Research On Photovoltaic Grid-Connected Inverter With APF Function

To solve the problems of traditional photovoltaic grid-connected inverter, such as intermittence, low utilization rate of equipment, and to save the cost of single active power filter (APF), the photovoltaic grid-connected inverter with active filter function is studied in this thesis. Compared with the common photovoltaic grid-connected inverter and APF, the functions of photovoltaic grid-connected and APF are realized at the same time, under the condition that the topology structure is almost unchanged and the hardware costs are almost not increased. It not only improves the power quality of the power grid, but also reduces the application cost and improves the utilization rate of the device. The main work of this thesis can be summarized as follows.(1)Selection of the topological structure and analysis of the feasibility for the system.The working principles of photovoltaic grid-connected inverter and APF are analyzed. Compared the topological structures of photovoltaic grid-connected inverter and APF, the single-stage three-phase photovoltaic grid-connected inverter and shunt APF are chosen as the research objects to carry on the unified control. The feasibility and the working principle of the system are analysed.(2)Research and design of the core control algorithm for the system.As the system needs to track the fundamental wave and each harmonic current simultaneously, the advantages and disadvantages of five current inner loop controllers are studied in this thesis. Repetitive and PI controller is used in the system, to improve the dynamic response of repetitive controller. Meanwhile, the strategy of output current limition and the transition of the working modes are studied. In addition, The core algorithms of the system are designed, such as software phase locked loop based on double second-order generalized integral filter, harmonic detection algorithm based on id-iq method, voltage outer loop controller of DC side, etc.(3)Application of the model based design.In view of the shortcomings in traditional development process of power electronic equipment, such as research and development cycle long, time-consuming for code test, the method of model based design is used to generate software code for TMS320F28335 in this thesis. The software in the loop test is made in MATLAB environment, and the correctness of the codes automatically generated is verified. With the integration of the underlying code written manually, software design of the whole system is completed, and the software development cycle is shortened.(4)Research on the method of measureing the harmonic compensation effect.The compensation effect can not be accurately analyzed only with the value of the current total harmonics distortion (THD), when the system works in grid connected and APF mode at the same time. The method of harmonic compensation rate is researched to measure the harmonic compensation effect of the system. The harmonic compensation performance of the system can be observed clearly.The simulation and experimental results show that the system can operate in three modes, and the steady state performance of repetitive +PI controller is better than that of deadbeat controller. The THD of grid connected current is less than 4% and the power factor is approximately equal to 0.99 when the system operates in the grid-connected mode. The THD of power grid current is less than 5% in the APF mode. Within the system capacity,77.24% of the power grid harmonic current can be compensated in the grid-connected and APF mode. The expected result is achieved. The feasibility and practicality of the system and related algorithms are verified.