Research On The Pulse Width Modulator For A Three-Level Inverter

The combination of new high voltage high power switching devices, advanced control strategies and multilevel topology is an effective approach and tendency to make high voltage and large capacity converters. In this paper, the Three-level Neutral Point Clamped (NPC) inverter and its pulse width modulation (PWM) technique is discussed. Space vector modulation algorithm and random PWM for Three-level inverter in the purpose of noise suppression is investigated, which is proved the feasibility by simulation.First of all, the application prospect of multilevel inverter is reviewed. The developing tendency of PWM technology is summarized. The SVPWM method for Diode-clamped Three-level inverter is introduced. Besides, several PWM methods are simulated, and the SPWM in-phase and SVPWM control methods are especially compared by characteristic of harmonic.Secondly, the traditional random number generator is improved and optimized, and a novel multi-level pseudo-random number generator is proposed to increase the uncertainty of random sequence .The uniformity and independence of the random sequence is verified. As the traditional RPWM is analyzed and assessed, a randomized space vector switching scheme for three-level inverters is reported. Based on the traditional space vector concept for PWM inverter, the sequence of the space vector is randomized in this scheme. The overall vector is mathematically identical to that of the traditional space vector. At the same time, the steady-state link capacitor voltage balancing algorithm is discussed.Finally, the most important statistic characteristic of random signals- power spectral density (PSD) is derived. The effectiveness of subdividing basic vector RPWM is confirmed at variable voltage variable frequency. Characteristic of the line voltage's PSD for several random space vector modulation algorithms are comparatively analyzed. Furthermore, the complexity of random modulation in single period is discussed.