Effects of Temperature and Voltage on Dielectric Breakdown Strengths of PET and FRP under Mechanical Stresses

The electric insulators for superconducting magnet coils to be used in a large fusion reactor are subjected to large compressive and tensile stresses to a maximum level of about 400 MPa, and these stresses should pose a significant electrical insulating problem in practice. In this paper, dc and ac dielectric breakdown characteristics of PET and FRP under mechanical stress at liquid nitrogen (LN2) temperature are presented as part of a comprehensive study for electrical insulating design of superconducting magnet coils to be used in a large fusion reactor. The results presented are complementary to those already reported for ac at room temperature. The dielectric breakdwon strength (DBS) of PET and FRP under compressive stress at 77 K shows similar variations to those obtained with ac at room temperature: the DBS increases at first, reaches a maximum value, and falls thereafter. The maximum DBS is little influenced by the temperature variation, but the compressive stress showing the maximum DBS depends strongly on the temperature regardless of voltage sources used. Under tensile stress the DBS of PET and FRP at 77 K is higher than at room temperature at a given tensile stress.