Novel Circulating Current Suppressing Control For Modular Multilevel Converter

With the continuous development of society in our country, practical problems such as the pollution of environment and the shortage of energy are becoming increasingly serious.Changing the consumption structure of energy and reducing the pollution become the development direction of the mainstream in present world. Under the background of expanding investment in new energy technology, the research in the high voltage direct current technology in China is also developing very fast, and the modular multilevel converter of high voltage direct current(MMC-HVDC) technology has been widely studied in recent years.Firstly, analyzing the development of electric power transmission technology, and then introduces the reason of MMC-HVDC inherit the traditional direct current technology,meanwhile point out that the circulation current is aroused in the three-phase bridge arm due to the structure of modular multilevel converter, which may cause the harmonic wave in three phase bridge arm current, thus will increase the cost of the system and power loss. By establishing on the basic structure of MMC and making a detailed analysis of the origin and the nature of circulation current.Then, in view of the traditional circulating current suppressing control had a high cost in hardware structure, then through the analysis of the basic principle of MMC and the mathematical model of MMC control system, designs a kind of MMC control system without phase-locked loop structure, which not only satisfies the MMC main control circuit design,but also may be applied to MMC circulation inhibition of controller design. The control system without phase-locked loop can reduce the system parameter selection effectively, and realize the independent decoupling control of active and reactive power. Simulation analysis verifies the effectiveness of the proposed control strategy and reduces the complexity of the MMC system.Finally, in view of the traditional mathematical model always had a ideal characteristics and a linear characteristics, the differential flatness theory(DFT) based MMC circulating current suppressing controller is deduced. According to the analysis about the nature of the interphase circulating current and the definition of the DFT, it chooses the input, output andintermediate quantity of the control system, proving that the design of circulating current suppressing controller can meet the conditions of DFT. The newly designed controller could reduce the interphase circulating current in the bridge arm current, and make it closely approximate the sine wave. At last, the MMC model is simulated based on Matlab/Simulink.In the case of no impact on MMC external voltage and current, the result show that the control model is effective, which provides a new nolinear idea for the design of circulating current suppressing controller.