Research On A Novel Three-Level Converter Topology And Its Control Strategy

Recently, neutral-point clamped three-level converter has become a significant subject in medium and high voltage frequency conversion field. Among many researches on the converter, improving power factor, reducing harmonics and balancing neutral-point potential are the key problems. In order to solve these problems, a novel three-level converter topology and its control strategy are proposed in this paper. The main research purpose of this paper is to improve power factor, reduce harmonics and balance neutral-point potential.The proposed converter, with half power switches at rectifier, improves the conventional'back to back'diode-clamped three-level converter. In order to improve power factor, reduce current harmonics and stabilize dc voltage, current and voltage loops control structure and hysteresis current PWM control strategy are presented in this paper.Neutral-point potential fluctuation must be controlled, or else converter can not work normally. The conventional solution is to implement neutral-point potential balancing (NPB) at inverter, whose drawback is the output performance will be decreased. To solve the problem, a NPB strategy with hysteresis current PWM is proposed in this paper. Because the proposed NPB strategy is implemented at rectifier, it will not decrease the output performance of converter.To improve output performance further, space vector pulse width modulation (SVPWM) is adopted at inverter, and sector division, vector selection, vector time fast calculation method of three-level SVPWM are researched. At the same time, space vector groups are optimized, the optimized SVPWM can guarantee output performance of converter does not fall when switching frequency properly decrease.Lastly, a three-level inverter based on TMS320F2812 DSP controller is designed and accomplished. Simulation and experiment results show that the proposed three-level converter and its control strategy are effective, can reach the goals of improving power factor, reducing input and output harmonics at medium and high voltage level.