Research Of Current Control And Efficiency Analysis Of Grid-connected Voltage Source Inverter Based On Neutral-Point-Clamped Three-level VSI

In recent years, the situation of environment pollution and global energy crisis hasbeen more serious, and the whole world pays much more attention on exploiting andutilizing the solar energy. The neutral-point-clamp three-level inverter has emerged as asignificant alternative in the area of high-power medium-voltage energy control, which hasmany attractive features, such as lower withstand voltage level for power semiconductorsand extremely lower distortion of output voltage compared with conventional two-levelinverters.This thesis has studied the topology of three-level-inverter, and discussed the and thecontrol principle of the three-level-inverter. According to principle of two-level inverter,this paper has discussed some kinds of pulse width modulation for three-level-inverter andemphasized on Space Vector Modulation algorithm, which is proposed by employing thenearest three vectors as reference voltage vector. The chief factor which influencesneutral-point voltage balance of three-level inverter is deeply studied by analyzing theinfluence of all kinds of vectors to neutral-point voltage and the neutral-point current. Theconclusion which figure out the balance of neutral-point voltage is introduced. A transfermodel of the three-level inverter was established by applying the dual-close-loop controland the state feedback decoupling to three-level inverter, which has favorable dynamic andsteady state performances and sinusoidal output current as well as unity power factor. Bycomparing conduction losses, switching losses of power semiconductors betweenthree-level and two-level inverters, the total losses and efficiency of three-level andtwo-level is contrasted. They are all simulated by Matlab/Simulink.The flow chart of the main program and the timer interrupt subroutine is given as wellas detail process of algorithm. In the end, a6KW prototype is designed and the controller isbased on DSP. The parallel-in control algorithm and balancing methods are applied in theexperiments for verification and comparison. The experiment results showed in the articleauthenticate the feasibility and effectiveness of the control strategy and the neutral-pointvoltage balance algorithm.