Research On The Control Strategy Of Static Synchronous Compensator Based On Modular Multilevel Converter

With the increase of the capacity and voltage level of the modern power system,a large number of medium and high voltage and high power nonlinear electrical equipments are connected to the power grid,which makes the power factor degradation and other power quality problems become more and more serious.As an important part of flexible AC transmission system,the static synchronous compensator(STATCOM)plays an important role in reactive power compensation and power quality improvement.However,limited by the topology,switching frequency,voltage and other factors,the traditional STATCOM has some limitations in the application of high voltage and high power applications.As a kind of novel multilevel converter structure model,modular multilevel converter(MMC)uses a modular structure,which is not only convenient for the capacity expansion of devices,the increasement of the redundancy of system operation,but also can realize four-quadrant operation of devices.Therefore,it is of great significance to use MMC as the main topology of the STATCOM to extend the applications of the STATCOM.Based on this,this thesis focuses on the study of modular multilevel STATCOM.Firstly,based on the analysis of working principle of STATCOM,the thesis compares two control strategies of STATCOM.Then,several current detection methods are analyzed and summarized,and a dq0 current detection method based on improved PLL is adopted to eliminate the influence of unbalanced system on reactive current detection.Secondly,according to the topology of the modular multilevel converter(MMC),the thesis analyzes the circuit principle.Then,the modulation strategy,the strategy of circulating current suppression and the capacitor voltage balancing control strategy of MMC are studied.In the study of the circulation suppression strategy,a quasi PR control strategy for loop suppression is proposed in the thesis.The quasi PR control strategy is more concise,and effectively suppresses the circulation no matter the power grid voltage is balance or unbalance,which has a better applicability.Thirdly,the thesis establishes the mathematical model of the modular multilevel STATCOM in the dqO coordinate system,and proposes a control strategy of modular multilevel STATCOM.Then,for the unbalanced three-phase grid voltage conditions,the thesis analyses and establishes the mathematical model of STATCOM under three-phase unbalance of power grid.Based on this,the thesis proposes a decoupling double coordinates separation of positive and negative sequence control strategy.The proposed strategy can realize the reactive power and negative sequence compensation of the modular multilevel STATCOM in the three phase unbalanced condition accurately and quickly.At last,the simulation models of control strategies aforementioned have established in Matlab/Simulink.The simulation results verify the effectiveness and feasibility of the proposed control strategy.