The Research And Development Of Three-Phase Photovoltaic Grid-Connected Inverter

With the increasing exhaustion of fossil energy and more serious greenhouse effect, governments around the world are trying to change the energy consumption structure and promote the development of renewable energy industries actively. The photovoltaic power generation has become the focus of research in recent years due to its inexhaustible energy source, environment-friendly power generation, wide applicability and convenient transportation. But the solar cell conversion efficiency is relatively low, and its output power also varies with the light intensity. So, we must use some control strategies to ensure that the photovoltaic array is always working at the maximum power point. In addition, because the energy output of photovoltaic array is DC power, only by using the inverter to turn it into alternating current and removing the high frequency harmonics with the help of filter can we transmit the power into the grid.Based on the “Development of 30 kW Photovoltaic Grid-connected Inverter” project, this paper discusses the control technique for inverter, maximum power point tracking control and device development deeply.Firstly, this paper analyzes various photovoltaic power generation system circuit topologies, such as power frequency isolated inverter, high frequency isolated inverter, single stage non-isolated inverter and multistage non-isolated inverter. And considering the system structure, device cost and work efficiency,this paper chooses the multistage non-isolated three phase photovoltaic grid-connected inverter as the main topology.Then, the mathematical models of grid-connected inverter with L filter and LCL filter are established in the three-phase static coordinate system. The filter characteristics of their output filter are also analyzed and compared to provide a theory basis for the research on inverter control strategy.For the resonance problem of photovoltaic grid-connected inverter with LCL filter, this paper studies the control models of inverter output current feedback and grid-connected current feedback, and investigates their open loop transfer functions. Based on the theoretical analysis, an inner current loop control strategy using inverter output current feedback and quasi-proportional-resonant control is proposed.After a brief introduction to several typical MPPT control strategies like constant voltage method, short circuit current proportional coefficient method, disturbance observer method and incremental conductance method, this paper analyzes the solar cell output characteristic curve and the working principle of the boost circuit, then puts forward a novel MPPT control method to solve the slow tracking speed and power oscillation of closed-loop MPPT control method.Finally, the simulation model of multistage non-isolated three phase photovoltaic grid-connected inverter is built and a 30 kW experimental prototype is designed to verify the above theories. Simulation and experimental results show that the control method researched in this paper could make the PV array work at the maximum power point quickly, suppress the resonance of LCL filter, reduce the steady-state error of inner current loop, and improve the power factor and grid-connected current quality effectively.