Performance Evaluation of Single and Three Phase 21-Level Symmetric Cascaded H-Bride 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 Inverter were analyzed. In designing these inverters, a suitable level shifted high frequency pulse width modulation technique (POD PWM) was selected to meet IEEE519 total harmonic distortion requirement (of at most 5% for system having bus voltage up to 69KV) which necessary for inverter deployment for renewable energy applications. The analysis was carried out using high frequency Phase Opposition and Disposition (POD) level shifted modulation technique where a 50Hz sinusoidal modulating signal is superimposed on 1kHz triangular carrier wave both of which have same peak amplitude but different levels. The effect of variation of modulation index (MI) on percentage total harmonic distortion (%THD) was also carried out using Discrete Fast Fourier transform (DFFT) in MATLAB SIMULINK and the result shows that the system best performance occurred when it was modulated at 100% as minimum THDs were obtained for single phase (of about 5.65%) and three phase (of about 3.68%) application. Also, it was observed that the THD increased as the modulation index varies above and below it optimum value (100%). It was also validated that the Three phase 21 level inverter has better THD (of about 3.68% for A-phase, 3.55% for B-phase and 3.64% for C-phase,) as compare to single phase 21 level inverter (having a THD of about 5.65%), which is due to natural cancellation of triple odd order harmonics due to the symmetry involve in the inverter output voltage waveform as reported in the literatures.