Research On Two-stage Single-phase Topology Switching Grid-connected System

Recently, interest in electrical power generation from renewable energy sources, such as photovoltaic systems, have had an enormous increase. A number of scientists regard solar energy as the ideal replacement of traditional fossil sources, as it is clean, renewable and abundant. On the other hand, it is still necessary to investigate possible solutions for the challenges encountered during grid integration of photovoltaic systems, like the intermittent character of renewable sources and low working efficiency. Therefore, research on the aforementioned aspects is quite meaningful and essential for large-scale application of photovoltaic systems.In view of the characteristics of PV energy sources, a novel two stage topology switching grid-connected inverter was proposed in this thesis to widen the range of power generation and improve operating efficiency. Furthermore, the working principle of the inverter, the math model of the PV systems, the control strategy and efficiency optimization were all analyzed in this thesis.First, The specific structure of online topology switching grid-connected inverter based on cascaded inverter was obtained. Two working modes of this inverter were analyzed respectively. Then based on carrier phase-shifting SPWM strategy, multiplex pulse width modulation strategy and online topology switching scheme, adapting to two different operation modes, were proposed.The expression of the total loss and current THD of the online topology switching grid-connected inverter on DC bus voltage and grid current amplitude were deduced. Based on the theoretical results, topology switch ing principle was proposed, which makes the system not only meet the harmonic constraints but also operate with a maximum efficiency.The idea of power independently control was elaborated with its advantages. Based on two operation modes of the system, independently controller schemes were designed. Then the MATLAB models were built, to verify the steady-state and dynamic characteristics of the system.The experimental platform for the grid-connected system was built and the program of DSP and FPGA was designed. Furthermore, the steady-state and dynamic characteristics of the grid-connected system was verified by the experimental results.