Improved Real and Reactive Power Balance in a Nine-Level Grid-Tied CHB Inverter Using Carrier-Reassignment PWM

In a cascaded H-bridge (CHB) multilevel inverter, achieving uniform power distribution across modules remains a well-known challenge. The commonly used level-shifted carrier pulse width modulation (LSPWM) results in a wide disparity in the power distribution between modules, leading to premature failures in over-stressed modules. Two new Carrier Reassignment PWM (CRPWM) techniques (Type-A and Type-B) improve upon these shortcomings for operation over the full power factor range with minimal computational overhead and can be easily integrated into existing control schemes. This article presents the complete hardware implementation and performance validation of Type-A and Type-B carrier reassignment PWM techniques, along with FIFO-CRPWM and LSPWM under identical conditions for comparison. A 9-level CHB inverter prototype is built and operated in closed-loop grid-connected mode with synchronous reference-frame based control (dq control) over the entire power factor range, from unity power factor, down to zero power factor. The real and reactive power delivered by each module is measured for each operating condition to demonstrate the efficacy of the proposed schemes. The carrier reassignments are also formulated as generalized matrix transformations suitable for fast digital implementation with an inexpensive device such as a programmable logic device or field programmable gate array.