Research On Modular Multilevel Converter With Capacitor Voltage Self-balancing Ability

In order to keep the IGBTs in different sub-modules under similar voltages and reduce power loss while depressing circuiting current,it is essential to guarantee the sub-module capacitors well balanced with periodical current flowing by.As the sub-module capacitors are independent of each other,voltage-balancing control is necessary for the voltage balancing of conventional modular multilevel converter(MMC).With the number of sub-modules increasing,the complexity of voltage balancing control,involving massive computation as well as demand for critical high speed and accuracy of signal acquisition,stands out.Based on the research of capacitor voltage ripple principle,a novel MMC topology with voltage self-balancing ability is proposed.With the extra port added to sub-module and auxiliary diode connecting adjacent sub-module capacitors,the capacitors are no more floating,consequently the proposed MMC can automatically keep the capacitor voltage balanced through the operation of sub-modules,which will hopefully benefit the MMC projects both in reducing investment and simplifying control system.Considering that voltage-balancing process of the proposed MMC varies a lot from the conventional one,corresponding modulation strategies should be put into place to active its voltage self-balancing ability.So both nearest level modulation(NLM)and carrier phase shifted SPWM(CPS-SPWM)are somehow transformed to integrate the modulation part and the triggering part.As a result,the control process is greatly simplified with the new modulation strategies.To validate the voltage self-balancing ability of the proposed MMC and effectiveness of the corresponding modulation strategies,simulation models and relevant low-power physical platform have been built,results verified the ability of the MMC and indicated its potential in reducing capacitance of sub-module capacitor.