Research On Circulation Restraining Of MMC Based On Ant Colony Algorithm

Modular multilevel converter is a new topology of voltage-source converter,it received a great recognition in the field of direct current(DC)transmission because of its highly modular topology and the character of expansibility,it is becoming the researching hotspot for domestic and foreign scholars.Though there are many advantages of MMC such as the great fault-tolerant ability,easy to expand for sub-modules and so on,the use of large quantum modules in MMC also give rise to many problems,among them,the circulation caused by the inconsistent switching of sub-modules has a great negative effect on the converter,the peak current of bridge arm is increased and the total harmonic distortion is increased,based on DC transmission,this paper focuses on the problems related to the internal system of MMC,in this paper,the internal dynamics of MMC and the generation and suppression of circulation problems are studied from the aspects of mathematical model building,system simulation verification and analysis,it mainly includes the following:(1)The research background and significance of this paper are briefly expounded,the development and present situation of converter and transmission system are introduced,explain the topology and working principle of MMC,the topological structure of MMC,the operation mechanism of sub-module and the basic modulation strategy of converter are expounded in detail;(2)The inner loop current controller and outer loop controller of MMC are build,combined with the mathematical model of the MMC the structure diagram of inner loop controller is established,based on Matlab/Simulink simulation environment,a 5level MMC rectification system was built,which verified the correctness of the design strategy;(3)The circulation problem in MMC is discussed.Based on the theory of energy conservation of the bridge arm,the generating mechanism of the circulation is deduced,the result shows that it mainly contains DC component and double frequency component,aiming at the circulation problem,based on the existing MMC capacitive voltage equalization control system,the application of fractional proportional integral control based on ant colony algorithm in MMC circulation suppression is proposed,the integral criterion of time times absolute error is taken asthe objective function,introduction of fractional order enhances the controller's ability to track input instructions,capacitive voltage equalization ability and circulation inhibition ability are enhanced by the optimized parameters optimized by ant colony algorithm,effectiveness of the proposed strategy is verified by simulation.(4)The simulation of power disturbance such as voltage surge and sudden drop on the grid side is carried out to verify the robustness of the proposed strategy.Finally,the main hardware selection and design of the MMC hardware system are carried out,and the design flow charts of the software are given.