Research On Distributed Photovoltaic Grid-Connected Micro Inverter

It is imminent to solve energy crisis and environmental problems which relates to the social sustainable development and the survival of mankind. It is a necessary to keep developing renewable energy. Since solar energy is a kind of clean, enormous, almost inexhaustible energy, it has gotten the support from the society. And photovoltaic power generation is one of the main approaches to utilize solar. In addition, the rise of global energy Internet has higher technical requirements of photovoltaic power generation in secure operation of distribution network.Photovoltaic power generation is divided into off-grid and grid connected power generations. The former requires energy storage technology, and the latter requires high quality grid-connected current. The power range of micro grid-connected photovoltaic(PV) inverter is between 180-1000W, which is specific to other forms of low MPPT efficiency PV grid-connected inverter caused by occlusion. The micro-inverter can ensure that each device is tracking the maximum power point, and does not interfere with each other, which maximizing the efficiency of whole system. Compared with the megawatt photovoltaic power inverter, micro-inverter is easy to install plug and play and installed locally. Each micro-inverter installed of communication systems that can monitor the operation of each device and the entire system, so you can quickly identify the faulty micro-inverter and is high reliability with high data redundancy.In this paper, micro-inverter adopted two-stage topology. The aims of the front DC/DC circuit are low voltage-input in wide range, high voltage-gain, high efficiency. Full bridge LLC resonant circuit realizes primary side of the transformer ZVS opening, and vice side rectifier operate without diode reverse-recovery, which improves the efficiency generally. Then front DC/DC circuit applies full bridge LLC resonant circuit. Firstly, the FHA (Fundamental Harmonic Approximation) method is applied to analyze LLC resonant characteristics. Secondly, a new resonant parameter design method was proposed to improve the efficiency of system through building the small signal modeling and simulation by Simplis software. A single voltage control loop is designed. The next part of this paper analyze post-stage circuit of micro inverter. The single polarity modulation of full bridge inverter exists leakage current problems, and single polarity modulation H6 bridge has current zero crossing distortion problem. In order to solve the two problems and realize the reactive power compensation, this paper puts forward a hybrid modulation based on H6 bridge inverter circuit. Then simulation is given to verify the theory. Then this paper analyzes several grid phase-locking strategy and compares their advantages and disadvantages by simulation. The following discussion is specific to current loop controller of the inverter. Maximum power point tracking technology and island detection technology are introduced which optimize single-phase PV grid-connected system, and analyzes the several solutions to DC voltage breakdown. Finally, the paper designs the whole system design scheme, and the experiment result of the LLC resonant circuit is given which verified the theory.