Bubble Behavior and Its Influence on Insulation Properties of Liquid Nitrogen for Superconducting Power Apparatus

In superconducting power apparatus, electric insulation is one of the most important deciding factors on their performance. Bubbles produced during quench condition of superconducting tapes will degrade the insulation performance of liquid nitrogen and reduce the electrical breakdown voltage of the apparatus. This paper presents the bubble behavior and the influence on insulation properties of liquid nitrogen ( LN₂) for superconducting power apparatus. The result will provide useful information for superconducting power devices in transmission voltage level, such as fault current limiter, transformers, and cables. Measurements are made to get the ac breakdown voltages in LN₂ using spherical brass electrodes as quasi-uniform electric field. Different structures of HTS tapes are used as electrodes to simulate real working conditions. Bubbles are created by heaters, and their influences on the breakdown voltage are measured. How the breakdown strength would be affected by the bubbles is studied by changing the length of the gap of electrodes and the amount of bubbles. Breakdown voltage of G10 slice is also tested. Experiments show that the breakdown strength of LN₂ is sensitive to the behavior of bubbles. Different structures of HTS tape have different breakdown properties under bubbling condition. G10 material is a good choice to provide insulation strength between HTS tapes.