Reliability and Performance of Composite Electrical Insulation at Cryogenic Temperature for Superconducting Power Apparatus

世界上最大的超导线圈用于“大型螺旋装置”。其电气绝缘系统可能会暴露在相当严重的多重应力，包括低温温度，大的机械应力和强磁场。因此，研究其电绝缘性能以建立线圈的可靠性是非常重要的。如果超导体从超导态失超到正常态，液体冷却剂很容易蒸发，并在低温下变成高密度气体，这可能会降低其耐受电压。此外，很难从隔热空间完全去除外来颗粒。本研究利用模拟绝缘系统的电极系统，研究小间隙和异物对绝缘击穿特性的影响。在液氦中具有小间隙的绝缘系统的击穿电压如图1所示。当电压极性为正时，击穿电压随电压上升速率的减小而增大，而当电压极性为负时，击穿电压随电压上升速率的减小几乎不变。假设正电荷可以沿着与金属电极接触的间隔物表面移动，则施加电压的较低上升速率将在间隔物上带来更显著的电荷积累，导致降低击穿电压的场增强。还变得清楚的是，导电外来颗粒影响击穿电压，而电介质外来颗粒不影响。

The world's largest class superconducting coil is used for the "Large-scale Helical Device". Its electrical insulation system might be exposed to considerably severe multiple stresses including cryogenic temperature, large mechanical stresses and strong magnetic fields. It is therefore very important to study its electrical insulation performance in order to establish the reliability of the coil. If a superconductor quenches from superconducting state to normal state, the liquid coolant vaporizes very easily and turns into high-density gas at cryogenic temperature, which may reduce its withstanding voltage. Furthermore, it is very difficult to completely remove foreign particles from the insulated space. So it is required to clarify the influence of foreign particles and electrification on the insulation performances.This research was conducted using electrode system that simulated the insulation system to investigate the behavior of small gaps and foreign particles with the breakdown characteristics of insulation. The breakdown voltage of insulation system that has a small gap in liquid helium is shown in Fig.1. The breakdown voltage increased with decrease of voltage rising rate when the polarity of voltage was positive, but the breakdown voltage was almost constant with decrease of voltage rising rate when the polarity of voltage was negative. Assuming that positive charge can move along the spacer surface that is in contact which the metallic electrode, lower rising rate of applied voltage would bring more significant charge accumulation on the spacer, leading to the field enhancement which reduces the breakdown voltage. It became also clear that conductive foreign particles affect the breakdown voltage while dielectric foreign particles do not.