Simulation Study Of Power Quality Of HVDC Transmission System Based On Modular Multilevel Converter

Electric energy is an indispensable energy in today's society.With the large-scale application of power electronic devices,the problem of power quality is concerned.Solve the power quality problems in general from two aspects,on the one hand to restrain the harmonics in the system to improve the utilization of equipment,on the other hand to provide users with a high quality and reliable power supply environment.With the rapid development of power electronic technology in the direction of high power and high voltage,especially the voltage source converter is widely used in flexible HVDC system.And the study of high power converter is more and more important.However,the traditional two-level and multi-level flexible HVDC system has many disadvantages,such as high loss,high cost,complex structure,low voltage level,low reliability and so on.Some defects of the traditional two-level and multi-level structure make the application of the system in direct current transmission system(DCTS)limited.For this purpose,novel modular multilevel converter(Modular Multilevel Conberter,MMC)has become the focus of research.MMC uses the modular structure design,and the modules are independent of each other,thus greatly reducing the rate of change of the bridge arm voltage and current and the stress of the switching device.The modularization of MMC is high,which is beneficial to the integration,improves the reliability in DC transmission system,and ensures the output qualified electric energy.Therefore,MMC is especially suitable for the application in the project of DC transmission.In this paper,the method and mechanism of suppressing circulating current based on MMC are studied,and a fault redundancy protection method is proposed to reduce the distortion and failure rate of the operating current and improve the output power quality.Firstly,this paper introduces the topology structure of MMC and the working principle of the three-phase MMC converter,and the operation characteristics of the MMC-HVDC system.Secondly,the selection method of the MMC main circuit parameters is introduced.On this basis,the modulation and control technology of MMC is studied.According to the sub-module capacitor voltage control method of MMC,an improved sub-module capacitor voltage balance control strategy is designed based on the traditional sorting method.This method improves the sub module frequent switching problem,effectively reducing the switching frequency of the device,so that the output voltage is well controlled.After that,the mechanism of circulating current generation and its suppression principle are analyzed.Meanwhile,the circulation suppression controller based on discrete model is designed,which can effectively restrain the inner three-phase doubling frequency circulation in the MMC without changing the external characteristics of MMC,greatly reducing the distortion degree of bridge arm current.Finally,the fault of single sub-module in MMC can cause the oscillation of DC voltage and DC current,which will endanger the normal operation of the converter.For this configuration,the sub-module fault redundancy protection is proposed,which is suitable for the MMC sub-module fault redundancy protection method,and which can make the converter quickly return to normal in case of failure of the sub-module.In this paper,the simulation circuit of the DC transmission system based on modular multi-level converter is built in the MATLAB/Simulink.The proposed improved sub-module capacitor voltage balance control strategy,the circulation suppression controller based on discrete model and the sub-module fault redundancy protection method are simulated respectively.And the effectiveness of various control strategies is verified by the comparison of simulation experiments.