| With the vigorous development of smart grid and global energy interconnection,the grid-connected capacity of the clean and renewable energy is still increasing,high voltage direct current based on voltage source converter(VSC-HVDC),especially modular multilevel converter(MMC),has also developed rapidly.The modular multilevel converter has become a research hotspot of flexible DC transmission technology,owing to its low power loss,high voltage waveform quality and strong fault handling capability.However,the topology of MMC is unique since its storage capacitors are distributed in submodules,which is doomed to the complicated internal dynamics.In addition,when the large-scale doubly-fed induction generator(DFIG)-based wind farm connected to grid through MMC,subsynchronous oscillation may occur,which will endanger the stable operation of the connected-system.Therefore,research on the inner dynamic characteristics of flexible DC grid based on MMC is of great significance.This thesis mainly contains the inner circulating current study of MMC,and the small signal stability analysis of MMC-based flexible DC grid,as well as the subsynchronous oscillation analysis of DFIG-based wind farm connected to grid via MMC.The research of this thesis provides a theoretical basis for the actual flexible DC grid operation.The main research work and achievements of this thesis are as follows:(1)Research on the inner circulating current analysis and circulating current suppressing strategy of MMC.First,the inner circulating current is analyzed via the more detailed phase-unit instantaneous energy derivation,and it is revealed that the quadruple-line frequency components of the inner circulating current will be increased,which results from the interaction between the double-line frequency components and the output voltage of the suppressing controller.Then,a double-loop circulating current suppressing strategy is proposed.By reasonably tuning the controller parameters,the suppression ability can be enhanced significantly.Finally,a three-phase MMC detailed simulation model is built and the correctness of the circulation analysis as well as the effectiveness of the proposed suppression strategy get verified.(2)Small signal stability analysis of the MMC-based flexible DC grid.Firstly,considering the dynamics of the the capacitor voltage fluctuation and the inner circulating currents,a small-signal model of the MMC-based flexible DC grid is established.Then,the eigenvalue analysis is applied to judge the small signal stability.Moreover,the participation factor and eigenvalue sensitivity analysis are utilized to determine the parameters which are strongly related to the potential unstable mode of the system.Adjust the parameters and the small signal stability of the MMC-based flexible DC grid can be enhanced.Finally,a simulation case based on a four-terminal flexible DC grid is built in Matlab/Simulink to verify the correctness of the theoretical analysis.(3)Research on subsynchronous oscillation caused by DFIG-based wind farm connected to grid via MMC.First,the impedance model including a DFIG-based wind farm and the MMC at wind farm side is established.Then,the impedance analytical method and the series resonant circuit theory are adopted to analyze the subsynchronous oscillation phenomenon of the grid-connected system.Meanwhile,the reasonable range of the parameter is obtained,which belongs to the inner-loop controller at rotor side of the DFIG.Finally,a time-domain simulation model of the grid-connected system is established in Matlab/Simulink and the theoretical analysis gets verified. |
PDF Full Text Request