| The stability of power system is the most basic problem in the power industry.In order to ensure the reliable operation of the power system,the voltage of the system must be operated within the required range.The voltage level is controlled by the generation,absorption and transmission of reactive power in the system,as the reactive power can not be transmitted over a long distance,the voltage can only be controlled through the whole system.As one of the most important achievements in the practical application of flexible AC transmission technology,Static synchronous compensator(STATCOM)has become a research hotspot in the field of reactive power compensation with excellent dynamic compensation performance.Modular multilevel converter(Modular Multilevel Converter,MMC)is the latest research results of multilevel voltage source converter technology,the standard modular structure is not only convenient for redundant design,but also can improve the ability to deal with the fault,reduce the switching frequency and improve the output performance.The application of MMC structure to STATCOM,as a new dynamic reactive power compensation device MMC-STATCOM,has a good prospect.This paper focuses on the design of MMC-STATCOM parameters and control strategies,the main work is as follows:(1)the working principle of MMC-STATCOM is analyzed.According to the structural characteristics of MMC with dispersed capacitance and bridge arm inductance,the equivalent mathematical model of abc static coordinate and dq rotating coordinate is established.On this basis,the operating characteristic of the main MMC-STATCOM(bridge arm inductance,connection inductance,sub-module capacitor and power device)is analyzed,and the range of parameter is obtained in order to ensure reliable and stable operation of MMC-STATCOM.At the same time,the effects of inductance and capacitance on the dynamic response of the system is analyzed,the relationship between the open loop response time of the system and the inductance,inductance and DC capacitance of the bridge arm is derived,to provide the basis for the design of parameters to meet the dynamic performance of the system.(2)the overall control strategy of MMC-STATCOM is studied.In the modulation of AC side,the realization method of Carrier Phase-Shifted PWM(CPS-PWM)applied to MMC-STATCOM is analyzed,according to the difference of the sub modules number in MMC-STATCOM bridge arm.In the part of capacitor voltage control,based on the principle of capacitor voltage balance by modifing CPS-PWM modulation wave,the method of grading voltage control is presented.In the part of Circulation inhibition,repetitive control is introduced,in order to eliminate the even harmonics in the circulation.Aiming at the shortcoming that the traditional parallel repetitive controller is difficult to design parameters independently,the mathematical characteristic of the transfer function of the repetitive controller is analyzed,and the parallel structure is simplified reasonably.Combined with the characteristics of even harmonics in the circulation,an improved method is proposed to optimize the performance of the controller.(3)a simulation model is built on the MATLAB/Simulink platform,to verify the correctness and effectiveness of the MMC-STATCOM control strategy,respectively on the capacitor voltage balancing method,harmonic suppression method and circulation comprehensive compensation performance analysis.On this basis,the hardware circuit and software algorithm of the drive control system are designed. |
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