Pressure Dependence and Size Effect of LN2 Breakdown Characteristics Under Transient Thermal Stress

Electrical insulation of liquid nitrogen (LN
₂
) for a high-temperature superconducting (HTS) power apparatus can be categorized into two breakdown (BD) modes. One is the intrinsic or conventional BD of LN
₂
. The other is the BD of LN
₂
with transient bubbles, such as a quench. In this paper, the BD characteristics of LN
₂
with transient bubbles were investigated for different LN
₂
pressures and electrode diameters. Experimental results revealed that the BD strength of LN
₂
with transient bubbles was lower than the intrinsic BD strength of LN
₂
. The difference between the intrinsic BD strength and the BD strength with transient bubbles in LN
₂
became smaller with the increase in the LN
₂
pressure and electrode diameter. Furthermore, the BD characteristics of LN
₂
with continuous bubbles were investigated and compared with the BD characteristics of LN
₂
with transient bubbles. As a result, the insulation design of an HTS power apparatus in consideration of their quench condition will be rationalized than the design based on the continuous thermal stress.