Research On Control Strategy Of Modular Multilevel Converter STATCOM

In recent years,with the gradual increase of high-power power electronic equipment in the power system,the power quality problems such as reactive power,harmonic and three-phase imbalance in the power grid are becoming more and more serious.Static Synchronous Compensator(STATCOM)is widely used due to its ability to solve a variety of power quality problems,among which STATCOM with Modular Multilevel Converter(MMC)structure is characterized by its modular design,harmonic.The advantages of less content and block of response speed have become the focus of current research.This thesis mainly studies the control strategy of MMC-STATCOM in reactive power compensation and unbalanced compensation modes,the research contents are as follows:(1)According to the circuit structure and the working principle of MMCSTATCOM,the equivalent mathematical model of MMC-STATCOM is deduced and established.The working principle of two common MMC modulation methods is analyzed in detail,and the advantages and disadvantages of the two methods are compared,the nearest level modulation method is selected as the modulation method of MMC-STATCOM studied in this thesis.(2)In order to solve the shortcomings of traditional model predictive control algorithm with many control parameters,difficult parameter design and multiple tuning of weight factors,a hierarchical model predictive control strategy of MMC is proposed.Modeling control is carried out at each level,which simplifies the design process and greatly reduces the computational complexity of the control algorithm.In view of the high switching frequency of model predictive control,which is easy to cause large switching loss of equipment and short service life of the device,the traditional sorting method is optimized by introducing a dispersion factor to limit the variation range of the capacitor voltage of the sub-module,so that the switching frequency of the submodule is reduced while the capacitor voltage of the sub-module is balanced.The simulation results show the effectiveness of the hierarchical model prediction and the improved pressure equalization control strategy.(3)Aiming at the problem of unbalanced three-phase voltage in power system under unbalanced load conditions,a positive and negative sequence separation technology based on Improved Double Complex Coefficient Filter(IDCCF)is proposed.The DCCF forward filtering link enables it to effectively separate the positive and negative sequences when the unbalanced grid contains DC offset and harmonic components.Then,based on IDCCF,a phase-locked loop that can stably provide grid phase and frequency under unbalanced conditions is designed.The positive and negative sequence sub-sequence compensation strategy is adopted,and the generated positive and negative sequence compensation components act on the MMC-STATCOM as a modulation signal to achieve the balance of three-phase voltage and current at the common connection point.The effectiveness of the compensation strategy is verified by building a simulation model in Simulink.Based on the RTDS(Real Time Digital Simulator)simulation experimental platform,the MMC-STATCOM system model of hierarchical model predictive control strategy and three-phase imbalance compensation strategy is constructed,and the feasibility of the control strategy proposed in this thesis is verified by experiments.There are 73 figures,5 tables and 99 references in this thesis.