Surface spark-over voltage on solid insulator in sub-cooled liquid nitrogen

Surface spark-over voltages on solid insulators in boiling and sub-cooled liquid nitrogen are studied under ac, dc and impulse voltage. The insulator structure used is a model of a power cable end. In the structure, static capacitance is formed between outer surface and inner conductor of the insulator. The spark-over voltages for several insulators were measured at gap length 60 mm or less under each voltage waveform. The spark-over voltages increase slightly with increasing gap length in boiling and sub-cooled liquid. In range of shorter gap length spark-over voltages in sub-cooled liquid are higher than those in boiling liquid nitrogen. The differences between spark-over voltages in both liquids are caused by bubbles generated due to discharge at low voltage. The spark-over voltage for the insulator with the larger static capacitance is lower than that for one with the small capacitance.