Research On Topology And Control Of Dc-module-based Photovoltaic Power System

Due to the shortage of energy and pollution of environment, solar power, which is abundant and clear, has attracted lots of attention recently. Building integration photovoltaic generation system is one of hot research areas because it takes no land resources and can improve indoor temperature. It can realize"green building"and stands for the trend of photovoltaic generation. The research work in this thesis mainly focuses on the paralleled DC-module-based photovoltaic generation system, which including topology of the system, control strategy of MPPT, grid-connected technology and two-mode transfer technology.Paralleled DC-module-based photovoltaic generation system has the advantages of high efficiency in energy transformation, strong robust to mismatch of parameters, and easy implementation of capacity improvement. It demands high voltage gain, low input ripple voltage and current in the former DC-DC stage. In this thesis, a current-fed half bridge converter is analyzed in detail. By deducing working principle in a switching cycle, key parameters including dead-time, voltage gain, and expression for voltage and current stress are obtained, which provide guide for the system design.MPPT is necessary in order to utilize the PV module with high efficiency. By introducing and comparing several typical MPPT methods, this thesis discusses mechanism of misjudgment in MPPT, especially the effect to incremental conductance method caused by sampling error, sampling time and step variation. Experiments are carried out by adopting the combination of constant voltage method and incremental conductance method.Inverter is utilized as the interface between system and grid. Firstly, this thesis analyzes the leakage current problem by adopting different modulation scheme in non-isolated single phase full bridge topology. Then, a grid connected control strategy with filter capacitor current compensation and grid voltage feed forward is analyzed and designed. The stability range of controller is deduced when considering zero-holder and one beat delay. Parameters of controller are obtained by pole assignment and experiment results validate the effectiveness of the method. Lastly, in order to realize smooth transfer between two mode working conditions, two loop control strategy is adopted when the system is working independently. The process of two mode transfer is analyzed. In order to analyze the time when output voltage is out of control during the transfer from grid-connected working to independently working, this thesis compares the two control methods: naturally current commutating and constrainedly current commutating. According to the simulation results, conclusion can be got that constrainedly current commutating based transfer strategy can cut off the switch quicker thus it reduces the time when the output voltage is out of control.