| With the rapid development of renewable power generation,modular multilevel converters(MMCs)play an increasingly important role in power transmission system,however,it has been reported that oscillation phenomenon had happened in many voltage source converters based high voltage direct current transmission(VSC-HVDC)systems.Therefore,it is so urgent to analyze the factors affecting the stability of MMC based transmission system and figure out the strategies to restrain the potential oscillation.The stability analysis method based on impedance model is widely used and effective in analyzing the oscillation phenomenon in complicated power electronics systems.Then,founded on this impedance-based analysis method,this thesis would like to derive the mathematical impedance model of MMC,working in AC voltage control mode,considering the influence of various control blocks and propose some pertinent suggestions on improving the stability of MMC systems.First,the capacitor voltage control strategy(CVCS)is introduced and the equivalent simulation model of MMC connected to active AC load based on bridge-equivalent model is conducted.CVCS is detailed derived and turn out to be effective in increasing the overmodulation ability of MMC to some degree,which is also validated by the simulation.The simulation achieves the goal of controlling AC side phase voltage and the benefits brought by circulating current control block is also validated.This lay the first stone for the following studySecond,the AC and DC side impedance model of MMC including the influence of AC load are derived.Based on the harmonic linearization method,the small signal model of power stage is built firstly and combined with the small signal model of control loops,thus the impedance of MMC working in AC control mode is developed.Then,the terminal impedance scan tool,based on MATLAB/Simulink software,developed in this paper is introduced and validated,which shows the effectiveness of this tool to get the characteristics of electric system.Comparation between mathematical impedance model and impedance scanned by simulation tool mentioned above proves the accuracy of the mathematical impedance model of MMC working in AC voltage control mode derived before.The impedance of AC load has little influence on the impedance of MMC once the circulating current control loop is applied,so the influence could be ignored in general.Finally,a doubly-fed induction generators(DFIGs)wind farm is studied as an example to analyze the stability of MMC connected to active AC load.Based on the impedance model derived above,the influence on stability of MMC brought by the number of DFIGs,the AC voltage controller,the AC current controller,the circulating current controller and the capacitors voltage controller is well discussed.Three dimensions(3D)impedance bode plots are used to depict the impedance characteristics of MMC when the parameters of controller changed within a small range,then further oscillation phenomenon are analyzed with the impedance of wind farm scanned by simulation tool.Time domain simulations are also conducted to verify the theoretical analysis result.The results show that the stability of MMC connected to wind farm would be reduced as the number of DFIGs increases,the integral coefficient of AC voltage controller and the proportional coefficient of AC current controller have big influence on the stability of system,the circulating current control controller is effective in improving the response speed and suppressing the oscillation especially the proportional coefficient.The capacitor voltage controller makes the AC impedance less changed than DC impedance.The capacitor voltage control loop has little influence on the stability of MMC systems when the circulating current controller is applied. |
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