Conceptual Design Of A Spacer For A 154 Kv Hts Apparatus

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 SF6 gas for its excellent dielectric strength. However, SF6 condenses into LN2 because of its high boiling point (approximately 209 K) during operation. The condensed SF6 into LN2 may result in problems such as dielectric degradation and system pressure decline. In order to prevent the condensation of SF6 into LN2, the termination of an HTS apparatus is divided into two parts by using a spacer: an upper part pressurized by SF6 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.