Power Coordinated Control Of MMC Series Micro Grid System In Grid Connected Mode

In traditional AC microgrid,multiple microsource inverters are connected in parallel,which has problems such as large loop current and serious harmonic pollution to the distribution network.Modular multilevel converter(MMC)seri es structure microgrid can solve some of these problems structurally.In this dissertation,we study the system power coordination control method to maximize the utilization of microsources and realize the efficient operation of MMC series microgrid under grid-connected operation mode.First,the topology of MMC series structure microgrid in grid-connected mode is analyzed,and the system equivalent circuit is established.The output power of the system generation unit,the output power of the three-phase system and the power transfer characteristics of the MMC series structure microgrid system are obtained through the equivalent circuit analysis.Secondly,the coordination principle of "those who can do more" is used to control the microsources of each brid ge arm,so that the microsources can give full play to their advantages according to their respective power output capacity.The output power of each generation unit is controlled independently by changing the amplitude of the delta carrier wave.Meanwhile,to avoid the periodic coordination error caused by the signal time delay phenomenon,the finite control set model predictive control algorithm is combined with the carrier wave variable amplitude phase shift modulation method to predict the output power of the next cycle and use the value for power sequencing and output control to improve the coordination control accuracy.Then,the real-time power balancing mechanism of the system is analyzed based on the bridge arm microsource power coordination control.The system needs the participation of hybrid energy storage system for energy exchange with the grid.When the system provides energy to the grid and the load,the DC component of the inductor current in the bidirectional DC/DC converter is adjusted to r ealize the energy release effect of each energy storage element.The output power value of the hybrid energy storage system is controlled to be equal to the power difference between the grid and the system,thus realizing the coordinated power control of t he whole system.Finally,the feasibility of the coordination strategy proposed in the dissertation is verified by building a simulation model of grid-connected MMC series structure microgrid.