Control And Protection Strategy For The MMC-HVDC Under DC Faults

Recently, the technology of high-voltage direct-current (HVDC) transmission based on modular multilevel converter (MMC) draws much concerns. As a novel topology of VSC converter, modular multilevel converter (MMC) using a stack of identical submodules (SM) instead of the switching series of the conventional two or three levels VSC converter, and the application of MMC providing a multi-level output waveform much closer to the sine wave with smaller step at each voltage level is a significant medium of resulting in a reduced switching loss and a high quality waveform. Furthermore, as the superiority of MMC’s topology, MMC-HVDC system possesses a stronger fault through capability compared with the conventional VSC-HVDC systems.First, this paper describes the main circuit topology of MMC and the operation modes of the sub-modules, and the basic working principle of MMC is illustrated. Then, it discusses the two start schemes for MMC, and ultilizes the control method based on auxiliary DC power to precharge the capacitances of the sub-modules. Besides, considering the topology of MMC, the CPS-SPWM modulation strategy is selected to receive a better quality waveform. Second, under the estabilished framework of control and protection system, this paper analyzes the control strategy of MMC, and designs the corresponding control algorithm. In order to bring out the stable operation characteristics, the Simplex algorithm is proposed to optimize the PI controller parameters, and in the PSCAD/EMTDC, the21-levels MMC-HVDC system is modeled, the simulation results verify the justifiability of the control strategy and the optimization algorithm. Finally, comparing the faults characteristics of DC transmission system based on MMC with VSC, this paper analyzes the impact of different DC faults of MMC-HVDC and disigns the grounding method of the converters. And more, according to the variation of the electrical parameters in the AC-DC system and the discharge mechanism of submodules, the basic design requirements for protection with the corresponding control and protection strategies are proposed. The simulation platform of DC faults of MMC-HVDC is built in the PSCAD/EMTDC, and the simulations included single-line to ground fault, line break fault and line-to-line fault are carried out to verify the correctness of the analysis on the faults’mechanisms and the validity of the control and protection strategies.