Research On Repetitive Control With Adaptivity To Grid Frequency For Grid-tied Inverters

With the exploration and utilization of renewable energy, distributed generation system has been paid great attention. As the grid-tied inverter plays an important role in distributed generation systems, its performance determines the output power quality directly. Among the various current control methods used in the gird-tied inverter, repetitive control becomes a research focus because of its high control accuracy. In this thesis, control structure, design method and improvement against the grid frequency variation of repetitive control for grid-tied inverters was discussed.In the grid-tied inverter, the common repetitive control structure was investigated. In this thesis, the reason of unstability of a direct repetitive controlled grid-tied inverter was revealed, which explained the importance of a feedforward path on the stability of the repetitive control system. Two classic repetitive control structures, namely the connection with PI control in cascade and in parallel, were compared. The result is that the defect in parallel structure's magnitude-frequency characteristics was pointed out. While using cascade compound controller, according to the stability condition of the system, advantages and disadvantages of using different parameters such as inner mode, compensator and gain coefficient were analyzed and a design method was obtained. The effects of grid frequency variation on repetitive control were analyzed, and a frequency adaptive repetitive control based on fractional order theory was introduced.Using the mathematical relationship between repetitive control and resonant control, the idea of qausi-resonant control was adopted to improve the conventional repetitive control method, which effectively expands the resonant bandwith of repetitive controller. Unfortunately, the method will decrease the magnitude of the system. To this, a method was introduced by analyzing every parameter's effect on the stability of the system that increases the repetitive control gain coefficient. A parameters design procedure was also concluded.Finally, the proposed parameter design method was verified through both simulation and experimental results. The results showed that the classic repetitive controller has higher control accuracy, but the control performance will be degraded under the condition of grid frequency variation. On the contrary, the improved repetitive controller can maintain high control accuary even with the grid frequency variation.