| In recent years,the popularity of power electronic devices within the scope of power system research has been greatly increased,and the pollution of power quality has becoming more and more serious.As a key equipment of modern dynamic static reactive power compensation device,SVG has been concerned by more and more professional researchers.MMC has the advantages of low running loss,highly modular design,strong fault handling capability,and high output waveform quality,etc.Using modular multilevel technology to develop high-voltage and large-capacity static-var-generator is an effective way to improve the universality,the technical performance and the cost-effectiveness of equipment,which has become a main direction for contemporary scholars to study the development of SVG In this paper,MMC-SVG is chosen as the research object,and its working principle,mathematical model,capacitor voltage equalization and output control are studied respectively.Based on the current-development situation of the Power Grid with the purpose of Strong Power Grid,this paper outlines the harm caused by the lack of reactive power,therefore expounds the development status of reactive compensation technology and the research overview of hybrid-system theory.Then,the principle of main circuit and control circuit of the MMC-SVG is also discussed in this paper,and this paper proposes a frequency domain mathematical model of the MMC-SVG positive and negative sequence component for dq coordinate system based on instantaneous reactive power theory.The capacitor voltage balance mechanism of MMC-SVG is analyzed based on the theory of capacitor charge and discharge,and the influence factors of capacitor voltage unbalance have been theorey studied and simulation analysised.However,there are many bad phenomena,such as the frequent capacitor switching times,the large amount of computations and the large switching power dissipation,etc,which the capacitor voltage balancing based on traditional strategy.To solve the problem mentioned above,the improved merge sort is proposed.According to the features of the stable operation with MMC-SVG,these two aspects optimize the method from the capacitor discrete character and the voltage admissible deviation,and the optimization for evaluation indexes of the voltage equalization method are realized,including voltage voltage balancing,operation complexity of the controller and switching frequency,etc.The hybrid operation character of MMC-SVG is discussed based on the hybrid automaton control theory,besides,this paper has been analyed the eight operating modales of the main circuit,and constructed the corresponding hybrid automaton model.Therefore,a hybrid automaton controller of the main circuit is designed according to the output control rules of MMC-SVG At last,this paper builts the 21 level MMC-SVG simulation model on the basis of the Hybrid-automaton control strategy according to Matlab/simulink simulation platform.the thesis discusses these two aspects respectively from the voltage equalization performance and the external characteristics for the complete machine by simulation analysis,and discussed the main circuit parameters and the preliminary hardware&software design of the control system.MMC-SVG is a high-voltage,large-capacity reactive power compensation device,which has the advantages of flexible control and high-efficiency operation.Its operation process has both discrete behavior and continuous behavior.By using the hybrid automaton theory and combining the voltage sequencing method of DC side capacitance voltage,the system load changes can be quickly responded and the demand of reactive power compensation is precisely controlled.The simulation results under steady state and power step conditions further validate the effectiveness and correctness of the Hybrid-automaton control method. |
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