PSO-Optimized SHE-PWM Technique in a Cascaded H-Bridge Multilevel Inverter for Variable Output Voltage Applications

One of the problems of the selective harmonic elimination pulsewidth modulation (SHE-PWM) method is a limited range of feasible solutions. In many practical applications, the inverter is required to produce a variable output voltage within a wide range (e.g., 0.1 to 1). When the inverter operates in a low modulation index (low output voltage), the output harmonic distortion increases. This article introduces a method to use the SHE-PWM technique for a wide range of modulation indices, which allows achieving a high-quality output waveform in a cascaded H-bridge multilevel inverter. In a five-level case, a dc-dc converter regulates the dc-link voltage of only one bridge. In this way, the extra hardware requirement is reduced, and also a higher number of voltage levels is achieved, which improves the waveform quality. The particle swarm optimization algorithm is used to solve the SHE problem for different output voltage values. Two strategies are proposed to obtain the variable dc-link voltage; one of them is suitable for the lower output voltage values and the other one is more suitable for high values of the output voltage. The proposed strategies have been tested through simulation and experimental studies in different conditions. The results indicate a considerable improvement in the output waveform quality.