Optimal Design And Research On Control Strategy Of Grid-Connected Photovoltaic System

The problem of conventional energy crisis and environment in the world makes it necessary to exploit the renewable and green energy sources, in order to cover the future demands. Meanwhile, solar "photovoltaic" power has made great progress and become one of the most effective clean and renewable energy with the advance of solar cell and power electronics. Recently, the photovoltaic system constantly develops with the tendency of lower cost, higher efficiency and higher power density techniques, following the higher and higher performance requirement of key equalizer equipments. The main application of solar energy will be the grid-connected photovoltaic system. With coming into using of the high-performance Digital Signal Processor (DSP) chip, it is possible that some advanced control strategies can be applied on the grid-connected PV system.The thesis addresses the solar grid-connected photovoltaic system. With considering the cost, reliability and efficiency, it mainly focuses on the optimal design and control strategies of grid-connected photovoltaic system. Therefore, it develops new and cheap concepts for converting electrical energy, from the PV module to the grid.One method, among many, to make PV power more competitive is achieved by developing inexpensive and reliable inverters. According to the standards of grid interface and PV module interface determined by the project, research has been conducted on thirteen kinds of popular inverter topologies. By generalizations and comparisons, a dual full bridge inverter topology is presented. On such basis, the inverter, with belonging auxiliary circuits, is designed. And an optimization method of LCL output filter is proposed. As a whole, the inverter is developed with low cost, high reliability and high efficient structure.The research contains an analysis of the PV module, a specification based on the analysis and inner P-N junction principle. A mathematical model for the PV module is built, and therefore the relationship between sunlight irradiation and generated current is determined. Theory research is also carried on other characteristics of PV module such as partial shadow and ripple at the PV module terminal. The module is established in simulation software and all kinds of associated properties are simulated. Based on the developed model, a novel maximal power point tracking (MPPT) algorithm, fixed-frequency quasi-sliding control algorithm based on switching surfacezero averaged dynamics, is developed, which guarantees the grid-connected photovoltaic system get most amount of energy from PV module. The precision and efficiency are verified by simulation and experiment.The selection of the DC-link voltage reference is carefully analyzed, in order to keep it as low as possible. It is desirable for lowering the switching losses of the MOSFETs, and protecting DC-link electrolytic capacitor. Based on this result, one cycle discrete PI controller is proposed to control the DC-link voltage, for which the current reference is the one achieved from PV's current. Therefore, the working point of PV system can be adjusted easily.The grid current controller is designed, in order to inject a sinusoidal current, with low harmonics into the grid. This controller is designed by using selective harmonic elimination pulse width modulation (SHE-PWM) base on Walsh transform, because the switching frequency is lower compared with other controllers under the same current harmonic distortion. Some additional parts to enhance the quality of the grid current are also discussed, such as blanking time compensation and detection of islanding operation by means of voltage and frequency monitoring presented, which guarantees the grid-connected PV system operates in safety.On the basis of theoretical research, a prototype of grid-connected PV system is realized, and the difference between the designed goal and actual system is shown. The evaluation includes measurements on the interfaces between the grid and inverter, and between the PV module and the inverter. The test results show that the system is close to keep all the demands, and with correction, feasibility and efficiency.Finally, this paper makes some conclusions, and presents some issues for future research.