A Fault-Tolerant Approach for Simultaneously Maximizing Post-Fault Voltage and Minimizing Common-Mode Voltage in Cascaded H-Bridge Multilevel Converters

This paper introduces a new fault-tolerant method for cascaded H-bridge (CHB) multilevel converters based on Selective Harmonic Mitigation (SHM) technique. In the proposed method, balanced line-to-line voltages are achieved by injecting a common-mode voltage (CMV) into the unbalanced phase voltages when the number of healthy cells differs among phases. The injected CMV allows maximum phase utilization using minimum number of healthy cells. The suggested method imposes new constraints on SHM switching angles to avoid over-modulation. It also maximizes line-to-line voltage by injecting an appropriate CMV to the modulation waveforms. However, the fundamental component of CMV (FCMV) can cause uneven power distribution among CHB phases, leading to power flow-back in the inverter, which is problematic for non-regenerative CHB inverters as it increases the dc-link voltage of the cells. To avoid power flow-back during faulty conditions, minimizing FCMV is included as an additional objective in the SHM modulation technique. Simulation and experimental results confirm the effectiveness of the proposed fault-tolerant method.