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

世界上最大的一类超导线圈被用于“大型螺旋装置”。其电气绝缘系统可能面临相当严重的多重应力，包括低温、大机械应力和强磁场。因此，研究其电气绝缘性能对于确定线圈的可靠性是非常重要的。如果超导体从超导态熄灭到正常状态，液体冷却剂很容易蒸发，在低温下变成高密度气体，这可能会降低其耐压。此外，要完全清除绝缘空间中的异物颗粒是非常困难的。本研究利用模拟绝缘体系的电极系统，研究了小间隙和带电颗粒对绝缘击穿特性的影响。液氦中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率的减小而增加，而在电压极性为负时，击穿电压随电压上升率的减小而几乎不变。假设正电荷可以沿着与金属电极接触的绝缘子表面移动，较低的外加电压上升速率会在绝缘子上产生更显著的电荷积累，从而导致电场增强，从而降低击穿电压。同样清楚的是，导电异物颗粒影响击穿电压，而电介质异物颗粒不影响击穿电压。

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.