Solid State Transformers: A Comprehensive Review of Technology, Topologies, Applications, Research Gaps, and Future Directions

Solid-State Transformers (SSTs), or Power Electronic Transformers (PETs), are emerging as transformative components in modern electric grids, capable of intelligent power flow control, AC/DC interfacing, and multi-level voltage regulation. While SSTs promise substantial advantages over conventional Low-Frequency Transformers (LFTs) in terms of compactness, bidirectional power flow, and integration with renewable energy sources and electric vehicles, their adoption necessitates a critical reevaluation of grid protection paradigms and communication infrastructure. Unlike passive LFTs, SSTs contribute minimal fault current due to fast-switching semiconductors, challenging conventional protection schemes based on overcurrent detection. Furthermore, their deployment requires robust, low-latency communication frameworks to coordinate with utility systems, raising pressing concerns regarding protocol standardization and cybersecurity resilience. This review advances the state of SST literature by offering a thematic and evaluative perspective—one that synthesizes converter-level advancements with system-level integration challenges. Specifically, we critique current SST architectures through a multi-criteria lens involving efficiency, cost, protection compatibility, and fault ride-through, supported by comparative matrices and taxonomy frameworks. A novel contribution of this work lies in identifying the disparity between component-level maturity and system-level readiness, especially in fault isolation, thermal resilience, and coordinated control. Rather than a broad technical survey, this paper adopts a focused perspective on SSTs as enablers of hybrid AC/DC smart grids. It emphasizes key innovations—such as advanced modulation for fault limitation, grid-compatible communication protocols, and modular multilevel topologies and maps them against evolving utility requirements. In doing so, we bridge the gap between technical feasibility and operational viability and propose a future research roadmap aligned with practical deployment milestones. The synthesis culminates in a revised classification of SST readiness for distinct grid applications and outlines unresolved technical bottlenecks that warrant targeted investigation.