MPPT study on photovoltaic microinverter using hybrid LLC resonant converter
In order to solve the problem of energy shortage and environmental pollution, solar energy as a renewable, pollution-free new energy has been widely concerned around the world. As a kind of photovoltaic power generation system architecture, AC module architecture adopts single panel power supply, has the advantages of strong anti-local shadow ability, high panel utilization, plug and play and high reliability, has become one of the important photovoltaic power generation architecture. Due to the characteristics of AC modular architecture, it has strict requirements on cost and efficiency and wide input voltage range. Therefore, one research direction of AC modular photovoltaic power generation is to reduce the cost of converter and improve energy efficiency. LLC converter with hybrid control strategy is one of the ideal topologies of AC modular photovoltaic power generation architecture: it can meet a wide input voltage range, ZVS of primary side switch tube and ZCS of secondary side diode, with high efficiency, wide gain range and electrical isolation. Photovoltaic cells have nonlinear characteristics, and their output characteristics are affected by lighting, temperature, load and other operating environment parameters. Therefore, maximum power point tracking (MPPT) technology is one of the key technologies of photovoltaic power generation, which constantly adjusts the output power point of photovoltaic cells according to external conditions so that it always works near the maximum power point. Based on the research of maximum power point tracking technology and LLC converter with hybrid control strategy, a pre-stage iterative MPPT algorithm is proposed to collect the back-stage power variables. Under the premise of not using the pre-stage isolated sampling circuit to sample the input variables, the maximum power point tracking of photovoltaic panels is realized. Firstly, the basic principles of photovoltaic panel and LLC converter with hybrid control strategy are described and modeled. The LLC converter is combined with the photovoltaic panel to analyze the working state of the LLC converter as the photovoltaic cell at the power supply end, and analyze the influence of frequency and shift on the output power point of the photovoltaic cell. Then, various MPPT algorithms are analyzed and compared. According to the characteristics of LLC converter, the iterative MPPT algorithm adopted in this paper is proposed, and its advantages and disadvantages are analyzed. On the basis of the above analysis, the iterative MPPT algorithm is further improved to improve the tracking speed and reduce the power loss in the tracking process. The iterative MPPT algorithm and its improved algorithm based on hybrid control LLC converter are simulated and verified by using simulation software. Finally, according to the design requirements, a full digital control micro-power photovoltaic inverter with maximum power point voltage range of 25V ~ 39V and full load power of 250W is developed. The experimental prototype is applied to the photovoltaic power generation field with wide voltage range. The prototype has excellent performance and good MPPT effect, and the effectiveness of the adopted method is verified by experiments.