【作者】 杜华珠；

【导师】 文习山；

【作者基本信息】 武汉大学 ， 高电压与绝缘技术， 2013， 博士

【摘要】 随着我国特高压直流输电工程的蓬勃发展,直流接地极在设计上也更加复杂。由于直流输电系统电压等级的不断升高以及直流系统共用接地极情况的增多,使得流过直流接地极的电流不断增大,接地极的温升将会更加严重。在传统温升的计算过程中,认为接地极周围土壤各参数为恒定值。而实际情况下土壤参数对温度变化十分敏感,温度升高将会改变极址土壤参数,进而影响接地极及其附近土壤的温升情况。本文针对上述情况开展了直流接地极温升试验和数值仿真计算研究,建立了考虑土壤参数变化情况的直流接地极温度场计算模型,并在1000kV特高压交流试验基地开展了垂直型和水平型两类直流接地极的温升试验。本文的主要研究工作如下：(1)直流接地极电流场与温度场的耦合计算。模型以电流场计算结果为依托,采用有限元法对直流接地极温度场的暂态过程和稳态分布进行了计算。为解决有限元法在计算开域问题时剖分量巨大甚至无法计算的问题,计算模型中采用有限元、无限元相结合的方法,用无限元来模拟远场效应,用有限元模拟近场效应。将有限元与无限元的耦合算法计算结果与有限元算法计算结果进行比较,证明耦合算法能减少剖分量且不影响计算精度,并由于其计算上考虑了无限远边界,减少了由人为定义截断边界所带来的误差。(2)不同性质土壤中直流接地极温升特性的计算。实际情况下土壤分散性很大,文中考察了不同土壤性质对接地极温升的影响,具有较大热导率的土壤中接地极的稳态温度较低且温度分布更均匀,土壤热容率与热导率比值较小的土壤达到热稳定所需时间较短,且接地体整体达到热稳定的时间相差较小,选择极址时应考虑热导率与热容率大的土壤。(3)土壤电阻率变化时不同性质土壤中直流接地极温升特性的计算。土壤电阻率对接地极温升影响巨大,接地极上的散流分布随着土壤电阻率增大变得越来越不均匀,并导致接地极的稳态温度分布也更不均匀。实际情况中,土壤电阻率是一个随温度变化而改变的量,当温度高于某一临界值时,土壤电阻率会迅速增大,电极温度也随之上升,且不会达到稳定,而与时间呈指数增长的关系。文中在计算中考虑了这一情况,建议在进行接地极设计时应将土壤电阻率这一变化特性纳入考虑,且接地极允许运行温度不应超过65℃。(4)垂直直流接地极的模拟试验。基于场的相似理论,开展了垂直直流接地极的模拟试验,试验采用霍尔电流传感器与热电偶温度传感器分别对三根接地极和单根接地极在持续注入直流情况下的电流散流和温度进行记录,得到接地极散流与温升规律。实验发现,由于端部效应,垂直接地极上的电流散流从接地极顶端到接地极底端逐渐增大,下端部10cm散流的密度要远远大于整个接地极的平均散流密度。越靠近接地极的底端温升开始的时间越早,上升的速率越高,幅值也越大,随着时间的推移,接地极温升的上升速率会减小；接地极底端具有最大温升。(5)水平直流接地极的模拟试验。基于场的相似理论,开展了水平直流接地极的模拟试验,试验采用霍尔电流传感器与光纤光栅感温光缆对单根水平直线型接地极在持续注入直流情况下的电流散流和温度上升进行记录,得到接地极散流与温升规律。实验发现,接地极上的散流呈现明显的端部效应,散流大小由两端向中间部分依次递减,并基本呈对称分布；接地极温度分布符合两端高,中问低的规律,在试验初始阶段,温度上升较快,随着电流加载时间的增大,温升速率逐渐变慢。试验同时发现,因土壤电阻率随温度上升发生变化,电流注入点的温升曲线在试验期间出现尾部曲线上扬的情况。(6)试验选址处土壤热参数选定与土壤电阻率的拟合。试验发现,因土壤电阻率随温度上升发生变化,电流注入点的温升曲线在试验期间出现尾部曲线上扬的情况。文中根据实测资料选取土壤热参数,并将其随温度变化的关系拟合成曲线加载到仿真计算过程中。根据Campbell土壤电导率经验公式对试验选址处土壤的土壤电阻率进行拟合,给出了拟合公式,并据此计算了接地极温升。通过对仿真结果与试验结果的比较,试验和计算具有较高的吻合程度。更多还原

