Evaluation of Electrical Characteristics of Polymer/Liquid Nitrogen Composite Insulation System in Liquid Nitrogen Flowing

研究了液氮流动对低温绝缘系统电性能的影响。比较了人工流动液氮和自然循环液氮在局部放电条件下的击穿电压、聚合物棒表面闪络电压和聚合物膜(S)的击穿寿命。主要结果如下：1.流动液氮的击穿电压略高于自然循环液氮中的击穿电压。随着液氮流量的增加，聚丙烯酸酯棒沿面闪络电压逐渐增大。液氮流动导致聚乙烯膜在液氮中局部放电导致的击穿寿命较长。在流动的液氮中，局部放电的起始电压和熄灭电压较高。寿命试验中的局部放电测量表明，流动液氮的累计荷电和累计放电次数较小。在热流入自然循环的液氮中观察到连续的局部放电活动。然而，在流动的液氮中，局部放电有时会停止。在双层聚乙烯薄膜中也观察到了类似的现象，认为液氮的流动可能会产生爆炸放电。通过液氮流动抑制放电的开始和/或传播可以改善电特性。

The effect of liquid nitrogen flowing on electrical characteristics of cryogenic insulation systems was studied. Breakdown voltage, surface flashover voltage along polymeric rods, life time to breakdown of polymeric film (s) subjected to partial discharges, were investigated in artificially flowing liquid nitrogen comparing with those in naturally circulating liquid nitrogen by heat inflow.The main results obtained are as follows :1. Breakdown voltage of flowing liquid nitrogen increased slightly compared with that in liquid nitrogen naturally circulated by heat inflow to the chamber.2. Surface flashover voltage along acrylic rods increased gradually with increasing flow rate of liquid nitrogen.3. Liquid nitrogen flowing resulted in longer lifetime to breakdown of a polyethylene film caused by partial discharges in liquid nitrogen.4. Partial discharge inception and extinction voltages are higher in flowing liquid nitrogen.5. Partial discharge measurement in the life tests showed that cumulative charge and cumulative number of discharge are smaller in flowing liquid nitrogen.6. Continuous partial discharge activity was observed in liquid nitrogen naturally circulated by heat inflow. In flowing liquid nitrogen, however, partial discharge sometimes stopped.7. In the case of two-layred polyethylene films, similar phenomena were observed.It is considered that flowing of liquid nitrogen may blast discharge. Restraint of inception and/or propagation of discharges by liquid nitrogen flowing may improve electrical characteristics.