Quench-induced Dynamic Insulation Performance of Cryogenic Liquids for Superconducting Power Apparatus

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In this research project, we investigated dynamic breakdown characteristics of liquid helium (LHe) and its mechanism under quench environment to be considered for practical insulation design of superconducting power apparatus. Synthetic tests for quench and breakdown have been carried out for superconducting wire (or coil) -to-plane electrode configurations immersed in LHe. Major results obtained are as follows :1.Quench of superconducting wires by overcurrent generates plenty of thermal bubbles associated with ohmic heats out of the wires, leading to the disturbance in gap space of LHe exposed to high electric field. In such disturbance of thermal bubbles, the quench-induced dynamic breakdown voltage of LHe may be reduced below 20% of the static breakdown voltage.2.Thermal bubble behavior after quench-onset of superconducting wires was observed optically by using high speed video system, and also analyzed theoretically. Under non-uniform electric field for superconducting wire-to plane electrode configuration, gradient force acting on bubbles in the high stress region around the wire exceeds 100 times of buoyancy, and controls the bubble behavior.3.There exists a delay time of the order of ms from the quench-onset to the dynamic breakdown. Under the non-uniform stress, partial discharges as the prebreakdown phenomena take place in bubbles at the high stress region in the vicinity of the superconducting wire. The partial discharges grow with the propagation of bubbles, resulting in the dynamic breakdown.4.Quench-induced dynamic breakdown for superconducting coil-to-plane electrode configuration does not always occur at the minimum gap, and exhibits complex characteristics compared with the wire-to-plane system. Quench propagates 2-dimensionally on the superconducting coil and dynamic breakdown is induced at the quench inception point where the largest ohmic heat is generated.