Phase-Shifted Suboptimal Pulse-Width Modulation Strategy for Multilevel Inverter

Multilevel inverter has been widely investigated for high-power applications in recent years. Various topologies and modulation strategies have been proposed for utility and drive applications in the literature. There are three main types of transformerless multilevel inverters proposed so far, diode-clamped inverter, flying-capacitor inverter and cascaded inverter. The cascaded inverter has drawn tremendous interest. The most attractive features of cascaded multilevel inverters are as follows: firstly, they require the least number of components to achieve the same number of voltage levels. Secondly, modularized circuit layout and packaging is possible and there are no extra clamping diodes or voltage balancing capacitors. Various multilevel PWM methods have been proposed during the past two decades. They can be classified into two catalogues, carrier-based PWM method and space vector PWM (SVPWM) method. Carrier-based PWM is superior to SVPWM for five-level converter and above. In order to overcome the disadvantages of low modulation index and fundamental component existing in subharmonic PWM (SHPWM) and phase-shifted PWM (PSPWM), a new triangular carrier-based phase-shifted suboptimal PWM (PS-SUB-PWM) suitable for cascaded multilevel converters is proposed in this paper. The triangular carriers in this method are same as that in the PSPWM method, but the modulation waveform is modified by adding the 3^rd , or the 6^th, or the 9^th harmonics and so on to it. Injecting harmonics into modulation waveform makes modulation index and fundamental component increase. Through analysis of PS-SUB-PWM strategy, the parameters of optimal wave and switching angles are derived. The simulation results indicate that the proposed method is valid