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

本研究旨在探讨失超环境下液氦（LHe）的动态击穿特性及其机理，为超导电力设备的绝缘设计提供理论依据。对浸没在液氦中的超导导线（或线圈）-平面电极结构进行了失超和击穿的综合试验。主要结果如下：1.超导导线过电流失超时，在导线外产生大量与欧姆热有关的热泡，导致高电场作用下LHe间隙空间的扰动。在这种热泡扰动下，LHe失超后的动态击穿电压可降低到静态击穿电压的20%以下。2.利用高速摄像系统对超导线材失超后的热泡行为进行了光学观测，并进行了理论分析。在非均匀电场作用下，超导线-平面电极结构的高应力区中，作用于磁泡的梯度力超过100倍浮力，并控制磁泡的行为。3.磁泡从失超开始到动态击穿存在ms量级的延迟时间。在非均匀应力作用下，超导线附近高应力区的磁泡中发生了作为预击穿现象的局部放电。局部放电随着气泡的传播而增长，导致动态击穿。4.超导线圈-平板电极结构的失超动态击穿并不总是发生在最小间隙处，与线-平板结构相比，表现出复杂的特性。失超在超导线圈上二维传播，在失超起始点产生最大的欧姆热，引起动态击穿。

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.