| In recent years,High Voltage Direct Current(HVDC)transmission technology has showed broad application prospects in important fields such as new energy power generation,clean energy integration,urban distribution network,multi terminal HVDC transmission,isolated island power supply and alike.Modular Multilevel Converter(MMC)has become one of the preferred topology of HVDC engineering because of its low switching frequency,low harmonic content and high modularity.In this paper,an MMC based HVDC transmission system(MMC-HVDC)is used as the research object,the control theory and the transient performance of AC and DC side faults are analyzed,the fault current characteristic of MMC bipolar short circuit fault is the main analysis content and corresponding fault current suppression measure is proposed.The following major aspects have been studied:(1)Detailed analysis of the topology and working principle of MMC main circuit,the nearest level approximation modulation strategy,submodule voltage balance control strategy based on capacitance voltage ordering and double frequency negative sequence circulation suppression strategy for MMC-HVDC system are given.(2)The mathematical model of MMC is established,then the whole structure of MMC controller is designed by the double loop decoupling control strategy,which establishes the system steady state model.(3)The 11 levels MMC-HVDC system AC and DC side fault models are built in the Matlab/Simulink software,and the processes of faults are simulated.It indicates that MMC output voltage is biased by Udc/2 when DC side monopolar grounding fault,the power transmission is terminated with overvoltage and overcurrent when the line breaks down or AC side faults,which composes a threat to the system.(4)The transient current analytical model is established for DC side bipolar fault and the main parameters affecting the fault current are analyzed.It shows that inductor L0 and capacitor Co respectively inhibit and promote the fault current,resistance Req accelerates the attenuation of the fault current.Finally,an overcurrent suppression method is proposed,and its effectiveness in fault current suppression is simulated and analyzed by 30%of peak fault current. |
PDF Full Text Request