Simple Vector Calculation and Constraint-Based Fault-Tolerant Control for a Single-Phase CHBMC

The single-phase cascaded H-bridge multilevel converter (CHBMC) stands out as a prominent topology in medium- and high-voltage applications due to its scalability and modularity. However, the CHBMC may encounter multiple open-circuit (OC) switch faults, leading to significant degradation in both dc voltages and grid current. To facilitate fault-tolerant operation postidentification of multiple OC switches, conventional approaches typically involve adjusting phase angles in the pulsewidth modulation. This, however, results in complex control loops for dc voltage balancing and lacks flexibility. This article proposes a simple fault-tolerant control (FTC) technique based on online vector calculation and a constraint mechanism to simultaneously address multiple objectives, including dc voltage balancing, reduction of grid current total harmonic distortion, and even power distribution following OC switch faults in the CHBMC. Depending on the specific OC switches involved, the voltage levels of healthy or faulty power cells are automatically calculated and adjusted using vector constraints to maintain balanced dc voltages and mitigate current harmonics. Subsequently, the resulting vectors are flexibly determined to meet the requirements of reduced voltage stress and even power distribution. This technique is independent of voltage levels and easily extended to the single-phase CHBMC with more faulty power cells. Experimental results validate the effectiveness of the proposed FTC technique.

I. Introduction

The cascade H-bridge multilevel converter (CHBMC) is widely employed in high-voltage/power applications such as photovoltaics, solid-state transformers, and storage systems due to its modularity, flexibility, and high fault tolerance [1], [2], [3], [4], [5]. However, the system faces substantial challenges due to power switches with a heightened risk of failure. Unlike short-circuit switch faults that necessitate hardware detection and promptly lead to an immediate system breakdown, open-circuit (OC) switch faults, mainly caused by the bond wire lift-off or rupture and the gate driver failure [6], usually make the voltage or current distorted. To reduce the damage to the remaining switches and improve the system reliability, numerous online diagnostic techniques for single or multiple OC switch faults have been developed [7], [8], [9], [10]. To minimize downtime costs, the systems are expected to continue functioning even after OC switch faults while preserving the control performance if possible. Consequently, the fault-tolerant control (FTC) design based on diagnostic results is of paramount importance.