Study On Photovoltaic Grid-connected Control Strategy Based On Mmc Power Electronic Transformer

With the continuous development of economy,the demand of electric energy in the electricity market is increasing,and China continues to promote the construction of new energy fields including photovoltaics.Because the power electronic transformer has a rich AC and DC bus,it provides more possibilities for photovoltaic power grid connection.Based on the MMC(Modular Multilevel Converter)power electronic transformer,the topology of the MMC power electronic transformer is introduced and analyzed the models of construct simulation.The photovoltaic array is connected to the grid through the low voltage DC bus of Power Electronic Transformer.The control strategy of photovoltaic grid connection is designed and simulated,thus verifiing its feasibility.Firstly,the working principle and topological structure of power electronic transformer are introduced.The core module MMC in power electronic transformer is modeled and analyzed,and the working principle of MMC is analyzed from the sub-module.The mathematical models of MMC are established in abc three-phase coordinate system and dq twophase coordinate system respectively.With choosing DC voltage as the control target,the control strategy of MMC is designed by decoupling control and combined with capacitor voltage equalization and loop suppression control,which decoupling control is combined with internal and external loop controllers.A 21-level MMC module is built in the PSCAD/EMTDC software to verify its control characteristics,including startup characteristics,steady-state characteristics and loop suppressor characteristics.Secondly,for the DC-DC module in the power electronic transformer,the working principle and mathematical model of the DAB(Dual Active Bridge)and CLLC topologies are analyzed,thus obtaining the topology output characteristics under different parameters.According to the characteristics of power electronic transformers,DAB is selected as the topology used in this thesis;the model of ISOP-DAB(Input Series Output Parallel)is established and the corresponding control strategy is designed.Then,for the full-bridge inverter topology of the inverter output stage,the droop control strategy is designed to ensure the stability of the output characteristics for the feature of the low-voltage side power grid.Finally,for the photovoltaic grid-connected system,the mathematical model of the photovoltaic array is established,and the main factors affecting the output characteristics of the photovoltaic array are obtained.Selecting the photovoltaic power generation grid-connected topology,designing a grid-connected control strategy based on MPPT(Max Power Point Tracking)combined with constant power control to achieve stable power transmission of the photovoltaic power generation system.On view of the above analysis and research,a detailed model of power electronic transformer and photovoltaic power generation grid-connected system are established in PSCAD/EMTDC to obtain the working characteristics of photovoltaic system based on power electronic transformer.The system operation characteristics under the conditions of PV on-site consumption and PV energy transmission are analyzed,which verifies the feasibility of this study and provides strong support for practical engineering design.