Hybrid Cascaded Three-Level H-Bridge Converter for Solid-State Transformer With Enhanced Fault-Tolerant Capability of Submodule Failures

Solid-state transformers (SSTs) based on three-level converter cells feature fewer semiconductor devices and achieve higher power density, making them attractive for medium-voltage photovoltaic and energy storage systems. This paper proposes a hybrid cascaded SST topology composed of T-type and Vienna three-level cells, which combines the control flexibility of the T-type cell with the cost advantages of the Vienna cell, thereby enabling low-cost operation under non-unity power-factor conditions. However, the high count of switching devices in SSTs increases the risk of faults. To address this, three degrees of freedom for fault tolerance are first identified: (1) output-voltage reconfiguration of faulty submodules; (2) coordinated voltage and power control of the front-end and DC-DC stages; (3) front-end modulation strategies to match reconfigured faulty submodules. Based on these, a unified fault-tolerant method is proposed, enabling the Hybrid SST to maintain fault-tolerant operation under various modes and fault types. Notably, the proposed multi-degree-of-freedom coordination strategy is applicable to the Hybrid SST with arbitrary ratios of the two submodule types. Furthermore, the proposed SST is compared with an SST utilizing only Vienna cells, which proved that the Hybrid SST has enhanced fault-tolerant capability. Experimental results validate fault-tolerant operation under different faults without capacity loss or capacitor-voltage imbalance.