XLPE绝缘材料对交流和直流电压耐受的互易性研究

It is globally recognized that it is an effective way for improving the reliability and upgrading the capacity of an existing cable line to convert its operating voltage and current from AC to DC. Although this technique has attracted world-wide attentions, it lacks strong theoretical support and mature engineering applications. Aiming at the fundamental technical problems for an existing XLPE AC cable line to be converted to a DC operation, this project intends to investigate the reciprocity between AC and DC voltage endurance characteristics of XLPE insulating materials. First of all, an appropriate accelerated electrical aging test is set up, and carried out to explore the long-term electrical aging law of the XLPE insulation under DC voltages. Combining it with the electric field simulation under steady state and transient process, and with the introduction of the space charge effect effectively characterized by a field distortion coefficient, the insulation design theory for XLPE DC cables is established. The design scheme of DC operating voltage for AC cables of different voltage level is proposed as well. Secondly, the thermal field and thermal circuit models are set up for the DC cable lines, and the design theory of permitted cable rating is established, as well as the corresponding reciprocity relationship between AC and DC current rating for a certain cable circuit. Thirdly, the tree initiation characteristics are tested for XLPE insulation samples under grounded DC conditions, and the discharge voltage decays of actual cable sections are also measured after DC pre-stressing. Based on these experiments, the design theory of grounding systems for XLPE DC cable lines is established. The research is of vital significance for the exploration into electrical aging performance and failure mechanism of DC solid insulation, for the upgrading transformation of existing AC cable lines, and for the rational design and reliable operation of DC XLPE cables.

现役电缆线路由交流改为直流运行是提升系统可靠性及传输容量的一个有效手段，这已是全球共识，但目前仍缺乏理论支撑及工程验证。本项目针对交流电缆线路应用于直流运行的基础性问题，就XLPE绝缘材料对交流和直流电压耐受的互易性展开研究。通过设计合理的加速电老化寿命试验，探索XLPE交流绝缘材料在直流电压下的长期电老化规律，结合稳态、暂态直流电场模拟，以及对空间电荷电场效应的有效表征，提出XLPE直流电缆的绝缘结构设计理论，建立不同电压等级交流电缆对应的直流运行电压设计方案；通过合理的热场及热路模拟分析，提出直流电缆线路的额定载流量设计理论，建立同一线路交、直流载流量的互易性关系；通过绝缘试样直流接地电树引发特性及实际电缆直流预压后放电的试验，提出XLPE直流电缆线路的接地系统设计理论。研究成果对于绝缘材料的直流电老化机理研究、现役交流电缆线路的增容改造以及直流电缆的产品设计具有重要的理论及工程意义。

现役电缆线路由交流改为直流运行是提升系统可靠性及传输容量的一个有效手段，但缺乏理论支撑及工程验证。本项目针对交流电缆线路应用于直流运行的基础性问题，就XLPE绝缘材料对交流和直流电压耐受的互易性展开研究。.研究获得了XLPE绝缘试样在相同条件下的交、直流E-t关系曲线，确定了直/交1小时击穿场强比2.86，并首次观察到直流击穿场强与传统电老化反幂定律相背离的现象。建议按照绝缘设计的平均场强法确定电缆本体直流运行电压，并推荐了相关参数。.直流XLPE电缆绝缘的稳态电场沿径向呈近似线性分布，采用电阻率场强关联系数大、热活化能小的绝缘材料，有利于抑制电缆高负荷下直流绝缘的电场反转，降低最大电场。推荐采用绝缘松弛时间常数的2~3倍作为升压时间，以兼顾直流升压时场强和时间的要求。暂态电场可采用阻性与容性电场的叠加求解，绝缘内侧易出现瞬时高场。.搭建单芯及三芯电缆的交、直流额定载流量评估试验系统，完成对解析算法的准确性验证，确定了典型敷设方式下不同结构电缆的交、直流载流量对应关系，两者偏差未超过6.5 %。.开发可精确控制的直流预压后瞬时接地试验系统，首次研究了接地电阻和温度对直流预压接地电树枝引发率的影响；开展了电缆样品放电试验；确定直流电缆的接地损伤由XLPE绝缘中电荷快速释放引起，初始放电功率或电流是关键参数。直流电缆长线路的放电时间常数可通过电缆电容与放电电阻的乘积来估算。电缆系统自然放电过程缓慢，在剩余电压降至额定工作电压之后，可引入具有远程控制功能、由多个电阻串联的放电装置加速电缆放电。.提出了同轴复合绝缘结构空间电荷畸变电场评估技术，确定了不同电压等级硅橡胶及乙丙橡胶预制接头中的最大实际场强，推荐了交流设计接头在直流下的安全运行电压，其显著低于电缆本体的设计值。.给出了现役交流配网XLPE电缆线路改为直流运行的基础设计方案，确定了电压、电流运行参数，完成了传输容量对比。