A New Modulation Technique for A Three-Cell Single-Phase CHB Inverter with Un-Equal DC-Link Voltage for Improving Output Voltage Quality

Abstract

This paper investigates a new modulation technique for a three-cell single-phase CHB (Cascaded H-Bridge) inverter with un-equal DC link voltage. The proposed PWM technique is verified using Matlab/Simulink. The applied DC-link voltages of inverter are 6 V, 12 V and 24 V respectively (DC-link voltage ratio of 1: 2: 4). The result is compared with the CHB inverter with equal DC-link voltage (using DC-link voltage of 12 Volt). The simulation results show that the output voltage using DC-link voltage ratio of 1: 2: 4 produces 15 levels DC voltage, while equal DC-link can only generate a 7 voltage levels. As the number of level in the output voltage increases, the quality of the voltage improves significantly. This is indicated by the reduction of THD percentage from 13.22% to 5.265% that can be seen from the harmonics spectrum of the three-cell CHB inverter using conventional and new PWM. The harmonics spectrum also shown that the output voltage of the three-cell CHB inverter using new PWM is almost free from harmonics, while using the conventional PWM, the 3rd, 5th and 7th harmonics and some higher harmonics are still appeared. Another indicator is carried out by comparing the percentage of low order harmonics content up to the 19th harmonic (950 Hz) of the output voltage. It is noted that there is a significant reduction of the low order harmonics from 3.25% to 0.518% achieved using the new PWM method. To sum up, the new modulation technique enables the inverter to produce better output voltage quality.