Control Strategy Of Modular Multilevel Converter In High-Voltage DC Transmission

Modular multilevel converter(MMC)is a kind of voltage source converter topology.Due to its good scalable performance,it is more and more widely used in the field of direct current transmission.What's more,it has become one of the highlights in the academic research and industrial fields in recent years.In the background of direct current transmission in electrical power system,this paper researches the internal dynamic characteristics and modulation strategy in steady state and the control method in the condition of AC voltage unbalance from aspects of theory analysis,simulation and experimental validation.Firstly,this paper expounds its working principle and establishes its control model on the basis of three-phase MMC mathematical model.Meanwhile,A improved modulation algorithm combining carrier phase shift control with capacitor voltage balancing control is proposed.This algorithm not only satisfies fields of many modules,but also applies to situations with fewer modules.Secondly,Aiming at the voltage asymmetry fault of three-phase AC power supply,the phenomenon of AC current imbalance and DC voltage oscillation in the converter is analyzed in detail,and improved the traditional unbalanced control method,which adds DC voltage stability control module to control AC current.This method can not only effectively reduce the negative sequence current of AC side,but also restrain the fluctuation of DC voltage.Thirdly,this paper researches the formation mechanism of circulating current between the bridge arms of MMC,and designs circulating current suppression controller based on trap and proportional controller to suppress the second harmonic circulation component.The controller can effectively reduce the fluctuation of the circulating current and improve the efficiency of the converter without affecting the performance of the AC side system.Finally,the five-level small power experiment platform based on FPGA controller is built.The experimental results show that the three-phase MMC DC transmission converter system based on the above control method has good steady and dynamic response,high waveform quality,small circulation ripple and the system works stability when the voltage is unbalanced,which validates the correctness and effectiveness of the control method in this paper.