Research On The Harmonic Control Technology For Multi-level Inverter

Multi-level inverter can improve the quality of voltage waveform, and also can decrease the harmonic content of output current and the switching losses of device. So it has been widely used. As the interface between the photovoltaic system and the power grid, the inverter is the core of energy conversion and control in the gridconnected PV system. In order to improve the waveform quality of PV system and reduce the harmonic distortion, effective harmonic control method is the key to solve the problem. In this paper, we study harmonic control method of multi-level inverter of photovoltaic system.Firstly, the topology structure and working principle of multilevel inverter is analyzed in the paper. And the output voltage waveform of typical topology structure is analyzed on harmonic. It provides a theoretical basis for simulation analysis. Harmonic control strategy of the traditional inverter has the defect of high harmonic content of output voltage, large total harmonic distortion. Aiming at these problems, the introduction of carrier phase cascade PWM modulation method based on harmonic control strategy of the three-loop control algorithm can eliminate harmonic and improve the output waveform. Harmonic control model of three-level and five-level inverter based on the three-loop control algorithm are built in MATLAB/SIMULINK. The validity of the multi-level inverter and the algorithm is validated by simulation. It illustrates that the five level inverter based on the three-loop control algorithm has a good effect of harmonic control so that harmonic content of the output voltage and current is reduced by comparing the total harmonic distortion of output voltage and current of three-level and five-level inverter.And then, aiming at photovoltaic grid connected system using a three phase fivelevel inverter, this paper use the five-level NPC inverter and carrier phase cascade PWM modulation method to minimize harmonic of the grid current, enhance utilization of the DC bus voltage and improve the waveform quality. The three-loop control method consisted of fuzzy MPPT power outer ring, DC voltage middle ring and the current inner ring is adopted in the system, which can eliminate the DC/DC chopper link. The simulation results proved that it can reduce the total harmonic distortion(THD) of the current of grid system, realize regulator control of the DC bus voltage and fast tracking of MPPT. The validity and feasibility of the proposed system and control method is verified by simulation experiment.Finally, the harmonic control experiments for multilevel inverter are studied and the corresponding detection monitoring interface based on the test bench of photovoltaic solar detection is designed. The module of harmonic control algorithm is integrated into the controller. It validates the generality and practicability of the harmonic control method based on the three-loop control algorithm in multi-level inverter photovoltaic system by controlling multi-level inverter solar power generation system.