Research On High-efficiency PV Grid-connected Microinverter With Wide Input Voltage

Renewable energy generation has been becoming one of effective measures to solve energy crisis. Among a variety of the renewable energy, solar energy is predicted to become the biggest contributor to take the place of the traditional fossil fuels because it is an inexhaustible clean energy. Grid interactive PV is an important way of utilizing solar energy. The AC-Module has become a future trend for grid-connected photovoltaic(PV) power system for its removal of energy yields mismatches between PV modules and "plug and play" concept.Photovoltaic micro-inverter(PVMI) is a key apparatus of AC-Module system. PVMI is strict in efficiency and power density, moreover it is often supplied by a wide and low voltage input, which requires a high-voltage step-up ratio to produce desired output AC voltage. Therefore, the paper’s PVMI is composed of two stages. A full-bridge LLC resonant converter is used at the first stage to step up the PV voltage to the high voltage dc-link. A full-bridge inverter is used at the second stage to output synchronized sinusoid current to the gird.LLC resonant converter commonly uses variable-frequency control. However, the wide voltage input will lead to wide frequency range. It is a problem to design circuit and build control system. The paper proposes a hybrid control strategy which combines variable-frequency(VF) with phase-shift(PS). The operational principles and the basic character of LLC resonant converter are analyzed under VF control and VF-PS control, and gain formula of input and output voltage is deduced. The paper also discusses the proportion of VF factor and PS factor in control process.The full-bridge inverter output synchronized sinusoid current to the grid with deadbeat control method. The deadbeat control is analyzed deeply, and the switch duty cycle control equations are deduced. To the problem that deadbeat control only be used under continuous current mode of inductor, and considering the THD of output current and CEC efficiency or EU efficiency of PV grid-connected inverter, in the paper we propose a control method to ensure the inductor operate at continuous current mode, which requires change the switching frequency accord to the value of the inductor current. The inverter can balance between efficiency and current distortion.Finally, a 250W PVMI prototype with Dspic33fj32gs606 controller is designed and tested in the lab. Experiment results are provided to verify the control strategy is effective and performance of the circuit output current, power factor and the switching frequency range. Meanwhile, the peak efficiency of prototype reaches 94.5% and the CEC efficiency reaches 93.5%.