A Single-phase Mirco-inverter For Off-grid PV Generation System

Micro-inverter is the core part of the distributed photovoltaic(PV) power generation system, whose work efficiency, output waveform quality, longevity and other factors will directly influence the performance of the system. A micro-inverter, which outputs pure sine wave, has been designed in this thesis.Basing on the PV micro-inverter basic principles, a voltage controlled two-stage structure inverter has been designed. The two-stage structure is divided into two parts, which included DC-DC converter and DC-AC inverter. The former boosts the voltage, and the latter inverts the DC voltage to AC voltage. Firstly, working principles of the pre-stage DC-DC converter and the rear-stage DC-AC inverter have been analyzed in this thesis. Control strategies for the DC-DC converter and the DC-AC converter have been designed, which suits the two-stage circuit structure. Simulation models, including the DC-DC converter, the DC-AC inverter and the integrated micro-inverter, have been established. At last, simulation models have been simulated by the Matlab software.Hardware circuits have been achieved, and control program has been programmed. A circuit structure of an active clamp flyback transformer and a voltage doubling rectifier circuit has been used for the DC-DC converter, and a structure of a full-bridge inverter circuit and a series LC filter circuit has been used for the DC-AC inverter. Driving circuits have been achieved to drive the power switch in the hardware circuits. Detecting circuits, which detecte the PV module output voltage, the DC bus voltage, the AC output voltage and the AC output current, have been achieved to obtain the closed-loop control of the DC-DC converter and the DC-AC inverter. A DSP controller has been used and a C program for the pre-stage DC-DC converter and the rear-stage DC-AC inverter has been programmed to achieve the closed-loop control of the micro-inverter. The control program mainly includes the main program of the pre-stage DC-DC converter and the rear-stage DC-AC inverter, the AD sampling program, the PWM generation program and the SPWM generation program.At last, experiments has been carried in the laboratory, and the output waveform has been analyzed.