【作者】 王升；

【导师】 毛承雄； 陆继明；

【作者基本信息】 华中科技大学 ， 电力系统及其自动化， 2010， 博士

【摘要】 随着高压直流输电(HVDC)技术在国内电网愈来愈多地应用,由于其输送容量大、输送距离远、调节迅速、运行灵活,HVDC在远距离大容量输电、区域电力系统互联中起到了十分积极的作用,但同时也带来了一些新问题。实际运行发现,当直流输电系统采用大地回路运行方式时,从大地中流过的巨大直流电流会对附近的交流系统产生影响,特别是会对接地极附近中性点直接接地变压器产生比较严重的直流偏磁,这不仅增加了变压器的无功消耗,影响变压器的正常运行,引起继电保护误动,而且引起的谐波大幅升高也会对其他电气设备产生较大影响,从而直接影响到变电站、发电厂的安全运行。随着我国高压直流输电工程的日益增多,直流输电工程单极投运时间和每年直流输电线路的检修造成的单极大地运行总时间也越来越长,而且直流输电系统大地回线运行方式作为故障应急方式,基本上不可避免,因此,由直流输电引起的变压器直流偏磁的相关研究在近几年越来越受到重视。本文采用了场路结合的办法,将直流输电单极大地回路方式下地表电位(Earth Surface Potential,简称ESP)分布计算分为电网和地下两部分来计算；推导出了复镜像法在水平多层、垂直多层土壤模型上的计算公式;通过对海洋、山脉、湖泊、河流等地形单独存在时的仿真计算,分析了地形因素对接地极的接地性能和接地极附近的地表电位的影响；由于土壤结构对地表电位分布和流入变压器中性点直流电流影响很大,通过合理选择换流站站址的办法可减小直流接地极电流对交流电网的影响。本文从变压器的理论模型出发,研究和分析了变压器在直流偏励磁情况下,励磁电流和谐波分量的变化情况；建立变压器的有限元模型,通过有限元仿真分析了变压器发生偏磁的机理；并提出了一种实用的方法来检验变压器耐受直流偏磁电流的能力。本文设计了一种新型变压器中性点电容器隔直装置(Neutral DC Current Blocking Device,简称NCBD),与一般的电容器隔直装置相比,由于不需要额外的整流回路和晶闸管关断回路,主回路的拓扑结构得以大量简化,降低了装置的成本,同时该NCBD采用纯硬件的集成电路保护和微机保护的完全双重化冗余结构,提高了系统的可靠性和智能性。通过分析其拓扑结构和工作原理,讨论了装置的电容器保护和直流电流抑制能力。仿真结果、大电流试验以及装置在广东电网的应用,表明该NCBD能有效解决由直流输电单极大地回路运行方式下造成的接地变压器直流偏磁的问题。更多还原

【Abstract】 As the development of High Voltage Direct Current Transmission (HVDC) technology in China, it plays an important role in power transmission and also brings some new problems.During Monopolar operations of the HVDC electrode, significant DC currents can be observed in the transformer neutrals of substations in the vicinity of the electrode of the HVDC converter. There are no doubts on the origin, cause and mechanisms of the DC currents that appear in the neutral of the high voltage transformers. A portion of the DC current that is flowing between the two HVDC electrodes is captured by the grounding systems of the transmission and distribution lines and substations located in zones where the earth potentials are high and is discharged back to soil by the grounding systems of the electric network at the locations where the earth potentials are lower. In other words, Part of the DC current injected into the electrode finds an easier return path through the AC system via the grounded neutrals of the transformers. This DC current through transformer neutrals may make the transformer DC bias and therefore, increases the noise of transformer. Furthermore, it can cause many problems due to excessive core vibration, overheating regarding the integrity and longevity of the transformers and other related instrumentation problems. So the abnormal operation of transformer will cause the unsafe operation of substation and power plant. The voltage total harmonic distortion of 220kV and 500kV AC transmission network will increase sharply and these will impact other equipments operation. Sequentially, it may damage the transformers and the whole network. In a word, the HVDC ground electrode current makes the transformer with earthed neutral point as a harmonic source.With the increasing of HVDC project of China, the total time of monopolar operations caused by new projects puting into operation and annual overhaul is getting longer. And as an emergency maintenance mode, monopolar operations mode is inevitable. Therefore, the research of transformer DC bias caused by the DC transmission in recent years has become a very active and important research topic.By using field-circuit coupled method, the distribution of the earth surface potential (ESP) calculations are divided into two parts, including the ground and power grid. The formulas of complex image method (CIM) in the horizontal multi-layer model and vertical multi-layer soil model are deduced. The simulation results of the seas, mountains, lakes and rivers in the vicinity of HVDC grounding electrode show that soil structures have a great impact on the surface potential distribution and the size of DC current through transformer neutral point. Selecting the converter station site is an effective way to reduce the impact of HVDC grounding electrode current on AC power grid.Proceeding from the theoretical model of the transformer, the excitation current and its harmonic components of transoformer in the DC bias case are analyzed. By using the finite element mothod (FEM), the model of the transformer in the DC bias case is established. The results show that the mechanism of transformer DC bias. A practical method to test the DC bias current withstanding capacity of transformer is proposed.In this paper, a novel transformer neutral DC current blocking device (NCBD) based on inverse parallel thyristors protection is proposed and developed to solve DC bias problem. Based on the analysis of the topology and operating principles of NCBD, the main circuit is simplified by removing the front-end rectifier circuit and the turn-off circuit of thyristor. Thus, the cost of the device can be cut down a lot. Moreover, the reliability and intelligence of NCBD are improved by its dual redundant structure, which is made up of a set of hardware integrated circuit and a set of microcomputer measure-controlling system. Simulation results, high current tests and field operation results confirm that the NCBD can resolve the demand of DC bias restraint effectively, and provide a practical based engineering solution.更多还原