【Abstract】 Because of the development of UHV DC transmission, the DC grounding electrodebecomes more and morecomplex. The current flowing through the DC grounding electrode is increasing by the higher voltage level and more common and the tempturerise of grounding electrode will be more serious. The soil parameter considered as aconstant value in traditionaltemperature rise calculation, but in fact, the soil parameters are very sensitive to changes in temperature and will change bytemperature rise, thereby affecting the temperature of electrode and the nearbysoil. Thispaper established a model of DC ground electrode temperature field considering changes in soil parameters and carry out two types ofDC grounding electrode temperature rise testincluding vertical grounding electrode and horizontal grounding electrode in1000kV UHV AC test base. The main research works are as follows:(1) Acoupling calculation model of DC grounding electrode current field and temperature field is established, which is based on the results of current fieldcalculation. Thetransient process and steady-state temperature distribution in grounding electrodeare calculated. In this paper, the finite element (FE)-infinite element (IFE) coupling method is proposed to calculate current field, whose coupling method simulates far-field effect withinfinite element method and simulates near-field effect withfinite elementmethod. The comparison results indicate that the amount of subdivision is decreased with FE-IFE coupling method than that with FE method, and it reduced the error caused by artificially defined truncation boundary at infinity with FE method.(2) Temperature rise of DC grounding electrode is calculated numerically in miscellaneous soils. In practical cases, parameters vary with different typeofsoil. In the numerical calculation, considering the influence of soil parameters, the calculation results of temperature rise show:with higher soil heat conductivity, the steady-state temperature of DC ground electrode is lower, with more uniform temperature distribution; with smaller ratio of soil thermal capacity and thermal conductivity, it has smaller thermal time constancy. Therefore, it’s better to select places with high thermal capacity and thermal conductivity to set the DC grounding electrode.(3) The influence of soil resistivity on temperature rise of DC grounding electrode is researched with soil of different type. The soil resistivity has a significant influence on temperature rise of DC grounding electrode. The current diffusion tends to be more non-uniform with higher soil resistivity, resulting in a more non-uniform steady-state distribution of temperature. In practical cases, soil resistivity is a function of temperature. And it increases rapidly when the temperature is higher than some critical value. In this case, the temperature increases exponentially with the soil resistivity. Considering this case in the calculation, it’s proposed that the variation of soil resistivity should be considered in the design of ground electrode and the temperature should not exceed65℃.(4) Temperature rise tests based on field similarity theory were conducted for vertical ground electrode. DC current was applied to the ground electrode and the temperature rise and current diffusion? of ground electrode were measured by thermocouple and hall current sensors. With the influence of end effect, the disperse current density is gradually increasesfrom head to toe, and disperse current density in the bottom of10cmis far greater thanthe average disperse current density of the whole electrode. Closer to the bottom of the electrode, temperature rise is earlier, rising rate is higher,amplitude is larger, With the passage of time, the temperature rising rate decreased and the largest temperature rise in the bottom ofelectrode.(5) Simulation tests based on field similarity theory were conducted further to study temperature rise of horizontal ground electrode. Single linear grounding electrode was applied, and the temperature rise and current diffusion of ground electrode were studied. There’s a significant end effect on the ground electrode, resulting in an symmetrical current divergence distribution that the current divergence decreased from two ends to the middle of the electrode; the temperature distribution of grounding electrode is high at both ends, low in the middle.At the beginning of the test, temperature rise is faster and it is gradually slow downwith the increase of current load time. A special phenomenon occurred in tests that the temperature curve at the injecting point tended to be upturn, was mainly caused by the evaporation of water with high temperature.(6)It’s fitted soil resistivity and determined soil thermal parameters oftest location. Tests’ results show that the soil resistivity rises with the temperature rise and the temperature rise curve upturns at the injecting point. Soil thermal parametersvary with temperature was determinedbased on the measured data and loaded into simulation.Soil resistance of test soil was fitted according to Campbell empirical equation of soil electric conductivity and a fitted equation is given out in this paper. And the simulation results with the fitted equation are much closer to practical tests.更多还原