GN 2 with cryogenic temperature. The termination part should be operated in a high-pressure condition in order to ensure safe dielectric characteristics of HTS apparatuses. In this paper, dielectric characteristics as well as mechanical characteristics of a spacer under pressurized gas are conducted. It is well known that electrical breakdown may occur in the form of creepage discharge along the surface of a solid material rather than in the form of penetration sparkover through a solid or gaseous material. Experiments on the creepage discharge characteristics according to several solid materials under various pressures are conducted and a computer simulation using a finite element method is performed to analyze electric field distribution and mechanical strength of a spacer. Finally, the specifications of a spacer for 154 kV superconducting fault current limiters such as diameter, thickness, shape, and material, are suggested according to various safety factors.

Solid insulation materials need to be robust in terms of both mechanical and electrical performance for application in a high-voltage superconducting apparatus. It is known that creepage discharge characteristics of solid insulation materials are inferior to their electrical breakdown characteristics with penetration. Therefore, electrical breakdown accidents in a superconducting apparatus could occur as a form of creepage discharge rather than electrical breakdown with penetration. In this study, we conduct dielectric experiments on the penetration and creepage discharge characteristics of epoxy resin and polypropylene laminated paper under cryogenic temperature by using three kinds of electrode systems. The first is for creepage discharge experiments along the surfaces of solid insulation materials in liquid nitrogen. The second is for creepage discharge experiments across the interface between two different solid insulation materials making contact in liquid nitrogen. The third is for experiments of electrical breakdown with penetration on solid insulation materials in liquid nitrogen. Dielectric experiments are conducted with an absolute pressure of 0.1 MPa under alternating current and lightning impulse voltages. Experimental results for electrical breakdown characteristics of solid insulation materials with penetration are compared with the creepage discharge characteristics on the surface and interfaces between solid insulation materials. Moreover, the criteria for electric field intensities at creepage discharge voltages in various conditions are deduced as empirical formulas.

This paper deals with the electrical characteristics of no-insulation coil wound with surface-processed HTS tape. The bypassing current path through turn-to-turn contacts within a coil is formed in the no-insulation coil, and this bypassing current path determines two characteristics: 1) self-protection and 2) charge-discharge delay. The amplitude of bypassing current is determined by contact resistance between the turn-to-turn contacts of the no-insulation coil. The surface roughness of the HTS tape is one of the parameters to change the contact resistance. The HTS tapes were processed to roughen by bead blast and abrasive paper, and the no-insulation coil is fabricated using processed HTS tape. We have studied the charge-discharge delay and self-protecting characteristic of each no-insulation coil by 1) sudden discharge tests and 2) overcurrent tests. The FEM simulations of contact resistance of no-insulation coil were carried out. The contact surface resistance of a case processed by abrasive paper has almost three times larger than that of reference no-insulation coil, and a case processed by bead blast presents almost same contact surface resistance with reference no-insulation coil.

Superconducting cables are one of the most promising applications among several high voltage superconducting power apparatuses owing to its low energy loss and high power capacities. Superconducting cables system consists of four parts: the termination, the joint box, the core, and the cooling system. The core is responsible for power transport, and is insulated with polypropylene laminated paper (PPLP) to ensure a high dielectric strength in the presence of liquid nitrogen (LN 2 ). The insulation layer of the core is fabricated by winding PPLP in multiple layers. It is inevitable that butt gap filled with LN 2 is generated in this process. Currently, many studies on the dielectric characteristics of PPLP according to various intervals between butt gap under 0.1 MPa and those with fixed butt gap according to various pressures have been conducted. A study on determining a proper interval between butt gap according to pressure in order to develop reliable superconducting cables under high pressure should be performed because superconducting cables system in power grid is operated under high pressure. This study focuses on the evaluation of the dielectric characteristics of PPLP with interval between butt gap at various pressures. In addition, dielectric experiments on V-t characteristics for a mini-model cable are conducted and experimental results are analyzed by the inverse power model.