Research On Control Strategy And Sliding Mode Observer Of Capacitor Voltage In Modular Multilevel Converter

Modular multilevel converter(MMC)as a new multilevel technology proposed in recent years has been widely used in high-voltage and high-power applications by virtue of considerable modular structure,common DC bus and great output harmonic performance.The research on operating mechanism,key elements parameter design,control strategy and sliding mode observation of capacitor voltage and design scheme of MMC system is developed in this thesis.Single-phase equivalent mathematical model is established based on the analysis of topology and operating principle of MMC.According to the variation of energy fluctuation in bridge arm,the selecting method of sub-module capacitor is deduced.The formula for calculating inductor in bridge arm under specified second harmonic component in the circulating current is concluded by analyzing the inner circulating current in MMC.A modified sorting method is proposed to cope with the contradiction between control performance of capacitor voltage and switching frequency in traditional ones.By setting a couple of voltage limits and introducing regulatory factors,sub-modules of which capacitor voltage exceeds and within the limits are addressed in different sorting voltage adjustment.Simulation results in Matlab/Simulink demonstrate that modified sorting method could not only reduce average switching frequency and switching loss prominently,but also maintain maximum deviation of capacitor voltages within an acceptable range.Sliding mode observer(SMO)for capacitor voltage observation is developed by measuring switching signals and arm currents to deal with complicated hardware design,cost increase and inferior system reliability caused by plenty of voltage transducers.Besides,in order to suppress high frequency chattering caused by sign function,a smooth and continuous hyperbolic tangent function is also introduced.Simulation results show that this SMO could accurately observe capacitor voltage and turn into an effective substitution of voltage transducer.Design scheme of MMC system is investigated at the end of this thesis,mainly including circuit design and device selection of main circuit,control architecture establishment,hardware circuit design and relative software program design of control system.