Pressure Dependence and Size Effect of LN2Breakdown Characteristics Under Transient Thermal Stress

Electrical insulation of liquid nitrogen (LN2) for a high-temperature superconducting (HTS) power apparatus can be categorized into two breakdown (BD) modes. One is the intrinsic or conventional BD of LN2. The other is the BD of LN2with transient bubbles, such as a quench. In this paper, the BD characteristics of LN2with transient bubbles were investigated for different LN2pressures and electrode diameters. Experimental results revealed that the BD strength of LN2with transient bubbles was lower than the intrinsic BD strength of LN2. The difference between the intrinsic BD strength and the BD strength with transient bubbles in LN2became smaller with the increase in the LN2pressure and electrode diameter. Furthermore, the BD characteristics of LN2with continuous bubbles were investigated and compared with the BD characteristics of LN2with 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.