| The distribution of power generation resources and electricity demand in Chinaare extremely uneven. Coupled with rapid economic and social development, thecurrent extra-high votage network has failed to meet the needs of our developingsociety. As a result, emphasis on UHV (Ultra High Voltage) transmission technology,especially the UHVDC transmission technology, is not only an significant approach tosolve the power transmission problems of ultralong distance, ultralarge scale, low lossand trans-regional interconnection but also an irreplaceable method to validate UHVtransmission technology and evaluate UHV equipments. Undeniably, it will upgradethe power distribution from regional balance to overall balance on the country level oreven larger scale, thus fundamentally changing the mode of China power development.This paper, combining with the national key project—Shanghai-Xiangjiaba UHVDCTransmission Project that the author joined in, has conducted a systematic explorationof some actual engineering application problems of HVDC transmission technologyconcentrating on the structure and function of the converter station, the structuralfeatures of the converter transformer and its relay protections and the charging testtechnologies.This paper has briefly and systematically summarized the history of thedevelopment of UHVDC transmission and its present application status in China,elaborated the main characteristics of UHVDC transmission and its technical andeconomic advantages. Systematic introduction to the structure and function of theconverter substation was carried out on five aspects to stress the analysis of convertertransformer`s basic structure and its characteristics. The five aspects were bushings,on-load tap-changers, OLTC (On-Load Tap Changer) oil flow protection relays, gasrelays and pressure relief valves. As one of the most important primary devices inconverter station, converter transformers differs from those conventional powertransformers in various aspects such as installation, testing, and operating conditions.Through the transformer`s insulation structure, short-circuit impedance, harmoniccharacteristics, on-load tap changing and pilot projects, the special structural featuresand operating characteristics of the converter transformer were systematicallyanalyzed. And key issues in the field test were summarized.Converter transformers account for a large proportion of the converter station investment, and their reliable and safe operation serve as the basis of reliable and safeoperation of the DC transmission system. Therefore, providing a sound protection tothe converter transformer is of great importance to the well operation of the DCtransmission system. Through the comparison with ordinary transformer protectionsystem, this paper has analyzed the scope and configuration principles of theconverter transformer protection, briefly introduced its protection requirements andconfiguration schemes, described the working principle of the converter transformerelectrical protection; emphatically analyzed the working principles andcommissioning methods of the converter transformer primary protection and backupprotection. Besides, the paper has also described the configuration schemes andon-site testing method of the non-electricity protections of converter transformer,summed up the problems found in the on-site commissioning, proposed solutionsbased on theoretical analysis, and described the constituent characteristics of theprotection control system and the DC protection configuration of the whole DC areaof the converter station.From the perspective of the importance of the converter substation charging test,the paper has outlined the purposes and procedure requirements of the charging test.Combined with the actual engineering construction project the author participated, theprinciple method and basic steps of the converter transformer charging test wereintroduced; Integrated with the system debugging of the Fulong Converter Stationproject, problems in the charging test were systematically summarized and analyzedwith basic methods and recommended practices proposed for solutions; At the sametime, systematic exposition of the characteristics of the inrush current occurred whenthe load-free converter transformer in another polar was injected into the charging testand its possible impacts on a DC transmission system were illustrated.The paper, guided by power system protection and high voltage test theory, hascarried out applied research with close combination of engineering practice. Itstheoretical analysis, solutions and proposed measures for relevant problems found inthe protection commissioning and charging test of the converter transformers wereseriously validated in the field application test, thus bringing a certain reference valueto similar problems of practical engineering. |
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