Modulation Strategies for Thermal Stress Control of CHB Inverters

Cascaded H-bridge inverters have attractive features in terms of availability as well as superior performances of power quality and power conversion efficiency in comparison to nonmodular topologies. However, as the operation time goes by, basic cells of a modular architecture have uneven remaining useful lifetime because of slightly different electrical and thermal parameters causing uneven lifetime degradation of individual cells. From this viewpoint, the lifetime of cells needs to be individually managed, and this shall bring further improved reliability of the entire system and reduced maintenance costs. This article proposes discontinuous modulation strategies, which have ability to reduce the thermal stresses of more aged cells, consequently enabling the individual lifetime management of cells. The proposed modulation strategies are analyzed in terms of capability of loss control and are implemented in a feedback controller to validate the feasibility. In addition, the proposed methods are validated on an experimental test bench.