【作者】 李伟；

【导师】 文习山；

【作者基本信息】 武汉大学 ， 高电压与绝缘技术， 2016， 博士

【摘要】 随着我国电力事业的飞速发展,直流输电由于其输送距离远、容量大、损耗低等诸多优点得到了越来越多的应用。当直流输电工程单极大地运行时,大量的直流电流入地导致大范围地表电位分布的不均匀,从而导致了交流电网变电站变压器中性点通过直流电流,产生直流偏磁现象,对电力系统的安全稳定运行造成了极大的危害。要对直流电流分布问题进行计算和分析,大地模型的建立是极为重要的一步。由于直流输电工程输送距离远,直流电流穿透深度大,因此必须考虑深层土壤电性特征对直流电流分布的影响。目前国内外的电力系统中,大地电阻率的勘测主要采用四极法,大地模型大多局限于浅层土壤。即使有个别考虑深层土壤模型的,也大多基于经典大地模型,缺乏实际的大地电阻率勘测结果的支持,数据缺乏说服力。因此,研究适合直流输电接地极极址大地电阻率勘测的方法,通过实地勘测获取充分的深层大地电阻率数据,在此基础上研究直流电流的分布和直流偏磁抑制措施的研究,具有重大的理论价值和工程实际意义。本文针对以上问题,在对直流电流在地中的分布理论研究的基础上,进行了大地电阻率勘测方法的理论研究,并通过实地勘测和反演得出典型地质条件下的的层状大地模型,在此基础上对各大地模型表现出的电性特征进行了计算和分析,提出了直流偏磁电流的主动防御方法。本文主要研究工作如下：首先,对水平多层土壤的格林函数求解进行了推导,对于考虑结构复杂的深层大地时格林函数求解困难的问题,可以采用智能复镜像法有效解决,并具有极高的计算精度精度。同时,建立了交流电网直流电流分布计算的地下电场模型与交流电网地上电路模型,共同形成了对直流偏磁电流进行计算和分析完整的场路耦合模型。理论和模型的研究结果为下文的分析和计算奠定了基础。其次,利用格林函数与复镜像法推导了直流电流在水平多层大地中的穿透深度,计算了理想情况下的穿透深度,结果表明由于直流输电工程输送距离远,大量的直流电流趋向于深层大地分布。基于地球的电性特征,讨论了适用于直流接地极极址大地电阻率勘测的四极法和大地电磁法勘测与反演方法,以及结合两种勘测方法获取完整大地模型的方法。特别的,对于四极法测量极距较大时引线间互感造成的误差问题,提出了采用相对误差进行互感消除的方法,在极距较大时可以比传统方法更有效的进行四极法测量结果的修正。此外,基于直流电流穿透深度的理论,利用经典大地模型初步计算和分析了四极法与大地电磁法在直流接地极极址勘测时测点范围的选择,以为下一步的实地勘测工作提供指导。通过实地勘测和分析,分别在青海、湖北和广东三个分别代表青藏高原、内陆平原与冲积平原的地质条件迥异的地区选择了共计11个测点进行了大地电阻率的勘测工作,通过数据处理与反演计算得出了上述地区的完整土壤模型。基于测量结果对不同地质条件下的大地电阻率分布特征,以及表现出的直流电流穿透深度、地表电位分布等电性特征进行了计算、对比和分析。将湖北地区的直流电流分布实际勘测结果与基于深层大地模型的仿真计算结果进行了对比和验证,误差小于2.7A,相对误差小于21%,结果的一致性证明了本文模型和方法的有效性。在实测的不同地质条件下,根据地中不同深度下的直流电流比例对大地电阻率勘测的广度进行了研究,修正了测点选取的范围：对于四极法的勘测,应当在直流接地极附近进行,测点与直流接地极之间的距离不应超过6km；大地电磁法的勘测在距离直流接地极10km～30km范围内进行最佳,测点与直流接地极的距离不应超过100kmm。在不同交流电网拓扑结构和不同直流接地极的情况下,以交流电网总体不平衡电流作为评价标准,对大地电阻率勘测的深度进行了计算和分析。结果表明,在直流接地极附近进行的四极法勘测只需在接地极附近6km内,大地电磁法勘测只需在接地极附近100kmm内；要达到95%的直流电流分布计算精度,大地电磁法的测深应当达到91.36km。基于目前的勘测数据,达到95%直流电流分布计算精度所需的测深为66.99km-120.28km。根据初步的拟合公式,提出对未知地区的勘探,达到95%直流电流分布计算精度所需要的测深为91.36km。测深拟合公式在后续研究中还需要大量的不同地区测量数据进行修正和完善。最后提出了分布式接地极与支援型入地电流控制策略两种直流偏磁防御措施。对分布式接地极的理论模型进行了推导,对不同拓扑结构下的分布式接地极对交流电网直流偏磁电流的抑制效果进行了计算和分析。基于混沌粒子群法,在给定的区域内对分布式接地极进行了选址优化计算,无论是哪种拓扑结构,在经过选址优化后都可以达到良好的直流偏磁抑制效果。以网状结构为例,单个变压器绕组最大直流电流和交流电网总体不平衡电流分别为0.51A和9.24A,降低了81.39%和56.20%。支援型入地电流控制策略对直流偏磁电流的抑制效果与目标函数直接相关,单目标、多目标和加权优化的目标函数对交流电网直流电流抑制的侧重点不同,抑制效果也有所差别。将分布式接地极与支援型入地电流控制策略相结合,使不平衡电流在分布式接地极系统内互相抵消,可以实现入地电流最小化的目标,从而从根本上抑制直流偏磁风险。以网状结构为例,与优化前相比,采用联合应用优化后电网的单体最大电流下降37.18%-91.58%,总体不平衡电流下降39.04%-91.39%。更多还原

