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 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.