Research And Desige Of Micro-inverter Grid-connected Photovoltaic Power Generation Systems

As the world is more and more concerned with fossil fuel exhaustion andenvironmental issues, photovoltaic (PV) power generation systems are becoming afocal point of the utilization of renewable resources, both politically andeconomically. Particularly, the micro-inverter, which is considered as an attractivesolution for the grid-connected PV systems, has become a hot research topic due toits inherent advantages, such as the energy harvest maximization, long life-span,enhanced reliability and installation labor costs reduction. Focusing on the presentcharacteristics and main problems of the micro-inverter, this thesis makes a deepresearch on the micro-inverter’s topology structure and control methodology.Based on the technical requirements of grid-connected PV inverters, this paperproposes a three-stage micro-inverter topology, which consists of the BOOSTconverter, the active-clamped flyback converter and the SCR full-bridge converter.This topology has the features of electrical isolation and utilizes film capacitors toprolong the micro-inverter’s lifetime. The detailed operation principle and designconsiderations of key parameters are presented. In order to guarantee the ZVScondition of the main switch, a non-complementary active-clamped control methodis applied. Depending on the grid voltage, an optimized control method of theactive clamp switch is adapted so as to further improve the conversion efficiency.Because of the small capacitance of film capacitor, the voltage regulation ofdc-link capacitor is much more difficult than traditional inverters with electrolyticcapacitors. In this paper, a new voltage regulation strategy based on hysteresismethodology is proposed, which can keep the dc-link voltage stable both in steadystate and dynamic state, and export a smooth current with low oscillation to the grid.In order to inject a high quality ac current to the grid, this paper determines thecontrol parameters by establishing small-siganl model of the active-clampedflyback converter. To overcome the energy wastage caused by misjudgment andslow searching speed, a three-point weight comparison and variable step MPPTalgorithm combined with a dc current sensorless calculation method is adopted,which can eliminate the dc current hall sensor and reduce the cost of micro-inverter.Finally, the experimental study of the micro-inverter PV power generationsystem is developed based on a200W prototype. Experimental results verify thevalidity of the theoretical analysis and the feasibility of the control strategy.