| Increasingly serious energy shortage and environmental pollution problems makeclean, safe, sustainable wind power has been extensively used throughout the world. Butits dispersion, far from the load center make the use of AC transmission technology orconventional current source converter-type DC transmission technology (CSC-HVDC)seems not very practical and economical. With the development of power electronicdevices, the flexible HVDC technology based on voltage source converter (VSC)becomes more widely in wind farm power transmission; modular multilevel converter(MMC)-based flexible HVDC system (MMC-HVDC) design modular, its advantage isthat can have a higher level of voltage output, reduces the switching frequency of theswitching components, output voltage harmonic content is low, is gradually replaced theearlier two-level or three-level flexible HVDC technology. Therefore, steady andtransient analysis of the wind farm associated networks via flexible HVDC systems isvery urgent and necessary.Firstly, This article research and analysis the current AC/DC Hybrid Power FlowAlgorithm in computing DC wind farms grid: the Jacobi Matrix irreversible, notiterative calculation; proposes a decoupled Power Flow algorithm of hybrid system withVSC-HVDC (the AC/VSC-HVDC). This algorithm decouples the AC and DC systems,respectively iterative algorithm, overcomes the disadvantages of the current AC/DChybrid system power flow calculation method: has inherited poor AC procedures, thepreparation of a large amount of code, extensions multi-variable, calculation slowshortcomings. Also solved flow algorithm problems of the wind VSC-HVDCinterconnection system. Standard example calculation results verify the validity of theproposed method in this paper, which can be used to analysis wind farm dcinterconnection system’s steady trend.Based on doubly-fed induction generator and modular multilevel converter (MMC)Flexible DC transmission system (MMC-HVDC), established a complete wind farm DCtransient network system model; Designed the doubly-fed induction machine controlstrategies with LVRT capability and MMC-HVDC system’s level, stations, valve threecontrol strategies. Studied and analyzed the current characteristics of the wind farm DCtransient network system short-circuit fault, the simulation results show that: HVDCsystem can reduce the harmonic content of three-phase short circuit fault, especially thetransient content of the DC component, also can reduce the impact of short-circuitcurrent during short circuit fault to the wind turbine.Finally, this dissertation analyzed the relationship of the wind speed changing andwind farm DC grid system fault currents. Analyzed the impact of wind farms via DCconnected grid to the transformer relay of the wind farm side: there are differences between AC and DC transmission system in fault current, the short circuit transientcurrent and short circuit steady-state current provided by DC transmission system aredifferent from AC transmission system, and it can produce great influence on overcurrent protection of transformer.The analytical and experimental results has a guiding significance and practicalvalue, and are very useful for analysis of power flow, analysis of transient short-circuitcurrent and transformer relay of wind farm in the system of wind farm transmissionwith VSC-HVDC.This research work is supported by the National High Technology Research andDevelopment of China (863Program)(NO.2011AA05A105). |
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