The relationship among AC surface spark-over voltage, specific capacitance of solid insulator and bubbles in LN2 and LHe

The characteristics of AC surface electrical discharge on solid insulators in liquid nitrogen (LN₂) or liquid helium (LHe) are studied. The insulation structure used is a model of a power cable end. Bubbles are generated due to discharge at a lower voltage than that of spark-over. However, it was experimentally and computationally confirmed that the bubbles do not influence the spark-over voltage. In the structure, static capacitance is formed on the insulator surface. The surface spark-over voltage depends on the static capacitance. The relation between surface spark-over voltage and the static capacitance is experimentally considered for several insulators. The results show that the spark-over voltage is lower for larger static capacitance. The relation between the spark-over voltage and the capacitance can be expressed by an equation V=A·C_sp^-B