Study On Hybrid-link Multi-terminal UHVDC Transmission Systems Feeding In Receiving Power Network

UHVDC transmission is an economical and efficient transmission mode for large area, long distance and large capacity, which is an important technical means to realize the optimal allocation of resources. The high voltage, large capacity UHVDC transmission feeding into a power grid, has a huge impact on this AC power grid system, at the same time, the receiving end power grid is generally located in load center, and the huge footprint of HVDC converter station exacerbates the tension problem of transmission corridor. In order to broaden the application range of HVDC transmission system, which is not only suitable for the strong AC system, but also for the passive system and the weak AC system. By combining the advantages of LCC converter and VSC converter, the traditional "point to point" two ends of transmission system transformation as multi-infeed UHVDC transmission is particularly necessary.Based on the existing domestic and foreign related research on proposing hybrid HVDC topologies, this paper proposes several hybrid multi-infeed UHVDC transmission structures, and analyses the technical and economic qualitative comparison of the proposed structure schemes, and then selects two topological optimum of these structure schemes.The quantitative technology and economic comparison between the two better topologies is analysed. From the point of view of reliability, the forced energy available rate and the forced energy non-available rate of the system are calculated. The economic cost of the converter stations and transmission lines, the operation cost analysis and investment return analysis are introduced to calculate some relevant economic indicators.Finally, Using PSCAD/EMTDC simulation software, the two topologies models are builted and the operating characteristics comparison are analysed by simulation. The operating characteristics include steady state, dynamic and transient characteristics. Simulation study on dynamic characteristics is through current step response. And the transient characteristics are studied by setting single-phase and three-phase grounding fault and DC line short circuit fault of all converters during simulation. Acording to the presented comparison results about technology and economic of the two kinds of topologies, the optimal topology is introduced, and the application range of the optimal topology is presented.