特高压换流变压器阀侧出线绝缘结构电场分析

Electric Field Analysis for Valve-side Lead-out Insulation Structure of UHVDC Converter Transformer

阀侧出线绝缘结构是决定换流变压器安全性的关键部位,包括套管油端、均压电极和多层纸板围屏系统,以及至阀绕组引线的绝缘。该出线结构承受着严酷的交直流电场应力,历来换流变压器在试验中和运行中的绝缘故障很多都发生在这里。为此基于以往设计和试验、特别是对试验中故障的调查分析,聚焦于均压电极区域,综合、归纳了阀侧出线装置的3种典型绝缘结构,即单环电极“半罩”结构、单环电极“全罩”结构、双环电极无绝缘罩结构,提出了一种新的绝缘结构,即“Ω”型绝缘罩结构。用有限元方法建立模型进行交流和直流电场计算和安全性评价。指出了各种结构的风险点,并从理论上指出了改善电场分布和提高安全性能的方向。计算结果和故障案例一致表明,半绝缘罩的端部油中有很高的沿端面直流电场,可引发局放进而引起沿大绝缘筒闪络;全绝缘罩端部油中直流场强很高,沿引下线头部绝缘表面的电位梯度也很高,可引起引下线头部油中沿面闪络或纸绝缘击穿;双环电极无绝缘罩绝缘电极两侧至大绝缘筒的斜向第一油隙很大,易发生交流放电;“Ω”型绝缘罩可能是兼顾交流和直流绝缘的优化解决方案。

The insulation structure of valve side lead-out is the key area affecting safety of converter transformer, which suffers severe stresses from both AC and DC electrical fields. Most insulation failures either in factory tests or in operation happened here. Based on experiences gained in a lot of UHVDC project design and test practices, focus on the shield electrode areas, the paper summarized 3 typical insulation structures, namely ‘semi-covers’ ‘full covers’ ‘double-ring’,provide a new type of ‘Ω’ shape-cover, all located in a multi-layer cylinder. The finite element method is used to calculate AC and DC electric fields and make safety assessment. The critical area of the structures and the improvement direction of field distribution and safety are theoretically pointed out. The calculation results and failure cases are matched and both indicate that: high DC field stress concentrate in the oil at the ends of ‘semi-covers’ where partial discharge is likely to occur and causing flashover along the inner surface of the multi-layer pressboard cylinders;the DC field stresses in the oil at the ends of ‘full covers’ are very high and the voltage gradient along the insulation surface of down-lead is also very high, which means high risk of flashover in oil or breakdown in paper;the non divided oblique oil gap from electrode to cylinder of ‘double ring without covers’ is rather large and has a high risk of discharge under AC;‘Ω’shape-cover may be a proper solution for both AC and DC insulation.