Application And Research Of MMC In HVDC Systems

Modular Multilevel Converter (MMC) is one of the most promising converter topologies for its high waveform quality, low total harmonic distortion, high modularity and can output high voltage level. However, the study of MMC is still in its infancy, the technology is still not mature enough and has many problems. On the basis of basic structure and operation principle of the MMC-based HVDC system, some research results on modulation strategy, sub-module balance control strategy and fault handling strategy is introduced.Firstly, a new variable integration modulation method is proposed on the basis of introducing to existing Nearest Lever Modulation (NLM) and Carrier Phase-Shifted Pulse Wide Modulation (PSPWM) method. The new method based on segmented modulation which has the advantages of high waveform quality, clear thinking and high expandability is proposed. Then, the paper introduces the traditional sub-module balance control strategy based on sorting and proposes the new Virtual Loop Mapping (VLM) and Selectivity Virtual Loop Mapping strategies (SVLM). These methods can respectively function well in parameters balance and unbalance systems and have the advantages of low calculated quantities, easy to realize. The concept of transposition margin is also introduced to improve the SVLM strategy and reduce the switching frequency. Simulation results in MATLAB/Simulink environment confirm the performance of the proposed scheme.Secondly, the paper explains the cause and influence of the sub-module failure in MMC. Then, the traditional redundancy fault-tolerated schemes of sub-module is analyzed and combined with SVLM strategy to create a new scheme. The new scheme plunge the sub-module in hot standby into loop mapping and improve the speed of replacement. This paper also proposes some fault-operating scheme named lack of level operating scheme and line voltage constant operating scheme if the hot standby sub-module is not exist. The first scheme, though causes the distortion of AC-side voltage, could handling the balance of system well and usually be used in single phase system. The second scheme is running in three-phase system and could maintain line voltage invariability. Both of the schemes could maintain the steady of the system after sub-module fault and make the system keep on working well. Simulink results in MATLAB/Simulink environment conform the availability and validity of the new schemes.Finally, a three phase five-level MMC-HVDC hardware platform is built and a series of experiments are conducted:the grid-connected experiment of three-phase MMC-HVDC verify the availability of the MMC structure in HVDC system; the constant frequency integration pulse-width modulation strategy based on VLM and SVLM experiments confirm the practicability of the new modulation strategy and submodule balance schemes; the lack of level operating scheme and line voltage constant operating scheme experiments show the affirmative impact of the new fault-operating schemes. The results of the experiments meet well with the results of the simulation, which ulteriorly verifies the correctness and validity of the proposed theories.