SPWM Control Scheme for CHB-MLI with Minimal Voltage THD

Abstract

Various level shifting-sinusoidal pulse width modulation (LS-SPWM) control schemes for the five-level cascaded H-bridge multilevel inverter (CHB-MLI) are presented in this paper. A five-level multilevel inverter working principle is explained with switching table. Phase disposition (PD), phase opposition disposition (POD), and alternate phase opposition disposition (APOD) modulation schemes are compared among them. Output voltage total harmonic distortion has been taken as a quality measuring constraint and the THD is measured in two ways, those are Nyquist form and maximum to maximum frequency form. As two constraints can be varied in the SPWM control schemes, they are modulation index and carrier frequency. First the carrier frequency has been kept constant at 3150 Hz and modulation index has been varied from under modulation to over modulation and noticed that at what modulation index the obtained THD is minimum for each control scheme. In a second step at critical modulation the carrier wave frequency has been varied and found that at which frequency the obtained THD is minimum for each control scheme. All the results are executed on Simulink platform and are tabulated and compared. With SPWM-APOD the minimum THD is obtained as compared among all the modulation schemes. Among the all carrier-based pulse width modulation (PWM) schemes level shifted PWM schemes are the best suitable for the CHB-MLI. These control schemes produce output pulses which is symmetrical to sinusoidal. Phase disposition (PD), phase opposition disposition (POD), and alternate phase opposition disposition (APOD) modulation schemes are well-known control schemes in the level-shifted PWM schemes. In this paper, a single-phase five-level CHB-MLI is simulated in MATLAB/Simulink software and a detailed comparison of these three modulation schemes simulated at various modulation indexes with fixed carrier frequency and simulated with different carrier frequencies with fixed modulation index at 1 (m = 1) with reference to total harmonic distortion (THD) in the output voltage. It is observed that all the modulation schemes produce the sinusoidal output voltage with minimum THD with under modulation index only (m < 1) and the APOD scheme produces minimum THD as compared with other two schemes. All the modulation schemes produce minimum THD at higher carrier frequencies.