Research On The Grid-connected Photovoltaic System Based On Cascaded Multilevel Inverter

As a kind of clean, well-resourced and green renewable energy, solar energy has become the focus of human exploitation. Grid-connected photovoltaic (PV) generation technology can transform solar energy into grid-connected electricity, and it is development trend of PV generation in the world.This thesis analyzes the study status and development prospects of PV system in the world, and compares the topology and control strategy of inverter in PV system. Then PV grid-connected system with cascade multilevel inverter is selected as the study object. The maximum power point tracking (MPPT), the cascade inverter control strategy, and islanding detection method of the system are all further studied in this thesis. Cycle Carrier Disposition PWM (CCDPWM) control method for cascade inverter is proposed, and it can reduce the total harmonic distortion (THD) of the output voltage and avoid unbalanced power among units which is caused by common Carrier Disposition PWM (CDPWM) method. The main contents and achievements are as follows:(1) Based on the V-I (voltage-current) characteristic of PV arrays, this thesis focuses on the Incremental Conductance control method of the maximum power point tracking, and proposes a kind of multi-connection DC/DC converter which is suitable for PV system with a cascaded multilevel inverter.(2) Based on studying cascaded multilevel topologies and control strategies, Cycle Carrier Disposition PWM (CCDPWM) control method is proposed, and Matlab/Simulink is used to simulate this method. It can reduce the THD of the output voltage and avoid power imbalance among units which is caused by common CDPWM.(3) For the structure of two-stage PV system and the characteristics of cascade multilevel inverter, it uses an improved double-loop control strategy in the grid-connected PV system with cascade multilevel inverter, including the inner current loop, the outer voltage loop, and MPPT tracking control loop. This control method can reduce the output current harmonics, and make the output current and grid voltage with the same frequency and phase.(4) In this paper, TMS320F2812 is the main control chip, but it can only generate 16 PWM control signal, which cannot meet the needs of 11 level cascaded inverter in PV grid-connected system. In order to provide multiple PWM control signal, a combination of DSP and FPGA method is designed, and software solutions of cycle multi-carrier control method is designed.