Research On Grid-Connected Photovoltaic Micro-Inverter Based On Interleaved Flyback

The use of renewable energy has been attached more and more importance due to the energy shortage and environmental pollution worldwide, solar energy is clear and widespread which is highly valued and photovoltaic power generation is increasing rapidly. Compared to centralized photovoltaic technology which is not that suitable in business and residential application, modular photovoltaic inverter meets the requirements of the market which is boosted by the policy of distributed power generation. With its remarkable superiorities of high conversion efficiency, flexible extend, simple installation and maintenance, redundant design, micro-inverter is expected to provide supports of technologies and devices for distributed power generation.Photovoltaic grid-connected micro-inverter base on interleaved flyback is researched in this thesis and related features and control strategies are analyzed under CCM. The main work is as follows.(1)Analytical expression of primary and secondary current in switching period is established under CCM and critical continuous output power is calculated. Analytical results indicate that micro-inverter runs under both DCM and CCM in half grid period. The concept of Continuous Rate(CR) is defined to weigh the proportion of CCM areas in global areas.(2)Parameters of key components in the main circuit are designed and selected, including power decoupling capacitor in the DC side, flyback transformer, power electronics and grid filter, which are the parameter bases of the control strategies designing.(3)Duty cycle to output current transfer function is established base on arbitrary quasi steady state point, Analytical results show that the worst effect to control performance of RHP zero happens at the situation of nominal power alongs with peak grid voltage and a Q-PR controller of current loop is designed base on this quasi steady state point.(4)A kind of improved MPPT algorithm is presented by means of division and combination, namely, open-circuit voltage algorithm is applied at the startup, variable step P&O algorithm is applied when the power fluctuation of PV panel is small and to the contrary short-circuit current algorithm is applied. Both steady state tracking accuracy and dynamic tacking performance are taken into account by this improved MPPT algorithm. A kind of dual-mode control strategy and mode selection controller is proposed to deal with high THD of output current at light load caused by the degraded adaptability of system controller of which designed under CCM. System control mode is switched to DCM control mode at light load, thus improving output current quality.(5)Simulation model by Matlab/Simulink and250W micro-inverter prototype are built, both simulation and experiment results verify the correctness of analysis and effectiveness and feasibility of system control strategy.