| The geographical distributions of energy and load are extremely uneven in our country:energy mainly locates in the west region, while a higher level of economic development and large electricity load are in the east. Thus, the demand for long distance power transmission grid has been increased. With the advantages of cross region, large capacity, long distance, low loss in power transmission and valuable land resources saving, UHVDC transmission with its unique technical superiority has been widely used in our country in recent years, which also plays an important role in the national economic construction and social development. The construction of Nuozhadu-Heshan±800kV UHVDC project, delivering the western hydropower resources to Guangdong province, is not only the promotion of the Great Western Development and West-East Power Transmission strategies, but also an important measure to realize win-win development in both west and east areas. In the AC and DC power transmission system, rich harmonic voltages and currents, which will result in a greater impact on the electrical equipment and power quality, will be produced due to the nonlinear devices such as transformers and converter station. Therefore, the research on the harmonic characteristics of UHVDC system has both important theoretical value and referential meaning in engineering.Based on the background of the transmission system of Nuozhadu-Heshan UHVDC project, this article analyzes the generation mechanism of the harmonic. Ideally, current of AC only contain12K±1times, DC voltage containing12K times harmonic. Moreover, with the analyses of the reasons and influencing factors of the non-characteristic harmonic, and research of the harmonic interaction and transfer mechanism of AC/DC system based on modulation theory, the related rules of harmonic transmission is obtained.Combining with the related parameters of Nuozhadu-Heshan±800kV transmission project, the simulation model of±800kV UHVDC transmission system has been built with PSCAD/EMTDC, and the accuracy of the simulation model has also been verified. Then based on the above research platform, the simulation study of harmonic characteristics in the two cases of both steady state and transient state has been made. Besides, the simulation analyses of bipolar full voltage operation, unipolar earth return, unipolar metal return, voltage decompression operation under steady state conditions; and AC single-phase grounding fault, two-phase grounding fault, and the DC line grounding fault under transient state conditions have been completed. What’s more, the harmonic characteristics under different working conditions have also been covered in this research.According to the analysis of AC and DC filter circuit structure and working principles, the characteristics of harmonic in UHVDC transmission system, and the distribution scheme of reactive power of converter station, the selection and parameter calculation of the AC and DC filters have been realized. Then, with the application of MATLAB, the researcher simulates the impedance characteristics of the designed filter, and contrastively analyzes the AC side harmonic between with and without filter. The simulation results show that the designed filter group can effectively restrain the harmonic, and the validity of the design parameters and the rationality of the filter scheme have also been verified in this research. |
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