Research On Main Circuit Structure And Control Strategy Of MMC-STATCOM

With the gradual development of the power system,harmonic and reactive power in asymmetry power system is becoming more and more prominent,the power quality requirements are more stringent,in order to ensure the power system safe and economic operation of static synchronous compensator(STATCOM)is proposed for the new power electronics device with response speed fast,small size has become one of hot issues in power quality research.However,the traditional two level inverter is simple and widely used,but its inherent topology limits the voltage of the inverter.Modular multilevel converter(MMC)is a new type of topology,through a series of sub modules in theory can meet the power and voltage level of the infinite,the application of this kind of topology in STATCOM can meet the high power demand,improve the quality of electric energy.In this paper,we first introduce the topology structure of several converters,and analyze the structure of MMC,and point out the advantages and application prospect of MMC structure.The switch state of the module of the modular multilevel converter is analyzed,and the mathematical model of MMC is established.The modulation strategy of MMC carrier phase shift technique is analyzed,and the emphasis is on reducing the output distortion rate according to different number of sub modules.Secondly,the problem of sub module capacitor voltage is studied.MMC converter in operation at the beginning of the initial voltage is zero,if the direct access system,the current impact and impact on system voltage,the control strategy will be sub module is charged to the rated voltage,reduce the impact of access.In order to maintain the stable operation of MMC,it is needed to ensure the stability of the capacitor voltage of the sub module,so the corresponding DC side control strategy is designed,and the factors that affect the different voltage difference between the sub modules are studied.In MM operation,the phases have large circulation,the stability of ring affects the system through Congress,leading to the output of the system power quality is poor,this paper design and circulation controller stable circulation waveform.Finally,this paper will be applied to STATCOM MMC converter system,first through dq0 control and feedforward controller under ABC system model transformation of active and reactive power decoupling model,respectively control the system of compensation by the reactive current output device;the second phase model is established by control system,using the current repetitive controller,capacitor control the double loop control strategy to ensure the sub module capacitor voltage stability,effectively compensate the harmonic current and reactive power asymetry.