A hybrid modulation scheme for cascaded h-bridge inverter cells

Project Abstract

This research project proposes a novel hybrid modulation scheme for cascaded H-bridge inverter cells in order to improve the performance of multilevel inverters. The cascaded H-bridge inverter structure is widely used in high power applications due to its scalability and ability to generate high-quality output waveforms. The proposed hybrid modulation scheme combines the advantages of both phase disposition (PD) and level shifted pulse width modulation (PWM) techniques to achieve better harmonic performance and reduced switching losses. The PD modulation technique is efficient in terms of harmonic distortion reduction but can lead to increased switching losses, especially at higher modulation indices. On the other hand, level shifted PWM helps to balance the voltage levels of the H-bridge cells and reduce common-mode voltage, but it may not be as effective in harmonic suppression as PD modulation. By integrating these two modulation techniques in a hybrid scheme, the proposed method aims to mitigate the drawbacks of each method while enhancing their benefits. The hybrid modulation scheme operates by dynamically switching between PD and level shifted PWM based on the modulation index and load conditions. At low modulation indices, PD modulation is favored to minimize harmonic distortion, while at higher modulation indices, the level shifted PWM is employed to reduce switching losses and maintain balanced voltage levels across the H-bridge cells. The switching between these two modulation techniques is seamlessly controlled to ensure optimal performance under varying operating conditions. Simulation studies have been conducted to evaluate the proposed hybrid modulation scheme compared to conventional PD and level shifted PWM techniques. The results demonstrate that the hybrid scheme achieves superior harmonic suppression performance compared to individual modulation methods while also reducing switching losses, thus improving the overall efficiency of the cascaded H-bridge inverter. Additionally, the proposed scheme helps to maintain balanced voltages across the H-bridge cells, leading to improved reliability and reduced stress on the power devices. Overall, the hybrid modulation scheme presents a promising approach to enhance the performance of cascaded H-bridge inverters, making them more suitable for high power applications requiring low harmonic distortion and high efficiency. Further experimental validation is warranted to confirm the effectiveness of the proposed scheme in practical applications.

Project Overview

<p> This work proposes a switching technique for cascaded H-Bridge (CHB) cells. Single carrier Sinusoidal PWM (SCSPWM) scheme is employed in the generation of the gating signals. A sequential switching and base PWM circulation schemes are presented for this fundamental cascaded multilevel inverter topology. With these proposed concepts, it is now possible to generate equal average switching signal patterns in all the constituting power semiconductor switches. This results in equal switching loss dissipation and equal power sharing in CHB multilevel inverter modules; and therefore technically modularizes the cascaded system. A 4-cell cascaded structure has been used to exemplify the proposed switching technique. Outlines with switching functions are given for the proposed modulation strategy. For a modulation index of 0.9, a Total Harmonic Distortion (THD) value of 14.61% has been achieved in the output voltage waveform of the exemplanary 4-cell cascaded configuration. Simulations for single phase cascaded multilevel inverters (five-level, seven-level and nine-level) and their Total Harmonic Distortion (THD) is compared. The THD for the five-level, seven-level and nine-level are 21.92%, 9.51, and 5.30% for different topologies with different modulation techniques for single phase. When compared with the cascaded H-Bridge Cell, there is an improvement compared with the single phase cells. Verification of the performance of the proposed control technique is done through simulations. <br></p>