Research On Grid-connected Control Strategy Of Modular Multilevel Converters Strategy Fault

With its advantages of easy scalability,high voltage level and small output current harmonics,modular multilevel converter(MMC)has drawn more and more attention.With the research of many scholars in recent years,Urban power supply,island power transmission and other fields have broad application prospects.However,due to the complexity of the MMC topology,there are still many problems,such as the second harmonic circulation in arm current,,arm energy balance control and the independent control of the sub-module.First,the paper analyses the hardware circuit structure and commutation path of MMC.Through the mathematical expression of the composition of the circulation analysis of bridge arm between the energy balance control method for the normal operation of the system to provide protection.At the same time,aiming at the new PV system with PV modules directly connected in parallel with the sub-module capacitors,a sub-module capacitor voltage independent control method is proposed.Finally,according to the theoretical analysis and simulation verification of the specific design of the energy and energy,circulation,sub-module independent controller.In the following,the control methods under unbalanced grid faults are studied,and under unbalanced grid faults,asymmetrical grid-connected current flows on the AC side,double-frequency fluctuations on the DC side and zero-sequence double-frequency fluctuations in the circulating current the reason.Based on the analysis of AC-DC side equivalent equations,a method of AC side negative sequence current suppression and a method of circulating current suppression are proposed,which can realize the system running off of the grid under unbalanced network faults.Finally,Simulink simulation is used to verify the system.To verify the correctness of the theoretical analysis,a two-layer control system experiment platform based on Ethercat technology of industrial Ethernet was designed and built.Among them,the upper control uses Beckhoff PLC(Programmable Logic Controller),the lower control uses TI's 28335 series DSP chip.On the basis of the experimental platform,a grid-connected experiment based on carrier phase shift modulation strategy,upper and lower arm energy and energy difference control algorithm and independent control of sub-module capacitor voltage is carried out.The experimental results show the feasibility of the scheme.