MMC-HVDC Operating Characteristics Analysis And Line Internal Fault Identification

Modular multilevel converter(MMC),as a new type of multilevel converter topologies widely used in high voltage direct current(HVDC)system,has many advantages and become the crucial technology of VSC-HVDC with the rapid development of modern power electronics.The studies on MMC-HVDC at present are mainly focused on the control design,so the further research on the MMC-HVDC protection and fault simulation analysis is needed.This thesis uses the PSCAD/EMTDC electromagnetic transient simulation software to build the MMC-HVDC simulation model.It aims to analyze the overall operation of MMC-HVDC system,based on the topological structure and module mechanism of MMC converter,the protection strategy of MMC-HVDC transmission system is analyzed thoroughly.Moreover,the dynamical responses of MMC-HVDC has been simulated.The simulation of different number of level converters on the output waveform signal,has verified the characteristics of the MMC of the harmonic filter.At the same time,this thesis has discussed the protection strategy of MMC-HVDC system,the type of system protection,the protection scheme based on the circuit breaker protection and the self-healing fault capability based on converter topology.This research has important significance to the operation mode and fault identification of MMC-HVDC line.In the aspect of fault characteristics of MMC-HVDC system,the number of submodule in the MMC-HVDC system simulation,the capacitance of the submodule and the reactance calculation on the MMC bridge has been given.In this thesis,the characteristics of the transient current in the MMC submodule and the single-line grounded fault of the DC line is analyzed.In addition,the simulation results of the system have also been analyzed.In the process of MMC-HVDC simulation,the lightning fault is increased rapidly,and the interference and fault caused by lightning strike in MMC-HVDC system are simulated and analyzed.Meanwhile,a single-ended protection method for MMC-HVDC system based on high and low energy ratio of wavelet coefficients is proposed.The results of frequency characteristics of MMC indicate that MMC has obvious attenuation effect for high frequency in fault signal.Hence,the identification of AC and DC fault simulation could be realized through wave characteristics and wavelet energy difference fault signal.The simulation has verified this method of fault recognition effect is obvious and effective.