Research On Intelligent Interface And MPPT Of DC Micro - Network In PV System

As a clean and green energy, the development and utilization of solar energy has received more and more attention. An important source of energy of DC micro Grid is photovoltaic power. However, generally the output voltage of the solar panel is low that can not be connected to the high voltage DC bus of DC micro network directly. Therefore, an intermediate interface is needed to connect to the DC bus which is power electronic conversion element. In photovoltaic systems, DC/DC converter is often used as the intermediate interface. DC/DC converter can not only raise the low voltage of photovoltaic cells output to the demand high voltage, but achieve the maximum power tracking of the solar cells, thereby improving the utilization of the battery.In this thesis photovoltaic intelligent interface is mainly composed of solar cells, power circuit, sensor circuit, conditioning circuit, A/D conversion circuit, CPU control board, handheld device and Zigbee interface, etc. Power circuit mainly includes inverter circuit, high frequency transformer and rectifier circuit. The signal acquired from the sensor circuit go to A/D conversion after conditioned by conditioning circuit. A/D conversion output to the CPU for the corresponding operation and control.In the thesis, the current situation and development of the DC micro grid, the characteristics and the development of photovoltaic DC/DC controller and analyzes the working principle of phase shifting full bridge ZVS PWM DC/DC full bridge converter are first described. On the basis of the analysis and research on the topology and control method of the phase shifting full bridge DC/DC converter, the PSIM phase shift simulation modules is set up to prove it.Aiming at the magnetic bias resulted from the DC component produced in the primary winding of transformer due to the inconsistent of characteristics in the phase-shifted full bridge DC_DC converter, the digital suppression of magnetic bias has been proposed. And the asymmetric digital phase shifting method in DSP has been presented as well. Simulation and experiment results show that good results are obtained.In addition, the system first need to stabilize the voltage to the set value 600V before incorporating into the DC bus to effectively prevent grid voltage to be driving down at the moment that incorporating into the DC bus, the inertial link is added to the boost phase to effectively control the voltage overshoot which makes the whole system operating safely and effectively.Finally, on account of the issues that the voltage characteristics of photovoltaic cells has a strong nonlinearity, the load adaptability is poor and the output power of the solar cell changes with the light intensity and temperature, etc. A nonlinear-control maximum power point tracking(MPPT) algorithm with sine perturb based on incremental conductance method is proposed. Simulation and experimental results show that this method works well.After the completion of design and debugging the CPU control board based on TMS320F28335DSP, the power board, condition board and the sensors sampling board are made. The PWM phase-shift program, the A/D conversion program and the grid-connected program are designed and debugged. Based on the requirements of Zigbee communication in DC microgrid, corresponding communication protocol and serial communication program are designed.