Research Of Single-phase Photovoltaic Grid-connected Power Generation System Based On Double-frequency Converter

Accompany with reduction and growing desire of first energy which is the main power currently, new energy power generation will stand a more important way in the future energy structure, which make its research and application more strategically vital. In all kinds of new energies, photovoltaic (PV) power generation is the most potential substitute.High-frequency operation could improve system responding speed and power density, so it is a trend of converter development. In the same while, it brings more switch wastage that takes down efficiency and increase electromagnetic interference. Double-frequency converting theory gives attention to both high-frequency and efficiency, and derives series of double-frequency converters which have flexible topology and high control performance, suitable in application for middle and high power transform.The thesis researches on application of double-frequency converter in PV power generation system, analyses double-frequency theory and do simulation for test, finally proves it could improve the system power density, converting efficiency, dynamic responding speed and steady precision.Firstly, the double-frequency single-phase PV system construction is confirmed and designed as two-stage system. The front stage applies Boost converter to make a steady DC voltage output; while the latter stage design a three-bridge double-frequency Buck inverter to transform power to the grid. Then, the paper researches on every part of the PV generation system. Simulation of PV cells are based on engineering model, which is capable to simulate real PV cell output under changing light intensity and temperature. The paper analyses the working state of Boost converter, supplies the design method of parameters, founds small-signal model, design double-circle control strategy to maintain a steady DC voltage output. According to requirement of PV grid-connected generation system, three-bridge double-frequency Buck inverter is chosen. After operating mode analysis and parameters design, controlling strategies are respectively run on high-frequency unit and low-frequency unit, instantaneous current feedback for former, hysteresis current control for latter.PV power generation system outputs energy in a huge variable range caused by environment change, the paper designs variable topology of PV system based on the designed double-frequency PV grid-connected generation system. For front stage, a new Buck-Boost converter with four switches is designed to implement voltage increase, decrease or direct transmission due to different PV cell output voltage, which makes the control more flexible. For the latter stage, variable inverter topology is developed based on double-frequency inverter, which could change working unit, switch frequency and filter inductance to fulfill transformed power variety, that also improve flexibility of system control and power transform.