| The large-scale development and utilization of various renewable resource power generation technologies have promoted the development of high-voltage and high-power power transmission technology.However,due to the limitation of the characteristics of power electronic switching devices,the traditional two-level converter has been unable to meet the current needs,and multilevel technology has attracted people's attention.In particular,modular multilevel convertor(MMC)has been favored by scholars at home and abroad as soon as it was proposed.The research and application of MMC have been made great progress in the field of distributed power generation,flexible DC transmission,and solid state transformer,et al.Based on the research of MMC system control strategy,the thesis presented a hybrid modulation method combining the nearest level modulation and carrier modulation strategy.At the same time,a sub-module voltage balanced control algorithm suitable for hybrid modulation strategy was proposed.The main contents are as follows:Firstly,the thesis compared and analyzed the topology of several traditional multilevel converters with MMC.It shows that MMC is highly modular,easy to expand,simple to control,and high quality in output waveform,et al.The working principle of MMC was analyzed in three-phase stationary coordinate system,and the topologies of several common sub-modules were analyzed from the point of view of working principle and output feature.In order to realize the decoupling control for the input phase current of the convertor,a mathematical model of MMC in two-phase rotating coordinate system was established.Secondly,in order to realize the power decoupling control of MMC system,a double closed-loop system controller with fixed DC voltage outer loop and current inner loop was built in two-phase rotating coordinate system.Based on the analysis of the mechanism of the bridge arm circulating current,the corresponding circulating current suppressor was studied.Finally,based on the analysis of traditional nearest level modulation and carrier phase shift pulse width modulation strategy,a hybrid modulation strategy was studied,and simultaneously,a sub-module voltage balanced control algorithm suitable for the hybrid modulation strategy was designed.A nine level MMC system was built on the platform ofMatlab/Simulink.Combined with the improved sub-module voltage balanced control algorithm,a hybrid modulation strategy studied in the thesis was simulated and verified.It compared with the simulation result of traditional nearest level modulation strategy.The results show that under the control of the system controller and the improved sub-module voltage balanced control algorithm,MMC with hybrid modulation strategy has higher system conversion efficiency,lower sub-module voltage fluctuation and lower input phase current THD rate. |
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