For the development of superconducting (SC) power apparatus, it is necessary to establish the high voltage technology with consideration of quench phenomena peculiar to SC environment. From the point of view, we have investigated so-called “dynamic” breakdown characteristics of liquid helium (LHe) under different quench conditions using ac SC wires and coils. As a result, electrical insulation performance of LHe was found to be drastically degraded in the thermal bubble disturbance by the quench of SC wires and coils.
In this paper, in order to systematize the insulation performance of LHe under non-uniform electric field, we obtained breakdown characteristics for various gap lengths, pressures of LHe and post-quench thermal energy injected into LHe. Experimental results revealed that the dynamic breakdown voltage increased and approached to the static breakdown voltage with increasing the gap length and pressure, because of the suppression of thermal bubble disturbance in LHe. All of the experimental data enabled us to establish the 3-D systematized diagram of the dynamic breakdown voltage of LHe as functions of post-quench thermal energy, gap length and pressure. This diagram is quite useful to determine which combination of gap length and pressure should be chosen under arbitrary post-quench thermal energy for the practical electrical insulation design of SC power apparatus.