Enhanced Fault-Tolerant Operation Strategy for Cascaded Vienna Front-End Based Solid-State Transformer

Solid-state transformer (SST) based on Vienna cell features with less semiconductor devices and high-power density, providing a promising concept for super charging stations and uninterruptible power supply for data centers. However, the presence of numerous switching devices in SSTs raises the possibility of faults. Bypassing faulty cells not only adds to the cost by requiring additional bypass switches but may also result in a loss of system power capacity. To address the above problems, this article proposes a fault-tolerant operation strategy for Vienna-based SSTs. The proposed strategy allows all faulty cells to continue operating, thereby avoiding system power capacity loss. First, an improved zero-sequence voltage injection method, conforming to Vienna characteristics, is presented to address issues of three-phase unbalance and current distortion. Second, an extra power control mechanism for the dc–dc stage is developed to regulate the capacitor voltages of the faulty cell, ensuring that the capacitor voltage of SST can be balanced to the rated value. Furthermore, an enhanced modulation based on a sort algorithm is introduced to match the configuration of faulty cells, preventing the mismatch between the reference voltage and the output voltage. Experiments are presented to prove that SST can operate continuously with the proposed fault-tolerant control.