【Abstract】 With the rapid development of China’s power industry, high-voltage direct current (HVDC) transmission has gained more and more applications because of its long transmission distance, large capacity and low loss. In the ground-return mode (GRM) of HVDC links, the ground-return current (GRC) may achieve high magnitude, which leads to the uneven distribution of large-scale surface potential, which, in turn, causes DC currents passing through transformer neutrals and leads to a number of adverse effects to transformer and power apparatus.To calculate and analysis the DC current distribution, the establishment of the earth model is a very important task. Because of the long transmission distance of HVDC transmission project and the large penetration depth of DC current, it is necessary to consider the effect of deep earth electrical characteristics on DC current distribution. At present, four-point method is mainly used to measure earth resistivity in domestic and foreign power system, and the earth model is mostly confined to the shallow earth. Even if individual models considered deep earth resistivity, they are mostly based on classical eath models, without the support of deep earth resistivity measurements, consequently the model is not convincing enough. Therefore, it is of great theoretical and practical significance to study the method of whole earth resistivity measurement around the HVDC electrode, and obtain sufficient deep earth resistivity data through field measurenment to study the distribution of DC current and the DC bias suppression measures. In this paper, based on the theoretical study of the distribution of DC current in the earth, the method of large-scale earth resistivity measurement is carried out, and the earth models of Qinghai, Hubei and Guangdong are obtained by field measurements and investigation. On this basis, the electrical characteristics of each earth model are caulated and analyzed, and active defense methods of DC bias current are proposed. The main work of this paper is as follows:Firstly, the solution of Green’s function of horizontal multi-layer earth is derived. The problem of Green’s function is difficult to solve when considering the complicated deep earth structure, which can be solved by the intelligent complex image method, whose precision of the calculation is very high. Meanwhile, the under ground electric field model and the AC power grid circuit model for calculation of DC current distribution in AC power grid are established. The two models form a complete field-circuit coupling model for DC bias current calculation and analysis. The theoretical analysis and modeling results provide the basis for the follow up calculation and analysis.Secondly, the penetration depth of DC current in horizontal multi-layer earth is derived and the penetration depth under ideal conditions are calculated. The results show that a large proportion of DC current tends to be deeply distributed due to the long transmission distance of DC transmission project. Based on the electrical characteristics of the earth, the measurement and inversion of the four-point method and the magnetotelluric (MT) method are discussed, and the method of combining the two kinds of measurement methods to obtain the complete earth model is also discussed. In particular, aiming at the measurement error of four-point method caused by the mutual inductance between the leads when the pole pitch is large, a method of eliminating mutual inductance with relative error is put forward. When the pole pitch is large, the correction of the four-point method measurement result can be more effective than the traditional method. In addition, based on the theory of penetration depth of DC current, the location of the four-point method and MT method measurement around the HVDC electrode is calculated and analyzed using the classical earth model.Through field investigation and analysis, the earth resistivity measurements are carried out in Qinghai, Hubei and Guangdong, respectively, representing the Qinghai-Xizang Plateau, the inland plain and the alluvial plain, with a total of 11 points measurements. By data processing and inversion, the complete earth models of the above areas are obtained. Based on the measured results, the distribution characteristics of the earth resistivity under different geological conditions, and the penetration depth of DC current and the distribution of surface potential are calculated, compared and analyzed. The comparison between the actual measured results of the DC current distribution in Hubei power grid and the simulation results based on the deep earth model shows that the error is less than 2.7A and the relative error is less than 21%. The consistency of the results shows the validity of the simulation model.Under the different geologic conditions, the breadth of the earth resistivity measurement is studied, and according to the proportion of DC current at different depths, the location of the measurement was revised. It is suggested that the measurement of the four-point method should be carried out in the vicinity of the HVDC electrode, and should not be farther than 6km away from the HVDC electrode; for MT measurement, it should be between 10km and 30km away from the HVDC electrode and not farther than 100km away from the HVDC electrode. In the case of different AC grid topologies and different HVDC electrodes, the requried depth of ground resistivity measurement is calculated and analyzed based on the total DC current of AC grid. Based on the current measured data, the measuring depth required to achieve 95% DC current distribution calculation accuracy is 66.99 km to 120.28 km. According to the preliminary fitting formula, it is proposed that the measuring depth required for the 95% DC current distribution calculation accuracy is 91.36km for the measurement of unknown area. The fitting formula of the measurement needs a lot of measurement data to be revised and perfected in the follow-up study.Finally, two kinds of DC bias current protection measures are proposed, which are distributed HVDC grounding electrodes and current support strategy. The theoretical model of the distributed HVDC grounding electrodes is deduced, and the effect on the DC bias current of the AC power grid under different topologies is calculated and analyzed. Based on the chaos particle swarm optimization, the optimal location of the distributed HVDC electrode is calculated in a given area. The results show that for what ever kind of topology, it can be achieved a good DC bias suppression effect after the optimization of the location. Taking the mesh structure as example, the maximum in the transformer neutrals and the total DC currents of AC grid are 0.51A and 9.24A, which are 81.39% and 56.20% lower respectively. The suppression effect of the current support strategy on DC bias current is directly related to the objective function. The objective function of single-objective, multi-objective and weighted optimization has different emphasis on the DC current suppression of AC power grid, so the suppression effect is different. The combined application of the distributed HVDC grounding electrodes and the current support strategy can eliminate the imbalance current in the distributed HVDC grounding electrode system and minimize the DC current into the earth, fundamentally reduce the risk of DC bias. Taking the mesh structure as example, compared with the former, the optimized results of maximum current of the transformer neutrals decreased by 37.18%～91.58% and the total DC current of AC grid decreased by 39.04%～ 91.39%.更多还原