Research On The Control Strategy Of Modular Multilevel Converter

With the development of power electronics,multilevel converter attracts more and more attention especially in the field of high-voltage and large-capacity.Modular Multilevel Convert(MMC)is an emerging multilevel topology which has broad prospect in medium and high voltage applications.The modular structure of MMC not only ease of integration and installation,but also let power devices work at lower voltage level,thus greatly improving the installed capacity and voltage rating.Until now,the stud,of MMC around the world is still in its initial stage.Although researchers have made some progress,there are also many inherent problems need to be solved,such as the mechanism of its nature and the capacitor voltage control method as well as the inner circulating current control method.The working principle of sub module and the three phases are introduced,and the mathematical models are established.Based on this analysis,proposing the equivalent voltage source circuit and its four-quadrant operation mechanism.In addition,several common modulation strategies of modular multi-level converter are analyzed and compared,such as N LM,CPS-PWM,CD-PWM.The method of calculate MMC device parameters are given,such as the leg inductance and sub-module capacitor.The precharging strategy of capacitor is presented,and the relationship between circuit parameters such as current-limiting resistance and charging time is deduced in detail.Focus on research of precharging strategy in rectifier state,and the disadvantages of low efficiency,complicated control process are improved.And it is verified through simulation result.Proposed two improved MMC capacitive voltage balance methods.Points out the disadvantages of capacitance voltage balance control based on energy equalization control.In this paper,the control strategy is optimized by improving the quality of ac side output voltage and restraining circulating current.The optimized control method not only reduces the error of the system output and the expected waveform,but also restrains circulating current to a certain extent.For the low-frequency carrier phase shift modulation strategy,the harmonic components of the switch function and the amplitude of each harmonic are analyzed by double Fourier transform.In addition,the switching function and the variation rule of capacitor voltage in a fundamental period are simplified.Then simplified relation between capacita nce voltage increment,power factor angle and different driving pulse is obtained.The purpose of balancing capacitor voltage is achieved by sorting algorithm.Simulation and experimental results show that the two kinds of capacitive voltage balance contro l methods are correct.Based on the analysis of the defects of the ideal repetitive controller and the traditional parallel repeat controller,a Circulation Suppression Strategy Based on IRC+LF is proposed.The weight factor ?,? is introduced in the repetitive control branch and the proportional control branch,and ?+?=1 is specified.The introduction of weight factor optimizes the steady-state and dynamic performance of the system.The relationship between the weight factor,the inner membrane filter,the compensator,and the load resistance is obtained by simplifying the analysis system characteristic equation,then the rule for setting weight factor is obtained.In addition,lyapunov function is constructed to improve the stable performance of the system.Finally,the simulation and experimental results show that this circulation suppression method is effective and correct.