A Novel Grid-connected PV System Based On MMC

With the rapid development of global economy, energy shortage and environmental pollution become more and more serious. In order to solve the problem of energy supply and improve the environment, countries around the world are competing to develop green renewable energy, such as water energy, biological energy, solar energy, wind energy, geothermal energy and ocean energy, etc. These energy consumption can be recovered after the supplement and rarely produce pollution. Among the new energy, solar energy has a wide range of resources, easy to use, which makes it get more and more attention. Photovoltaic technology is the main form of the use of solar energy. The optimization and improvement of efficiency of photovoltaic under partial shading condition is a hot and difficult problem of this technology. Photovoltaic grid-connected topology optimization is an effective method among the solution. In this paper, a new modular multilevel photovoltaic grid connected topology is designed,which takes advantage of the structural characteristics of modular multilevel topology.Firstly, the basic structure and operation mechanism of MMC is introduced, and the output characteristics of photovoltaic cells are analyzed.the basic structure and operation mechanism, and analyzed the output characteristics of photovoltaic cells. Then, the photovoltaic branch and array model are established, to analysis of the reasons for the power mismatch of photovoltaic arrays under partial shading. Furthermore, the structure design of photovoltaic system based on MMC and the analysis of the basic operation mechanism is completed by combining the characteristics of them.Secondly, according to the operation characteristics of the proposed topology, the control strategy is designed, which includes the maximum power tracking control, redundancy control, power control and grid connection control. The maximum power point tracking control captures the MPP voltage of each pv’s, redundant control and voltage control keep the energy of each bridge balance. According to the analysis of selection basis of the system modulation strategy, carrier phase shift modulation strategy is adopted in the system.Thirdly, according to the control method of the new topology structure, the simulation is carry out comparing with the conventional group string structure. Draw the conclusion according to the simulation results, the new photovoltaic grid connected topology is able to operate stably under the condition of sub module fault and partial shading. What is more, compared to the conventional group string structure, the topology also has a higher utilization of solar energy and lower harmonic.Lastly, the experimental platform of photovoltaic system is introduced, including the main chip of the control board; the selection of parameters of the capacitor and the inductance, the design of the PCB, as well as the design of the voltage and current protection circuit. Then the flow chart software and relevant experiment results is given.