Research On The Key Problems Of SVPWM For NPC Converters

The neutral point clamp three-level converter has emerged as a very important alternative in the area of high-power medium-voltage energy control. It has many attractive features for high-power energy control, such as lower withstand voltage level for power semiconductors, extremely lower distortion of output voltage compared with conventional two-level converters. But more switches and control demand cause complicated control method. So the PWM control is the key technology in three-level converters.Three key problems of SVPWM for thee-level converters are mainly focuses in this article, including simplifying SVPWM, balancing neutral point potential and designing vector generation. The tree parts are based on introducing operation principle of three-level converters, and traditional SVPWM algorithm. The paper presents a simple three-level SVPWM algorithm with reference voltage vector decomposition, avoiding complex and time-consuming calculations of the traditional algorithms. Neutral point potential unbalance is an inherent problem in neutral point clamped three-level converters. A detailed analysis by vectors and neutral point current about the regularity and principle of the problem is presented. Then some control methods of the neutral point potential are studied. And a neutral point potential balancing control method with balancing factor is proposed in the paper, which is applicable for the simple PWM algorithm. In the method, the balance can be perfectly controlled by the balancing factor. Otherwise, an idea about a vector generator is defined for implementing SVPWM of three-level converters. And then, two schemes of vector generation are compared for selection.The simple three-level SVPWM algorithm and the studied balancing methods are all simulated by Matlab/Simulink. The major subject is comparison among different balancing schemes. The results prove that the strategy based on the balancing factor is outstanding in the methods. Following the conclusion, an exact and minute analysis is performed for the strategy based on the balancing factor in the paper.In the end, a 1.5KW prototype is designed for a three-level inverter. The controller selects a vector generator based on DSP. The simple algorithm and balancing methods are applied in the experiments for verification and comparison. The experiment results show in the article, and verify the feasibility and effectiveness of the global algorithm and the control strategy.