| 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%. |
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