Research On Photovoltaic Grid-connected Structure And Control Based On Modular Multilevel Converter

At present,the main form of using solar energy still depends on photovoltaic power generation.Therefore,enhancing the efficiency of photovoltaic power generation technology and improving the overall power quality has always been a hotspot and difficulty in this technical field.One of the difficult and effective methods.Consider the characteristics of modular multi-level converter(MMC):present the advantages of modular topology,easy expansion,convenient redundancy,low proportion of overall harmonic content of AC output,and low-frequency switching capability,combined with current large-scale photovoltaic power After more than ten years of research by scholars from various countries,MMC has made good progress in the direction of flexible DC power transmission and new energy utilization.To this end,this paper combines the distribution type structure and expansion characteristics of modular multi-level topology with the application of distributed type energy to form a unique advantage.Three photovoltaic grid-connected structures based on modular multi-level converters are designed to be applicable For high-power,high-voltage photovoltaic grid-connected occasions.First of all,this article analyzes the structure and mathematical model of Modular Multilevel Converter(MMC),analyzes the internal structure of the indispensable photovoltaic module,and combines the output combination characteristics of the photovoltaic array circuit part and MMC Combined with the working characteristics of the system,a structure of photovoltaic power generation system based on MMC is generated,and a bridge arm power controller is designed according to its structure.According to the design needs,in a variety of modulation strategies,according to the analysis basis,finally select the carrier phase shift method that conforms to the MMC inverter output characteristics.Second,the single-level MMC photovoltaic grid-connected system with sub-module protection capability is first analyzed,especially to improve the operation mechanism of sub-modules.Secondly,the dual closed-loop control strategy based on MPPT(Maximum Power Point Tracking)technology,photovoltaic grid-connected control strategy and the modulation method of MMC converter are introduced.Finally,the PSCAD/EMTDC power system simulation software was used to construct a single-level MMC photovoltaic grid-connected model with an output of 9 levels to verify the effectiveness of the system.Third,referring to the advantages and control methods of the maximum power point tracking(MPPT)link and modular multi-level converter(MMC)structure in photovoltaic inverters,a two-stage grid-connected structure that connects photovoltaics is designed,and it can For the control of a single MMC sub-module,combined with the analysis of each link control,PSCAD/EMTDC builds a two-level power generation model based on the MMC-based photovoltaic power distribution connection,by comparing the output waveform of the bridge arm power control link with the unadded output waveform,Explain the necessity of designing bridge arm power control in this paper.Fourth,firstly introduces the distribution type single-stage photovoltaic grid-connected structure and operation theory part based on the modular multilevel converter,and secondly combines the analysis of the proposed topology operation mechanism to build the corresponding control structure,which mainly includes three links:MPPT tracking link,sub-module capacitor voltage fine-tuning voltage stabilizing link and current decoupling and grid-connecting link.Finally,the superiority of the control is verified by comparing the harmonic distortion rate of the added bridge arm power control link with the output harmonic distortion rate not added.