Modified wavelet-based pulse width modulation technique for cascaded H-bridge multilevel converter

Abstract

Power electronics (PE) converters are crucial for providing a cost-effective, reliable, and efficient solutions for integrating renewable energy sources (RES) into the electrical grid. Over the years, many converter topologies have been developed for low, medium, and high voltage applications. However, they work more efficiently when appropriate modulation techniques are used. Many pulse width modulation (PWM) methods have been presented to decrease output harmonics, such as carrier-based (PWM methods, selective harmonic elimination (SHE), and nearest-level modulation (NLM). Regardless, these methods raise the complexity and expense of the converter, reduce the fundamental component, and increase high-frequency harmonics in the output signal. In the present work, a novel wavelet-based PWM (WPWM) method is developed for a multilevel converter. This mathematical modulation technique reduces the harmonic content at both low and high frequencies, improves the fundamental component in the output, and reduces switching losses. However, in multilevel converters, the gate pulses generated by WPWM are designed to only shape the output voltage without considering the load balancing between the DC sources or split capacitors. Hence, an additional load-balancing algorithm is necessary. This work proposes a new phase-shifting WPWM method that naturally balances load sharing between all the DC sources or split capacitors in the multi-level converter. This method operates at a low switching frequency, thereby keeping switching losses low, and reducing total harmonic distortion (THD). Moreover, since the proposed method is a mathematical closed-form PWM method, it can be evaluated within a finite number of iterations as compared to the open-ended SHE method. The proposed method is simple and runs efficiently in real-time, which enables fast system dynamics during transient conditions. Also, it does not depend on a minor computational time-step. Hence, it can be implemented on a low-cost digital controller. The validity of the proposed method is validated both by MATLAB/Simulink model, LTSpice model, and experimental tests. The results are discussed and compared against other PS-PWM methods to demonstrate the advantages of the proposed method.