Research On Single Stage Three-Phase Grid-Connected Photovoltaic Inverter

With the development of photovoltaic industry, the photovoltaic power generation system gets wide concern and substantial support from urge governments. Developing three-phase grid-connected solar inverters of great efficiency, stability and security is the technical foundation for building high-capacity solar power station, which is an important part of photovoltaic power generation. This paper mainly conducts the research on control system for single-stage three-phase grid connected photovoltaic inverters.By building the mathematical model under synchronization reference frame, the traditional PI control principle for the inverter was introduced, including the design of control parameters. Due to drawbacks of PI current controller, which mainly lie in the slow transient response, a deadbeat current controller suitable for the vector control system was proposed. A current prediction algorithm was introduced to compensate the control delays introduced by digital controller, and the effectiveness of this deadbeat controller is verified by the simulation and experimental results.Maximum power point tracking (MPPT) technology is the key to improvement of photovoltaic inverters'efficiency. According to the PV array's electric model, a PV array simulation model was established by using its output characteristics, which could mimic the nonlinear characteristic of PV array output. The principles of traditional MPPT algorithms, whose advantages and disadvantages are both introduced, were demonstrated, future more, a novel variable incremental conductance algorithm was proposed. Finally, fuzzy control theory was introduced into the MPPT algorithm, which guaranteed the robustness, rapidity and high-precision of the MPPT controller, and its effectiveness was verified by simulation results.Fast and accurate detection of islanding conditions, which is a common phenomenon for distributed generation systems, is the necessary function for grid connected PV inverters. This paper introduced the principles of traditional island detection algorithm. A novel active frequency drift (AFD) algorithm, which could introduce frequency passive feedback by changing current vector angle, was proposed for three phase inverter to eliminate the non-detection zone of passive island detection method. The simulation and experimental results proved that this algorithm could detect island even under power match conditions.A single stage three phase PV inverter was established, which was functionally tested with DC voltage source and islanding device, and both the software and the hardware of the inverter were introduced in details. The grid generation experiment was conduced to realize the power decoupling control, and the effectiveness of variable step MPPT algorithm was also verified experimentally. An islanding experiment was designed according to island detection standard, which finally proved that the novel AFD algorithm works in the real PV system.