Performance Evaluation of Single and Three Phase 21-Level Symmetric Cascaded H-Bridge Multilevel Inverter using Level Shifted POD-PWM Technique

Abstract

In this paper, the performance of simulated Single Phase and Three Phase 21-level Symmetric Cascaded H-bridge Multilevel Inverters was analyzed. In designing these inverters, a suitablelevel-shifted high-frequency pulse width modulation technique (POD PWM) was selected to meet the IEEE 519 total harmonic distortion requirement (of at most 5% for systems having bus voltage up to 69 kV), which is necessary for inverter deployment in renewable energy applications. The analysis was carried out using the high-frequency Phase Opposition Disposition (POD) level-shifted modulation technique, where a 50 Hz sinusoidal modulating signal was superimposed on a 1 kHz triangular carrier wave. Both signals had the same peak amplitude but different levels. The effect of variation in the modulation index (MI) on the percentage total harmonic distortion (%THD) was also carried out using the Discrete Fast Fourier Transform (DFFT) in MATLAB SIMULINK. The results show that the system's best performance occurred when it was modulated at 100%, as minimum THDs were obtained for single-phase (approximately 5.65%) and three-phase (approximately 3.68%) applications. Also, it was observed that the THD increased as the modulation index varied above and below its optimum value (100%). It was also validated that the Three-phase 21-level inverter has better THD (approximately 3.68% for A-phase, 3.55% for B-phase, and 3.64% for C-phase) compared to the single-phase 21-level inverter (having a THD of approximately 5.65%). This is due to the natural cancellation of triple oddorder harmonics, attributed to the symmetry involved in the inverter output voltage waveform, as reported in the literature.