Rearch On Topology And Other Key Technologies Of Hybrid HVDC Transmission System

China’s primary energy and electricity load centers showing the reverse distribution,"West to East" and "North-South" are important features of China’s power delivery.Under these conditions, the use of AC power transmission to achieve large-scale and long-range transport is limited by inadequate corridors and stability issues.Therefore, HVDC technol-ogy (LCC-HVDC) has been generally considered to be an important technique to solve the long-distance transmission of electricity.Further, with the new worldwideincreasing inte-gration demand of wind energy, photovoltaic power generation, thevoltage sourceconverter basedHVDC (VSC-HVDC)is considered to be a better solution to the access and transport of these clean energy.Hybrid HVDC is able to combine the advantages of both LCC-HVDC and VSC-HVDC technology. And the topology of hybridHVDC system is the key technical is-sue to give full play to its advantages. Hybrid HVDC system is the main object of study, its topology andapplications in the new energy grid integration is the research focus of this paper. The main research work of this paper includes the following sections:(1) The history and construction cases of LCC-HVDC and VSC-HVDC were introdu-cedbriefly. And three research aspects on the topology of hybrid HVDC, hybrid MTDC and hybrid DC technology in wind power applications were summarized and analyzed(2)A hybrid multi-terminal HVDC system model with VSC and LCC in parallel was established. Wind farm side VSC with AC voltage control and grid side LCC with DC vol-tage droop control was designed. And an extra frequency support control by droop control of LCC and fast power modulation of DFIG was proposed. The operation flexibility of the system and effectiveness of the proposed control method was verified by time domain si-mulation.(3) The topology of is proposed to connectDC systems with different voltage and con-verter types. The control method and operating principle were designed and analyzed. A typical multi-voltage, multi-terminal hybrid HVDC system based on hybrid auto-DC con-verter was proposed. Its steady-state properties and effectiveness of the control method to restart after a fault was verified by time domain simulation. Besides, two kinds of topolo-gies in wind power integrating applications were also tested.(4) An improved topology of LCC and VSC in series was proposed. The paralleled VSCswere connected to the middle of positive and negative grounding points of a LCC sta-tion via DC lines. The technical advantages and operational principles of the topology were introduced. The application method in large-scale wind power integratingwas proposed and verified by the time domain simulation.(5)An accurate power loss evaluation method of MMC based on fast simulation of MMC is proposed. Mathematical equations for accurately evaluating the power loss are firstly deduced. A fast power loss evaluation platform is then proposed by linking the PSCAD/EMTDC and Matlab files. Relationship between the power loss and power transfer level and the relationship between the power loss and operating frequencies are presented and fitted.(6) Two-level VSC experimental device development process was introduced, includ-ing its main circuit structure, detection and control system design, hardware and software protection. A three-terminal VSC-HVDCexperimental test system was developed. Control methodsfor each VSC were designed. The rationality and feasibility of the test system and operating characteristics of the control method were verified by experimental results.Through this paper, the study results wish toprovide references to HVDC technology development and engineering applications of hybrid HVDC.