Research On The Control Strategy Of Modular Multilevel Converter

It's not a long time since module multilevel converter(shorted for MMC)was invented.As a new multilevel topology,module multilevel converter has been widely used in high level and high current occasion especially in convert station of HVDC project with its advantage of highly modularized design and expandable circuit.This paper mainly research and analyze relevant control strategy of MMC,doing some deep research about sub-module capacitor voltage balance strategy and circulating current suppression strategy.At last,those control strategy was verified in MMC experimental platform.The operation principle of MMC and its mathematical model was introduced.Commonly used modulation strategy was analyzed and compared.A lot of work was done about nearest level modulation and carrier phase shifted modulation with their theoretical research and relevant simulation analysis.These two kinds of modulation were compared in aspect of output voltage waveform and analysis of fundamental content in harmonic current.The sub-module capacitor voltage balancing strategy was analyzed including building mathematical model about bridge arm energy and analyzing precharging strategy of sub-module capacitor.Some commonly used sub-module capacitor voltage balancing strategy was introduced.A lot of research was done about analyzing sub-module capacitor voltage balancing strategy of optimization sorting method through comparing its capacitor voltage of simulation results with traditional sorting method.The simulation also verified that optimization sorting method can balance sub-module capacitor voltage as well as reduce switch frequency and switch loss.The formation and mathematical model of circulating current was analyzed.Relevant simulation was done after analyzing the relationship between circulating current and output current.The result of simulation shows out that circulating current mainly consists of direct current and second harmonic current.Some commonly used solution of circulating current suppression was introduced and a lot of research was done about circulating current suppression strategy based on PR controller.Simulation about this strategy was done which shows out that circulating current suppression strategy based on PR controller can reduce three phase circulating current effectively as well as polishing up the voltage waveform of sub-module capacitor.An experimental platform of MMC was built to finish some deep research.Details about the experimental platform was introduced and the selecting principles of hardware was analyzed.The control system of this experimental platform mainly consists of DSP and FPGA,which work together to ensure the operation of MMC.Some experiments were done to test the control strategy that is mentioned above.The results verified that optimization sorting method can balance sub-module capacitor voltage and circulating current suppression strategy based on PR controller can reduce three phase circulating current effectively.